e:> Pergamon Waf. Sci. Tech. Vol. 40. No.3. pp. 283-289. 1999IC1999

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DESIGN OF A HYBRID REED BEDSYSTEM TO ACHIEVE COMPLETENITRIFICATION AND DENITRIFICATIONOF DOMESTIC SEWAGE

Paul Cooper", Paul Griffin**, Stuart Humphries*** andAdrian Pound t• WRc plc, Frankland Road. Blagrove, Swindon, Wiltshire. SN5 8YF. UK•• Severn Trent Water. Avon House. St Martins Road. Finham, Coventry. CV3 6PR. UK••• Severn Trent Water. Alpha House. Warwick Technology Park. Heathcote Road.Warwick. CV34 3DA. UKt Carl Bro. 13/23 King Square Avenue. Bristol. BS2 8HU, UK

ABSTRACT

The design of vertical-flow (VF) reed beds is reviewed and the performance of the few worldwide existinghybrid systems, combining both horizontal- and vertical-flow beds, is assessed. Horizontal-flow (HF) bedsare good for suspended solids removal and will remove BODs up to a set loading. Vertical-flow beds canachieve BODs removal at much higher loading rates and they are capable of complete nitrification as tertiaryor secondary treatment systems. It is possible to achieve biological denitrification in horizontal-flow beds.By combining horizontal- and vertical-flow systems in the appropriate process sequence it is thus possible toproduce a system which removes BODs, TSS and achieves complete nitrification as well as substantialremoval of nitrate and hence a lowered Total N.

The paper discusses the possible process options for combining horizontal- and vertical-flow systems. Oneexample of these systems will be described in more detail. This is a flexible hybrid system being built bySevern Trent Water. Designed to treat the flow for a small village with a population of 129 it will havevertical-flow beds followed by horizontal-flow beds. The design allows for flexibility such that the size ofthe vertical-flow and horizontal-flow stages can be altered (for experimental purposes) to allow the designarrangement to be stressed and performance evaluated with the intention of defining the optimum loadingrates for each of the stages.

The same site also has sludge drying reed beds for treating the primary sludge. Cl 1999 Published by ElsevierScience Ltd on behalfof the IAWQ. All rights reserved

KEYWORDS

Constructed wetlands; denitrification; design; environmental impact; habitat enhancement; horizontal-flowsystems; hybrid system; nitrification; reed beds; sewage treatment; vertical-flow systems.

INTRODUCTION

Stoulton Hawbridge is a small rural village in the English Midlands 10 Ian south east of Worcester,comprising medium quality housing and supporting a public house ('pub') within the catchment. Sewers areof the combined type and there are no combined storm overflows provided, so all flows must be lifted by the

283

284 P. COOPER et al .

inlet pump ing station to the biological filters and associated settlement tanks . The present wastewatertreatment facility serves a population of 129 and is in poor asset condition , particularly the inlet pumpingstation. Asset renewal of the works has been approved to rectify the situation. The standard of

75 mgBODsfl125 mg TSS/l

as 95 percentiles

for a Dry Weather Flow (DWF) of 21 m3/d is generous, but it is expected to be tightened in future, with anammonia standard considered likely. Consideration was given to how to improve the work's performance. Itwas clear that a new pumping station was necessary. The biological filters were old and could have beenrefurb ished and the permit met but Severn Trent Water decided that it would be appropriate to test a newdesign of reed bed capable of achieving much higher effiuent qualities to assess its application for useelsewhere in future designs. Severn Trent use a process selection procedure to select processes which hasbeen developed from their experience. They have standardised on the use of rotating biological contactors(RBCs) at sites between 50 and 2000 p.e. with tertiary treatment reed beds provided if standards morerestrictive than 25 BODs and 45 TSS need to be achieved. Whilst effective, RBCs for very small populationsare expensive on a per capita basis and priority has been given to investigating alternative processes capableofachieving high standards but with lower whole life costs. The target effluent quality set was:

25 mgBODsfl45 mgTSS/110 mg NH4-N/I

)))

as 95 percentiles

The equivalent mean values would be approximately 12, 23, 2. The system was to be a hybrid systemcapable of achieving BODs removal, nitrificat ion and partial-denitrification. Severn Trent Water haveconsiderable experience with secondary and tertiary reed beds since they operate about 180 systems (Griffin,1998). However, they had limited experience with vertical-flow reed beds or the hybrid systems whichcombine both vertical and horizontal-flow beds . They invited WRc, who had designed and operated theirown vert ical-flow system (Cooper et al., 1997), to help with process design and evaluation of the system.Carl Bro were contracted to do the overall design and construction. The aim with this system was to test asystem capable of higher qualities without in any way putting at risk the permit. The system would haveflexibility built into it to allow: a) units to be closed off to reduce the size ofbeds, or b) beds to be bypassed.

The use of wetlands appeared to fit well with the company's policy of using constructed reed beds wherepossible, albeit taking the company into new areas by using hybrid reed bed treatment systems. Since thescheme was to be funded from capital it was necessary to have confidence in the ability of the processesused to meet the required permit standards. Stoulton Hawbridge with its generous consent was ideallypositioned within the capital works programme, had a population in the desired range and no apparentlimitations on space provided that adjacent land could be successfully purchased. The provision of generousamounts of treatment capacity, flexibly arranged, would allow an experimental programme which wouldidentify quite clearly the process configurations and sizes which would meet various effluent qualitystandards including total nitrogen limits whilst always meeting existing permit conditions. In keeping withthe wetlands theme, sludge treatment would be performed using sludge drying reed beds.

The intention was also to build the new system close to the existing treatment plant, which would continuein operation as the new hybrid reed bed system was constructed. Once the reed bed system was completethen the old biological filter plant would be closed down and the sewage fed to the new reed bed system. Bydoing this the treatment could be maintained throughout the construction phase and the effluent qualitywould not jeopardise the permit.

REVIEW OF HYBRID SYSTEMS

This review is deliberately kept brief because one of the authors is presenting a fuller review of thesesystems elsewhere in this issue (Cooper, 1999).

Complete nitrification and denitrification ofdomestic sewage 285

Full-scale experience over the last 13 years in Europe has shown that HF systems can reliably remove BODsand TSS but they do not transfer oxygen at a sufficient rate to be able to achieve full nitrification. VFsystems have been developed over the past II years to overcome these limitations in the HF systems. Theycan be designed to reliably oxidise ammoniacal-nitrogen and will also remove BODs but are less efficient atTSS removal. In a hybrid system the strength and weakness of the HF and VF beds balance each other outand it is possible to produce a system which can reliably remove BODs, NH4-N and TSS. If a fully nitrifiedeffluent is produced it is then possible to design for nitrate removal by biological denitrification. This can beachieved in HF beds which, since they are oxygen-limited, will have anoxic areas within the matrix wheredenitrification will take place. There are currently two schools of thought as to how the flowsheet should bearranged (Johansen and Brix, 1996; Cooper and de Maeseneer, 1996). Johansen and Brix (1996) proposedputting the VF stage after an HF system sized at 10 ml/pe. To denitrify they proposed to pump back effluentfrom the outlet of the VF bed to the inlet of the HF bed. This system allows the denitrification process to useexternal carbon sources as in the system with conventional biological sewage treatment processes usingactivated sludge. Cooper and de Maeseneer (1996) proposed using the VF stage before the HF stages partlybecause: a) the system can be made smaller (because the VF stages are more effective than HF for removingBODs as well as oxidising NH4-N; and, b) it should be possible to remove significant amounts of nitrate inthe tertiary HF system thus avoiding pump recycling. In this arrangement the carbon source fordenitrification is internal and the rate of denitrification is slower than for the case where the external sourcesare used, but with the long retention times in HF wetland systems it should be adequate.

At the time of the design ofthe Stoulton Hawhridge system, August 1997, it was unclear which was the bestapproach, but the system was designed using the experience gained from other UK systems at OaklandsPark (Burka and Lawrence, 1990) where the VF stages were placed first in the flowsheet.

DESIGN OF THE SYSTEM

The design was such that it would be safe and certain to reach the permit, but was built in sections (wherepossible) to allow for stressing of individual units by switching offor bypassing units. This was to allow thedesigners and operators to assess the performance under maximum loading and hence determine theoptimum arrangement.

When the system was designed the argument over whether to put the vertical-flow stages before thehorizontal-flow stages or vice versa was still in progress. The system designed for Stoulton Hawbridge wasPrimarily designed with vertical-flow stages first, followed by horizontal-flow, but an allowance was madein the design to alIow horizontal-flow units to be inserted ahead of the vertical-flow units. The flowsheet isshown in Figure 1.

There are several bypasses allowed for in the system. There are also several places where it is necessary topump. This is mainly because it is an experimental facility (as well as a working sewage treatment works).The final hybrid design which will derive from this work will not have as many places where it is necessaryto pump.

The basic flowsheet is 2 VF stages, VFI and VF2 in series followed by 2 HF stages, HF2 and HF3 in seriesOr either can be bypassed. In addition we have provided HFI which can precede the two VF stages.

IVDO

'"

:-a8

~~

!t

I,IIITIIII

r;:-- ---- .

'~-I.'- SDRB1 ,, f3"- - -- III '--- -­, II I

.. I

FinalEt!kJenltoSlream

L,_,_ ...r........- /"-------- i __ 0",..

'--'-'-'-'-H- '-'- 50'00 """,,---U ......eds I1-1-1-1 I 1 I -I 1 I I 1 1 I 1 I I1 I I -I 1 1- 1-1-1-- 1-- I1--1- 1--

Figure 1.Flowsheetof thehybrid systemproposed for Stoulton Hawbridge.

Basis for design

Complete nitrification and denitrification ofdomestic sewage 287

The VF stages were designed with information gained from the VF systems at Oaklands Park,Gloucestershire, UK (Burka and Lawrence, 1990; Cooper et al., 1996) and the nitrifying VF system at WRcMedmenham, UK (Cooper et al., 1996, 1997).

The two sets ofVF beds at Oaklands Park were sized with a total area of 0.97 m2fpeand achieved full BODsremoval but did not fully nitrify (average ILl mgNH4i'1 in 48 effluent samples taken over 2 years). Thesystem had two HF reed beds following and these achieved:

a) further TSS removalb) some BOD reduction (as a result ofTSS removal)c) some denitrification

These HF beds had a total area of0.43 m2/pe.

The work done at WRc Medmenham using two tertiary treatment VFs for nitrification showed that theoxidation rate was 7 to 13 gNl4_N/m3 ofbed.d or 10.0 to 18.6 gN"I-4_N/m2ofbed.d. This is equivalent to anoxygen transfer rate of 43 to 80 g02fm2.d for the oxidation of the ammoniacal nitrogen alone. In this workthe concentration of BODs was quite low, typically 10 to 20 mg/l, much lower than the average of258 mgBODsll at Oaklands Park.

A review of removal rates in hybrid systems has shown that the oxygen transfer capability of VF beds canbe 50 to 90 g02/m2.d.

Since the designers were uncertain as to whether the VF beds would oxidise all the ammonia as well as theBODs a conservative design of2 m2fpewas used. 1 m2fpewas used in each of the VFl and VF2 stages.

It was convenient to use four 6 m x 6 m square beds in each VF stage which gives 144 m2/stage and hencethe area is 1.11 m2/peper stage to make 2.22 m2fpein total. It will be possible to stress the system during theexperimental period by using only three beds in either VF stage which would give a sizing of 0.84 m2/peperstage - 1.68 m2/pe in total. Likewise it will be possible to bypass the VF2 beds and operate only one VFstage.

The VF stages will be dosed intennittently and only one bed in each stage will be used each day; the otherthree beds in each stage will be resting.

The HF2 and HF3 beds serve two main purposes:

a) Tertiary suspended solids removalb) Denitrification

The two beds can work in series or parallel or either can be bypassed. Each of the beds was sized at0.5 m2fr.e.d and they will be 8 m x 8 m. At Oaklands Park the first HF bed, which was sized at only0.12 m fpe.d, dropped the TON from 22.5 to 10 mgTON/l and the larger second HF bed, sized at 0.31 m2/pe,dropped it further to 7.2 mgTON/l. The denitrification achieved is as a result of endogenous denitrificationusing internal carbon sources. The design is for more than twice the area/pe provided at Oaklands Park withthe hope of achieving significantly more denitrification. As with the VF stages the system can be testedunder stress conditions (to assess the optimum area required) by by-passing either HF2 or HF3.

The HFI bed can be by-passed or used to pre-treat the feed to the VF stages. The purpose of the bed is totest whether or not it is beneficial to remove suspended solids in order to protect the VFI beds and preventany potential for clogging. It has been sized at 0.5 m2/pe and will treat screened settled sewage. The optionto use the HFI bed as the pre-denitrification stage (as in the Johansen and Brix design, albeit with a smaller

288 P. COOPER et al.

bed) is not included in the flowsheet but is easy to do with a portable pump taking effiuent from the outletsumpofVF2.

The sludge generated on the site in the primary settlement tanks will be treated in a Sludge Drying Reed Bed(SORB). This has been sized at 13 m x 10m using a design loading of 30 kg.dslm2.year, however, the bedcan be split into two halves (SDRBI and SDRB2) allowing a dosing rate of60 kg.dslm2.yr to be tested whenthe system is fully matured.

Media

For the VFl and VF2 beds the media will be layers of graded gravel (5-10 mm) topped off with a layer ofsand as previously used at Oaklands Park and WRc Medmenham (Burka and Lawrence, 1990; Cooper et al.,1996). The SDRB will use the same arrangement as the VF beds. The HF beds will be filled with 5-10 mmwashed gravel except at the inlet and outlets where 50-200 mm rock will be used in gabions.

All the beds will be lined with a low density polyethylene (LOPE) liner.

Plants

All the beds will be planted with pot-grown reed seedlings (Phragmites australis) with 4 pots/mz• The potswill be 10 em x 10 em.

Distribution and cotlection

The flow distribution to the HF beds will be the standard system used in UK. with an inlet gabion filled withlarge (50-200 rom) stones, the flow being fed to this via plastic pipework. Outlet collection will be via aperforated agricultural drainage pipe set into a stone gabion, Water level control will be via a flexible pipe inan outlet sump. The flow distribution to the VF beds will use the system which was in operation at WRcMedmenham (Cooper et al., 1996a; 1997) using paving stones onto which the flow is pumped intermittently.The beds will be rotated on a one or two day basis. Collection of the outflow of the beds is via agriculturaldrainage pipes to a central collection sump from which the flow is pumped to the next stage. The pumping iscontrolled via a set of float sensors.

ENVIRONMENTAUHABITAT CONSIDERATIONS

The receiving watercourse is a tributary of the Bow Brook, itself a tributary of the River Avon which isdesignated as a 'sensitive' watercourse and requires phosphorus removal at the larger works within thecatchment. While the watercourse has wooded banks supporting a variety of flowers and shrubs, the landadjacent to the existing works is heavily farmed and ecologically devoid of value.

In total some 568 m2 of reed bed will be provided. By designing the site to be intrinsically safe it would bepossible to avoid the need for visually intrusive high security fencing out of keeping with the ruralenvironment. Instead post and rail type fencing could be used, with hedges planted alongside for the longerterm. By using native species of shrub for the hedge, habitats would be created which would complementthe reed bed habitat, increasing biodiversity. It was pointed out that this highly desirable situation could befurther improved by purchasing an area of land contiguous with the proposed site and the river bank andproviding suitable landscaping. Approval was sought, and granted, for this extension to the scheme.

The Worcestershire Naturalists Trust was entrusted with the design which provisionally involves removal ofapproximately 450 mJ of soil. The major features will be: i) an extensive area of wet grassland covering2800 m2

; ii) a dragonfly pool; iii) a pool and marsh connected by a level control to the watercourse in thewinter; iv) a wet ditch; v) tall herb habitat; and, vi) a small planting of new scrub. Some bankside re­profiling would restore linear wetland habitat along the brook and 100 m of new hedgerow would demarcatethe landscaped area from agricultural activity. The entire scheme complements the strategy for reointroduction of the otter into the Avon catchment, primarily by providing suitable lying up areas.

Complete nitrification and denitrification of domestic sewage

CONCLUDING REMARKS

289

The Stoulton Hawbridge hybrid system is currently in the Severn Trent Water capital programme but thestart date has not yet been determined. Spring 1999 is a likely date to start construction.

A detailed experimental programme to test all the possible arrangements will be defined prior tocommissioning. The first process arrangement to be tested will be that of the 2VF stages in series, followedby the two HF stages in series.

ACKNOWLEDGMENTS

The authors wish to thank Severn Trent Water, Carl Bro and WRc for permission to publish this paper. Theopinions expressed are those of the authors and not necessarily their employers.

REFERENCES

Burka, U. and Lawrence, P. C. (1990). A new community approach to waste treatment with higher plants. In: ConstructedWetlands/or Water Pollution Control, P. F. Cooper and B. C. Findlater (eds), pp. 359-372, Pergamon Press, Oxford, UK.

Cooper, P. F., Job, G., Green, M. B. and Shutes, R. B. E. (1996). Reed Beds and Constructed Wetlands/or Wastewater Treatment.WRc Publications, Swindon, UK, June 1996.

Cooper, P. F. and de Maeseneer, J. (1996). Hybrid systems - What is the best way to arrange the vertical and horizontal-flowstages? lAWQ Specialist Group on Use 0/ 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 ora nitrifying vertical-flow Reed Bed TreatmentSystem. Wat. Sci. Tech., 35(5),215·221.

Cooper, P. F. (1999). Review of the design of vertical-flow and hybrid Reed Bed Treatment Systems. Wat. Sci. Tech., 40(3), 1-9(this issue).

Griffin, P. (1998). Severn Trent Water. Personal communication, May. 1998.Johansen, H.-H. and Brix, H. (1996). Design criteria for a two-stage constructed wetland. Paper presented to the 5th International

Conference on Wetland Systems for Water Pollution Control, Vienna. Austria, September 1996.