EPA Wastewater Treatment for A Single House

WASTEWATER TREATMENT MANUALS

TREATMENT SYSTEMS for

SINGLE HOUSES

ENVIRONMENTAL PROTECTION AGENCYAn Ghníomhaireacht um Chaomhnú Comhshaoil

P.O. Box 3000, Johnstown Castle Estate, Co. Wexford, Ireland.

Telephone : +353-53-60600 Fax : +353-53-60699Email: [email protected] Website: http://www.epa.ie/

© Environmental Protection Agency 2000

Although every effort has been made to ensure the accuracy of the material contained in thispublication, complete accuracy cannot be guaranteed. Neither the Environmental Protection Agencynor the author(s) accept any responsibility whatsoever for loss or damage occasioned or claimed tohave been occasioned, in part or in full, as a consequence of any person acting, or refraining fromacting, as a result of a matter contained in this publication. All or part of this publication may be

reproduced without further permission, provided the source is acknowledged.

WASTEWATER TREATMENT MANUALS

TREATMENT SYSTEMS FOR SINGLE HOUSES

Published by the Environmental Protection Agency, Ireland.

Mr. John Mulqueen, Teagasc and Dr. Michael Rodgers, NUI, Galway,are the external contributors to this manual.

Mr. Gerard O’Leary and Mr. Gerry Carty, EPA are the internal contributors.

06/00/1,000ISBN 1 84095 022 6Price IR£15.00

19.05

TABLE OF CONTENTS

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi

LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

1.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .91.2 CHARACTERISTICS OF WASTEWATER FROM A SINGLE HOUSE SYSTEM . . . . . . . . . . . . . . .101.3 CRITERIA FOR SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101.4 SEPTIC TANK SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101.5 MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141.6 POLISHING FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141.7 SITE DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141.8 SITE CHARACTERISATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

2. SITE CHARACTERISATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

2.1 DESK STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .172.2 ON-SITE ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .182.3 INTEGRATION OF THE DESK STUDY AND ON-SITE ASSESSMENT INFORMATION . . . . . . .25

3. TREATMENT OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

3.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .273.2 CHOOSING AN ON-SITE SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .273.3 CHOOSING THE OPTIMUM DISCHARGE ROUTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303.4 LICENCE REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

4. SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 31

4.1 SEPTIC TANKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .314.2 PERCOLATION AREAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .354.3 CONSTRUCTION REQUIREMENTS FOR PERCOLATION PIPES . . . . . . . . . . . . . . . . . . . . . . . . .384.4 MAINTENANCE OF SEPTIC TANKS AND PERCOLATION AREAS . . . . . . . . . . . . . . . . . . . . . . .384.5 FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .394.6 SOIL FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .404.7 SAND FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .404.8 PEAT FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .434.9 OTHER INTERMITTENT MEDIA FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .444.10 CONSTRUCTED WETLANDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .444.11 POLISHING FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

CONTENTS i

5. MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

5.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .515.2 BAF SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .515.3 RBC SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .525.4 SEQUENCING BATCH REACTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .525.5 OTHER TREATMENT SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .535.6 LOCATION OF MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .535.7 POLISHING FILTERS FOR MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . .54

REFERENCES AND FURTHER READING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

APPENDIX A: SITE CHARACTERISATION FORM . . . . . . . . . . . . . . . . . . . . . . . .59

APPENDIX B: PERCOLATION TESTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

APPENDIX C: EVALUATION OF TREATMENT SYSTEMS . . . . . . . . . . . . . . . . .67

APPENDIX D: SOIL/SUBSOIL CLASSIFICATION CHART . . . . . . . . . . . . . . . . .68

APPENDIX E: INDICATOR PLANTS OF DRAINAGE . . . . . . . . . . . . . . . . . . . . . .69

USER COMMENT FORM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70

ii WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES

PREFACE

The Environmental Protection Agency was established in 1993 to license, regulate and control activities forthe purposes of environmental protection. In Section 60 of the Environmental Protection Agency Act, 1992, itis stated that "the Agency may, and shall if so directed by the Minister, specify and publish criteria andprocedures, which in the opinion of the Agency are reasonable and desirable for the purposes ofenvironmental protection, in relation to the management, maintenance, supervision, operation or use of all orspecified classes of plant, sewers or drainage pipes vested in or controlled or used by a sanitary authority forthe.....treatment or disposal of any sewage or other effluent to any waters". The following is a list of themanuals published to-date:

• Wastewater Treatment Manuals - Preliminary Treatment;• Wastewater Treatment Manuals - Primary, Secondary & Tertiary Treatment;• Wastewater Treatment Manuals - Characterisation of Industrial Wastewaters; and• Wastewater Treatment Manuals - Treatment Systems for Small Communities, Business, Leisure

Centres and Hotels.

This manual has been prepared to provide guidance on the design, operation and maintenance of on-sitewastewater treatment systems for a single house. The National Standards Authority of Ireland publishedstandard recommendations in 1975 (revised in 1991) with the aim of achieving satisfactory practice in thedesign, construction and maintenance of septic tank drainage systems. This manual has been prepared havingregard to the above and will inter alia assist planning authorities, developers, system manufacturers, systemdesigners, system installers, system operators to deal with the complexities of on-site systems. Wherereference in the document is made to proprietary equipment, this is intended as indicating equipment type andis not to be interpreted as endorsing or excluding any particular manufacturer or system.

Chapter 1 of this manual contains an introduction to wastewater treatment and the types of on-site treatmentsystems available for a single house.

Chapter 2 outlines the steps which should be taken to characterise a site. Characterisation of a site is dividedinto a desk study followed by an on-site assessment. The on-site assessment is subdivided into a visualassessment, a trial hole and a percolation test. The significance of the information collected during the deskstudy and the on-site assessment is summarised at the end of this chapter.

Chapter 3 outlines a methodology for choosing the on-site treatment system and the optimum dischargeroute.

Chapter 4 includes information on the design, construction and maintenance of a septic tank,soil percolationarea, intermittent filters, constructed wetlands and polishing filters.

Chapter 5 includes information on mechanical aeration systems and polishing filters.

A site characterisation form for use with this guidance manual is included in Appendix A.

This manual was prepared following completion of a research study carried out under the direction of the EPAin the period 1995 to 1997. A seminar on the conclusions of the study was held on the 12th February, 1998.The Geological Survey of Ireland (GSI) in conjunction with the Department of Environment and LocalGovernment (DELG) and the EPA have developed a methodology for the preparation of groundwaterprotection schemes to assist the statutory authorities and others to meet their responsibility to protectgroundwater. Groundwater protection responses have been developed for on-site systems for single houses(DELG/EPA/GSI, 2000). These responses should be consulted when reading this document.

The Agency welcomes any suggestions which users of the manual wish to make. These should be returned tothe Environmental Management and Planning Division at the Agency headquarters on the enclosed UserComment Form.

PREFACE iii

ACKNOWLEDGEMENTS

In order to examine the current position in relation to on-site systems (in Ireland and internationally) and toproduce draft guidelines for their future use, a research project apropos on-site systems was part-financed bythe European Union through the European Regional Development Fund as part of the EnvironmentalMonitoring, R&D sub-programme of the Operational Programme for Environmental Services, 1994 -1999.The sub-programme is administered on behalf of the Department of the Environment and Local Governmentby the Environmental Protection Agency, which has the statutory function of co-ordinating and promotingenvironmental research.

The consortium awarded the project was led by the Civil Engineering Department, National University ofIreland, Galway. The project leader was Dr. Michael Rodgers, assisted by Mr. John Mulqueen and Mr. BrianGallagher. Other members of the project team were: Ms. Angela Casey, Mr. John Kenny, Mr. PadraicBallantyne, Mr. Eamonn Waldron (P.J. Tobin & Co.), Mr. Brendan Fehily (Fehily Timoney & Co.), Ms. MaryHensey (Hensey Glan Uisce Teo.), Ms. Sheila Davey (Neptune Labs) and Ms. Patricia Brannick (CentralMarine Services Labs, NUI Galway).

The project was monitored by a Technical Steering Group established by the EPA and included representativesof the EPA, the Department of the Environment and Local Government, the County and City Engineers’Association and the project consortium.

Members of the Technical Steering Group were (in alphabetical order):

• Mr. Gerry Carty, EPA• Mr. Tony Cawley, Department of Environment and Local Government• Ms. Lorraine Fegan, EPA• Mr. Frank Gleeson, Sligo Co.Co., representing the City and Co. Engineers’Association• Mr. John Mulqueen, Teagasc• Mr. John O’Flynn, Waterford Co.Co., representing the City and Co. Engineers’Association• Mr. Gerard O’Leary, EPA• Dr. Michael Rodgers, NUI, Galway, Project leader

As part of this research study a detailed questionnaire was issued to local authority and health board personnel.The co-operation of those who returned a completed questionnaire is gratefully acknowledged. The outputfrom the research study formed the basis for the development of this manual. The Agency wishes toacknowledge the assistance of Mr. Donal Daly, Geological Survey of Ireland and Mr. Billy Moore, MonaghanCounty Council in reviewing early drafts of the manual.

The Agency would like to acknowledge the National Standards Authority of Ireland for the use of materialand diagrams from SR6.

The Agency wishes to acknowledge the contribution of those persons listed below, who took the time to offervaluable information, advice, comments and constructive criticism on the draft manual.

• Mr. Martin Beirne, Environmental Officers’ Association.

• Mr. Dan O’Regan, National Standards Authority of Ireland.

• Ms. Louise Mulcair, National Standards Authority of Ireland.

• Ms. Yvonne Wylde, National Standards Authority of Ireland.

• Mr. Bruce Misstear, Trinity College Dublin.

• Mr. Paul O’Connor, Environmental Assessments.

iv WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES

• Mr. Garvan Ward, Biocycle.

• Dr. Eugene Bolton, Bord na Mona.

• Dr. Hubert Henry, Bord na Mona.

• Mr. Jer Keohane, Geotechnical and Environmental Services.

• Mr. John Molloy, John Molloy Engineering.

• Mr. Seamus Butler, Butler Manufacturing Services.

• Mr. Albert Sneider, Aswatec.

• Mr. Terry O’Flynn, Banks Douglas Environmental Science.

The Agency also wishes to acknowledge the contribution of the Engineering Inspectors of the Department ofthe Environment and Local Government, and the Sanitary Services sub-committee of the Regional Laboratory,Kilkenny, who commented on the draft manual. The authors would also like to acknowledge the assistanceof Ms. Margaret Keegan, Mr. Donal Howley and Ms. Jane Brogan.

ACKNOWLEDGMENTS v

LIST OF FIGURES

Figure 1: A typical septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Figure 2: Illustration of biomat formation on the base of a percolation trench . . . . . . . . . . . . . . . . . . . . . .12

Figure 3: Schematic diagram of a soil covered mound sand filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Figure 4: Types of constructed wetlands (Section) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Figure 5: Selecting an on-site treatment system for a single house . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Figure 6: Soil classification chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Figure 7: Types of soil structure illustrated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Figure 8: Relationship between structure type and water movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Figure 9: Flow diagram for choosing an on-site system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

Figure 10: Longitudinal section of a typical septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

Figure 11: Plan and section of a septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

Figure 12: Section of a percolation trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Figure 13: Plan and section of a conventional septic tank system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Figure 14: Plan and section of a typical distribution box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

Figure 15: Illustration of an intermittent filter or constructed wetland system . . . . . . . . . . . . . . . . . . . . . . .39

Figure 16: Schematic diagram of a soil covered intermittent sand filter for an impervious soil . . . . . . . . . .42

Figure 17: Sub-Surface (SFS) horizontal flow wetland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

Figure 18: Vertical flow wetland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

Figure 19: Intermittent filter overlying and loading a soil polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . .47

Figure 20: Secondary treatment unit followed by a soil polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . .47

Figure 21: Secondary treatment unit followed by a percolation trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Figure 22: Secondary treatment unit followed by a sand polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . .49

Figure 23: Schematic cross section of a sand polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

Figure 24: Mechanical aeration and polishing filter system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

Figure 25: Percolation test hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

Figure 26: Percolation test hole for shallow soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66

LIST OF TABLES

Table 1: Characteristics of domestic wastewater from a single house . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Table 2: Attributes of a septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Table 3: Factors to be considered during a visual assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Table 4: Minimum separation distances in metres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Table 5: Soil/Subsoil textures and typical percolation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Table 6: Factors to be considered during a trial hole examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

Table 7: Trial hole - site requirements which indicate adequate percolation characteristics . . . . . . . . . . .24

Table 8: Information obtained from the desk study and on-site assessment . . . . . . . . . . . . . . . . . . . . . . .26

Table 9: Typical capacities of septic tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

Table 10: Typical design features of a septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Table 11: Minimum gradients for drain to septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Table 12: Minimum percolation trench length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Table 13: Details of a typical percolation trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Table 14: Design criteria for intermittent sand filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

Table 15: Minimum trench lengths in a soil polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Table 16: Design criteria for the SBR process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

vi WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES

List of Abbreviations

C Capacity

°C Degrees Celsius

Agency Environmental Protection Agency

BAF Biofilm aerated filters

BOD Biochemical oxygen demand

BOD5 Five-day biochemical oxygen demand

COD Chemical oxygen demand

DELG Department of the Environment and Local Government

d Day

DO Dissolved oxygen

DWF Dry weather flow

EPA Environmental Protection Agency

FOG Fats, oils and grease

FWS Free-water surface

g Gram

GSI Geological Survey Of Ireland

h Hour

kg Kilogram

ISO International Organisation for Standardisation

l Litre

m Metre

m3 Cubic metres

m/s Metres per second

mg Milligram

mm Millimetre

NHAs National Heritage Areas

NUI National University of Ireland

p.e. Population equivalent

PFP Preferential flow paths

RBC Rotating biological contactors

s Second

SACs Special Areas of Conservation

S.I. Statutory instrument

SBR Sequencing batch reactor

SFS Sub-surface flow system

SS Suspended solids

TSS Total suspended solids

TWL Top water level

LIST OF ABBREVIATIONS vii

viii WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES

1. INTRODUCTION

1.1 GENERAL

In Ireland, the wastewater from over one third of thepopulation - principally those living in dwellings notconnected to municipal sewers - rely on systemsdesigned to treat the wastewater at or near thelocation where it is produced. These wastewatertreatment systems are called on-site systems.

Many on-site systems are available for the treatmentof wastewater from single houses and are designedto:

• treat the wastewater to minimise contamination of soils and water bodies;

• protect humans from contact with wastewater;

• keep animals, insects, and vermin from contact with wastewater;

• prevent direct discharge of untreated wastewater to the groundwater;

• minimise the generation of foul odours; and

• prevent direct discharge of untreated wastewater to surface water.

The biological treatment of the wastewater in on-sitetreatment systems occurs, in the main, under aerobicconditions. For example, in a soil percolation area,aerobic conditions are present due to the unsaturatednature of the soil.

Public health is threatened when on-site systems failto operate satisfactorily. System failures can resultin wastewater ponding or forming stagnant pools onthe ground surface when the wastewater is notabsorbed by the soil. In such circumstances ofsystem failure, humans can come in contact with theponded wastewater and be exposed to pathogens andfoul odours can be generated.

The three documents commonly used in relation tothe design of on-site systems in Ireland are:

• SR6: 1991, Septic tank systems:Recommendations for domestic effluent treatment and disposal from a single dwellinghouse (National Standards Authority of Ireland);

• BS 6297: 1983, Design and installation of small sewage treatment works and cesspools (British Standards Institution) deals mainlywith the design of small sewage treatment works serving small communities, not primarily concerned with septic tank systems;and

• US EPA/625/R-92/005 Manual: Wastewater Treatment/Disposal for Small Communities.

In order to examine the current position relating toon-site systems (in Ireland and internationally) andto establish guidelines for their future use, so as toensure sustainable development, a research studywas carried out between 1995 and 1997 (as part ofthe Department of the Environment OperationalP rogramme for Env i ronmental Serv i c e s , 1 9 9 4 -1 9 9 9 ). This study was co-ord i n ated by theD ep a rtment of Civil Engi n e e ri n g, The Nat i o n a lUniversity of Ireland, Galway under the direction ofthe Environmental Protection Agency (EPA) and wasfunded through the E nv i ronmental Monitori n g,Research and Development Sub-programme of theOperational Programme.

Some of findings of the research regarding singlehouse treatment systems were:

• conventional septic tank systems (septic tankand percolation area), properly installed and maintained, are satisfactory where suitable subsoil conditions exist;

• where suitable subsoil conditions do not initially exist for treatment by means of a conventional septic tank system, site development works may improve the subsoilconditions and make the subsoil suitable in certain circumstances;

• in certain situations such as when unsuitable subsoil conditions exist, other systems, whichinclude mechanical aeration or intermittent filters for secondary treatment and followed by a polishing filter can be used;

• all treatment systems including wastewater collection systems must be designed,constructed, commissioned and maintained inaccordance with recognised standards; and

1 INTRODUCTION 9

Parameter Typical concentration(mg/l unless otherwise stated)

Chemical Oxygen Demand COD (as O2) 400

Biochemical Oxygen Demand BOD5 (as O2) 300

Total solids 200

Total Nitrogen (as N) 50

Total Phosphorus (as P) 10

Total coliforms (MPN/ 100 ml)* 107 - 108

• all surface water and groundwater should be excluded from entering any treatment system.

1.2 CHARACTERISTICS OF WASTEWATERFROM A SINGLE HOUSE SYSTEM

For the purposes of this manual, a single housesystem refers to a dwelling house of up to ten peoplewith toilet, living, sleeping, bathing, cooking andeating facilities. Under no circumstances shouldrainwater, surface water or run-off from paved areasbe discharged to on-site single house systems. Toprevent the quantity of wastewater generated in ah o u s e h o l d, water reducing measures should beadopted. Such measures include: minimising the useof high water using equipment such as automaticwashing machines and dishwa s h e rs , the use ofshowers instead of baths, the use of dual flushcisterns in WCs, and the prompt fixing of leaks inhousehold plumbing system.

The s t re n g t h of the infl ow in terms of BOD( B i o chemical Oxygen Demand) into an on-sitesystem will largely depend on the water usage in thehouse; for example, houses with dishwashers mayhave a wastewater strength reduced by up to 35%due to dilution even though the total organic load tothe treatment system (kg/day) remains the same.Household garbage grinders can increase the BODloading rate by up to 30% and because theseappliances are becoming more popular their use is animportant consideration.

Other important constituents in domestic wastewaterinclude nitrogen, phosphorus and microorganismss u ch as colifo rms. Table 1 gives typicalconcentration values for a number of parameters indomestic wastewater.

Typical daily hydraulic loading to an on-site systemfor single houses is 180 litres per person.

1.3 CRITERIA FOR SELECTION

When selecting a tre atment system to tre atwastewater from single houses, the system chosen:

• should protect public health;

• should protect the environment;

• should be economical;

• should operate with minimal maintenance from the owner; and

• should have a long (> 20 years) lifespan.

On-site systems for single houses can be divided intotwo main categories:

• septic tank systems; and

• mechanical aeration systems.

1.4 SEPTIC TANK SYSTEMS

1.4.1 CONVENTIONAL SEPTIC TA N KSYSTEM

A conventional septic tank system comprises a septictank followed by a soil percolation area. The septictank functions as a primary sedimentation tank,removing most of the suspended solids from thewa s t ewater; this re m oval is accompanied by alimited amount of anaerobic digestion. It is in thep e rc o l ation area that the wa s t ewater undergo e ssecondary treatment and is purified. The wastewaterfrom the septic tank is distributed to a suitable soilpercolation area, which acts as a bio-filter. As thewastewater flows into and through the subsoil, itu n d e rgoes surface fi l t rat i o n , s t ra i n i n g, p hy s i c o -chemical interactions and microbial bre a k d ow n .

TABLE 1: CHARACTERISTICS OF DOMESTIC WASTEWATER FROM A SINGLE HOUSE

* MPN Most Probable Number

10 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES

After flowing through a suitable percolation area thewastewater is suitable for discharge.

A typical septic tank is illustrated in Figure 1 and theattributes of a septic tank are given in Table 2. Thetank, which should be two-chambered, allows thewastewater from the dwelling house time to settleout into three layers viz. the sludge layer, the liquidlayer and the scum layer (Figure 1). The sludge layeris a blanket of heavy solids and some coagulatedmaterials, which settle out on the tank floor. Theliquid laye r, while re l at ive ly free of coars esuspended solids, is high in decomposable dissolvedand colloidal organic matter and contains bacteria,viruses, worm eggs, larvae etc.; it is allowed to flowto the perc o l ation area through a tee-pipe fo rdistribution and secondary treatment. The scumlayer consists of greases, oils and gas-buoyed solidswh i ch accumu l ate as a layer on the surfa c e.Detention times should be in excess of 24 hours.

The subsoil through which the wastewater percolatesacts as an at t a ched growth medium fo rmicroorganisms. As the percolation trenches are

loaded with wastewater from a septic tank, a biomatlayer quickly develops along the base and wettedsides of these trenches (Figure 2). The biomat layerconsists of a deposit of microorganisms, slimes andsludge which coats the floor and walls of the trenchand enters the subsoil for a short distance inside theinfiltrative surface. The biomat drastically lowers thei n fi l t ration through the base and sides, c a u s i n gponding in the tre n ches. The ponding causeswastewater to flow over the entire trench base and ina short time leads to a uniform distribution of thewastewater over the total length of the trenches.Ponded wastewater gradually rises in the trenchesaccompanied by the development of a biomat alongthe wetted walls of the trench until an equilibrium isreached, causing flow through the sides and base. Anadequate depth of gravel aggregate in the trench isimportant for hydraulic function. The biomat layerthen determines the hydraulic loading. Therefore forl o n g - t e rm successful operation of a perc o l at i o nsystem, the system should be designed to cope withthe impedance caused by the development of thebiomat layer along the base and wetted walls of thepercolation trench.

OutletTWL

Sludge layer

Inlet

Liquid layer

CHAMBER NO. 1 CHAMBER NO. 2

Scum Layer

Manhole cover with ventilation Manhole cover with ventilation

SECTION A - A

FIGURE 1: A TYPICAL SEPTIC TANK

1 INTRODUCTION 11

initial operation - no biomat

Biomat formation

Biomat formation

Biomat formation with extension of clogging of

the base and adjoining walls

of trench

Distribution box

(Order of weeks)

A properly constructed septic tank will:

Retain and remove 50% or more solids; outflow from tank contains about 80 mg/l solids

Allow some microbial decomposition

Accept sullage (i.e. water from baths, wash hand basins etc.)

Accept water containing detergents

Reduce clogging in the percolation area

Not fully treat domestic wastewater

Not work properly if inadequately maintained

Not significantly remove microorganisms

Not remove more than 15 - 30 % of the BOD

Not operate properly if pesticides, paints, thinners, solvents, disinfectants or household hazardoussubstances are discharged to it

Not accommodate sludge indefinitely

Not operate properly if surface waters (i.e. roofs etc.) are discharged to it

TABLE 2: ATTRIBUTES OF A SEPTIC TANK

FIGURE 2: ILLUSTRATION OF BIOMAT FORMATION ON THE BASE OF A PERCOLATION TRENCH


A percolation area is considered "failing" when (i) itcauses a backing up of wastewater in the distributionbox or (ii) it does not keep untreated wastewaterbelow the surface of the land or (iii) it does not treatthe wastewater before it reaches groundwater orsurface water.

In Ire l a n d, a significant number of septic tanksystems do not function properly, mainly becauset h ey have been poorly constru c t e d, i n s t a l l e d,operated, maintained or, are located in areas withunsuitable subsoils, or percolation of the septic tankeffluent is through a soakaway. It is important tonote, however, that in the absence of a connection toa sewer system, one of the most appropriate and costeffective means of treating wastewater in a suitablesite is a pro p e rly constructed and maintainedconventional septic tank system.

1.4.2 FILTER SYSTEMS

Where the subsoil is unsuitable for treating thewastewater from a septic tank, filter systems may beused. These include intermittent soil filters, sandfilters, peat filters and other filters using materialssuch as plastic foam filters and geosynthetic strips.Intermittent soil filters comprise suitable soils placedoften in the form of a mound, through which septictank effluent is filtered and purified. Intermittentsand filters consist of one or more beds of gradedsand underlain at the base by a filter gravel orpermeable soil layer to prevent outwash or piping ofthe sand. Soil covered intermittent sand filters maybe undergro u n d, p a rt underground and partove rgro u n d, or ove rgro u n d. The latter twoconstructions are commonly referred to as moundsystems (Figure 3). Fibrous peat and plastic media

for the other fi l t e rs are usually installed inprefabricated containers (prefabricated intermittentfilters).

1.4.3 CONSTRUCTED WETLANDS

C o n s t ructed wetlands can also be used for thet re atment of wa s t ewater from single houses.Wetlands are areas with high water tables whichpromote aquatic vegetation or water tolerant plantssuch as reeds.

Primary treatment by a septic tank is used prior todischarge to a constructed wetland. In the wetland,the wastewater from a septic tank is treated by acombination of physical, chemical and biologicalprocesses that develop through the interaction of theplants (re e d s ) , the growing media (gravel) andmicroorganisms. These processes include settlementand filtering of suspended solids, biodegradation,plant uptake and chemical interactions.

There are two different types of constructed wetlandsand they are characterised by the flow path of thewater through the system (Figure 4). In horizontalflow constructed wetlands, wastewater is introducedat one end of a flat to gently sloping bed of reeds andflows horizontally across the bed to the outfall end.In the second type, called the vertical-flow wetland,the wa s t ewater is dosed unifo rm ly ove r, a n dintermittently onto the media, and gradually drainsvertically to a drainage network at the base of themedia. Constructed wetlands should be securelyfenced off to prevent access by unauthorised persons,especially children.

Distribution gravelGeotextile

Distribution laterals

Filter sand

Gravel, fractured bedrock, high water table or impervious soil

filter gravel or permeable soil

Soilcap

Top soil

Washed gravel

FIGURE 3: SCHEMATIC DIAGRAM OF A SOIL COVERED MOUND SAND FILTER

1 INTRODUCTION 13

1.5 MECHANICAL AERATION SYSTEMS

In recent years, many mechanical aeration systemshave come on the market; these offer a solution insome cases where a site may be unsuitable fortreating the septic tank wastewater, or an alternativeto the conventional septic tank system. Th e s esystems include the following:

• biofilm aerated (BAF) systems;

• rotating biological contactor (RBC) systems;and

• sequencing batch reactors (SBR) systems.

BAF systems may consist of a primary settlementtank, aerated filter media and a secondary settlementtank. RBC systems consist of a primary settlementtank, a biological treatment compartment and as e c o n d a ry settlement tank. These systems aresimilar to conventional trickling filter systems in thatthe micro o rganisms carrying out the secondarytreatment are attached to an inert media surface.Sequencing bat ch re a c t o rs (SBR) consist of aprimary settlement tank and a reactor in whichbiological treatment and clarification occur.

1.6 POLISHING FILTERS

Polishing filters should be used to treat wastewaterfrom intermittent filters, constructed wetlands andmechanical aeration systems. These filters consist ofeither soil or sand and are employed to reducemicroorganisms, phosphorus, and nitrate nitrogen.Soil polishing filters may comprise in-situ, improved

soil or imported soil, whereas sand polishing filterscomprise stratified layers of sand.

1.7 SITE DEVELOPMENT

Where a site is initially unsuitable for a septic tanksystem, site development works may improve thesite and make it suitable for the development of anon-site system.

Site development works could include lowering thewater table, raising the ground surface by filling withsuitable soil, part replacement of the subsoil bysuitable soil or subsoil loosening. After carrying outthe necessary improve m e n t s , the site should bereassessed to establish whether the improved soil issatisfactory.

1.8 SITE CHARACTERISATION

The objective of a site characterisation is to obtainsufficient information to determine if the site can bedeveloped for an on-site system. Characterising thesite involves a number of stage s . These shouldinclude:

• a desk study, which collects any information that may be available on maps etc. about the site;

• a visual assessment of the site, which definesthe site in relation to surface features;

• a trial hole to evaluate the soil structure, depthto rock and water table; and

• percolation tests.

Septic tank

Horizontal flow wetland

Septic tank

Vertical flow wetland

FIGURE 4: TYPES OF CONSTRUCTED WETLANDS (SECTION)


DESKSTUDY

SITEIMPROVEMENT

UNSUITABLE *

ON-SITEINVESTIGATION

VISUAL ASSESSMENTTRIAL HOLE

PERCOLATIONTESTS

UNSUITABLE**

NO SITE RESTRICTIONS

SITECHARACTERISATION

SUITABLE

* This option may not always be available

NOT SUITABLE

Or

Or

SITERESTRICTIONS

** Site may not always be suitable for anon-site system

Fi g u re 5 below summarises the protocol to befollowed to select and design an on-site system.

The concepts of ‘risk’, ‘risk assessment’ and ‘riskmanagement’ have recently become important toolsin environmental protection. Risk can be defined asthe likelihood or expected frequency of a specifieda dve rse consequence. Applied for example togro u n dwat e r, a risk ex p resses the likelihood ofc o n t a m i n ation arising from a proposed on-sitetreatment system (called the hazard). A hazardpresents a risk when it is likely to affect somethingof value (the target, e.g. surface water). It is thec o m b i n ation of the pro b ability of the hazard

occurring and its consequences that is the basis ofrisk assessment. Risk management involves siteassessment, selection of options and implementationof measures to prevent or minimise the consequencesand probability of a contamination event (e.g. odournuisance or water pollution). The methodology forselection and design of an on-site system in thismanual embraces the concepts of risk assessmentand risk management.

The remainder of this manual sets out how a sitecharacterisation should be completed and a choice ofon-site system made.

FIGURE 5: SELECTING AN ON-SITE TREATMENT SYSTEM FOR A SINGLE HOUSE

1 INTRODUCTION 15

FILTER SYSTEMAND

POLISHING FILTER

MECHANICAL AERATION SYSTEMAND

POLISHING FILTER

FILTER SYSTEM /MECHANICAL AERATION SYSTEM AND

POLISHING FILTER

CONVENTIONAL SEPTIC TANK SYSTEM

The purpose of a site assessment is to determine thesuitability of the site for an on-site treatment system.The assessment will also help to predict thewastewater flow through the subsoil and into thesubsurface materials.

The key to installing a reliable on-site system thatminimises the potential for pollution is to select anddesign a suitable tre atment system fo l l owing athorough site assessment. For a subsoil to beeffective as a medium for treating wastewater, itmust retain the wastewater for a sufficient length oftime, and it must be well aerated.

Only after a site evaluation has been completed canan on-site system be chosen. The info rm at i o ncollected in the evaluation will be used to select theon-site system.

In designing a soil perc o l ation area to tre atwastewater, three factors must be considered:

• the suitability of the site;

• the suitability of subsoil and groundwater conditions, and

• the permissible hydraulic load on the subsoil.

To determine these considerations a sitecharacterisation is undertaken. This includes:

1) a desk study; and

2) an on-site evaluation, consisting of :

• a visual assessment;

• a trial hole; and

• percolation tests.

2.1 DESK STUDY

The purposes of a desk study are to:

• obtain information relevant to the site, whichwill assist in assessing its suitability;

• identify targets at risk; and

• establish if there are site restrictions.

A desk study involves the assessment of availabledata pertaining to the site and adjoining area thatmay determine whether the site has any restrictions.Information collected from the desk study shouldi n clude mat e rial re l ated to the hy d ro l ogi c a l ,hydrogeological and planning aspects of the site,wh i ch may be ava i l able in maps or rep o rt s .Hydrological aspects include locating the presence(if any) of streams, rivers, lakes, beaches, shellfishareas and/or wetlands while hydrogeological aspectsinclude:

• soil type;

• subsoil type;

• bedrock type;

• aquifer type;

• vulnerability class; and

• groundwater protection response (refer to theDELG/EPA/GSI groundwater protection scheme and groundwater protection responses for on-site systems for single houses).

The Gro u n dwater Protection Schemes prov i d eguidelines for developers in assessing groundwatervulnerability and for the planning authorities incarrying out their functions, and a framework toassist in decision-making on the location, nature andcontrol of developments and activities (includingsingle house treatment systems) in order to protectgroundwater. The density of on-site systems shouldbe considered also at this stage. The protectionresponses required to protect groundwater from on-site systems should be satisfied. Where no schemeex i s t s , i n t e rim measures as set out in theGroundwater Protection Schemes should be adopted.Each site is specific and local factors should be takeninto account in using this guideline information.

Planning aspects include:

• zoning in the development plan;

• presence of significant sites (archaeological,natural heritage, historical etc.); and

• past experience of the area.

2 SITE CHARACTERISATION 17

2. SITE CHARACTERISATION

2.1.1 INTERPRETING THE RESULTS OF THEDESK STUDY

The information collected from the desk studyshould be examined and the following should beconsidered for all treatment options:

• zoning (including groundwater protection schemes): Zoning for groundwater protectionschemes outlines the aquifer classification inthe general area and the vulnerability of the groundwater. The groundwater protection responses will provide an early indication of the probable suitability of a site for an on-sitesystem. The on-site assessment will later confirm or modify such responses;

• presence of significant sites: Determine whether there are significant archaeological,natural heritage and/or historical features within the proposed site. To avoid anyaccidental damage, a trial hole assessment orpercolation tests should not be undertaken inareas, which are at or adjacent to significant sites (e.g. SACs, NHAs etc.), without prior advice from Duchas, the Heritage Service;

• nature of drainage: A high frequency of watercourses on maps indicates high or perched watertables; and

• past experience: Is there evidence of satisfactory or unsatisfactory local experiencewith on-site treatment systems?

2.2 ON-SITE ASSESSMENT

2.2.1 VISUAL ASSESSMENT

The purposes of the visual assessment are to:

• assess the potential suitability of the site;

• assess potential targets at risk (adjacent wells); and

• provide sufficient information to enable a decision to be made on the suitability of the site for the wastewater to be treated and the location of the proposed system within the site. The principal factors which should be considered are listed below.

Topography and landscape: Topography reflectsthe relief of the site. Landscape position reflects thelocation of the site in the landscape e.g. crest of hill,valley, slope of hill. Sites which are on level, well

d rained are a s , or on convex slopes are mostdesirable. Sites which are in depressions, or on thebottom of slopes or on concave slopes are lessdesirable.

The principal factors which should be consideredare, relief, shape and form, rock outcrops, wells andwat e rc o u rs e s , land use, vege t at i o n , t ra m p l i n gdamage to the soil by livestock, seepage, boundaryof property, and old building foundations.

Slope: It is more difficult to install pipework andensure that the wastewater will stay in the soil if theland has an extreme slope. Where there is surfacewater run-off and interflow, low-lying areas and flatareas generally receive more water. This accounts tosome extent for the occurrence of poorly drainedsoils in low-lying areas. Soils with poor drainage,however, may also be found on good slopes wherethe parent mat e rial or the subsoil is of lowp e rm e ab i l i t y. Provision must be made for theinterception of all surface run-off and seepage, andits diversion away from the proposed percolationarea.

P roximity to surface fe at u re s : M i n i mu mseparation distances as set out in the followingch ap t e rs should be maintained from specifi e dfeatures. The presence/location of surface featuressuch as wells/springs, watercourses, dwelling houseson adjacent sites, site boundari e s , ro a d s , s t e epslopes, etc. should be noted.

Wells: Wells should be considered as targets at risk.The groundwater flow direction, where it can beinferred; the number of wells; the presence of anywetlands, and presence of any karst features shouldbe noted.

Drainage: A high density of streams or ditchestends to indicate a high water table and potential riskto surface water. Low density of streams indicates afree draining subsoil and or/bedrock.

Type of vegetation: Rushes, yellow flags (irises)and alders indicate poor percolation characteristicsor high water table levels. Grasses, trees and fernsm ay indicate suitable perc o l ation ch a ra c t e ri s t i c s .Plants and trees indicating good drainage and poordrainage are illustrated in Appendix E.

Ground condition: The ground conditions duringthe on-site investigation should be noted. Tramplingdamage by livestock can indicate impeded drainageor intermittent high water tables, especially whereaccompanied by widespread ponding in hoof prints. The factors examined during a visual assessment and


their significance are summarised in Table 3 above.

2.2.2 INTERPRETING THE RESULTS OF THEVISUAL ASSESSMENT

The minimum separation distances that should beused in the visual assessment are set out in Table 4.These apply to any on-site system. If any of these

requirements cannot be met, on-site systems cannotbe developed on the site. The re c o m m e n d e dminimum distances from wells should satisfy there q u i rements of the gro u n dwater pro t e c t i o nresponse, which should have been reviewed duringthe desk study. In some cases, the requirements ofthe groundwater protection scheme and responsesmay be greater than the distances set out in Table 4.

Factor Significance

Water level in ditches and wells Indicates depth of unsaturated subsoil

Shape, slope and form of site May indicate whether water will collect at a siteor flow away from the site

Presence of watercourses May indicate low permeability or a high water table

Presence and types of rock outcrops Insufficient depth of subsoil to treat wastewaterallowing it to enter the groundwater too fast

Proximity to adjacent percolation areas and/or houses May indicate too high a loading rate for thelocality and/or potential nuisance problems

Land use and type of grassland surface (if applicable) Indicator of rate of percolation or groundwaterlevels

Vegetation type Indicator of the rate of percolation orgroundwater levels

Proximity to wells on-site and off-site, water supply Indicates targets at risksources, groundwater, streams, ditches, lakes,surface water ponding, beaches, shellfish areas,and wetlands

TABLE 3: FACTORS TO BE CONSIDERED DURING A VISUAL ASSESSMENT

Type of system Watercourse/ Wells/ Lake Any Site Road Slopestream springs* Dwelling boundary breaks/

cuts

Septic tank;prefabricatedintermittent filters; 10 10 50 7 3 4 4mechanical aerationsystems

In situ intermittentfilters; percolation 10 30 50 10 3 4 4area; polishing filters

TABLE 4: MINIMUM SEPARATION DISTANCES IN METRES

* This applies to wells down-gradient or where flow direction is unknown. For more information on wells alongside orup-gradient consult DELG/EPA/GSI ground water protection scheme1.

1 Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland2000. Groundwater Protection Responses for On-site Systems for Single Houses.


2.2.3 TRIAL HOLE

The purposes of the trial hole are to determine:

• the depth of the water table;

• the depth to bedrock; and

• the soil and subsoil characteristics.

The trial hole will also help to predict the wastewaterflow through the subsoil.

The trial hole should be as small as practicable, e.g.1.0 metre x 0.75 metre in plan, and should beexcavated to a depth of at least 1.2 m below the invertof the lowest percolation trench. In the case of a levelsite the depth of the trial hole should be a minimumof 2.1 m below ground surface. In the case of asloping site it is essential that an estimate of thedepth of the invert of the percolation trench be madebeforehand. The hole should remain open for 48

hours to establish the depth to the water table (ifpresent) and should be securely fenced. The soilch a ra c t e ristics assessed are : t ex t u re, s t ru c t u re,p resence of pre fe rential fl ow pat h s , d e n s i t y,compactness, colour, layering, depth to bedrock anddepth to the watertable. If items of suspecteda rch a e o l ogical interest are discove re d, c o n t a c tshould be made with the relevant authorities.

Depth to bedrock and depth to water table: Adepth of 1.2m of suitable free draining unsaturatedsubsoil, to the bedrock and to the water table belowthe base of the percolation trenches, must exist at alltimes to ensure sat i s fa c t o ry tre atment of thewastewater. Sites assessed in summer when thewater table is low, should be examined below theproposed invert of the percolation pipe for soilmottling - an indicator of seasonally high watertables.

Soil texture: Texture is the relative proportions ofsand, silt and clay particles in a soil after screeningthrough a 2 mm size sieve. The rate and extent ofmany important physical processes and chemicalreactions in soils are governed by texture. Physicalprocesses influenced by texture include drainage andmoisture retention, diffusion of gases and the rate oftransport of contaminants. Texture influences theb i o film surface area in wh i ch biochemical andchemical reactions occur. The soil texture may becharacterised using the chart in Figure 6 .

To classify a soil/subsoil, it should be wetted andsqueezed between the fingers. Soils/subsoils high insand feel sandy, soils/subsoils high in silt are silky tofeel and soils/subsoils high in clay are sticky andh ave tensile strength. A guide to assist thecl a s s i fi c ation of soil/subsoils is included inAppendix D. Va rious soil/subsoil tex t u reclassifications schemes exist; Table 5 compares threesuch classifications and indicates typical percolationrates.

FIGURE 6: SOIL CLASSIFICATION CHART

Soil Class Subsoil Unified Class Typical PercolationClassification Classification Classification Rate *

(min/25mm)

sand medium fine SAND sand; silty sand; clayey sand 1 - 5

loamy sand silty, clayey SAND sand; silty sand; clayey sand 6 - 10

sandy loam silty SAND silty sand; clayey sand 6 - 30

loam / silt loam sandy SILT silty fine sands - low plasticity 31 - 50**

TABLE 5: SOIL/SUBSOIL TEXTURES AND TYPICAL PERCOLATION RATES

* typical for soil in an uncompacted state and not indurated or hard.** upper limit of 50 may need to be reviewed in the light of on-going research findings.


Example x: 50% Clay30% Sand20% Silt

x

Structure: Soil structure refers to the arrangementof the soil particles into larger units or compoundparticles in the soil. The soil particles,sand, silt, clayand organic matter, are generally clumped togetherto form larger units called peds. The shape and sizeof the peds have a large effect on the behaviour ofsoils. A ped is a unit of soil structure such as anaggregate, a crumb, a prism, a block or granulesformed by natural processes. Soil texture plays amajor part in determining soil stru c t u re. Th es t ru c t u re of the soil influences the pore space,a e ration and dra i n age conditions. Types of soilstructure (shape of the ped) are illustrated in Figure7 and are:

• Crumb - peds have curved surfaces. The faces of peds do not fit into the faces of neighbouring peds. Commonly found in topsoils.

• Granular - peds composed of single grains with curved surface e.g. sands.

• Blocky - the faces of each ped are nearlyequal and fit into the faces of neighbouring peds. They are common in loamy soils.

• Prismatic - the soil particles are arranged about a vertical axis and are bounded byrelatively smooth vertical faces; the vertical faces are longer than the horizontal faces andfit into neighbouring peds; commonly foundin clayey soils.

• Platy - peds consist of thin flat plates and areformed where soils dry out rapidly (rare in Ireland).

• Structureless - massive - soil is not separatedinto structural units but occurs as one large(often plastic) mass; typical of clays and silts.

• Structureless - single grain - soil has no visible aggregation; on immersion in water,soil readily disintegrates into its single grainsof gravel, sand, silt and clay.

The rate of flow of water through soils of the variousstructures is in the following order:

crumb faster than blocky; blocky faster than structureless-single grain.

Structureless massive structure have very low flowrates and can often be regarded as impervious ( e.g.with a permeability < 10 mm/day). The relationshipb e t ween stru c t u re type and water movement is

illustrated in Figure 8. Where water is supplied to asoil at a rate less than its permeability, as in the caseof septic tank effluent, the r ate of flow through thesoil equals the rate of supply in soils of adequatepermeability.

The preferred structures from a wastewater treatmentperspective are granular (as fine sand), blocky andstructureless-single grain sandy loams, loams andsilt loams. Unstru c t u red massive plastic soilsindicate seasonal or continuous saturation and areunsuitable. Likewise soils with extensive, large andcontinuous fissures and thick lenses of gravel andcoarse sand may be unsuitable; this suitability willbe assessed in the percolation test.

Preferential flow paths: Preferential flow paths(PFPs) are formed in soils by biological, chemicaland physical processes and their interactions. Theymay be randomly distributed or their formation maybe systematic, reflecting the influence of agriculturalpractices. Research in recent years indicates thatPFPs can have a significant influence on them ovement of ponded or perched water insoil/subsoils where free (non capillary) water is indirect contact with PFPs. The presence of PFPsshould be noted during the trial hole assessmentbecause their presence may influence the percolationrate of the subsoil.

Soil density: this refers to how tightly the soil grainsare packed together. Dry bulk density is commonlyclassified as low, medium or high.

• Low - loose and easily disintegrated into structural peds when dry to moist; typical of many topsoils;

• Medium - dry bulk density of intermediate magnitude (e.g. 1.3 tonne/m3); typical of many permeable soils; and

• High - compact and strong and resistant to penetration; typical of some deep permeable soils.

Soils of low and medium dry bulk density are best aspercolation soils.

Colour: This is a good indicator of the state ofa e ration of the soil/subsoil. Free dra i n i n gunsaturated soils/subsoils are in the oxidised state atall times and exhibit brown, reddish brown andyellowish brown colours. Many free draining soils oflimestone origin with deep water tables are grey atdepth. Saturated soils/subsoils are in a reduced stateand exhibit dull grey or mottled colours. Mottling


FIGURE 7: TYPES OF SOIL STRUCTURE ILLUSTRATED

FIGURE 8: RELATIONSHIP BETWEEN STRUCTURE TYPE AND WATER MOVEMENT


(comprising a reddish brown or rusty staining) of thesoil layers can indicate the height to which the watertable rises in periods of high rainfall; mottling in agrey mat rix (grey with re ddish brown mottles)indicates aeration along old root channels and crackswhile the matrix remains reduced; this soil layer issaturated during part of the year.

Layering: This is common in soils, arising duringdeposition and/or subsequent weathering. In soils,that are free draining in the virgin state, weatheringcan result in downward movement of some of theclay fraction leading to enrichment of a sub-layer

with clay. In some areas a thin, hard, rust colouredimpervious layer can develop (iron pans) as a resultof the downward leaching of iron and manganesecompounds and deposition at shallow depth (lessthan 1m) . The underlying subsoil often has asatisfactory percolation rate. Enrichment with clayparticles and precipitation of iron and calcium andm agnesium compounds can lead to ve ry lowpercolation rates. Such soils can often be improvedby loosening or by breaking the impervious layer.

The factors that are evaluated from the trial hole andtheir significance are summarised in Table 6 below.

Factors Significance

Soil structure and texture Both influence the capacity of soil to treat and dispose of the wastewater; silts and clays are generally unsuitable

Mottling Indicates seasonal high water tables

Depth to bedrock Subsoil must have sufficient depth to treat wastewater

Depth to water table Wet subsoils do not allow adequate treatment of wastewater

Water ingress along walls Indicates high water table

Season Water table varies between seasons

TABLE 6: FACTORS TO BE CONSIDERED DURING A TRIAL HOLE EXAMINATION


2.2.4 INTERPRETING THE RESULTS OF THETRIAL HOLE TEST

Table 7 sets out the subsoil characteristics whichi n d i c ate sat i s fa c t o ry perc o l ation and other

ch a ra c t e ristics necessary for the tre atment ofwastewater. The percolation characteristics will beconfirmed later by examining the percolation testresults.

Subsoil Characteristics Requirements

Minimum depth of unsaturated permeable subsoil 1.2 mbelow base of all percolation trenches Percolation trench cross section

for a level site

Minimum depth of unsaturated subsoil to bedrock 1.2 mbelow invert level of all percolation trenches

Minimum depth to water table below invert of all 1.2 mpercolation trenches*

Texture of unsaturated soil/subsoil Sand (medium fine SAND),Loamy sand (silty, clayey SAND),Sandy loam (silty SAND),Loam and silt loam (sandy SILT);

Structure of unsaturated soil/subsoil Granular, blocky; and structurelesssingle grain

Colour of unsaturated soil/subsoil Greyish brown, reddish brown, and yellowish brown; grey in the case of manyfree draining limestone soils

Layering in the walls of a percolation trench or No gravel or clay layer should be presentbelow its invert

Bulk density of unsaturated soil/subsoil Low to medium

TABLE 7: TRIAL HOLE - SITE REQUIREMENTS WHICH INDICATE ADEQUATE PERCOLATION CHARACTERISTICS

Ground level

Topsoil

GravelGravelDistribution pipe &

Gravel

Unsaturated Subsoil

300 mm

150 mm100 mm

250 mm

1200 mm

450 mm2000 mm

*where the dimensions of the percolation trench and unsaturated soil are as shown, the minimum depth to the watertable is 2 m below ground surface.


2.2.5 PERCOLATION TESTS

A perc o l ation (perm e ability) test assesses thehydraulic assimilation capacity of the subsoil i.e. thelength of time for the water level in the percolationhole to fall from a height of 300 mm to 200 mmabove the base of the test hole in a percolation area.The permeability of each soil class may vary withinan order of 10 - 100 fold depending primarily on thep a rt i cle size grading wh i ch re flects the va ry i n gamounts of fine particles, the structure and void ratioin each soil class. The procedure for carrying out apercolation test is set out in Appendix B.

The results of percolation tests are expressed as a "T"value. This is the average time in minutes for thewater level to fall 25 mm in each of two percolationtest holes over the water depth range of 300 mm to200 mm in the proposed percolation area.

To carry out a percolation test (which should bewithin the proposed percolation area), a 300 mmsquare percolation test hole is excavated to a depth of400 mm below the invert of the proposed distributionpipe.

To establish the percolation value for shallow soilsthat may be used for polishing filters (discussedlater) a modification of the T test is required. Forthis, the test hole is 400 mm below the groundsurface as opposed to 400 mm below the invert of thedistribution pipe for the T test. To avoid confusionwith the previous test, this test is called a P test, andthe values are referred to as P values.

2.2.6 INTERPRETING THE RESULTS OF THEPERCOLATION TEST

A "T" value greater than 50 suggests that wastewaterentering such subsoils would cause ponding on-site.A "T" value less than 1 suggests that the retentiontime for the wastewater would not be long enough toprovide satisfactory treatment. If the percolation Tvalue is within the range 1-50* then the site shouldbe suitable for the development of a conventionalseptic tank system.

Where the "T" value is less than 1 or greater than 50

the site is not suitable for the treatment of septic tankwastewater by soil percolation. Other options shouldbe considered such as a constructed percolation area,mechanical aeration systems, intermittent filters orconstructed wetlands. Where mechanical aerationsystems, intermittent filters or constructed wetlandsare used, the treated wastewater from such systemsshould discharge to receiving waters (surface orgroundwater) through a polishing filter.

Where shallow or impervious soils exist, a soilpercolation area may still be possible by importingsuitable soil and placing it in lifts in the proposedp e rc o l ation area such that there is a minimu mthickness of 2.0 m of unsaturated soil with drainageover the bedrock or impervious soil. A trial hole andpercolation tests (T - tests) should then be carried out(see Appendix B - Percolation Tests for furtherdetails) in the same way as for in situ soils. Wheresuch soil filling is not feasible, alternative systemsfollowed by a polishing filter may be suitable.

Where an alternative system and a polishing filter aree m p l oye d, the nat u re of the soil or bedro cku n d e rlying the polishing filter determines thedisposal route of the treated wastewater. For apolishing filter overlying impervious soils or rocks,the treated wastewater is collected in a suitabledrainage system and discharged to surface waters.Polishing filters overlying permeable soils, gravelsor bedrock with a T/P value less than 50 mayd i s ch a rge the tre ated wa s t ewat e rs to thegroundwater. A flow diagram to assist in the choiceof an on-site system is shown in Figure 9.

2.3 INTEGRATION OF THE DESK STUDYAND ON-SITE ASSESSMENT INFORMATION

Table 8 summarises the information that can beobtained from the data collected from the desk studyand the on-site assessment. This information is usedto characterise the site and used later to choose anddesign an on-site system. An integrated approachwill ensure inter alia that the targets at risk areidentified and protected.

* upper limit of 50 may be reviewed depending on experience.


To assist in the selection of the on-site system and tos t a n d a rdise the assessment pro c e s s , a sitecharacterisation form has been prepared (AppendixA ) . The completed form should accompany all

planning applications for on-site systems for a singlehouse. A verification section is included at the endof the form and this should be completed by theplanning authority.

Information Collected Relevance Factor Determined

Zoning (County development plan,groundwater protection scheme,groundwater protection response etc.);

Hydrological features;

Density of houses;

Proximity to significant sites;

Experience of the area;

Proximity to surface features;

Depth to bedrock

Texture;

Structure;

Bulk density;

Layering;

Colour;

Mottling;

Depth to water table;

Drainage (permeability);

Percolation test;

TABLE 8: INFORMATION OBTAINED FROM THE DESK STUDY AND ON-SITE ASSESSMENT

Identifies planning controlsand targets at risk

Indicators of the suitability ofthe subsoil for percolationand of its percolation rate

A minimum thickness of 1.2 mof unsaturated soil is required

to successfully treat septictank effluent

Identifies suitable soils that haveadequate but not excessive

percolation rates(T or P value)

Depth of the water table

Unsuitability if prismatic,structureless-massive

silt or clay.

Sufficient subsoil to allowtreatment of wastewater

Depth to bedrock

Site restrictions


3.1 INTRODUCTION

The information collected from the desk study andon-site assessment should be used in an integratedway to determine whether an on-site system can bedeveloped, and if so, the type of system that isappropriate and the optimum final disposal route ofthe tre ated wa s t ewat e r. Depending on thecharacteristics of the site, more than one option maybe available.

3.2 CHOOSING AN ON-SITE SYSTEM

Figure 9 sets out how the information from the deskstudy and on-site assessment is used to choose an on-site system. The procedure for deciding how todispose of the treated wastewater, i.e., whether it canbe disposed of by soil percolation to groundwater orbe discharged directly to surface water, is set out inthe lower half of the diagram. The wastewater froman on-site system cannot be disposed of by soilp e rc o l ation to gro u n dwater unless the subsoilcharacteristics are suitable for this purpose.

The desk study information is first examined. Areaswith significant sites e.g. archaeological, naturalheritage or historical features should be ruled out offurther consideration. If past experience indicatesthat there have been problems with the proposedsystem in the locality, further investigation may bewarranted before proceeding to the next step. If theDesk Study conclusion is that the site is potentiallysuitable for an on-site system, an on-site assessmentshould be carried out.

As previously mentioned, the on-site assessmentconsists of a visual assessment, a trial hole andpercolation tests. The minimum separation distancesto pass the visual assessment are set out in Table 4 .These apply to any treatment system. If any of theserequirements cannot be met, on-site systems cannotbe developed on the site. Where a site is marginal,further investigation may be warranted to identifywhether or not site improvements will suffice. If thevisual assessment indicates that the site is potentiallysuitable, proceed to the trial hole stage of the on-siteassessment.

Table 7 sets out the subsoil characteristics whichindicate satisfactory percolation and other subsoil

ch a ra c t e ristics suitable for the tre atment ofwastewater. The percolation characteristics will beconfirmed later by examining the percolation testresults. It is important to remember that the subsoilcharacteristics (i.e. depth and type of subsoil) as setout in Table 7, and the control measures outlined inthe groundwater protection responses should bothbe satisfied. In some cases the requirements of thegroundwater protection responses will be greaterthan 1.2 m of subsoil below the inve rt of thepercolation trench.

3.2.1 SYSTEMS USED WHERE ON-SITEASSESSMENT IS SUCCESSFUL

If the site satisfies all the specified requirements andhas a T value between 1 and 50, the site is suitablefor the development of a septic tank with a soilpercolation area (conventional septic tank system).

A septic tank followed by either an intermittent filteror a constructed wetland, or a mechanical aerationunit can also be developed on sites that have suitablepercolation characteristics.

3.2.2 SYSTEMS USED IN THE EVENT OF ON-SITE ASSESSMENT FAILURE

For sites which fail the on-site assessment (refer toFigure 9), site improvement works may allow thed evelopment of an on-site system. Th e s eimprovement works may include lowering the watert able by dra i n age, i n c reasing the soil depth byi m p o rting suitable soil or replacing ex i s t i n gunsuitable soil. The conditions that give rise to ahigh water table are site specific; these includetopography, nature of soils, bedrocks and outfalls.Some of the pro blems resulting from theseconditions are re a d i ly solve d. Detailed designprocedures are available in drainage manuals2 .

I m p o rted soil may be placed in mounds - asillustrated in Figure 3 - or level with the groundsurface. The mounds may be constructed partially ortotally overground. Free draining unsaturated soilsas detailed in Table 5 should be used. The fill shouldbe placed in layers not exceeding 300 mm thick andlightly compacted. Great care should be taken not toovercompact the soil as this will lead to ponding.After each lift is placed, percolation tests should be

3. TREATMENT OPTIONS

2 Mulqueen, Rodgers, Hendrick, Keane, McCarthy (1999). Forest Drainage Engineering. COFORD Dublin.

3 TREATMENT OPTIONS 27

carried out. A 150 mm square hole is excavated to adepth of 150 mm in the placed soil. After presoakingto completely wet up the soil, 0.5 litres of water ispoured into the hole and the time in minutes for thewater to soak away is recorded. This time should bebetween 10 minutes and 2 hours. After these workshave been completed, a second test hole must beexcavated in an appropriate location in the improvedsoil and a percolation test carried out.

Where a minimum of 0.6 m of permeable soil ispresent or can be placed over the rock and/or thewater table (i.e. shallow soils) and all otherrequirements of the on-site assessment are satisfied,an on-site system can be developed using this soil asa polishing filter in anyone of the following:

• a septic tank and an intermittent filter followed by a polishing filter;

• a mechanical aeration system followed by a polishing filter; or

• a septic tank and constructed wetland systemfollowed by a polishing filter may be installedon the site.

The treated wastewater from systems other than aconventional septic tank system should be percolatedthrough a polishing filter to reduce microorganisms.

The polishing filter may comprise the in situ soil,wh e re there is adequate depth of suitable soil,imported suitable soil or a combination of the in situand imported soil. Polishing filters may also beconstructed from medium fine sands placed in layersalternating with layers of 10-20 mm clean washedgravels. Typical designs for polishing filters aregiven in Chapter 4.


Pass

PercolationTest

(T- Test)

VisualAssessment

Pass

Trial Hole

Pass

On-site Assessment

Desk studyFailDesk study

Conventional septictank system

Mechanicalaeration system

Intermittentfilter system

Wetlandsystem

Pass Fail

Unsuitable

Siteimprovement

Archaeologicalsite, NHA

Fai

l

Discharge togroundwater

Discharge**

soilpolishing

filter

sandpolishing

filter

PercolationTest

(P- Test)*

Fail

This option may notalways be available

Site may not besuitable for any on-

site system

FIGURE 9: FLOW DIAGRAM FOR CHOOSING AN ON-SITE SYSTEM

3 TREATMENT OPTIONS 29

* A P (or T) test is required to design a soil polishing filter. The hydraulic loading r ate depends on the soil or bedrockand recommended loading rates are as follows: up to 20 l/m2.d for P/T values of 20 or less; up to 10 l/m2.d for P/Tvalues from 21 to 40 and up to 5 l/m2.d for P/T values 41 -50.

** The treated wastewaters from the polishing filter may discharge to g roundwater or surface water depending on thenature of the strata underlying the filter. The discharge of treated wastewater from the polishing filters overlyingpermeable soils or bedrocks may go to groundwater. A soil or bedrock with a P/T value of 50 or less is suitable topercolate effluent from a polishing filter to groundwater. The treated wastewater from the polishing filters overlyingsoils or bedrocks with P/T values greater than 50 is collected in a suitable drainage system and discharged to surfacewaters.

Desk Study

On-site Assessment

Archaeologicalsite, NHA

3.3 CHOOSING THE OPTIMUM DISCHARGEROUTE

Once the on-site treatment system has been decidedupon, the disposal of the treated wastewater needs tobe considered. For septic tank systems with a soilpercolation area the treated wastewater will normallybe discharged to the groundwater. In the case offilters, mechanical aeration systems and wetlandsystems, treated wastewater from the polishing filtermay be discharged to the ground or to surface water(Figure 9).

In the case of a discharge to surface water a licenceto discharge is required from the local authorityunder the Water Pollution Acts 1977-1990. Wheresuch a licence is required, the final wastewaterquality from the on-site system should comply withthe requirements set out in the licence.

3.4 LICENCE REQUIREMENTS

The discharge of any sewage effluent to "waters3"requires a licence under the Water Pollution Acts1977-1990. Licence applications are processed bythe local authorities. Domestic sewage, however, notexceeding 5 m3/day, which is discharge d to anaquifer from a septic tank or other disposal unit, bymeans of a percolation area, soakage pit or othermethod is not subject to the licensing provisions ofthe 1977-1990 Acts. If an on-site system does notcomply with all the conditions above, a dischargelicence is required for the on-site system. However,it should be noted that a "soakage pit" or similarmethod is not an acceptable means for treating septictank effluent and does not comply with therequirements set out in this document.

Where it is proposed to discharge wastewater to"waters", local authorities should assess the impactof the discharge from the on-site system on thereceiving water. The parameters to be examinedshould include:

• Flow;

• BOD;

• Nitrates;

• Ammonia;

• Phosphates; and

• Microorganisms.

When assessing the impact of an on-site system,local authorities should consider the beneficial usesof the receiving water. The principal beneficial usesof surface water are, water intended for humanconsumption after treatment, agriculture, bathing,b o at i n g, c o a rse fi s h e ry, c o o l i n g, game fi s h e ry,general amenity, or industry. Principal beneficialuses of gro u n dwater are agri c u l t u re, d ri n k i n g,i n d u s t ry and raw water intended for humanconsumption after treatment.

Once the beneficial use of the water has beenestablished, local authorities should consult relevantRegulations, water quality management plans anda ny published standards to obtain the re l eva n tdischarge standard. The treated wastewater from theon-site system should comply with the water qualitystandard set for the receiving waters.

3 includes any (or any part of any) river, stream, lake, canal, reservoir, aquifer, pond, watercourse or other inland waters,whether natural or artifical.


4.1 SEPTIC TANKS

Septic tanks are primary settlement tanks providinga limited amount of anaerobic digestion. Septic tanksshould comprise two ch a m b e rs. For bestperformance, septic tanks should have the followingcharacteristics:

• septic tanks should be much longer than theyare wide to promote settlement of suspendedsolids;

• larger septic tanks are better than smaller tanks because of greater settlement of solids and larger storage volume for liquid and solids;

• properly designed baffles provide quiescent conditions and minimise the discharge of solids to the percolation area; and

• the inlet and outlet of the septic tank should be separated by a long flow path for the wastewater; if the outlet is too close to the inlet, solids settlement and grease separationmay be inadequate.

Septic tanks must be able to (i) withstand corrosion(ii) carry safely all lateral and vertical soil pressuresand (iii) accommodate water pressure from insideand outside the tank without leakage occurring.S eptic tanks must be wat e rtight to prevent (i)wastewater escaping to the soil outside, and (ii)surface water and groundwater entering the tank. Aleaking tank can cause pollution problems.

4.1.1 SEPTIC TANK CAPACITY

The septic tank should be of sufficient volume top rovide a retention time for settlement of thesuspended solids while re s e rving an adequat evolume for sludge storage. The volume required forsludge storage is the determining factor in sizing theseptic tank and this sizing depends on the potentialoccupancy of the dwelling which can be estimatedfrom the maximum number of people that the housecan accommodate taking into account the numberand types of bedrooms. The tank capacity may becalculated from the following formula:

where

C = the capacity of the tank (litres)

P = the design population with a minimum of 4persons

A minimum capacity of 2720 litres (2.72 m3) shouldbe provided. This assumes that desludging of theseptic tank is carried out at least once in every 12-month period. When kitchen grinders are installed,additional sludge solids are discharged with thewastewater and the capacity of the septic tank shouldbe increased by 70 litres for each additional person.Typical septic tank capacities and dimensions forrectangular tanks are shown in Table 9, Table 10,Figure 10 and Figure 11.

4. SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS

C = 180 . P + 2000

4 SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 31

Outlet T-Piece

OUTLETINLET

Cover

Scum layer

2200 1000

liquid layer

200Sedimentation

Gas BuoyedFlotation

Inlet T-piece

300 Freeboard

Sludge Layer

TWL75

350C

Ventilation cowl

350

550

SECTION A-A

No of Required Dimensions (m)Persons Storage Capacity

(litres)

length width depth

C = 180 . P + 2000 a* d* b* c*

3 2720 2.2 1.0 1.0 1.2

4 2720 2.2 1.0 1.0 1.2

5 2900 2.4 1.0 1.0 1.2

6 3080 2.5 1.0 1.0 1.2

TABLE 9: TYPICAL CAPACITIES OF SEPTIC TANKS

* refer to Figure 11

FIGURE 10: LONGITUDINAL SECTION OF A TYPICAL SEPTIC TANK (ALL DIMENSIONS IN MM)

INTLET


TWL

450

200

Section B-B

B

B

AA

10001000

2200

Floor Plan

d

c

a

b

FIGURE 11: PLAN AND SECTION OF A SEPTIC TANK (ALL DIMENSIONS IN MM)


4.1.2 CONSTRUCTION OF A SEPTIC TANK

The design features of a septic tank system areoutlined in Table 10.

S eptic tanks may be cast in situ or may beprefabricated from steel, reinforced concrete, glassfibre reinforced concrete or plastic. Two-chamberedtanks may be accommodated by having two separatetanks connected together using a tee-piece baffle ineach tank. The principles given for rectangular tanksshould be fo l l owed for cy l i n d rical tanks wh e rereasonably practical. Some upward adjustment tovolumes may be necessary. The roof, outer wallsand floors of the tank and all joints should bewatertight.

4.1.3 WATERTIGHTNESS OF SEPTIC TANKS

A septic tank should be watertight up to the top ofthe tank. Methods employed to test such tanksshould be in accordance with CEN E u ro p e a n

Standard EN 12566 watertightness test method4.

For concrete septic tanks the loss of water measuredafter 30 min. should be ≤ 0.1 litre per m2 of theinternal wet surface area of external walls. Forp o lye t hylene and glass re i n fo rced plastic (GRP)septic tanks, no leakage is permitted.

4.1.4 IN SITU TANKS

The fo l l owing construction standards arerecommended for in situ tanks:

• the floors should be of concrete with a minimum thickness of 225 mm;

• the walls should be a minimum of 100 mm thick reinforced concrete or equivalent suchas 225 mm solid block rendered wall;

• the roof should be either cast in situ or precast reinforced concrete; and

Tank Characteristics Recommended Requirements

Tank capacity 2720 litres for 4 persons

Tank length to width ratio 2 to 3 : 1

Number of compartments 2

Volume of inlet compartment 2/3 to 3/4 of the total tank capacity

Concrete compressive strength 35 N/mm2 at 28 days, minimum

Wall thickness 100 mm minimum reinforced concrete orequivalent

Roof thickness 125 mm minimum

Interior height 1.2 m minimum

Liquid depth 0.9 m minimum

Freeboard (roof height above liquid) 300 mm

Baffle wall liquid opening 450 mm to centre of opening from floor of tank(Figure 10 and Figure 11)

Inlet and outlet pipes Minimum internal diameter of 100 mm

Bottom end of T-piece 550 mm above floor of tank

Difference in elevation of inlet and outlet 75 mm

Joints Watertight joints required

Ventilation 100 mm diameter pipe in roof with a cowl ineach chamber

Access covers 600 mm x 600 mm (2 no.)

TABLE 10: TYPICAL DESIGN FEATURES OF A SEPTIC TANK

4 CEN/TC 165 "Wastewater Engineering" is preparing a series of European standards on small wastewater treatmentsystems.


• for safety, the roof should be strong and stableenough to prevent interference from childrenand others.

4.1.5 PREFRABICATED TANKS

Prefabricated tanks should be manufactured froms u i t able mat e rials (e. g. pre-cast concre t e, g l a s sreinforced plastic, glass reinforced concrete) and therequirements stated above for capacity, hydraulics,strength and water-tightness should be observed.Prefabricated septic tanks are to be preferred to thosecast in situ because quality control and testing can beundertaken at the factory. In the case of l ightprefabricated tanks, attention should be paid to therisk of fl o t ation of the tanks as a result ofgroundwater pressure or surface run-off gainingaccess to the excavation.

4.1.6 LOCATION OF SEPTIC TANKS

Recommended minimum distances of separation ofseptic tanks and percolation areas and filters from avariety of features are shown in Table 4. Provisionshould be made for access for a sludge tanker andmaintenance equipment to desludge the tank.

4.1.7 ANCILLARY CONSTRU C T I O NMATERIALS

All materials used in the construction of the worksshould comply with the requirements of the BuildingRegulations and the relevant Technical GuidanceDocument.

4.1.8 DRAIN FROM HOUSE TO SEPTIC TANK

The drain to the septic tank should be at least 100mm in diameter. It may be of earthenware, concrete,or uPVC or similar materials. It should be jointed togive a watertight drain and should be laid to theminimum gradients listed in Table 11.

It should be vented by means of a vent pipe abovethe eaves of the house (Figure 13). A manholeshould be provided for rodding the drain and shouldbe located within one metre of the septic tank. Thedrain should include, at an appropriate location anaccess junction, to facilitate a future connection to asewer network.

4.2 PERCOLATION AREAS

The most important component of a conventionalseptic tank system is the percolation area. Septictanks remove most of the suspended solids andgrease from the wa s t ewat e r, but it is in thepercolation area that the wastewater is treated. In thec o nventional perc o l ation tre n ch method, t h ewastewater is allowed to flow by gravity into adistribution box which distributes the flow evenlyinto the several distribution pipes in the percolationtrenches. Wastewater flows out through orifices inthe distribution pipes into a gravel underlay whichthen distributes it on to the soil, where it undergoesbiological, physical and chemical interactions thateliminate or reduce the contaminants. For effectivetreatment, the wastewater must enter the soil; if thebase or walls of the percolation trench are compactedor glazed or otherwise damaged during excavation,they should be scratched with a steel tool such as arake to expose the natural soil surface. It is equallyimportant that the wastewater remains long enoughin the soil; the residence time is controlled by thehydraulic loading and the rate of flow into the sidesand base of the trench.

4.2.1 HYDRAULIC LOADING RATES

The percolation rate through the trench base andsidewalls is controlled by the biomat on the floor andsides of the trench rather than by the subsoil itself inthe case of all suitable subsoils. The percolationrates, measured as they are on virgin subsoil usingclean water, cannot be used for the design of thehy d raulic distri bution system and length ofpercolation trench (Figure 2). A loading rate of 20l/m2.d is recommended to take into account the effectof the biomat. The minimum length of percolationtrench required is given in Table 12.

e.g. at a loading rate of 20 l/m2.d and a wastewateruse of 180 l/person.d, the invert area of percolationtrench required for a 4-person household is 36 m2 .If the width of the invert of the percolation trench is450 mm, then the length of perc o l ation tre n chrequired is 80 m.

Drainpipe Material Minimum

Earthenware 1 in 40

Concrete 1 in 40

uPVC 1 in 60

TABLE 11: MINIMUM GRADIENTS FOR DRAIN TO

SEPTIC TANK


4.2.2 GENERAL

No wat e rm a i n s , s e rvice pipes, access ro a d s ,driveways or paved areas should be located withinthe percolation area.

The layout of the distribution pipes should makeoptimum use of the available site and be consistentwith the recommendations in Table 4 and Table 13.Land drainage pipes have narrow slots and are likelyto clog; hence they are unsuitable as percolationpipes. A plan and section of a conventional septictank system layout is given in and a distribution boxis detailed in Figure 13 and Figure 14.

Number of People Required Lengthin the House of Trench (m)

3 60

4 80

5 100

6 120

7 140

8 160

9 180

10 200

TABLE 12: MINIMUM PERCOLATION TRENCH LENGTH

Percolation Trench Characteristics Recommendations

Length of distribution pipe 20 m maximumin each trench

Minimum separation distance 2 m (2.45 m centre to centre)between percolation trenches (Figure 13)

Diameter of pipe from septic tank 100 mm

Slope of pipe from tank to 1 in 40 for earthenware or concrete, 1 in 60 for uPVCdistribution box

Slope of percolation trench from 1 in 200distribution box

Distribution (percolation) pipes • 100 mm bore, perforated (typically at 4,6 and 8 o’clock)smooth wall PVC drainage pipes with perforations of 8 mm diameter at about 75 mm centres along the pipe;

or• pipes with similar hydraulic properties.

Width of percolation trench* 450 mm

Depth of percolation trench About 800 mm below ground surface depending on site(Figure 12)

Backfilling of percolation trench 250 mm of 20-30 mm washed gravel or broken stone aggregate(Figure 12) on invert; pipe laid at a 1 in 200 slope surrounded by 20-30mm

clean washed gravel or broken stone aggregate and with 150 mm of similar aggregate over pipe; geotextile layer followed by topsoil to ground surface.

TABLE 13: DETAILS OF A TYPICAL PERCOLATION TRENCH

* any compaction or glazing of the soil surfaces on the invert and sidewalls on the trench should be undone byscratching to expose a natural soil surface with a steel tool such as a steel rake or trowel.


Ground level

Topsoil

GravelGravelDistribution Pipe

Gravel

UnsaturatedSuitable Soil

300 mm

150 mm100 mm

250 mm

1200 mm

Geotextile

450 mm

Base of the percolation trench

800 mm

House 10 m (min) to percolation area

100 mm Ø100 mm Ø

7m (min) to septic tank

Access chamber (for rodding and possible later connection to public sewer)

PLAN

Distribution box 20 m (max.) length

Percolation area

2m

Vents2.45m

2.45m

Eaves vent

Septic tank Distribution box

1 in 200 slope1 in 40 slope for earthware orconcrete: 1 in 60 for uPVC

vents

SECTION

Percolation pipework

FIGURE 13: PLAN AND SECTION OF A CONVENTIONAL SEPTIC TANK SYSTEM

FIGURE 12: SECTION OF A PERCOLATION TRENCH


4.3 CONSTRUCTION REQUIREMENTS FORPERCOLATION PIPES

4.3.1 PERFORMANCE OF WORK ANDINSTALLATION OF THE SYSTEMS

Earth moving machinery should not circulate overthe percolation area after pipework and backfillingof trenches have been completed. Access manholesshould be located at ground surface so that they area c c e s s i bl e. The distri bution box (Fi g u re 14)comprises a chamber which divides the effluent fromthe septic tank equally between the distribution pipessupplying the percolation area. The distribution boxmust be designed and constructed to ensure equaldistribution among the various distribution pipes. Ifnecessary, special fittings may be used to facilitatethis.

4.3.2 GENERAL PRECAU T I O N A RYMEASURES FOR EXCAVATION OFTRENCHES

Earthworks should normally be carried out on dryground. Trenches should be backfilled as soon aspossible after excavation.

4.3.3 INSPECTION OF PERCOLATION PIPES

Cutting and drilling of pipes should be clean andsmooth. Befo re installat i o n , the holes in thepercolation pipes should be inspected to ensure thatthey are not blocked.

4.3.4 ACCESS AND INSPECTION

Access and inspection covers should be visible andflush with the ground surface without allowing thee n t ry of surface wat e r. All cove rs should be

accessible, and of suitable size (minimum 600 x 600mm) for maintenance and inspection.

4.3.5 VEGETATION

The growth of any type of tree or plant whichdevelops an extensive root systems should be limitedto a minimum distance of 3 m from the percolationarea. This restriction also applies to the cultivationof crops necessitating the use of machinery, likely todisturb the percolation trenches.

4.4 MAINTENANCE OF SEPTIC TANKS ANDPERCOLATION AREAS

R egular maintenance of the septic tank andpercolation area is very important for the satisfactoryperformance of the system. The septic tank should bedesludged a minimum of once per year or when:

• scum is noticeable in the second chamber of the tank; and/or

• the depth of sludge in the second compartment is greater than 400 mm.

The depth of sludge can be checked using thefollowing technique:

(i) use a 2 m pole and wrap the bottom 1.2 m with a white rag;

(ii) lower the pole to the bottom of the tank andhold there for several minutes to allow the sludge layer to penetrate the rag; and

(iii) remove the pole and note the sludge line,which will be darker than the coloration caused by the liquid waste.

50 mm100 mm

100 mm

A A

100 mm 600 mm

Varies to suit site

Effluent from tank

100 mm

100 mm

600 mm

100 mm

effluent from tank

SECTION PLAN

FIGURE 14: PLAN AND SECTION OF A TYPICAL DISTRIBUTION BOX


The percolation area should be inspected regularly.Signs of ponding indicate blockage or insufficientpermeability.

4.4.1 ADVANTAGES AND DISADVANTAGESOF CONVENTIONAL SEPTIC TA N KSYSTEMS

The advantages of a septic tank and soil percolationsystem include:

• easy management;

• no external power requirements;

• no noise emissions;

• natural treatment process yielding a high quality effluent;

• cost effective treatment system; and

• no need for polishing filter.

The disadva n t ages of a septic tank and soilpercolation system include:

• size of area required;

• greater depths of subsoil to treat the wastewater relative to other systems; and

• unsuitable for some subsoil types.

4.5 FILTER SYSTEMS

As discussed in Chapter 3, ap a rt from a soilpercolation area (conventional septic tank system) asalready outlined, the wastewater from a septic tankcan be treated by the following filter systems:

• a soil percolation system such as a placed soiloften in the form of a mound (intermittent soilfilter system) as already outlined;

• an intermittent sand filter followed by a polishing filter (intermittent sand filter system);

• an intermittent peat filter followed by a polishing filter (intermittent peat filter system);

• an intermittent plastic and other media filter followed by a polishing filter (other intermittent media filter systems); or

• a constructed wetland followed by a polishingfilter.

The typical layout for the treatment of wastewaterusing an intermittent filter or a constructed wetlandis illustrated in Figure 15. The site conditions willi n fluence the re q u i rement for pumping thewastewater through the different treatment units;h oweve r, i n t e rmittent fi l t e rs and most polishingfilters require pumping.

SEPTIC TANK

INTERMITTENT FILTER SYSTEM

orCONSTRUCTED WETLAND

POLISHING FILTER**

PUMPING CHAMBER* PUMPING CHAMBER*

* Pumping is always required for intermittent filters. If the topography or the design permits, gravity systems may be possible for constructed wetlands and polishing filters

** The intermittent filter and the polishing filter may be combined into one unit in certain cases

FIGURE 15: ILLUSTRATION OF AN INTERMITTENT FILTER OR CONSTRUCTED WETLAND SYSTEM


4.6 SOIL FILTER SYSTEMS

4.6.1 INTERMITTENT SOIL FILTER

An intermittent soil filter system is pri n c i p a l lyapplicable to a placed or improved soil with a1 ≤ T/P ≥ 50 but can also be used on in situ soil witha top layer re m oved to accommodate grave l .Wastewater from the septic tank is allowed to flowinto a pumping chamber, in which pump and flowcontrols are housed. The effluent is then pumpedintermittently, 3-4 times per day onto a manifoldwith lateral distribution pipes provided with orifices.These lat e rals are arra n ged in parallel andsurrounded by a gravel layer, resting on top of thesoil that often is a mound but can also be level withground surface; the gravel distributes the wastewaterevenly onto the soil surface. The wastewater doseenters the soil and percolates slowly downwards. Ifthe native in situ soil surrounding the placed soil is acoarse sand or gravel, the percolation soil and graveloverlay should be lined at the sides; if the native insitu soil is impervious, an underlay of gravel with apumping sump is required to remove the purifiedeffluent to a surface water body e.g. watercourse, forwh i ch a disch a rge licence may be re q u i re d.Hydraulic loading is 4 l/m2.d on the plan area of thesoil. The layout and arrangement of the pipeworkare described in greater detail in Section 4.7 underSand Filter Systems.

4.6.2 ADVA N TAG E S / D I S A DVA N TAGES OFSOIL FILTER SYSTEMS

The advantages of soil filter systems include:


• high quality effluent;

• high operational flexibility that can be used for nitrification, denitrification and phosphorus removal;

• stable treatment process; and

• no need for a polishing filter.

The disadvantages of soil filter systems include:

• pumping is required for influent distribution;

• odours may occur from open filters; and

• filter may clog giving rise to waterlogging.

4.7 SAND FILTER SYSTEMS

4.7.1 INTERMITTENT SAND FILTERS

Intermittent sand filter systems can be used to treatwastewater from a septic tank. They are normallyused wh e re the soil is unsuitable for a soilp e rc o l ation system. Intermittent sand fi l t e rs areeffective and easy to operate. They require onlysmall areas and are relatively cheap to construct.

Two types of intermittent sand filters are commonlyused, namely, soil covered and open. Soil coveredintermittent sand filters may be underground (Figure1 6 ) , p a rt underground and part ove rgro u n d, o roverground (Figure 3). The latter two constructionsare commonly referred to as mound systems. Openintermittent sand filters are constructed similar to thecovered sand filters,but without the soil cover i.e. thegravel distribution layer is exposed at the surface toa l l ow for inspection and periodic maintenance.They are preferably underground with the top of thegravel at ground surface.

Intermittent sand filters are single-pass slow sandfilters, which support biofilms. They consist of oneor more beds of graded sand commonly 600 - 900mm deep, underlain normally by a layer of filtergravel about 200 mm thick to prevent outwash orpiping of the sand. Septic tank wastewater is pumpedintermittently 3-4 times per day onto the surface ofthe sand bed through 25 mm diameter lateral pipeswith orifices, embedded in a 200 mm thick layer ofd i s t ri bution gravel. In soil cove red fi l t e rs , ageotextile is used to separate the soil cover from thedistribution gravel. The wastewater from the septictank flows through the sand bed where it receivestreatment. The wastewater treatment takes placeunder pre d o m i n a n t ly unsat u rated and aero b i cconditions. In a soil cove red fi l t e r, both thedistribution gravel over the sand and the drain filtergravel under the sand are vented; the vents areextended vertically above ground or mound level andcapped with a cowl or grid. In an open filter only thedrain filter gravel is vented.

In impervious soils, all surface run-off and seepagefrom the surrounding soil should be cut off bys h a l l ow interc eptor dra i n s , the depth of wh i chdepends on the depth to the impervious layer. Theinterceptor drain should be 2 m distant from the edgeof the sand fi l t e r. These drains comprise landd ra i n age pipes ove rlain to ground surface withpermeable gravel or broken stone aggregate. Thesei n t e rc eptor drains are brought to the neare s twatercourse or stream into which they outfall. In thecase of overground sand filters, the collector drains


to remove the filtrate are excavated in the soil toplayer at appropriate spacings into the top of theimpervious layer, piped and backfilled with filtergravel to original ground surface. The wastewaterfrom the intermittent filter is normally collected in achamber, from where it is discharged to a polishingfilter. In some cases, the in situ topsoil underneaththe intermittent filter may have sufficient depth (seesection 4.11) on its own or with placed imported soil(i.e. site improvement works) to act as a polishingfilter.

In very permeable (gravelly) sites, the filtrate fromthe intermittent sand filter, after passing through apolishing filter, may percolate to the groundwater;filter gravel at the base of the sand filter is notre q u i red wh e re the polishing filter is dire c t lyunderneath the intermittent filter. An impermeableliner is used to seal off the sides of the intermittentsand filter to prevent possible bypass into thegravelly soil when the filter is underground; thisbypass could occur when a flooding dose is appliedto the distribution gravel. Where the polishing filteris off s e t , the entire intermittent filter must beenclosed in a leak proof liner.

4.7.1.1 DESIGN CRITERIA

Sand selection is decided first and is based ongrading curve characteristics. Effective grain sizes(D10) for soil covered and open sand filters are in therange 0.7 - 1.0 mm and 0.4 - 1.0 mm respectively

with uniformity coefficients (D60/D10) less than 4(Table 14). The smaller the effective grain size, thehigher the level of tre at m e n t , the lower thepermissible hydraulic loading and the more frequentthe need for maintenance. The lower the uniformitycoefficient, the longer the filter's life-span and theless the potential for elutriation downwards of thefiner particles which could result in clogging. Allfilter gravels must be designed on filter principlesafter for example, viz.:

D15 Filter gravel/D85 Sand < 5;

4 < D15 Filter gravel/D15 Sand < 20;

D50 Filter gravel/D50 Sand < 25,

where D15, D50 and D85 are the particle sizes from agrading curve at 15, 50 and 85% finer by weightordinates, respectively.

4.7.1.2 LOADING RATES

Loading rates vary with the characteristics of thefilter sand and the type of design. Rates typicallyvary from 40 - 100 l/m2.d* . Soil covered filtersshould have the lowest hydraulic loadings. Higherrates than those above result in impaired wastewaterquality and an increase in the fre q u e n cy ofmaintenance. Design criteria are shown in Table 14.

* USEPA (1992). Wastewater Treatment/Disposal for Small Communities. Manual No. EPA/625/R-92/005


Effluent fromseptic tank

Threaded

100 mm perforatedunderdrain with 5 mm holes

Inspection lid

3 mm holes facing up

25 mm PVC lateral

Top soil Distribution pipes

Inspection/vent

Existing ground level

900 mm

Clean washed gravelor broken stone

Sand

75 mmFiltergravel

PLAN

SECTION

Grassembankment

Geotextile

FIGURE 16: SCHEMATIC DIAGRAM OF A SOIL COVERED INTERMITTENT SAND FILTER FOR AN IMPERVIOUS SOIL


4.7.1.3 APPLICATION OF SEPTIC TA N KWASTEWATER

The wastewater should be applied uniformly to thesurface of the filter sand at intervals such that thewastewater completely infiltrates the sand. Thisallows ample aeration of the filter sand to effecta e robic tre atment. Even distri bution may beobtained by pumping the wastewater through evenlyspaced lateral pipes with evenly spaced orificese m b e dded in a distri bution gravel. Dosingfrequencies are related to the type of filter sand. Adosing fre q u e n cy of 3 - 4 times daily isre c o m m e n d e d. Dosing tanks are sized for themaximum daily dose to be used.

In a typical design for a 4-person household with ahydraulic loading of 0.72 m3/day using a loading rateof 100 l/m2.d, a plan area of 7.2 m2 of intermittentfilter is required. This can be accommodated in atypical layout of 8 number 25 mm diameter lateralpipes each 1.5 m long and spaced 0.6 m apart, ands e rved by a central manifo l d. Ori fices with adiameter of 3 mm are drilled in the lateral pipes atequal spacings of 0.3 m to uniformly distribute thewastewater onto the filter media. The wastewater ispumped onto the intermittent filter in 4 doses, eachof 180 litres over a duration of about 2 minutes.

4.7.1.4 OPERATION AND MAINTENANCE

Intermittent sand filters require little control andmaintenance. The main tasks are servicing of thedosing equipment,monitoring of the wastewater, andpossible maintenance of the sand surface in opensand filters. Soil covered sand filters are expected towork without maintenance throughout their workinglife. When desludging the septic tank, the pumpsump should also be desludged.

4.7.2 ADVA N TAG E S / D I S A DVA N TAGES OFSAND FILTER SYSTEMS

The advantages of sand filters include:



• high operational flexibility that can be used for nitrification, denitrification and phosphorus removal; and

• stable treatment process.

The disadvantages of sand filters include:

• need for a polishing filter;

• costs are higher than natural percolation areas;

• pumping is required for influent distribution and in most cases for the distribution of the filtrate to a polishing filter;



4.8 PEAT FILTER SYSTEMS

Fibrous peat filters are used as intermittent openfilters to treat septic tank wastewater. The peat fibresa re placed in modules and compre s s e d. Th ethickness or depth of the compressed peat is about0.7 m and its dry density is about 200 kg/m3. Thesurface area of one commercial fibrous peat filter issized at 1 m2/person. The hydraulic loading rate onpeat filters varies depending on the type of peate m p l oye d. Commercial fi b rous peat fi l t e rs are

Design Factor Soil Covered Open

Effective grain size (D10), mm 0.7 - 1.0 0.4 - 1.0

Uniformity coefficient (D60/D10) < 4.0 < 4.0

Depth of sand filter, m 0.6 - 0.9 0.6 - 0.9

Hydraulic loading, l/m2.d 40 - 60 50 - 100

Dosing frequency, no./day 2 - 4 1 - 4

TABLE 14: DESIGN CRITERIA FOR INTERMITTENT SAND FILTERS

USEPA, 1992


presently designed at hydraulic loading rates inexcess of 100 l/m2.d. A layer of lightweight coarsegrained aggregate such as shells is placed on top ofthe peat to promote unifo rm distri bution andminimise disturbance. A pipe distribution networkfitted with orifices as illustrated in Figure 16 or aspray irrigation system is used to evenly distributethe wastewater that is pumped from the secondchamber of the septic tank or from a plastic sumpfitted with a baffle filter, downstream of the septictank. Each module of a modular unit should beprovided with a cover.

4.8.1 OPERATION AND MAINTENANCE

The surface of the peat filter should be examinedperiodically for signs of ponding. When desludgingthe septic tank, the pump chamber should also bedesludged.

4.8.2 ADVA N TAG E S / D I S A DVA N TAGES OFPEAT FILTER SYSTEMS

The advantages of peat filters include:



• high operational flexibility that can be used for nitrification, denitrification and phosphorus removal; and

• stable treatment process.

The disadvantages of peat filters include:


• costs are higher than natural percolation areas;

• pumping is required for influent distribution and in most cases for the distribution of the filtrate to a polishing filter;



4.9 OTHER INTERMITTENT MEDIA FILTERSYSTEMS

Other intermittent media filter systems may fromtime to time be introduced to treat wastewater. Suchfilter products could include geotextile strips andother media that can be used to attach biofilms.Many, such as geotextile strips will operate in amanner similar to fibrous peat filters while othersmay employ novel ways to attach biofilms. Wheres u ch products are intro d u c e d, i n d ep e n d e n teva l u ation should be carried out to ve rify themanufacturer’s design loadings. Other intermittentmedia filters should be followed by polishing filters.

4.10 CONSTRUCTED WETLANDS

A constructed wetland system is another option usedto treat wastewater from a septic tank ( Figure 17 andFigure 18)5. As mentioned in Chapter 1, constructedwetlands can be characterised by the flow path ofthe wastewater through the system. In horizontalflow constructed wetlands, wastewater is introducedat one end of a flat to gently sloping bed of reeds andflows across the bed to the outfall end. If the surfaceof the wastewater is at or above the surface of thewetland media, the system is called a free-waters u r face (FWS) hori zo n t a l - fl ow we t l a n d. If thesurface of the wastewater is below the surface of thewetland media, the system is called a sub-surface(SFS) hori zontal fl ow we t l a n d. As it fl ow s ,microorganisms attached to the reeds and supportmedia purify the wastewater. The media can consistof soil (free-water surface), gravel or other suitablematerial. In the vertical-flow wetland (Figure 18),the wastewater is distributed uniformly over, andintermittently onto the media, and gradually drainsvertically to a drainage network at the base of themedia; as the wastewater drains vertically, air re-enters the pores in the media. The media used in thevertical-flow wetland can consist of a layer of sandoverlying a layer of grave l . The sand must beprotected from erosion and piping.

5 Cooper, P.F., Job, G.D., Green, M.B. and Shutes, R.B.E. (1996). Reed beds and constructed wetlands for wastewatertreatment. Water Research Centre, Swindon, U.K.



Constructed wetlands should be inspected weekly.Flow distribution should be carefully examined forchannelling and orifice blockage. Sidewalls shouldbe maintained. Rabbits, weeds and plant diseases cancause damage to the reeds. Solids from thewastewater will reduce the pore space in the mediae s p e c i a l ly at the inlet end of a hori zo n t a l - fl owwetland and it may be necessary to replace some ofthe media after a period of time. Vegetation growth,flows, and influent and wastewater quality should bemonitored. The wetland should be securely fencedoff.

4.10.2 DESIGN CRITERIA

For a horizontal-flow sub-surface wetland, the planarea (Ap) requirement for BOD removal is about 5m2/person. The depth of the bed is about 0.6 mplaced on a base with a slope of 0.5-2.0%. The widthof the constructed wetland can be calculated fromthe following equation:

Ac = Q/( k.i)

where Ac is the cross sectional area (m2) of thewetland at the inlet, Q is the average wastewater flow

Inlet

Adjustable discharge

outlet

Levelsurface

Roots & Rhizomes

Gabion inlet zone

Typical depth 0.6m

Slope 1%

Gravel

Phragmites

Impervious liner

FIGURE 17: SUB-SURFACE (SFS) HORIZONTAL FLOW WETLAND

Outlet

liner drainagepipes

1% slope

Inlet

Intermittentdosing pipe Phragmites

Graded filter material

~ 80mm

Legend: 1

2

3

4

‘Sharp sand’

6mm washed pea-gravel

12mm round washed gravel

30-60mm round washed gravel

1

2

3

4~ 150mm

~ 150mm

~ 100mm

Perforated pipe

Solid pipe

FIGURE 18: VERTICAL FLOW WETLAND


Sand & Gravel

rat e (m3/d), k is the permeability (m/d) of thewetland media and i is the hydraulic gradient equalto the slope of the bed. For a four-person household,Ap is 20 m2; for a wetland media, with a depth of 0.6m and a permeability of 50 m/d (sand), laid at agradient of 1%, a suitable width is 2.5 m and thelength would then be 8 m.

4.10.3 ADVA N TAG E S / D I S A DVA N TAGES OFCONSTRUCTED WETLANDS

The advantages of constructed wetlands include:

• low construction and running costs;


• excellent reduction of biochemical oxygen demand (BOD5) and suspended solids (SS)from septic tank effluents; and

• secondary benefits in terms of potential wildlife habitat enhancement.

The disadvantages of constructed wetlands include:

• lack of agreed design criteria;


• systems remain unproven for other than BOD5 and SS removal;

• security and safety;

• concern about disease vectors; and

• difficulty in maintaining uniform distributionof flow at inlet.

4.11 POLISHING FILTERS

All intermittent filter systems, constructed wetlandsand mechanical aeration systems (discussed later)require a polishing filter following the secondarytreatment stage (Figure 15). The polishing filter canreduce micro o rga n i s m s , p h o s p h o ru s , and nitrat en i t rogen in otherwise high quality wa s t ewat e reffluents.

Figure 19, Figure 20, Figure 21 and Figure 22illustrate three possible loading arrangements forpolishing filters. Polishing filters may be of twotypes: soil or sand.

4.11.1 SOIL POLISHING FILTERS

Soil polishing filters may comprise in situ soil,improved soil or imported soil. Where grass growthis allowed, there can be a large reduction in NO3-N.These soils should have percolation values (P or T)in the range of 1-50. Dosing may be by gravity or bypumped arrangements.

In typical layouts, the polishing filter:

(i) may underlie an intermittent filter such as apeat filter with the effluent being spread outover a shallow gravel layer (Figure 19); anyexposed polishing filter area may be soil covered and grassed (Option 1);

(ii) may underlie or may be offset from a secondary treatment unit; loading may be by a pumped arrangement as in an intermittent sand filter (Figure 20) (Option 2); and

(iii)may be offset from the secondary treatmentsystem; loading may be by gravity into percolation trenches (Figure 21) (Option 3).

Recommended loading rates for polishing filters are:

• up to 20 l/m2.d for P/T values 1-20; and

• up to 10 l/m2.d for P/T values 21-40; and

• up to 5 l/m2.d for P/T values 41-50.

4.11.1.1 Option 1- Direct Discharge

In the case of spreading the secondary effluent overa polishing filter using a distribution gravel and withd i rect disch a rge from the polishing filter togroundwater through soil or bedrock the loadingrates on the soil or bedrock should not exceed(Figure 19); 20 l/m2.d for P/T values 1-20, 10 l/m2.dfor P/T values 21-40 and 5 l/m2.d for P/T values 41-50.


4.11.1.2 Option 2 - Pumped Discharge

In the case of hydraulic loading by pumping, in a soilwith a P/T value between 21 and 40 the minimumarea of polishing filter required for a 4 personhousehold (i.e. 4 x 180 l/person/d) is 0.72m3/0.01m= 72m2 (9m long x 8m wide). Figure 20 illustratesthe loading arrangement for such a soil polishingfilter. The treated wastewater from the secondaryt re atment unit is pumped to a manifold anddistribution pipes provided with 3 mm diameterorifices. The distribution pipes are embedded in a100-200 mm thick layer of gravel.

4.11.1.3 Option 3 – Gravity Pipe Discharge

In the case of loading through percolation trencheswith a P/T value of 1-50 (Figure 21), a greater areais required. For trenches 450 mm wide at 2 mspacing the minimum length required for a 4 personhousehold is 64m and the land area is 157m2, basedon a P/T value of 21-50 and a loading rate of 25l/m2.d on the trench base. The loading rate can beincreased to 50 l/m2.d for subsoils with a P/T valuein the range 1-20. The length of percolation trenchfor secondary treated wastewater for the differentpercolation values is give n i n Table 15. Treatedwa s t ewater from the secondary filter fl ows by

Secondary treatment unit e.g. intermittent peat filter

grassed soil overlay

distribution gravel

soil polishing filter (size dependent on P or T value)

to groundwater

Ground Level

FIGURE 19: INTERMITTENT FILTER OVERLYING AND LOADING A SOIL POLISHING FILTER

Secondary treatment

unit


Polishing filter

25 mm lateral

pumping chamber

FIGURE 20: SECONDARY TREATMENT UNIT FOLLOWED BY A SOIL POLISHING FILTER


gravity to a distribution box which distributes theflow evenly into the several trenches.

All soil polishing filters must have a minimumthickness of 600 mm of free-draining unsaturatedsoil between the point of infiltration of the effluentand the water table and bedrock. They may be atground surface or partially or totally above groundsurface (Figure 3). Where the native soil at the site

is impervious, a graded gravel layer with drainsshould underlie the polishing filter and these drainsshould outfall to a watercourse or stream. Where apolishing filter is constructed in contact with a verypermeable gravel or sand stratum in the soil and ispressure dosed into a surface gravel layer, the sidesof the filter must be enclosed by an impervious linerto prevent bypass of flooding doses directly to thegroundwater.

Secondary treatment

unit

Percolation trench

Vents

Distribution box

Gravityflow

2.45 m

FIGURE 21: SECONDARY TREATMENT UNIT FOLLOWED BY A PERCOLATION TRENCH

Estimated maximum Required length of Required length ofnumber of people in trench* (m) for trench* (m) forthe house based on T/P values T/P values

number of bedrooms 21-50 1-20(loading at 25 l/m2.d) (loading at 50 l/m2.d)

3 48 24

4 64 32

5 80 40

6 96 48

7 112 56

8 128 64

9 144 72

10 160 80

TABLE 15: MINIMUM TRENCH LENGTHS IN A SOIL POLISHING FILTER

* 450 mm trench


100 mm distribution gravel (10-20 mm)

200 mm sand (0.4-1.4 mm); D10 = 0.56

75 mm pea gravel (10-20 mm)

100 mm sand (0.2-0.7 mm); D10 = 0.29


200 mm sand (0.1-0.5 mm) D10 = 0.18


150 mm graded gravel (5-30 mm)

50 mm sand (0.2-0.7 mm); D10 = 0.29

4.11.2 SAND POLISHING FILTERS

Sand polishing filters comprise stratified sand filters( Figure 22 and Figure 23)6. In a typical layout, threelayers of sand decreasing in coarseness with depthare separated from each other by thin layers ofwashed pea-sized gravel. The surface laye rcomprises a pea-sized gravel, e.g. 10-20 mm gravelaggregate of 100 mm thickness in which pressuredistribution pipes are placed overlain by a geotextileand soil cover. The gravel layer serves to distributethe secondary effluent evenly over the underlyingsand layer. The top layer of sand is a 200 mm thicklayer of 0.4 - 1.4 mm coarse sand with a D10 of 0.56mm and a uniformity coefficient of 1.7. This rests

on a 75 mm layer of pea-sized gravel. The middlesand layer is 100 mm of medium-fine sand 0.2-0.7mm wi th a D10 of 0.29 mm and a uniformitycoefficient of 1.7. This layer rests on a 75 mm layerof pea-sized gravel. The bottom sand layer is a 200mm layer of fine sand 0.1-0.5 mm with a D10 of 0.18mm and a uniformity coefficient of 1.7. This layerrests on a 100 mm layer of pea-sized gravel, which isunderlain by a layer of graded gravel in whichdrainage pipes are placed. A thin (50 mm) layer ofsand below the graded gravel protects the liner fromdamage. The hydraulic loading should not exceed 601/m2.d. The filter can be soil covered and sown withgrass.

4.11.3 PAST EXPERIENCE (DESK STUDY)

All intermittent filter systems, constructed wetlandsand mechanical aeration systems require a polishingfilter following the secondary treatment stage. Thepolishing filter produces a high quality effluent. Thea dvice provided ab ove allows effluent from apolishing filter to discharge to ground provided thesubsoil has a P/T value less than 50. Otherwise thedischarge must be directed to surface water and alicence obtained from the local authority. However,where previous experience (Desk study) suggests,that a subsoil with a T/P value greater than 50 willnot result in ponding, consideration may be given ins u ch circumstances to permit the effluent todischarge to ground. In any case, where soil is usedas the polishing filter (as an alternative to a stratifiedsand filter), such soil must have a T/P value in therange 1-50.

Secondary treatment

unit


Sand polishing filter

25 mm lateral

pumping chamber

FIGURE 22: SECONDARY TREATMENT UNIT FOLLOWED BY A SAND POLISHING FILTER

FIGURE 23: SCHEMATIC CROSS SECTION OF A SAND

POLISHING FILTER

6 Nichols, D. J., Wolf, D. C. Gross,M. A.,and Rutledge, E. M., “Renovation of Septic Effluent in a Stratified Sand Filter".ASTM STP 1324. American Society for Testing and materials, 1997.


5.1 GENERAL

Mechanical aeration systems may be used to treatwastewater from a dwelling house where a site isunsuitable for a conventional septic tank system orthey may be used as an alternative to septic tanksystems on suitable sites. The effluent from allmechanical aeration systems should be treated on apolishing filter. Many systems are available on themarket and include the following:

• biofilm aerated filter (BAF) systems;

• rotating biological contactor (RBC) systems;and

• sequencing batch reactors (SBR).

Mechanical aeration systems should be assessedunder the following headings:

• Costs: these should include the capital,running and maintenance costs;

• Experience with proposed system:information sought from other users should include:

• durability of the components of the system;

• ease of operation and inspection; and

• frequency of maintenance.

Appendix C contains an evaluation form whichshould be completed to compare the available on-sitetreatment systems.

5.2 BAF SYSTEMS

Biofilm aerated filter (BAF) systems can be used tot re at wa s t ewater from single dwellings. A BA Fsystem may consist of a primary settlement tank, anaerated submerged biofilm filter and a secondarysettlement tank. Solids are sometimes returned fromthe secondary settlement chamber to the primarysettlement chamber to facilitate desludging and toavoid sludge rising due to denitrification. Thereshould be adequate sludge storage capacity in thep ri m a ry settlement ch a m b e r. Norm a l ly BA Fsystems which are used to treat wastewater fromsingle dwellings can be purchased as prefabricated

units, with all chambers in one unit. BAF systemsare normally constructed in either glass reinforcedplastic (GRP), concrete or steel.

The BAF system is a biofilm system. Th emicroorganisms are attached to the filter media in thesecondary treatment stage. The media normally havea high specific surface area (m2/m3) and can consistof plastic modules or a granular material. Wheregranular media are used the system may requirebackwashing to prevent clogging of pore spaces. Therequired surface area of the media can be determinedusing an organic loading rate of 5 g BOD/ m2.d ofsettled sewage. For a single house with 4 persons,therequired area is about 32 m2 based on a per capitaloading of 40 g BOD/d of settled sewage.

Normally the BAF system provides carbonaceousox i d ation but can be designed to prov i d enitrification. Grease should not be allowed to enterthe aerated zone.


In the case of BAF systems the owner normally takesout a maintenance contract with the manufacturer.This is advisable due to the high level of skilln e c e s s a ry to service and maintain pumps andcompressors. BAF systems require desludging, and aproperly designed BAF system should provide foradequate sludge storage capacity in the primarysettlement compartment. Sludge levels should neverbe allowed to rise, as any entry of sludge into themedia compartment will cause pro blems. A l lm e chanical and electrical components re q u i reperiodic checking. In many cases, manufacturersinstall an alarm system to alert the owner ofmalfunction.

5.2.2 ADVANTAGES AND DISADVANTAGES

In terms of tre ating wa s t ewater from singledwellings, the BAF system offers the followingadvantages:

• ease of operation;

• low noise level;

• can function under conditions of shockloading, which are common in single dwelling situations;

5. MECHANICAL AERATION SYSTEMS

5 MECHANICAL AERATION SYSTEMS 51

• possibility of nitrification and denitrification if properly designed; and

• low fly nuisance.

Disadvantages include:


• package plants may have small sludgestorage volumes that could lead to overloading of the biofilm;

• pumps and compressor, which are usuallyrequired, will need skilled maintenance; and

• grease, if allowed to enter the aeration zone may cause problems with media.

5.3 RBC SYSTEMS

A ro t ating biological contactor (RBC) systemconsists of a primary settlement tank, a secondarytreatment compartment and a secondary settlementtank. In this system the microorganisms are attachedto an inert media surface and the inert media aremounted on a shaft that is turned by an electricmotor. These media are partially submerged in thewastewater. A biofilm develops on the media overtime; it is this biofilm which treats the wastewater.The settled sludge in the secondary settlement tank issometimes returned to the primary settlement tank.There should be adequate sludge storage capacity inthe primary settlement chamber. RBC units can bepurchased as packaged treatment units for singledwellings; these units normally contain all threecompartments in one unit. The required surface areaof the media can be determined using an organicloading rate of 5 g BOD/ m2.d of settled sewage. Fora single house with 4 persons, the required mediasurface area is about 32 m2 based on a per capitaloading of 40 g BOD/d of settled sewage. Greaseshould not be allowed to enter the contactor zone.


In the case of the RBC system, the owner normallyt a kes out a maintenance contract with them a nu fa c t u re r. A pro p e rly designed RBC systemshould provide for adequate sludge storage capacityin the pri m a ry settlement compartment. RBCsystems re q u i re desludgi n g. All mechanical ande l e c t rical components re q u i re periodic ch e ck i n g.The structural condition of the RBC unit should bechecked periodically. Any unusual noise from theunit should be investigated.

5.3.2 ADVANTAGES AND DISADVANTAGES

The advantages of an RBC system for use in singledwelling situations include:

• ability to function under conditions of shockloading, which is common in single house situations;

• low noise levels;

• no fly nuisance;

• low head loss through the system; and

• possibility of nitrification and denitrification,if properly designed.

Disadvantages include:


• package plants may have small sludgestorage volume that could lead to overloadingof the biofilm;

• grease, if allowed to enter the contactor zonemay cause problems by coating the media; and

• skilled personnel are required to service and maintain motor and pumps.

5.4 SEQUENCING BATCH REACTOR

The sequencing batch reactor (SBR) process is aform of activated sludge treatment in which aeration,settlement, and decanting can occur in a singlereactor. The process employs a five-stage cycle: fill,react, settle, empty and rest. Wastewater enters thereactor during the fill stage; typically, it isaerobically treated in the react stage; the biomasssettles in the settle stage; the supernatant is decantedduring the empty stage; sludge is withdrawn fromthe reactor during the rest stage; and the cyclecommences again with a new fill stage. For singlehouse systems the reactor is preceded by a primarysettlement tank. Grease should not be allowed toenter the reactor.

Critical components of an SBR system include theaeration/mixing process, the decant process, and thec o n t rol process. SBR systems are cap able ofp roducing a high-quality effluent. Th ey can bemodified to remove nitrogen and phosphorus.

Since the SBR system provides batch treatment of


wastewater, it can accommodate wide variations inflow rates that are typically associated with singlehouses. The SBR technology is well established inother countries.

Design criteria for the SBR process are summarisedin Table 16.


In the case of the SBR system, the owner normallyt a kes out a maintenance contract with them a nu fa c t u re r. A pro p e rly designed SBR systemshould provide for adequate sludge storage capacityin the pri m a ry settlement compartment. SBRsystems re q u i re desludgi n g. All mechanical ande l e c t rical components re q u i re periodic ch e ck i n g.The structural condition of the SBR unit should bechecked periodically. Any unusual noise from theunit should be investigated.

5.4.2 ADVANTAGES AND DISADVANTAGESOF SEQUENCING BATCH REACTORS

SBR systems have the following advantages:

• simple and reliable;

• ideally suited for wide flow variations;

• high quality wastewater achievable; and

• high operational flexibility, which can be used for nitrification, denitrification and phosphorus removal.

SBR systems have the following disadvantages:

• complex control system;

• frequent sludge wasting required in the

reactor;


• package plants may have small sludgestorage volume that could lead to overloadingof the system;

• pumps and valves are used and these requireskilled maintenance; and

• grease may cause problems if allowed to enter the reactor.

5.5 OTHER TREATMENT SYSTEMS

Other treatment systems may be introduced fromtime to time to treat wastewater. Such systemsinclude other activated sludge systems, membraneb i o re a c t o rs and composting units. Wh e re suchp roducts are introduced independent eva l u at i o nshould be carried out to verify the manufacturer’sdesign loadings. In addition, the evaluation criteriaset out in Appendix C should be consulted.Polishing fi l t e rs should typically fo l l ow suchsystems to reduce micro - o rganisms to re q u i re dlevels.

5.6 LOCATION OF MECHANICAL AERATIONSYSTEMS

Recommended minimum distances of separation ofmechanical aeration treatment systems should be aslisted in Tabl e 4 . The recommended minimumdistances from wells should satisfy the requirementsof the gro u n dwater protection re s p o n s e, wh i chshould have been reviewed during the desk study. Insome cases, the requirements of the groundwaterprotection scheme may be greater than the distancesset out in Table 4. All mechanical aeration systemsrequire desludging, usually, at yearly intervals and

Parameter Range

Total tank volume 0.5 - 2.0 times average daily flow

Number of tanks Typically 2 or more

Solids retention time (days) 20 - 40

Aeration system Sized to deliver sufficient oxygen during aeratedfill and react stage

Cycle times (hr) 4-12 (typical)

TABLE 16: DESIGN CRITERIA FOR THE SBR PROCESS*

* USEPA, 1992

5 MECHANICAL AERATION SYSTEMS 53

provision should be made for access for a sludgetanker.

5.7 POLISHING FILTERS FOR MECHANICALAERATION SYSTEMS

The treated wastewater from mechanical aerationsystems should be tre ated in a polishing fi l t e rsystem, the primary purpose of which is to reducemicroorganisms numbers in the treated wastewaterto required levels. If the mechanical aeration system

does not produce an outflow with a low BOD andsuspended solids concentrations, the polishing filtermay clog.

Polishing filter systems should be designed inaccordance with the procedures outlined in 4.11-Polishing Filters. A typical layout for the treatmentof wastewater using a mechanical aeration system isillustrated in Figure 24.

* If the topography or the design permits, gravity systems may be possible.

MECHANICAL AERATION SYSTEM POLISHING FILTER

PUMPING CHAMBER*

FIGURE 24: MECHANICAL AERATION AND POLISHING FILTER SYSTEM


REFERENCES AND FURTHER READING

• Cooper, P.F., Job, G.D., Green, M.B. and Shutes, R.B.E. (1996). Reed beds and constructed wetlands for wastewater treatment. Water Research Centre, Swindon, U.K.

• Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland (1999). Groundwater Protection Schemes. Geological Survey of Ireland, Dublin.

• Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland (2000). Groundwater Protection Responses for On-site Systems for Single Houses.Geological Survey of Ireland, Dublin.

• EPA (1999). Wastewater Treatment Manuals: Treatment Systems for Small Communities, Business,Leisure Centres and Hotels.

• Mulqueen J., Rodgers M., Hendrick E., Keane M., McCarthy R., (1999). Forest DrainageEngineering. COFORD Dublin.

• Nichols, D. J., Wolf, D. C. Gross, M. A., and Rutledge, E. M., (1997). Renovation of Septic Effluent in a Stratified Sand Filter. ASTM STP 1324. American Society for Testing and materials,.

• US EPA, (1992). Wastewater Treatment/Disposal for Small Communities, Manual No. EPA/625/R-92/005.

REFERENCES AND FURTHER READING 55

GLOSSARYBiofilm:

Biofilm aerated filter(BAF):

Biochemical oxygendemand (BOD):

Biomat:

Chemical oxygen demand(COD):

Constructed wetlands(CW):

Conventional septic tanksystem:

Distribution box:

Groundwater protectionresponse:

Groundwater protectionscheme:

Mottling:

Organic Matter:

a thin layer of microorganisms and organic polymers attached to a medium suchas soil, sand, peat, and inert plastic material;

a treatment system normally consisting of a primary settlement tank, an aeratedbiofilm and, possibly, a secondary settlement tank. The system is similar to theconventional percolating filter system except that the media are commonlysubmerged and forced air is applied;

BOD is a measure of the rate at which microorganisms use dissolved oxygen inthe biochemical breakdown of organic matter in wastewaters under aerobicconditions. The BOD5 test indicates the organic strength of a wastewater and isdetermined by measuring the dissolved oxygen concentration before and after theincubation of a sample at 20°C for five days in the dark. An inhibitor may beadded to prevent nitrification from occurring;

a biologically active layer that covers the bottom and sides of percolation trenchesand penetrates a short distance into the percolation soil. It includes complexb a c t e rial poly s a c ch a rides and accumu l ated organic substances as well asmicroorganisms;

COD is a measure of the amount of oxygen consumed from a chemical oxidisingagent under controlled conditions. The COD is generally greater than the BODas the chemical oxidising agent will often oxidise more compounds thanmicroorganisms;

a wastewater treatment system which includes a septic tank, providing mainlyprimary treatment, followed by a wetland system supporting vegetation, whichprovides secondary treatment by physical and biological means. Constructedwetlands are also used for tertiary treatment;

a wastewater treatment system that includes a septic tank mainly for primarytreatment, followed by a percolation system in the soil providing secondary andtertiary treatment;

a chamber between the septic tank and the percolation area, arranged to distributethe tank wa s t ewat e r, in ap p rox i m at e ly equal quantities, t h rough all thedistribution pipes leading from it;

control measures, conditions or precautions recommended as a response to theacceptability of an activity within a groundwater protection zone as set out in theD E L G / E PA/GSI document G ro u n dwater Protection Responses for On-siteSystems for Single Houses;

a scheme comprising two main components: a land surface zoning map whichencompasses the hydrogeological elements of risk and a groundwater protectionresponse for different activities;

the occurrence of reddish/brown spots or streaks in a matrix of dark grey soil; thereddish/brown spots or streaks are due to intermittent aeration and the greycolours may be due to anaerobic conditions;

mainly composed of proteins, carbohydrates and fats. Most of the organic matterin domestic wastewater is biodegradable. A measure of the biodegradable organicmatter can be obtained using the biochemical oxygen demand (BOD) test;


Pathogenic Organisms:

Peat filter:

Perched water table:

Percolating filter system:

Percolation area(soil absorption system)

Preferential flow

Rotating biologicalcontactor (RBC):

Sand filter:

Sludge:

Soil structure:

Soil texture:

Soil (topsoil):

Subsoil:

Suspended solids (SS):

Total nitrogen:

Total phosphorus

these potential disease producing microorganisms can be found in domesticwastewaters. Organisms, such as E. coli, and Faecal streptococci, with the sameenteric origin as the pathogens are used to indicate whether pathogens may bepresent or not in the wastewater;

a filter system consisting of peat used to treat wastewater from a primarysettlement tank (usually a septic tank) by biological and physical means;

unconfined groundwater separated from an underlying body of groundwater byan impervious or perching layer;

a wastewater treatment system consisting of primary settlement and biologicaltreatment (effected by distributing the settled liquid onto a suitable inert mediumto which a biofilm attaches) followed by secondary settlement;

a system consisting of trenches with pipes and gravel aggregates, installed for thepurpose of receiving wastewater from a septic tank or other treatment device andtransmitting it into soil for final treatment and disposal. This system is also calleda drain field, seepage field or bed, distribution field, subsurface disposal area, orthe treatment and disposal field;

a generic term used to describe the process whereby water movement followsfavoured routes through a porous medium bypassing other parts of the medium.Examples include, pores formed by soil fauna, plant root channels, weatheringcracks, fissures and/or fractures;

a contactor consisting of inert plastic modules mounted in the form of a cylinderon a horizontal rotating shaft. Biological wastewater treatment is effected bybiofilms that attach to the modules. The biological contactor is normallypreceded by primary settlement and followed by secondary settlement;

a filter system consisting of sand used to treat wastewater from a primarysettlement tank (usually a septic tank) by biological and physical means;

the material which settles in the bottom of the primary/secondary settlement tank;

the combination or arrangement of individual soil particles into definableaggregates, or peds, which are characterised and classified on the basis of size,shape, and degree of distinctiveness;

the relative proportion of various soil components, including sands, silts, andclays, that make up the soil layers at a site;

the upper layer of soil in which plants grow;

the soil material beneath the topsoil and above rock;

includes all suspended matter, both organic and inorganic. Along with the BODconcentration, SS is commonly used to quantify the quality of a wastewater;

mass concentration of the sum of Kjeldahl (organic and ammonium nitrogen),nitrate and nitrite nitrogen;

mass concentration of the sum of organic and inorganic phosphorus;

GLOSSARY 57

Trench:

Unsaturated soil:

Wastewater:

Water table:

also referred to as a percolation trench, means a ditch into which a singlepercolation pipe is laid, underlain and surrounded by gravel. The top layer ofgravel is covered by soil;

a soil in which some pores are not filled with water; these contain air;

the discharge from sanitary appliances, e.g. toilets, bathroom fittings, kitchensinks, washing machines, dishwashers, showers etc.;

the level of the surface of the groundwater in a trial hole or other test hole.


1.0 GENERAL DETAILS (From planning application)

APPENDIX A: SITE CHARACTERISATION FORM

PLANNING APPLICATION Ref. no.:

NAME & ADDRESS OF APPLICANT:

SITE LOCATION AND TOWNLAND:

TELEPHONE NO:

MAXIMUM NO.OF RESIDENTS:

PROPOSED CAPACITY OF SEPTICTANK (litres) (if applicable):

PROPOSED WATER SUPPLY:(tick as appropriate)

FAX NO:

NO. OFDOUBLEBEDROOMS:

E-MAIL:

NO. OFSINGLEBEDROOMS:

NUMBER OF CHAMBERS:

mains private well/borehole group well/borehole

2.0 DESK STUDY

Soil type: Bedrock type:

Subsoil type: Aquifer type:

Vulnerability class: Groundwater protection response:

Presence of significant sites (archaeological, natural and historical):

Zoning in county development plan:

Past experience in the area:

Comments:(Integrate the information above in order to comment on: the potential suitability of the site, potentialtargets at risk, and/or any potential site restrictions).

APPENDIX A 59

Sketch of site showing measurement to Trial Hole location and Percolation Test Hole locations,wells and direction of groundwater flow, proposed house (incl. distances from boundaries) adjacenthouses, watercourses, significant sites and other features. North point should always be included.

[A copy of the site layout drawing should be used if available]


3.0 ON-SITE ASSESSMENT

3.1 Visual Assessment

TOPOGRAPHY: SLOPE:

LANDSCAPE: STEEP (>1:5) SHALLOW (1:5-1:20)

GEOLOGY: RELATIVELY FLAT (<1:20)

SURFACE FEATURES:

OUTCROPS

HOUSES

DITCHES*

WELLS*

SPRINGS

KARST FEATURES

ROADS

WATERCOURSE*

LAKES/SURFACE WATER PONDING/BEACHES/SHELLFISH AREAS/WETLANDS

SITE BOUNDARIES

EXISTING LAND USE* note water level

LOCAL DRAINAGE:

TYPE OF VEGETATION:

GROUND CONDITION:

COMMENTS:(Integrate the information above in order to comment on: the potential suitability of the site, potentialtargets at risk, the suitability of the site to treat the wastewater and the location of the proposedsystem within the site).

APPENDIX A 61

3.2 Trial HoleHole should be approximately 1m x 0.75m in plan and a minimum of 2.1 m deep

Date and time of Date and time ofDepth of Trial Hole (m):Excavation: Examination:

Depth from ground surface to bedrock (m):

Depth from ground surface to water table (m):

Soil type: Subsoil type:

0.1 m

0.2 m

0.3 m

0.4 m

0.5 m

0.6 m

0.7 m

0.8 m

0.9 m

1.0 m

1.1 m

1.2 m

1.3 m

1.4 m

1.5 m

1.6 m

1.7 m

1.8 m

1.9 m

2.0 m

2.1 m

2.2 m

2.3 m

2.4 m

2.5 m

Additional Soil/Subsoil Information

Texture Structure Bulkdensity

Colour* Preferentialflowpaths

Other information (e.g. depth of water ingress)

* All signs of mottling should be recorded


3.3 Percolation Test

Type of test: T-Test or P-Test

Percolation Test Hole 1 2

Depth from ground surface to top of hole (mm) (A)

Depth from ground surface to base of hole (mm) (B)

Depth of hole (mm) [B - A]

Dimensions of hole [length x breadth (mm)]

Each hole must be pre-soaked twice before the test is carried out (from 10.00 am to 5.00 pm andfrom 5.00 pm to next morning)

Date of test:

Date pre-soaking started:

Time filled to 400 mm

Time water level at 300 mm

Percolation 1 2Test Hole

No.

Fill no. Start Time Finish Time ∆t (min) Start Time Finish Time ∆t (min)(at 300 mm) (at 200 mm) (at 300 mm) (at 200 mm)

1

2

3

Average ∆t Average ∆t

Average ∆t/4 = [Hole No.1] _____(t1) Average ∆t/4 = [Hole No.2] _____(t2)

T value = (t1 + t2)/2 =_______(min/25 mm)

Result of Test : T =

Comments:

APPENDIX A 63

4.0 CONCLUSION:(Integrate the information from the desk study and on-site assessment (i.e. visual assessment, trial hole andpercolation tests) above and conclude the type of system that is appropriate. This information is also usedto choose the optimum final disposal route of the treated wastewater).

Suitable for (delete as appropriate):

(a) septic tank and soil percolation system (b) septic tank and intermittent filter system and polishing unit; or septic tank and constructedwetlands and polishing unit

(c) mechanical aeration system and polishing unit

and SUITABLE for discharge to surface water/groundwater (delete as appropriate)

5.0 RECOMMENDATION:

Propose to install:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

and discharge to surface water/groundwater (delete as appropriate)

Signed: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Qualifications . . . . . . . . . . . . . . . . . . . . . . . . . . . Date of Report: . . . . . . . . . . . . . . . . . . . . . . . . .

Phone: . . . . . . . . . . . . . . . . . . . . . Fax:. . . . . . . . . . . . . . . . . . . . . . E-mail: . . . . . . . . . . . . . . . . . . .

6.0 VERIFICATION (by Local Authority):

Site visit ❐ Date:

Inspection of Trial Hole ❐ Date:

Inspection of Percolation Test Holes ❐ Date:

Comments

SIGNED: Date:


The percolation test comprises the measurement of the length of time for the water level in the percolation testhole to fall from a height of 300 mm to 200 mm above the base of the hole.

PERCOLATION TEST (T Test) PROCEDURE

Day 1: Two percolation test holes should be dug in the proposed percolation area. Each hole should be 0.3 mx 0.3 m and 0.4 m deep below the proposed invert level of the distribution pipe (Figure 25). The bottom andsides of the hole should be scratched with a knife or wire brush to remove any compacted or smeared soilsurfaces and to expose the natural soil surface.

Clear water should be carefully poured into the hole at about 10.00 am so as to fill it to the full height of 0.4m. The water should then be allowed to percolate. At about 5.00 pm the hole should once again be filled tothe full height of 0.4 m and allowed to percolate overnight.

Day 2: The hole should be filled with clear water at about 10.00 am and the water should be allowed to dropsuch that there is 0.3 m of water in the hole. Thereafter, the time in minutes required for the water to drop 100mm, that is from 0.3 to 0.2 m, in the hole should be recorded. The hole should then be refilled to the 0.3 mlevel again and the water allowed to drain to the 0.2 m level and the time again recorded. The filling andmeasurement of the percolation rate through the hole should be repeated two times – three tests in all.

The average value in minutes of the three recordings should then be divided by 4 to give the time required fora fall of 25 mm or the percolation value "t". The same procedure should be repeated in the second hole in thepercolation area.

TEST RESULTS

The time for the 25 mm drop ("t") for each of the test results in the percolation area should be averaged togive the value "T". A proposed percolation area whose "T" value is less than 1 or greater than 50 should bedeemed to have failed the test.

APPENDIX B: PERCOLATION TESTS

0.3 m

0.4 m

0.3 m square

Water level

Invert of pipe

Ground level

proposed invert of distribution pipe

FIGURE 25: PERCOLATION TEST HOLE

APPENDIX B 65

PERCOLATION TEST (P Test) FOR SOIL POLISHING FILTERS

To establish the percolation value for soil polishing filters and to determine the discharge route for secondarytreated effluent where shallow subsoils exist, a modification of the percolation test as described above isrequired. The test procedure is identical to that outlined above but in this situation the trial hole dimensionsshould be 0.3 m x 0.3 m and 0.4 m deep below the ground surface (Figure 26). To avoid confusion with theprevious test, this test is called a P type percolation test and the values are referred to as P values.

TEST RESULTS

The time for the 25 mm drop ("t") for each of the test results in the percolation area should be averaged togive the value "P". A proposed percolation area whose "P" value is less than 1 or greater than 50 should bedeemed to have failed the P test.

0.3 m

0.4 mWater level

Ground surface

0.3 m square

FIGURE 26: PERCOLATION TEST HOLE FOR SHALLOW SOILS


Factor Treatment Option No. 1 Treatment Option No. 2

Capital cost £

Construction costs prior to delivery £

Additional costs prior to commissioning

Annual running cost £/annum

Installation and commissioning service available

Maintenance agreement available

Cost of annual maintenance agreement £

Design criteria*

Performance - % reduction in BOD, COD, TSS

Performance - % reduction Total P and Total N

Performance - % reduction Faecal coliforms

Beneficial uses of the receiving water

Guarantee available

AGRÉMENT certification

Recommendations from other users

Expected life of the system

Power requirements kW/d

Power requirements – single phase/three phase

Ease of operation

Daily, weekly and annual maintenance requirements

Licence required (Water Pollution Act Licence)

Access requirements for sludge removal

Sludge storage capacity (m3)

APPENDIX C: EVALUATION OF TREATMENT SYSTEMS

* in the case of biofilm systems the organic and hydraulic loading rates in g/m2.d and l/m2.d respectivelyshould be quoted

APPENDIX C 67

APPENDIX D: SOIL/SUBSOIL CLASSIFICATION CHART


1 - 5Sandy and rasping sound?

NO

NO

NO

NO

NO

Slighty sandy, faintrasping sound?

Smooth soapy feel,no grittness?

Very smooth, slightysticky to sticky?

Very smooth, stickyto very sticky?

START again orsoil is organic

6 - 10

6 - 30

31 - 50

>50

>50

Sand SAND

silty or clayeySAND

very sandy SILT

clayey, sandy SILT

silty, sandy CLAY

CLAY

Loamy Sand

Sandy Loam1

Silt Loam

Clay loam

Clay

YES

OR

YES

YES

YES

YES

START HERE:Rub moistSoil/Subsoilbetween thumband fingers Soil Classification

(Soil Scientist or Agricultural)Subsoil Classification

(Geotechnical)2

Range ofT values

1 Loam:A soil composed of a mixture of sand, silt and clay such that the properties of no one groupdominate its characteristics is called a Loam.

2 Classification system used in BS 5930:1981.

APPENDIX E: INDICATOR PLANTS OF DRAINAGE

The following illustrate plants which:

• indicate dry conditions throughout the year (good drainage); and

• indicate wet conditions through the year (poor drainage).

Some of the plates below illustrate the plants in flower, this aspect should be ignored. Plants in flower, orotherwise, do not change their indicator status. Note that an alder is a tree.

DRY CONDITIONS

WET CONDITIONS

Creeping Thistle Bracken

Alder IrisSoft Rush Big

Common Ragwort

APPENDIX D 69

USER COMMENT FORM

NOTE: Completed comments to be forwarded to: The Environmental Management and Planning Division,Environmental Protection Agency, P.O. 3000, Johnstown Castle Estate, Wexford.

Document Title: Wastewater Treatment Manuals: Treatment Systems for Single Houses

____________________________________________________________________________________CONTENTS:

____________________________________________________________________________________STYLE:

____________________________________________________________________________________INFORMATION:

____________________________________________________________________________________SUGGESTIONS FOR FUTURE EDITIONS:

____________________________________________________________________________________

NAME................................................................... ORGANISATION.......................................................

ADDRESS.....................................................................................................................................................

DATE.............................................. PHONE ....................................... FAX...........................................

EPA Wastewater Treatment for A Single House

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