~ Pergamon Wat. Sc«. Tech. Vol. 40, No.3, pp. 1-9, 1999C 1999

Published by Elsevier ScienceLIdon behalfof the IAWQPnnted m Great Britam. All nghlSreserved

0273-1223199 520 .00 + 0.00

PH: S0273-1223(99)00414-X

A REVIEW OF THE DESIGN ANDPERFORMANCE OF VERTICAL-FLOWAND HYBRID REED BED TREATMENTSYSTEMS

Paul Cooper

WRc pic. Frank/and Road, Blagrove, Swindon SN5 8YF. UK

ABSTRACT

The paper reviews the different options for the combination of vertical- and horizontal-flow beds used inhybrid reed bed/welland systems. The design and performance of these systems are briefly described. Theimportance of the oxygen transfer capacity of the different arrangements to their performance and their sizeis discussed. Alternative methods for denitrification are briefly described. C 1999 Published by ElsevierScience Ltd on behalfof the IAWQ. All rights reserved

KEYWORDS

Constructed wetlands; denitrification; horizontal-flow; hybrid systems; media selection. nitrification; oxygentransfer capacity; reed bed treatment; vertical-flow.

INTRODUCTION

The use of constructed wetlands/reed bed treatment systems has gradually developed over the past 20 years.Initially the main interest was in horizontal-flow (HF) systems because they were simple and promised lowconstruction and operational costs. There are now many fmc examples of HF systems for secondarytreatment and they proved very satisfactory where the standard required only BODs and TSS. However.there has been a growing interest in achieving fully-nitrified effiuents. Tertiary treatment HF systemsproduce well-nitrified effiuents (Green. 1997; Cooper et al., 1996) but secondary treatment HF systemscannot do this because of their limited oxygen transfer capacity (OTC). As a result of this there has been agrowing interest over the past 10 years in vertical-flow (VF) systems because (a) they have a much greaterOTC. and (b) they are considerably smaller (1-2 m2/pe) than the HF system (which need 5-10 m2/pe forsecondary treatment). Even more recently over the past 5 years there has been a growing interest in hybridsystems (also sometimes called combined systems). Many of these systems are derived from the originalhybrid systems of Seidel (1978). In these systems the advantages and disadvantages of the HF and VFsystems can be combined to complement each other. It is possible to produce an effiuent low in BOD. whichis fully nitrified and partly denitrified and hence has a much lower Total N concentration.

2 P.COOPER

ADVANTAGES AND DISADVANTAGES OF DIFFERENT TYPES OFBED

HF systems are:

good for:Suspended solids removal and bacteria removal because of their ability to filterBOD removal up to a set oxygen transfer capacityDenitrification (since it provides oxygen as a part ofthe nitrate).

poor for nitrification because oflimited oxygen transfer capability

VY systems are:

good for nitrification because of their high oxygen transfer capability which also leads to good removal ofBODsand COD. They can also remove some bacteria.

less good for suspended solids removal and can become clogged of the sand selection is not correct.

TYPES OF HYBRID SYSTEM

There are basically two main types depending on whether the HF stages of VF stages are placed at the frontof the system (Cooper and Maeseneer, 1996).

The first system is that described by Johansen and Brix (1996) and later built by Ciupa (1995. 1996). Thepaper by Johansen and Brix at the Vienna Conference in September 1996 is an excellent step-by-stepdescription of how to design such a system. The system is shown in Figure 1.

Recycle for... denitrification

if needed

HF

,,,,,,,,,,___________ 1

-+ - - - - - - - - -,I

I,II,,,,,

O.76mr/pe

8.8m"/pe

Figure J- System used by Johansen and Brix (1996) and Ciupa (1995. 1996).

The sizes used per person (pe) in Figure 1 are those used in the design by Johansen and Brix in their 1996paper. The paper presents the design for 55 pe with a 485m 2 HF bed followed by a 42m

2VF bed. The idea

behind the system is that the BOD is removed in the HF bed to prevent interference with nitrification in theVFbed.

The design recommends limiting the loading rate to the equivalent of a maximum of oxygen transfer rate of30 g02/m2.d for VF beds and 15 g02/m2.d for HF beds (Johansen and Brix, 1996; Birkedal et al.• 1993).

An example of the Johansen-Brix system was built in Poland by Ciupa (1995.1996).

The performance data from this system is shown in Table 1.

Design and performance of vertical-flow and hybrid reed bed treatment systems 3

Table I. Average performance of the hybrid system at Sobiechy, Poland (Ciupa, 1995, 1996)

mg/l Influent EffiuentPeriod March 1995BODs 110 17TSS 64 21~-N 48.5 6.6TotalN 56.6 24.2Total P 11.2 0.9Air temperature DC 2 2Water temperature DC 9 6Period June 1995BODs 370 21TSS 132 20~-N 44.3 4.7Total N 81.4 27.5Total P 16.2 4.2Air temperature DC 18 18Water temperature DC 12 13

It is clear from this table that the system is achieving satisfactory BODs and TSS removal but that thenitrification is not complete. Significant Total N reduction is taking place presumably by denitrification.

The alternative arrangement with the VF stage placed first has been used in France at St Bohaire (Lienard etaJ., 1990) and in the UK. at Oaklands Park (Burka and Lawrence, 1990). The Burka system is shown inFigure 2.

o.74fff/fJe

O.23m'/fJe

o.12m'/fJe

0.30fff/pa

VF

HF

HF

VF

Figure 2. The system used by Burka at Oaklands Park (Burka and Lawrence, 1990).

The Oaklands Park system for 65 pe contains two intennittently-Ioaded VF stages in series followed by twoHF stages. The two VF stages had a total area of63 m2 and the two HF stages a total area of28 m2

• This is atotal area of 1.4 m2/pe. A two-year study from August 1989 to September 1991 was carried out by WRc andthe mean results are shown in Table 2. The data is derived from 48 samples. These are long-term averagevalues and take into account performance during both winter and summer conditions.

4 P.COOPER

Table 2. Performance data from the Oaklands Park secondary treatment RBTS,August 1989 to September 1991

BODsTSSNH..-NTONOrtho P

Influent

28516950.51.7

22.7

Stage 15753

29.210.218.3

EffluentsStage 2 Stage 3

14 1517 11

14.0 15.422.5 10.016.9 14.5

Stage 479

11.17.211.9

The data in Table 2 shows that:

i) BODs and TSS removal was satisfactory in the VF stages and nitrification proceeded in parallel withBODS removal. There was significant nitrification taking place in the heavily-loaded 1st VF stage.This shown by the reduction in NH4-N and the corresponding rise in TON (Total OxidisedNitrogen).

ii) The VF stages (at 1 m2/pe) was just to small to achieve full nitrification.

iii) Significant denitrification (removal of TON) was taking place in the 3rd and 4th (HF) satges despitethe relatively low BODs, but perhaps aided by the long retention times, presumably by endogenousdenitrification.

iv) There was some denitrification taking place in the VF stages since the addition of the effiuent TONand NH..-N concentrations (36.5 mg Nn) does not add up to the NH..-N concentration (50.5 mg Nil)in the feed. In fact the NH..-N in the feed probably underestimates the total arnmonicalload on theVF beds. The KjN value would probably be in the range 70-100 mg/l.

An alternative to the combination of VF and HF stages has been used by Perfler and Haberl (1995) andLaber et al. (1997), in Austria. They have used the intermittently-loaded VF stages but have incorporated arecycle to improve the degree ofnitrification and denitrification achieved. Two systems were used, one withrecirculation (presumably allowing denitrification in the feed tank as well as the bed) and one with a dose ofmethanol added. The two systems (A & B) had approximately the same size (5 m2/pe) but system B wassplit into two equal stages with the system operated in series. The attempt here is to use external carbonsources and achieve the more rapid exogenous denitrification. The system achieved 72% removal of Total Nin the recirculation system (A) and 78% removal of Total N for the methanol dosed system. Table 3 showssome results from their study.

Table 3. Mean influent and effiuent composition for System A (Perfler and Haberl, 1995)(April 1993 to June 1994)

mg/lBODsCODTOCNH..-NTotalNTotal P

Influent10929882778910

Effiuent42596SS3

Design and performance of vertical-flow and hybrid reed bed treatment systems

DESIGN AND OPERATION CONSIDERATIONS

5

Many papers have been written on the HF stages and because of this. it is not discussed in any detail here.The design is detailed in several places.

In the UK we have used the following equation for sizing the area ofHF beds.

whereAh

QdCoCt

kBo D

Ah

= Qd (In Co - In C,)kBOD

= surface area ofbed, m2

= average daily flow rate of sewage, m3/d

= daily average BOD, offeed, mg/l= required daily average BODs of effluent, mg/l= rate constant, mid

(1)

The factor kBOD has been measured at:

a) 0.083:l: 0.017 for 49 systems in Denmark (Brix et al.• 1989);b) 0.067 to 0.1 in UK (Cooper, 1990; Findlater et al., 1990; Cooper et 01., 1996);c) 0.06 for secondary systems and 0.31 for tertiary systems derived from data in the UK Performance

Database (Job et 0/., 1996).

If the HF bed is for secondary treatment (taking settled sewage) this tends to produce an area of5 m2/pe andreduces the BODs and TSS to around 20 mg/l but removes little ~-N.

If the HF bed is placed in the tertiary position the bed are sized at 0.5 to 0.7 m2/pe and can sometimesachieve complete nitrification. For VF beds the information is very much less precise. I speculated in 1996that it would be reasonable to recommend:

1 m2/pe for BOD, removal only2 m2/pe for BOD, removal and nitrification

The systems at St Bohaire (France) and Oaklands Park (UK) have the following areas (Cooper, 1990):

1st Stage2nd Stage

St Bohaire

0.8 m2/pe0.4 m2/pe

Oaklands Park

0.74 m2/pe

0.23 m2/pe

In neither case is nitrification complete but BODs removal is satisfactory.

For small systems «100 pe) (Grant, 1995) recommends that the vertical-flow system is sized by thefollowing equation:

AI =3.6po.3S + 0.6P

whereAIP

=area of first vertical-flow bed, m2

=population equivalent

6 P.COOPER

A2the area of the second VF bed should be 50% of A.. the sewage is from a septic tank and 60% of the areaof A" ifno septic tank is used.

This results in A.. ranging from 2 m2/peat 4 pe to 0.78 m2/pe for 100 pe.

Distribution

It should be said at this point that most vertical-flow systems are designed with layers of graded gravel asthe media. This is usually topped up with coarse sand whose purpose is to allow the liquid to distribute overthe whole surface area ensuring even distribution and then to let it pass through. The selection of this layer(so that it does not clog) is absolutely essential to good operation of the VF systems (Grant, 1995; Cooper etal, 1996, 1997). See later section.

Oxygen transfer

The sizing of the beds is intimately linked to the oxygen transfer capacity of the system. This is linked to theintermittent dosing system used and the hydraulic loading rate.

I have made a crude estimation of the necessary total oxygen transfer rate. This estimate is made across eachvertical-flow stage. This has been made as follow:

{(BODin - BODOIn) + (NH. - N,n- NH. - NOI,,) X 4.3] fl I d..:..-_::......__.:::::..:~..:...---"---=-----''--.....:::::..:......_~X owrate ay

area of bed

The author accepts that this is a crude estimate because it does not allow for:

i) BOD, removal by settlement/filtrationii) Nl-4·N lost to a) the plants, b) the air, or c) adsorptioniii) BOD, removal by denitrification.

(2)

The value of 4.3 used to estimate the 02 needed for ammonia oxidation comes from work on nitrification inactivated sludge systems.

The Nl-4-N for a fresh sewage is also an underestimate of the potential Nl-4-N when all the organically­bound nitrogen in urea has been hydrolysed. KjN is a better measure but is not often reported.

The calculation is crude but it does indicate how much oxygen is being transferred.

Platzer (1998, 1999) has made a more accurate measurement for his systems. He uses:

[0.7 (CODin - CODDO, ) + (NH. - Nin - NH. - NOI,,) x 4.31 x flow I d (3)area of bed

This is similar to equation 2 since for many domestic wastewaters the BODsis 0.5 0.7 x COD.

Platzer takes account of the 02 recovered by denitrification. He has found 23 g02/m2oday on treatment unitswith a low hydraulic loading rate and 64 g 02/m2oday for units with a high hydraulic loading rate.

A comparison of the oxygen transfer rates calculated for those VF and hybrid systems reported in sufficientdetail in the literature is shown in Table 4.

The values found by Platzer (1998, 1999) and the author are similar and indicate that the value of 30 for VFsystems shown by Johansen and Brix (1996) may be conservative.

Design andperformance of vertical-flow andhybrid reedbedtreatment systems 7

Job (1992) in a study of agricultural wastewater treatment reports performance which indicate even higherrates ofoxygen transfer in the VF stage.

Table 4. Comparison ofoxygen transfer rates calculated for systems reported in the literature

Authors System Calculated oxygen Overall Notesconfiguration transfer rate m2/pe

kg 02/m2·dayBUTka & Lawrence (1990) 2VF+2HF 65 94 (VF beds only) 1.3 Long term averagesCuipa(1995,1996) lHF+ lVF 5.4 (combined beds) 10.3 Overall average for HF +

VFBedsJohansen & Brix (1996) lHF+ IVF 15 (HF) 30 (VF) 9.6 Design recommendationsCooper et al. (1996, 1997) 2VF 4-50 (VF only) 0.6 (tertiary) Tertiary nitrification onlyPerfler & Haberl (1995) VF + Recycle (A) 9.7 (B) 13.6 (A) 5 2 systems. System A with

2VF(B) (VFonly) I bed. System B with 2beds in series

Platzer (1998) VF+HF 23 64 (VF) Values for VF beds only

Sand/soil clogging

Another very important design consideration for VF beds is sand or soil clogging. This can be presented by:

a) correct selection ofthe sand (Grant, 1995, Cooper et aJ .• 1996)b) use of4 or more beds and rotating themc) limiting the organic loading rate to 25 g COD/m2.d, (platzer and Mauch, 1997).

I tdipa(1)

I td/pe

I tdipa (1)

HF

VF

VF

HF

TSS(ancIBOO)_

--

Figure 3. Another possible arrangement propose.

FINAL COMMENTS

I believe that the VF stage should be put at the front end of the system because if properly designed it canremove BOD, COD and bacteria as well as oxidise all the ammoniacal-nitrogen to nitrate. They will also

8 P.COOPER

remove some of the Total-N by denitrification (platzer, 1996, 1999). In another paper at the ICWSconference in Vienna, von Felde and Kunst (1996) showed that COD-removal and NH4- conversions tookplace simultaneously in the top 25 ern of the bed and that both nitrification and nitrate reductase activity inVF systems are highest in the upper layers. The same conclusion was drawn from work on Rapid Infiltrationsystems (Lance et al.• 1980). The presence of COD (and BOD) does not appreciably prevent the nitrifiers(am from doing their work.

There may be a case for putting in a small HF bed before the VF stage to remove TSS and prevent cloggingof the VF (see Figure 3). The design purpose of each stage is indicated alongside. The design needs norecycling but it depends upon endogenous denitrification to provide enough nitrate removal. This is notproven yet. A system to test this has been designed for Severn Trent Water in UK and is described inanother paper (Cooper et al, 1999) in this issue.

There are so few of these systems at the present that it is: lot possible to judge which is likely to be the mosteffective arrangement. I hope that the present and future conferences of the IAWQ Group will produce morepapers on hybrid systems and allow us to share information on the development of these systems.

ACKNOWLEDGMENTS

The author wishes to thank WRc pIc for permission to publish this paper. The opinions that are expressedare those of the author and not necessarily WRc pic.

REFERENCES

Birkedal, K., Brix, H. and Johansen, N. H. (1993). Wastewater Treatment in Constructed Wetlands; Designers Manual.Danish-Polish Post-Graduate Course on Low Technology Wastewater Treatment. Technical University of Gdansk,Poland.

Brix, H.• Schierup, H.-H. and Lorenzen, B. (1989). Design criteria for BODs removal in constructed wetlands. Poster paperpresented at the IAWQ Conference on Small Wastewater Treatment Plants. Trondheim, Norway.Tune 1989.

Burka, U. and Lawrence, P. (1990). A new community approach to wastewater treatment with higher 'Water plants. In:Constructed Wetlandsfor Water Pollution Control, P. F. Cooper and B. C. Findlater (eds). Pergamon Press, Oxford, UKpp. 359-371.

Ciupa, R. (1995). Results ofNutrient Removal in Constructed Wetlands in Sobiechy - North-Eastern Poland. Paper presented tothe Workshop "Nutrient Cycling and Reduction in Wetlands and their use for Wastewater Treatment". Trebon, CzechRepublic. September 1995.

Ciupa, F (1996). The experience in the operation of constructed wetlands in North-Eastern Poland. Paper presented at the 5thInternational Conference on Constructed Wetlands Systems for Water Pollution Control, Vienna, Austria, September,1996.

Cooper. P. F. (Ed) (1990). European Design and Operations Guidelines for Reed Beds Treatment Systems. Prepared by theEuropean CommunitylEuropean Water Pollution Control Association Expert Contact Group on Emergent HydrophyteTreatment Systems, December 1990. (WRc Report VI 17).

Cooper, P. F.• lob, G. D., Green, M. B. and Shutes, R. B. E. (1996). Reed Beds and Constructed Wetlands for WastewaterTreatment. pp, 206, WRc Publications. Medmenham, Marlow, UK.

Cooper, P. F. and De Maeseneer (1996). Hybrid Systems - What is the Best Way to Arrange the Vertical and Horizontal FlowStages. IAWQ Specialist Group on the Use of Macrophytes in Water Pollution Control, Newsletter No. IS, pp. 8-13,December 1996.

Cooper, P. F., Smith, M. and Maynard, H. (1997). The design and performance of a nitrifying vertical-flow reed bed treatmentsystem. Wat. Sci. Tech.•35(5). 215-221.

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WRc, Swindon, UK. June 1996.

Design and performance of vertical-flow and hybrid reed bed treatment systems 9

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Lance, J. C., Rice, R. C. and Gilbert, R. G. (1980). Renovation of wastewater by soil columns flooded with primary effluent.Journal ofthe Water Pollution Control Federation, 52(2), 381-388.

Lienard, A., Boutin, C. and Esser, D. (1990). Domestic Wastewater Treatment with Emergent Helophyte Beds in France. In:Constructed Wetlands In Water Pollution Control, P. F. Cooper and B. C. Findlater (eds). Pergamon Press, Oxford, UK,pp.183-192

Perfler, R. and Haberl, R. (1995). Reed Bed Systems for Water Pollution Control in Rural Areas. Paper presented to the workshop.The use of wetIandslreed beds for pollution control and nature conservation. Loughborough, UK. June 1995.

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Platzer, C. (1998). Entwicklung eines Bemessungsansatzes zur Stickstoffelmination In Pjlantzenklaranlagen. Ph.D Thesis,Technische Universitat Berlin, Germany.

Platzer, C. (1999). Design recommendations on sub-surface flow constructed wetlands for nitrification and denitrification. Wat.Sci. Tech., 40(3), (this issue).

Platzer, C. and Mauch, K. (1997). Soil clogging in vertical-flow reed beds - mechanisms, parameters, consequences and solutions.Wat. Sci. Tech., 35(5), 175-181.

Seidel, K( 1978).Gewlisserreinigung durch hohere Pflanzen. Zeitschrift Garten und Landschaft HI, 9-17.von Felde, K. and Kunst, S. (1997). N-and COD-removal in vertical flow wetlands. Wat. Sci. Tech., 35(5), 79-85.