Urban water systems9 Introduction to wastewater disposal© PK, 2005 – page 1 9 Introduction to...

Urban water systems 9 Introduction to wastewater disposal © PK, 2005 – page 1

9 Introduction to Wastewater disposal

9.1 Overview of wastewater system

9.2 Goals of wastewater disposal

9.3 Costs of sewers and wastewater treatment

9.4 Interface between sewer and wastewater treatment plant

9.5 The receiving water as a goal system

Technische Universität Dresden

Department of Hydro Sciences, Institute for Urban Water Management

Peter Krebs

Urban Water Systems


9.1 Overview of wastewater system

9 Introduction to wastewater disposal


Urb

an r

egio

n

Rain-runoff processSewage

retention tank

Wat

er

dist

ribut

ion

Reservoir

Sew

er s

yste

m

Combined sewer

Retention tank CSO structure

Wat

er

puri

ficat

ion

WW

TP

Rec

eivi

ng

wat

er

Gro

und

wat

er

Urban water system

Infiltration Overflow

Retention

Sedimentation

Sludge disposal

In-/Exfiltration

Clean water inflow

Treatment


The urban drainage system

Overflow structure

Receiving water

Overflow

Combined water storage

WWTP

WWTP effluent

Sewage retention


The reality at overflow structures…


9.2 Goals of wastewater disposal



Goals of wastewater disposal

Hygiene

Flood protection

Water protection

Hygienic disposal

No backwater effects in and from sewers

Minimising of pollutants impact

Minimising oxygen depletion

Maintaining hygienic water quality


Iit is the task of urban drainage to collect and remove all

kinds of wastewater from housing areas completely and as

quickly as possible, (…) without impacts on surface and sub-

surface waters.“

„Wastewater includes sewage from domestic and industrial

areas, rainwater, snow melt water, infiltration, effluent water

from fountains, enclosed running waters (…), irrespective

whether they are polluted or not.“

Hörler (1966)

„Classical“ drainage approach


„only these wastewaters should be collected and disposed

which cannot be infiltrated in the catchment without impact

on groundwater. Moreover, the runoff should be subject to

retention and deceleration in order to decrease the runoff

peaks.“

„Instead of purely technical approaches to solve the

wastewater disposal problem, it is the aim to consider the

entire water cycle in urban areas.“

VSA (1989)

Problem-oriented drainage


9.3 Costs of sewers and wastewater treatment



Connected inhabitants 5000 inh 50‘000 inh 200‘000 inh

Population density 50 inh/ha 100 inh/ha 200 inh/ha

Sewer Length per person (m/inh) 7 4.5 3

Costs per m (EUR/m) 400 500 600

Costs per inh (EUR/inh) 2600 2250 1800

Storage Volume / inh (m3/inh) 0.2 0.15 0.13

tanks Costs / inh (EUR/inh) 200 150 135

WWTPs Industry add-on (PE) 2500 25000 100000

(inh+PE) (PE) 7500 75000 300000

costs / PEtot (EUR/PE) 450 340 250

costs / inh (EUR/inh) 675 510 375

Investment costs for the wastewater system


Annual costs

Fixed costs

Operation costs

• Depreciation

• Payment of interest

• Personnel

• Energy

• Operation means, e.g. chemicals

• Repairs, spares

• Sludge disposal

• Administration


Depreciation and operation costs

Connected inhabitants 5‘000 inh 50‘000 inh 200‘000 inh

EUR / (inh · a)

Depreciation Sewer system (2%/a) 105 90 72

Storage tanks (3%/a) 13 10 9

WWTP (5%/a) 62 50 38

Total 180 150 119

Operation Sewer system 30 15 9

Storage tanks 4 3 3

WWTP 38 20 14

Total 72 38 26


9.4 Interface between sewer and wastewater treatment plant



0

100

200

300

400

0 4 8 12 16 20 24

Time (h)

Infl

ow

ra

te (

l/s)

Dry-weather flow Q t

Average inflow rate

Capacity of WWTPQ m = 2 Q S,max (85%) + Q f

Q s

Q f

WWTP capacityfor stormwater inflow

Q s,m

Capacity of WWTP


Capacity of WWTP

Combined water inflow according to DWA A131 (2000)

fsm QQQ 2

decisive Qs for design

One-hours peak dry-weather flow, which is matched or exceeded at 15% of days

„hidden“ extra capacity

• hourly flow rate below the daily peak value for 23 hours per day

• hourly peak value at 85% of days below design inflow

• Design for increasing wastewater flow in the future


Extraneous water flow Qf

• Groundwater infiltration

• Drainage

• Small rivers

• Water from fountains

• Cooling water

• Excess water from drinking water reservoirs

Extraneous water flow is variable

sf QQ 4.03.0Rough estimate, if no data available


Sewage flow: diurnal loads variation

0

10

20

30

40

50

60

70

00:00 04:00 08:00 12:00 16:00 20:00 00:00

Time (hh:mm)

CO

D-L

oa

d (

kg/h

)

0

1

2

3

4

5

6

7

NH

4-L

oad

(kg

/h)

Daily mean load COD and NH4

NH4-Load

COD-Load


9.5 The receiving water as a goal system



Emission

„Immission“

Approach of Water Framework Directive


Hydrology of receiving water

Flow rate

• Important with regard to dilution of wastewater input

Mean flow rate

Variations, minimum and maximum

Rain-runoff process

• Response time

• Quicker runoff process and CSO than flow increase in river


Response time of CSO and river

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 1 2 3 4 5 6

Time (h)

Flo

w r

ate

(m

3

/s)

River

CSO

Critical phase


Impact to rivers by CSOs: hydraulic effects

Changed hydrology

• Frequency of high flow rates

• Flow rate Gradients are steeper due to intense CSO events

River bed erosion

• Potentially more frequent

• Local erosion

• Effects on biocenosis

Intense events are decisive

bATV =Impervious area

Area of hydrologic catchment


Development of flies (Gammeter, 1995)A

nzah

l Tie

re p

ro P

robe

0

10

20

30

40

50

60

70

80

1.5. 6.6. 13.7. 18.8. 24.9. 30.10. 6.12. 11.1. 17.2. 25.3. 1.5.

Em

ergenz und Eiablage

Schlüpfen der Larven

1990 1991

LFO

LFU

(LP nicht untersucht)

Anz

ahl T

iere

pro

Pro

be

0

10

20

30

40

50

60

70

80

1.5. 6.6. 13.7. 18.8. 24.9. 30.10. 6.12. 11.1. 17.2.

Em

ergenz und Eiablage

Schlüpfen der Larven

Hochw

asser vom 12.M

ai

19921991

LP

LFO und LFU

LF town area

LP natural, upstream


Impacts to rivers by CSOs: polluting effects

Particulate matter

• Accumulation on catchments surface and in sewer

• First flush = f (dry-weather period, runoff rate)

Dissolved matter originating from sewage

Event Concentration Load

weak high small

medium medium high

intense low high

aATV =Number of inhabitants

Low-flow discharge


Oxygen depletion (eutrophication)bio-chemical

Heavy metals, org. Subs. in sedimentchemical

Flow regime, morphologyhydrologic

Floatables, oil, greaseaesthetic

Bacteria, viruses in sedimenthygienic

Oxygen depletion in sedimentbio-chemical

toxic Substances (NH3, NO2) in river bedchemical

Smell, floatablesaesthetic

Bacteria, viruseshygienic

Oxygen depletion in water bio-chemical

Suspended matter, turbidityphysical

Toxic substances (NH3) in waterchemical

Flow rate, sher rate, erosionhydraulic

IndicatorEffect

years)

(weeks,

accumulative

(days)

delayed

(hours)

acute

Time scale

Effects in rivers (Schilling et al., 1997)


Ecological river quality

Morphology • Layout • Shading • Erosion frequency • Flow shadow

Hydrology • Flow regime• Rain-runoff characteristics

Physics • Temperature and temperature variations • Conductivity

Chemistry • NH4+, NH3 , Nutrients

• Heavy metals

Biology • Species variety • Species numbers


River water quality in Saxony

(Source: LfUG (1998))

Urban water systems9 Introduction to wastewater disposal© PK, 2005 – page 1 9 Introduction to...

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