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Engineered reed-bed systems for

Workshop wastewater treatment

Over the past decade, major interest has developed in the concept of constructed wetlands for the treat- ment ofpoint sources ofpollution1-4. Although wetlands in parts of the UK have been used for treating domestic ‘dirty water’ since the Middle Ages, the catalyst for recent scientific studies has been the need to find low-cost, ‘environmentally friendly’ techniques for wastewater treatment. Wastewaters derive from many sources. Some contain only a single pollutant, but most contain a wide variety of pollutants. Materials can be pollutants either by adding excess nutrients to watercourses (e.g. carbon substrate, nitrogen and phos- phorus), or by causing toxicity (e.g. heavy metals and pathogenic organ- isms). Work carried out by SeideP6, closely followed by Kickuth’, showed that significant removal of pollutants was possible when con- taminated water was passed through beds of reeds planted in soil or gravel. These wastewaters can have a wide range of strengths (Table 1) and con- tain one or more key pollutants (Table 2).

Principles The main types of aquatic plants

used in constructed wetlands are the Common Reed (Phra@cs atrstrulis), the Common Reedmace (Typha luti-

@a) and the Common Club-rush (Schoenoplecttrs larustris). These plants have a root system of rhizomes, con- taining thick hollow air passages, from which tine hair roots hang down; vertical aerial shoots develop upwards from the rhizome. Oxygen from the leaves passes through the stems and rhizomes, and is exuded from the fine roots so that a thin, oxygenated, aqueous film less than 1 mm thick surrounds the hairs. Consequently, this ‘root zone’, or rhizosphere, can support a very large population of aerobic microorgan- isms, far larger than are found in comparable soil systems. Anaerobic microorganisms, with slower grow- ing lifestyles, predominate in spaces between the root hairs.

The microbial populations present on the supporting gravel media and

TIBTECH JULY 1995 (VOL 13)

in the rhizospheres of Pkragmites and Typka have been shown to be 2-3 orders of magnitude larger than those in unplanted gravels, emphasizing the importance of the vegetation (Table 3). The microbial population also appears to be dominated by bacteria. However, the bacteria have relatively low enzymic activities within the organic substrates testedx, and would, therefore, be expected to degrade most of the simpler organic materials, i.e. those contributing most to the biochemical oxygen demand (BOD). By contrast, the fungi and actinomycetes, although fewer in number, have a wider range of hydrolytic activities8. Conse-

Therefore, wastewater passing through the thickly developed reed- bed rhizosphere encounters alternate aerobic and anaerobic microbial populations. These convert carbon- aceous, and to a lesser extent nitrogen- ous and phosphatic, contaminants in the water to less-polluting materials (Table 4). In addition, root growth causes minor displacement of the media in which the plants are grow- ing; this opens up passages for water flow. The multiplicity of upright stems above ground (often up to 500per m*) provides a veritable jun- gle for water flowing over the bed surface. Microorganisms can fonn biofilms around the lower stems, which can then trap particles suspended in the wastewater by adsorption.

Horizontal sub-surface flow of wastewater through the rhizosphere

Table 1. Strengths of wastewatersa

quently, they would be expected to degrade many of the larger molecules that contribute to the chemical oxy- gen demand (COD).

Strength BODmg I-’

Weak <45

Medium 45-300

Strong 300-3000

Very strong >3000

Example

Effluent from secondary sewage treatment

Effluent from primary sewage treatment

Farm ‘dirty water’, industrial effluents

Farm manure slurries, silage liquor, industrial effluents

“A simplified approach to measuring wastewater strength is based on defining biochemical oxygen demand (BOD) in mg 1-l.

Table 2. Key pollutants

Pollutant

Carbon

Available form

Readily biodegradable compounds [measured as biochemical oxygen demand (BODII

Nitrogen in several forms

Slowly biodegradable plus readily biodegradable compounds [measured as chemical oxygen demand (COD11

Measured as total-N, organic-N, NH,-N, NO,-N and NO,-N

Phosphorus in several forms Measured as total-phosphate and orthophosphate

Suspended solids 6%

Heavy metals Such as Fe, Mn, Pb, Zn

Pathogens Measured in colony-forming units (CFU) g-1 dry weight or wet weight

0 1995, Elsewr Science Ltd 0167 - 7799/95/$9.50

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Table 3. Comparison of microorganisms found in gravel matrices and rhizospheres of sub-surface flow wetland9

Substrate Bacteria Actinomycetes Fungi

Unplanted gravelb 0.6 x 106 3.1 x 104 1.0 x103

Typha gravelb 1.6x 106 1.4x105 4.0 x 103 Typha rhizosphereC 3.5 x 109 2.5 x 106 2.8 x 104 Phragmites gravelb 1.2x 107 2.8 x 105 1.1 x 105 Phragmites rhizosphereC 0.6 x 109 1.3 x 106 1.6 x 106

aData from Ref. 8. ‘Expressed as colony forming units KNJ g-1) of dry weight. ‘Expressed as CFU g-1 of wet weight.

is used for tertiary treatment of sewage and other weak wastewaters’. Downflow through a multi-layered aggregate bed is used for treating stronger wastes l”.ll. Above-ground flow between plant stems is used for the removal of metals, and to bring about a pH change; this is particularly useful for the treatment of acid mine- discharges”. Water transpiration and ‘root zone’ oxidation is used for stabilizing and drying sludges and slunies13.

Phvagmiter has most frequently been used in European applications, whereas Typha has been extensively used in the USA, and is now starting to be employed in schemes in the UK that use above-ground flow for the removal of metals”. These two

species have proved able to tolerate weak, medium and strong waste- waters (see Table 1). So far, there have been no major attempts to use reed beds for very strong eflluents; these are likely to put the plants under stress. Moreover, the simple technology of reed beds cannot reasonably be ex- pected to treat this type of material.

Treatment systems Horizontal-flow beds consist of

either a soil or gravel matrix into which the reeds are planted (Fig. 1). The lnatrix is kept flooded, with the water surface less than 5cm below the top of the bed’4. A basis for design, developed from experience gained in sewage treatment, has been established’“-“.

Table 4. Removal mechanisms in reed-bed treatment systems

Contaminant

Suspended solids

B~;~arn~al oxygen

Nitrogen

Removal mechanism

Sedimentation, filtration, adsorption

Sedimentation Degradation to CO,, H,O and NH, by microorganisms attached to plant and sediment surfaces

Main removal by nitrification-denitrification Ammonia oxidized to nitrate by nitrifying bacteria in aerobic zones

Nitrates converted to N, gas by denitrifying bacteria in anoxic zones

Phosphorus

Heavy metals

Pathogens

A little removal by plant uptake and ammonia volatilization

Adsorption, complexation, precipitation reactions within the bed matrix, particularly with Al, Fe, Ca and clay minerals. Very little plant uptake

Precipitation reactions after pH changes, sedimentation and adsorption onto biomass films on plant stems

Sedimentation and filtration. Competition and natural die-off. Excretion of antibiotics from roots of plants and from composting of plant litter on bed surface

The length ofhorizontal-flow beds for the tertiary treatment of sewage and for other weak wastewaters is short (5-1Om). For the secondary treatment of sewage having a BOD of about 300mgl-‘, the bed length can be about 70m, but the bed may need to be subdivided into terraces. The residence time in these horizon- tal-flow beds is several days.

Downflow beds are constructed of sharp sand, pea gravel, coarser gravel and stones in layers of appropriate thickness”‘,1 (Fig. 2). Oxygen from the atmosphere can diffuse into the bed via perforated pipes set within the layers, as well as from the roots of the reeds. Either one, two or three stages of downflow treatment may be required, according to the strength of the initial wastewater. Each stage comprises several ident- ical beds, with perhaps six in the first stage, four in the second and three in the third’“,“. The downflow beds undergo alternating periods of opera- tion and rest. During an operating period of several days, the surface of the sand top layer gradually becomes choked with fine faltered solids, lead- ing to flooding conditions. By resting the bed for about a week, the solids are oxidized and the bed becomes permeable again. Downtlow beds have considerably more potential for oxy- genation than have horizontal-flow ones. Owing to their self-draining nature, the residence time in them is only a matter of minutes, as opposed to several days in horizontal-flow beds. So far, these beds have been designed essentially on hydraulic con- siderations, i.e. to handle the water flow. Experience is slowly being acquired in designing these beds so that they will be able to process organic loads in terms of kg BOD d-l.

Overland-flow beds aim to pre- cipitate metal salts from solution by pH change and aeration. The pre- cipitates are then adsorbed onto biomass on the plant stems, or settle onto the matrix surface”. Sub-surface flow is impossible because the pre- cipitates would soon choke the bed. These beds are mainly designed on residence-time considerations.

The effectiveness of each of these three types of bed for pollutant removal is indicated in Table 5.

Sludge treatment beds comprise a sand/gravel matrix on to which the sludge/slurry is poured about once a fortnight. Water is removed by drainage and evapo-transpiration. If the sludge has a carbonaceous con- tent, then oxidation and stabilization

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Outlet chamber

Inlet wastewater

coll6ction pipe tmpewious liner

Limestone chippings

Figure 1 Horizontal-flow reed-bed treatment system showing Phragmites australis planted in a matrix of limestone chippings. Also shown are the inlet wastewater-distribution and outlet purified water-collection zones of rocks, and the water-level control device. The bed is constructed in a shallow exca- vation, with an impervious liner to prevent seepage of polluted water into the subsoil.

can occur. The beds are sized essen- tially according to residence time. As solids build up on the bed, the reeds and their roots grow up with it. The sludge solids are allowed to accumu- late for several years before they are removed; this is most easily done when the reeds have died down in

Advantages and disadvantages The advantages of constructed

wetlands systems include an initial capital cost that is normally cheaper than that of its mechanical counter- part. For example, Green reports a capital cost in the range UKA700-1600 per head of popu-

tank and horizontal-flow reed bed is used for secondary sewage treatment in small rural worksao. This range of costs, related to the size of the unit and to site-specific factors, compares with costs of about UKA2000 per head of population for the alternative mechanized rotating biological con-

winter. lation when a combination of septic tactor installation.

Solid pipe

Perforated 1 /.‘I :’ . b ,f 1 . A pipe

ICI I /p- (’ ^̂ “̂ ”

I I . . .

~ I

I,. ‘I ,

/I+ ,‘I’;~. / f ,’ ‘:. ::

:. Q,”

4 \ ~-ii . * I. Retaining walls ~

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. * - . : . . * . *

. * . . - ** . . ..*. - -.

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-“--------~---o-----o l-l l-3

*a . . . ^^

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Pea gravel

Gravel

Stones layer

Outlet

Figure 2 Downflow reed bed with Phragmites at&a/is planted in a multi-layered matrix of various-sized aggregates. A system of perforated pipes allows counter-current diffusion of oxygen into the bed and carbon dioxide out of it. The wastewater is flushed on to the bed in pulses large enough to allow even distribution over the bed-top surface. These beds are often built above ground between retaining walls.

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Table 5. Percentage removal of pollutants by reed beds

Bed type Removal type Refs

Horizontal sub-surface flow

Downflow

Overland flow

Results from a large number of beds in Europe indicate that biochemical oxygen demand (BOD) removal is 80-90%, with a typical outlet concentration of 20 mg I-1, total-N removal is only 20-30% and total-phosphate removal is 3040%

Using a system of two downflow stages to treat domestic sewage achieved removals of 93% BOD, 90% suspended solids (SS), 75% NH,-N and 37% orthophosphate

Passing the effluent from a lead-zinc mine through a bed of Typha latifolia achieved reductions of SS of 99%, Pb of 95% and Zn 80%

15,16

10

23

In suitable locations, gravity flow ofwastewater is possible and an elec- tricity supply is not needed. Main- tenance and supervision require- ments are also low, hence operating costs are far less than those of mechan- ized units. These cost advantages have led to reed beds being con- structed in significant numbers for the treatment ofweak- and medium- strength wastewaters (Table 6). In addition, reed-bed treatment systems are aesthetically pleasing and provide wildlife habitats for birds and inver- tebrates, particularly where they are carefully landscaped and include poolsa’.

fore, the calculation of bed sizes h: tended to err heavily on the side of caution. This has the benefit of help- ing the bed to cope with occasional overload conditions.

Sewage and farm wastes normally provide adequate nutrients for growth of the reeds, but with many industrial eflluents these nutrients need to be added to the system through the inflow. Concern was originally expressed over the possi- bility of the transmission of pol- lutants, particularly heavy metals, along the food chain from waste- waters, via plants, to higher trophic organisms. In practice, heavy metals tend to be concentrated in the root systems and sediments, so that the danger to birds and animals eating the leaves is small. However, studies

With this new technology, the frontiers are still being pushed back and problems exposed. Reliable data on flows are often lacking and, there-

Table 6. Applications of wetlands in wastewater treatment

Wastewater type

Municipal sewage Sewage treatment following septic tanks

Runoff

Farm

Landfill

Industrial

Mine

Sludges (particularly organic ones)

Application

Secondary and tertiary treatment

Houses, hotels, stately homes, caravan parks, leisure centres, golf courses

From roads, airports and vehicle servicing areas (trains and coaches)

‘Dirty water’ on dairy units and vegetable washing water

Leachate From chemicals manufacture, oil refineries, food and beverage manufacture, paper mills, tanneries

With metal contamination from operating and derelict coal/metal mines

Dewatering and stabilization

will need to be carried out to estab- lish the effect on invertebrates con- tinuously exposed to sediments, especially those in overland flow beds which accumulate heavy metals. To date, there has been no report of an increase in toxicity due to biotrans- formation of pollutants in wetlands. However, there is a possibility of pollution where sulphur compounds are only partly broken down and are discharged as sulphides, rather than being fully oxidized to sulphates.

Reed beds are proving capable of handling wastewaters with BOD levels up to about 500 mg l-l, when a combination of downflow and hori- zontal-flow beds is used (Fig. 3). They have not yet been proven for wastes with higher BOD levels, such as ‘dirty water’ from farms, which has a BOD up to about 3000mgl-* (Ref. 22). The concentration of ammoniacal nitrogen in farm wastes and other e!&rents is also high, often up to 500 mgl-i. This is proving difficult to break down in simple reed- bed systems using the normal pro- cesses of nit&cation-denitrification and is delaying the application of wetlands to such wastesaa. The evi- dence gained from horizontal-flow beds indicates that, although BOD removal is normally in the range 80-90%, the removal of total nitro- gen is only 20-30% (Refs 15,16). Once the problems of nitrogen removal have been solved, that of phosphorus removal will become important, because this element is heavily implicated in the eutrophica- tion of water masses.

Conclusions The technology of constructed

wetlands is still relatively new. Their application to the tertiary treatment of sewage in horizontal-flow beds has proved technically and economically sound, at least in small rural works. The use of downflow beds in super- vised situations for slightly more demanding effluents is showing merit. However, their application to strong wastewaters with BODs of up to 3000 mgl-i has not yet been resolved, and the removal of am- moniacal nitrogen to satisfactory levels is a major drawback in farm waste treatment. The use ofwetlands for the removal of metals has great potential, and will be needed for the treatment of seepage from aban- doned coal and metal mines in the UK. At present, sludge stabilization and drying on reed beds has not made much progress.

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a

Figure 3 (a) View of the Unrversity of Birmingham (UK) experimental reed-bed facility for treating farm ‘dirty water’ at Rugeley, Staffordshire. The unit comprises downflow beds, horizontal-flow

beds, nitrifying beds and frnal lagoon. (b) The corner of a downflow reed bed showing flush- ing box, wastewater and air distribution pipes, and drainage sump with pump.

References

2 Cooper, 1’. F. and Fmdlater, B. C. (rds) (1YYO) Cons/rurred Wrfhds it1 Water IN-

/ution comr, Prrgamon Press 3 Moshlri, G. A. (cd.) (lYY3) Comtrnrtr~d

W&zndx jbr Water Qualiry Inrprovemenf,

Lrws Pubhshrrc 4 Bavor, H. J, and Mitchell, D. S. (eds) (1994)

Wufhd Sysms ia Warer Pollrrrim Control,

Water S&w md Tethrolo~y 29, l-336 5 Sridrl, K. (1973) IJS P&M XJ~ 3770623 6 Sridrl, K. (1976) in Biolqiral Confrol of

Water Pdbrtion (Tourbm, J. and Pierson,

R. W., rdr), pp. 109-122, Univenity of Pennsylvania Press

7 Kickuth, II. (lYX4) Lmdsisrh. Stadt. 16,

145-153 8 Hatano, K., Trettin, C. C., House, C. H.

and Wollum, A. G. (1993) in Constructed

W&nils jh Water Quality Inrpnwrm~nt

(Moshiri, G. A., cd.), pp. 541~547, Lewis Publishers

9 Green, M. B. and Upton, J. (1994) Writer Erwirm. Kes. 66, 188-I 92

10 Burka, U. and Lawrence, P. C. (1990) in Cowstrucred W&nd.c irz Water Pollution

Conrrvl (Cooper, P. F. and Find&r, B. C., cds), pp. 359-371, Pergarnon Prets

11 Gray, K. I<., BiddIestone, A. J., Job, G. and Galanos, E. (1 YYO) in Colzstnrrf~d

Wetlands in Water Pohtiow CovM

(Cooper, P. F. and Findlater, B. C., ed$), pp. 333-346, l’ergamon Press

12 NRA (1994) W'7mlJnw A Cknr Way For-

ward, National Riven Authority South Western Region, Exeter, UK EX2 7LQ

13 Reed, S. C., Middlebrooks, E. J. and Cntes, R. w. (1988) Natwal Systrrnsfor

Waste Matq~ewlerzt artd Treatment,

McGraw-Hill 14 Boon, A. G. (1985) Kqporr $a !&it by Mm-

hers and Sra/f‘of WRc fo Grmany to Iwesti-

gate the Root Zone kkthod~fir Treatment LJJ

Wastewaters, Report 376-S/1, Water Research Centre, Swindon, UK SN5 8YR

15 Cooper, I’. F. (1990) Ewopran Dccip and

Opmtirwzs Goidc/inrs,fiw Reed Brd Trcatmmt

Systems, Report Ul 17, Water Research Centre, Swindon, UK SN5 8YR

16 Cooper, I’. F. (1993) in Comtrurted

Wetlmds fw Wafer Qualify Improtwmwf

(Moshiti, G. A., ed.), pp. 203-217, Lewis Publishm

17 Co&y, L. M., Dick, R. I. and Lion, L. W. (1991) Res. J. WPCF63, 239-247

18 Biddlestone, A. J., Gray, K. R. and Thurairqan, K. (1991) J. Biotechol. 17,

209-220

19 Job, G. I~., Biddlestone, A. J. and Gray, K. R. (1991) Tram. I&. Clwm. En<?. 69, Part A, 187-18’)

20 Green, M. B. and Upton, J. (1993) in Conrrvuctcd Wethdr for Water Qtraliry

Impr~wwvnt (Moshiti, G. A., 4, pp. 517-524, Lewis Publishers

21 Merritt, A. (1994) Wetlands, Industty arid

Wildl$k A Mmua/ of Principles und

Practices, The Wildfowl & Wetlands Trust, Slimbridge, Gloucester, UK GL2 7BT

22 Nicholson, R. J. (lYY4) Treotmwt of Dilute EJuct~ts by the Roar Zone (Reed

Bedi A4cfhd. WA0501 Fmal Report, Ministry of Agriculture Fisher& and Food, Envmnmental Protection DI~I~I~I~, London, UK SWlP 3JR

23 Lan, C., Chen, G., LI, L. and Wang, M. H. (1990) in Conrtructcd Wetlands

in Water Pollution Covmd (Cooper, P. F., and Findlater, B. C., eds), pp. 419-427, Pergamon Press

Kenneth Roy Gray Anthony Joseph Biddlestone

School qf Chemical Engineering, The University

qfBirmiqhn, Bimtirz&m, LJK B15 ZTT.

Students

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TlBTECHJULY1995P/OL13)