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583VASUDEVAN et al : LOCALIZED DOMESTIC WASTEWATER TREATMENT: PART I - CONSTRUCTED WETLANDSJournal of Scientific & Industrial ResearchVol. 70, August 2011, pp. 583-594

*Author for correspondenceE-mail: [email protected]

Localized domestic wastewater treatment: part I - constructed wetlands(an overview)

Padma Vasudevan1*, Paul Griffin3, Alan Warren4, Alka Thapliyal2 and Mamta Tandon1

1Centre for Rural Development & Technology, 2Centre for Energy Studies, Indian Institute of Technology, Delhi, India3Severn Trent Water Limited, PO Box 5309, Coventry, CV3 9FH, UK

4Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD, UK

Received 04 May 2011; revised 09 May 2011; accepted 13 May 2011

Constructed wetlands (CW) are an alternative localized treatment technology suitable for reducing BOD, COD, NPK andpathogens to acceptable levels for subsequent use of treated water, especially for irrigation. Dissolved chemicals and heavy metalsare also reduced to an extent. CWs are extensively used in developed countries for secondary and tertiary treatment of domestic

sewage and treatment of surface run off as well as industrial effluents. But CWs are yet to be commercialized in developingcountries. Also detailed mechanisms involved in the treatment, especially role of macrophytes and microorganisms, need to beunderstood. Precautions must be taken in the use of CWs to prevent proliferation of mosquitoes. This review presents prospectsand problems in propagating CWs for different types of wastewater, with special focus on domestic wastewater treatment.

Keywords : Constructed wetland, Domestic wastewater, Macrophytes, Microorganisms, Nutrients

Introduction

Constructed wetlands (CW) are gaining importanceas an effective and low-cost alternative to conventionalwastewater treatment plants that involve large capitalinvestments and operating costs. In developed countries,

CWs have been commercialized for treatment of a varietyof effluents. In many Asian and African cities, populationgrowth has outpaced improvements in sanitation andwastewater infrastructure, making management of urbanwastewater a tremendous challenge1-3. In less developedcountries, many villages now have piped water andamount of wastewater generated is increasing. Faecalmatter is deposited in septic tanks or soak pits whereavailable. Domestic sullage (grey water) flows throughopen channels and collects in low lying areas and getscontaminated by faecal matter and other solid wastes.

Thus a lot of wastewater is being generated in cities,villages and industrial units by human activities, butnumber of centralized facilities and their treatmentcapacity is far from adequate.

Due to a shortage of water for irrigation, farmers inperi-urban areas of many developing countries are using

domestic and other wastewater effluents for raising cropsand often use undiluted wastewater to provide nutrients4,5.This practice can severely harm human health andenvironment due to associated pathogens, as well asallowing heavy metals and other undesirable constituents

from water to enter food chain6. Use of CWs for initialtreatment could greatly reduce adverse environmentaland health impacts from wastewater irrigation7,8.

This review presents types of CWs, wastewatertreatment mechanism in CWs, performance efficiencyand costs involved in operating CWs.

Constructed Wetlands (CWs)

CW comprises a bed of soil, sand or gravel, whichtogether treat wastewater. Root system of plants andmedia (soil and stone) act as filters and support biofilms,

which help in removing contaminants. In addition, plantsutilize nutrients and bioaccumulate contaminants suchas metals. First experiment using wetlands withmacrophytes for wastewater treatment was carried outin Germany during 19509. Various European countriesincluding UK adopted this technology during 1980s andfirst European Design Guidelines were publishedfollowing International Conference on the use of CWsin water pollution control, Cambridge, UK in 1990.

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584 J SCI IND RES VOL 70 AUGUST 2011

Guidelines for Municipal water treatment by CW havebeen developed in several countries including Australiaand USA.

CWs are classified based on the basis of vegetationtype (emergent, submerged, floating leaved, free-floating)and hydrology (free water surface and subsurface flow)10.

Subsurface flow wetlands are further classified accordingto flow direction [vertical (V) or horizontal (H)]. Forbetter performance, different types of CWs aresometimes combined into hybrid systems11,12.

Types of Constructed Wetlands (CWs)

Free Water Surface Constructed Wetland (FWS CW)

A typical FWS CW with emergent macrophytescomprises a shallow sealed basin or sequence of basins,containing 20-30 cm of rooting media, with a water depthof 20-40 cm. Emergent vegetation covers a significantproportion of surface, usually > 50%. Naturally occurringspecies may also be present in addition to plantedmacrophytes. Litter provides organic carbon necessaryfor denitrification, which may proceed in anoxic pocketswithin litter layer. Besides municipal wastewater, FWSCWs are used to treat storm water run off, landfillleachate, agricultural and drainage run off and industrialeffluents. Sizing of FWS CWs is usually based either onvolume or area. Area-based methods assess pollutantreduction using overall wetland area, while volume-basedmethods use a hydraulic retention time to assess pollutantremoval. For achieving target effluent concentration of

BOD5 (20-30 mg/l), TSS (20-30 mg/l) and TKN (10 mg/ l), loading rates recommended13,14 are of the order of BOD5 (3-6 g/m2d), TSS (3-7 g/m2d) and TKN (1.5 g/ m2d) respectively. FWS CWs are efficient in removal of organics through microbial degradation and settling of colloidal particles. Suspended solids are removed bysettling and filtration through dense vegetation. Nitrogen(N) is removed primarily through nitrification (in watercolumn) and subsequent denitrification (in litter layer).Ammonia volatilization occurs under higher pH causedby algal photosynthesis. Phosphorus (P) removal isusually low due to limited contact of water with soil

particles, which are responsible for absorption orprecipitation of P. Uptake of nutrients by macrophytesrepresents only temporal storage because these nutrientsare released back into water when plants decay11,15.

Horizontal Flow Constructed Wetland (HF CW)

HF CWs consist of a bed of media, usually gravel orsoil, sealed by an impermeable layer and planted withwetland vegetation. Wastewater is fed at inlet and flows

through porous media under the surface of bed down asmall gradient at floor level until it reaches outlet zone,where it is collected. In initial stages of HF CWdevelopment, soil media was extensively used16 andmarketed as root zone treatment. Subsequently, soil wasreplaced by washed gravel (grain size, 5-20 mm).

Widespread use of common reed (Phragmites australis)has led to HF CW being called ‘Reed Beds’, though inpractice any pollution-tolerant deep-rooted emergentmacrophyte can potentially be used. In media, pollutantsare removed by microbial degradation and chemical andphysical processes in a network of aerobic, anoxic andanaerobic zones with aerobic zones being restricted tothe surface and areas adjacent to roots where oxygenleaks to substrate17. Organic compounds are microbiallydegraded under anoxic/anaerobic conditions asconcentration of dissolved oxygen in filtration beds isvery limited18. Suspended solids are retained essentiallyby filtration and sedimentation and removal efficiency isusually very high15. Major removal mechanism for N inHF CWs is denitrification. Removal of ammonia is limiteddue to lack of oxygen in filtration bed due to permanentwaterlogged conditions19. Phosphorus (P) is removedprimarily by ligand exchange reactions, where phosphatedisplaces water or hydroxyls from the surface of ironand aluminum hydrous oxides. Siliceous gravels are themost popular media, so unless special materials are used,removal of P is usually low19.

Most important role of plants in HF CWs is provision

of hydraulic pathways through media to maintainhydraulic conductivity. Secondary roles are provision of surfaces (roots and rhizomes) for the growth of attachedbacteria, radial oxygen loss (oxygen diffusion from rootsto rhizosphere), nutrient uptake and insulation of bedsurface in cold and temperate regions 20. HF CWs arecommonly used to treat domestic and municipal watersand area requirement for UK conditions and per capitaloadings is 5 m2PE (person equivalent). To achieve BOD5and TSS of 30 mg/l in outflow, USEPA recommendsloadings of 6 g/m2d and 20 g/m2d respectively.

Vertical Flow Constructed Wetlands (VF CW)In VF CWs, wastewater is pumped in large batches

to the bed and allowed to percolate through media. Anew batch is fed only after all water has drained fromthe bed. This allows for diffusion of oxygen from air intobed and makes VF CW efficient for nitrification andachievement of ammoniacal nitrogen standards21. VFCWs can provide some denitrification if design allowspart of the bed to be left flooded22. VF CWs are also

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585VASUDEVAN et al : LOCALIZED DOMESTIC WASTEWATER TREATMENT: PART I - CONSTRUCTED WETLANDS

effective in removing organics and suspended solids.

Removal of P is low unless media with high sorptioncapacity are used15. As compared to HF CWs, VF CWs

require less land (1-3 m2 PE-1)23-25. Early VF CWs werecomposed of several stages with beds in the first stage

fed in rotation. More recently, many small VF CWs

treating domestic only flows are being built with one beddesigned for long resting intervals and these are marketed

as compact VF CWs 24 . In upflow vertical CWs,wastewater is fed at the bottom of wetland. Water

percolates upward and then it is collected either near

surface or on the surface of wetland bed26. Removal of organics is high in all types of VF CW.

Hybrid Constructed Wetland (VF – HF)

VF - HFs that combine VF and HF stages27 are inoperation in many countries around the world and are

used especially when removal of ammonia-N and total-

N is required15.

Design of CWs

Important parameters in wetland design are surface

area, depth and pollutant load in wastewater. Loadingrate of wastewater is also dependent on type of

wetland28. Shallow depths that help in better aeration of

water are beneficial. CWs having water depths of 30±50cm have been evaluated for efficacy in treating septic

effluents prior to disposal on land. Other studies29-31 onevaluation of subsurface flow wetlands included wetlands

that were 60±100 cm deep.Plants used in purification process need to satisfy

following criteria: 1) ability to grow in permanently or

intermittently saturated ground or media; 2) must be

tolerant of wastewater contaminants; 3) aggressive

plants, which are not readily displaced by other less

suitable species, are preferred but they should not

proliferate as weeds; 4) should grow readily from seed

to facilitate establishment; (5) should have extensive and

deep root systems to maximize surface area for biofilm

growth and maintain hydraulic conductivity within media.

For floating species, deep root systems promote

flocculation and sedimentation; if possible they should

promote treatment process as with oxygen ‘leakage’ from

roots of common reed (P. australis). Species, which act

as hyper-accumulators of nutrients or other contaminants,

especially heavy metals, may be favoured for specific

applications and they should be of local provenance to

increase biodiversity and avoid introduction of unwelcomealien species.

At present, most common aquatic plants used in

subsurface wetlands are bulrush (Scirpus), cattail(Typha) and reeds (Phragmites). Many other types of

macrophytes such as Vetiveria (Chrysopogon

zizanioides) are gaining importance. Ornamental plants

could function as an important part of a wastewatertreatment system and also provide economic benefits.Wolverton32 reported that calla lily ( Zantedeschia

aethiopica), canna lily (Canna flaccida), and threeother ornamental plant species planted in a rock filter

used to treat septic tank effluents were able to add oxygen

and increase biological activity in septic bed. Neralla et

al33 concluded that ornamental plants, including canna

lilies, were as effective as cattails in improving quality of effluents from septic tanks, but noted that use of cattails

led to 200% more water loss through evapotranspiration

in comparison to flowering plants. Hydroponic cultivationis also being integrated in CWs34.

Commercial CWs for Domestic Wastewater Treatment in UK

Among various types of CWs, horizontal sub-surfaceflow type is most common. A cross section of a typical

horizontal sub-surface flow reed bed (Fig. 1) from UK

comprises a shallow excavation with a plastic liner toprevent partially treated water from leaking out into

groundwater, or groundwater from entering excavationif water table is high. No liner required if low permeability

clay is present. There is an inlet distribution section where

pipes or a trough distribute flow evenly over the width of excavation onto large stonesc:, which distribute flow down

into the media having sufficiently high hydraulicconductivity to permit water to flow under media surface.

This facilitates contact of water with plant roots and

rhizomes and their associated microfauna. A large specificsurface area for biofilm development is also essential.

Among many types of media being used, most popular iswell rounded siliceous gravel (size, 4-15 mm).

Fig. 1—Longitudinal section through a HF CW

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Final section of HF CW is for effluent collection andfor controlling water levels within the system. Typically,effluent collection zones comprise large stones that overliean agricultural drainage pipe running full width of thebed and terminating in a level control chamber with anadjustable outlet height, which, in its simplest form, may

be no more than a plastic pipe with a chain attached tochamber surface. Optimum water depth is just belowthe media surface. Level control pipe should be capableof completely draining the bed to maximise hydraulicgradient as media clogs with accumulated solids overtime, and to facilitate maintenance and repair. As suchthere is a significant head loss, most of which is associatedwith the depth of media used. Bed depth should be atleast the average depth of root penetration and a figureof 0.6 m is widely used. Greater depths increase headloss correspondingly. A bund depth suitable foraccumulation of plant litter and sludge solids over manyyears is also provided. A typical system might have 1.1m total head loss from invert of inlet pipework todistribution system. UK practice is to use P australis, orsometimes Typha latifolia , and a minor industry hasgrown up providing pot-grown seedlings for planting of new or refurbished systems.

Secondary treatment HFCWs are dimensioned at 5m2 PE-1 or higher, where a hydraulic PE is considered tobe 200 l per person per day, a typical value in WesternEurope. Such systems are most commonly used for smallrural populations. Life of such systems is typically 5-10years, depending on how generously they are sized. Highsuspended solids loads may lead to rapid clogging and areduced asset life. Tertiary treatment application iswidespread in UK in order to provide additional BODand suspended solids removal from good quality secondarytreatment effluents where very high standards arerequired for protection of receiving watercourse. Nitrateremoval (30-40%) may be expected. In such cases,HFCWs are typically dimensioned at 0.7-1.0 m2 PE-1

and are economically viable for populations up to 2000.Reed beds at higher end of dimensioning criteria

quoted may also be used for treatment of dilute storm

water, thereby having a combined function. Storm wateronly HFCWs vary in size greatly and may be used eitherfor stormwater treatment, or as part of sustainable urbandrainage systems (SUDS) (Fig. 2). Special arrangementsare needed to allow addition of water to ensure plantsurvival during dry periods when water levels are reduceddue to evapo-transpiration.

Nominal retention time in a secondary treatmentreed bed (size, 5 m2 PE-1) is 5 days. This means that

pathogenic organisms must face an extended period in ahostile environment, and this leads to high percentageremovals of many species of concern to human health.Subsurface flow CWs do not provide suitable conditionsfor proliferation of mosquitoes due to absence of freesurface water, on which mosquitoes rely for egg-layingand for development of juvenile stages.

HF CWs are oxygen limited and to achievesignificant removal of ammoniacal nitrogen, VF CWsare needed. These are shallow trickling filters with smallmedia (an upper sand layer) and rely upon a pulsed inputof wastewater to flood surface of VF CW, followed by

a period of drainage, when bed becomes aerobic andtreatment occurs. Pulsed input is achieved by either apump or a high capacity siphon. VF CWs are typicallydimensioned for a total area of 3m2PE-1 or higher. SeveralVF CWs in series may be required to achieve requisitelevel of treatment.

Performance Efficiency

Nutrient content of wastewater is a valuable resourceif irrigation is primary use, but if discharge towatercourses occurs then nutrient content may lead toeutrophication of water body with consequent ecological

damage35,36

. Vymazal10

compiled data on treatmentefficiencies of a large number of CWs and found averageremoval efficiencies for BOD, TSS and nutrients[N, total nitrogen (TN), ammoniacal nitrogen (NH4N)and P] in FWS, HF and VF (Table 1). Additional studies37-

44 on BOD and nutrient removal by CWs from domesticwastewater have also been carried out (Table 2).Presence of detergents could be problematic due to theirpoor biodegrability and surface activity45. Average

Fig. 2—Aerial photograph of a combined tertiary treatment andstormwater treatment CW following a rotating biological contactor

(courtesy Paul Griffins)

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Table 2—Studies related to nutrient removal from domestic wastewater (% efficiency in paranthesis)

Types of wastewater Parameters studied Plant spp

Black/grey domestic TSS, COD, BOD5, N- -

wastewater treatment37

NH4

+

, N-NOx, Ntot, TC

Domestic effluent38 TSS & total BOD5, N & P Lemna gibba L.

Domestic wastewater23,39-42 TSS, COD (80%) removal Typha angustifolia

of NO3- & NH

4(50%)

BOD (75%), TSS (88%) T. latifolia L. or Festuca

arundinacea Schreb.

BOD (80±90%), TSS, Canna accida, Cyperus

VSS, NH4N, total P alternifolius, Sagittaria

lancifolia, Scirpus sp.,

T latifolia, Collocasia

esculenta, Gladiolus sp., Iris

sp., & Thalia sp .

NH4+ (18.1-46.2%) and T. latifolia

TKN (39-68%)

BOD (95%) and Scirpus cyperinus,

Nitrification (90%) Phragmites australis

Mix of domestic and 1.5- BOD (70-80%), COD (50- Typha sp. & Phragmites sp .

3% industrial wastewater43 75%) and TSS (80-90%)

Mix of domestic and COD (80-90%), total N & P. australis

agricultural wastewater44 total P

treatment efficiency of anionic tensides removal in CWswas 67%. Varying rates in reduction of differentsurfactants from wastewater using wetlands or dry land-based treatment systems have also been reported46-49.Belmont & Medcalfe47 showed that subsurface CWscan reduce nonylphenol ethoxylate surfactants in treated

wastewaters. Mori et al48 reported that rhizospheremicrobes around giant duckweed (Spirodela polyrrhiza)

play an important role in degradation of surfactants.Waterborne pathogens50,51 of serious risk to humans

include bacteria (Salmonella typhi, Campylobacter sp.,

and Escherichia coli), viruses (enteroviruses and

Hepatitis A) and protozoans ( Entamoeba histolytica,

Giardia intestinalis and Cryptosporidium hominsi).Ability of CWs to reduce pathogenic microorganisms inwastewater effluent is well established50,52-56. Removalrates for faecal coliforms (>99%) generally equal orexceed those described for conventional biologicalwastewater treatment processes57-59 (Table 3).

Comparable high removal efficiencies have also beenreported for bacteriophages, coliphages and viruses insubsurface flow CWs64. However, removal of parasiticprotozoa and helminths from wastewaters in CWs hasbeen only rarely investigated. Nevertheless removalefficiencies appear to be relatively high53, 64-66.

Metals are persistent in environment and accumulatein sediments and plants within CWs. Some of these metalsare hazardous to public health, even at ppm levels. For

example, arsenic can cause disorder of dermal andnervous systems, cadmium, mercury and chromiumcause kidney damage, and lead causes anemia and mental

retardation. Some studies on removal of metals 67-75 fromdomestic wastewaters in CWs are listed (Table 4).

Wastewater Treatment Mechanisms in Constructed Wetlands

(CWs)

Wastewater is treated in CWs by adsorption(chemical attachment and chelation to active surface of

Table 1—Average performance efficiencies (% removal) by

different types of constructed wetlands (CWs) for BOD, TSS andnutrients

Types of CW BOD TSS TP TN NH4N

FWS 73 72 40 48 45

HF 75 75 50 38 35

VF 90 89 56 43 73

FWS, free water surface CWs; HF, horizontal flow CWs; VF, vertical

flow CWs; BOD, biological oxygen demand; TSS, total suspended

solids; TP, total phosphorus; TN, total nitrogen; and NH4N,

ammonical nitrogen

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media), filtration (by planting substrate and biofilmnetwork), precipitation, redox chemistry (reduction andoxidation due to redox gradient present in the system),predation (grazing by micro fauna), sedimentation(settling and deposition of large solids at low velocities),phytoremediation (active uptake of pollutants into rootsand leaves of plant) and biological degradation (biomassproduction by nutrient and organic matter mineralization).

Role of MacrophytesRole of plants in wastewater treatment by CWsinclude: i) providing attachment sites for microorganismsresponsible for many of the treatment processes; ii)uptake of nutrients (N & P) and metals, althoughharvesting may be needed in order to realize benefits,otherwise decay may release what is accumulated; (iii)improving hydraulic conductivity of media, and extendingperiod, in which wastewater is in contact with plants

and can be treated by growth of roots and rhizomes thatform a network of underground channels; iv) roots of floating plant species promote flocculation andsedimentation of suspended material; and v) degradationof plant material and trapped organic matter releasessoluble carbon, which allows natural denitrification tooccur in oxygen limited conditions.

Role of Microorganisms

Macrophytes play a secondary role in degradation of organic matters in wetland systems76. CWs rely primarilyon microorganisms to perform treatment processes, whichinclude degradation of organic matter (OM), removal of N and precipitation of metals77. Microorganisms may alsobe involved in removal of parasites and pathogens. Moststudies of microbial communities focused on bacteria anda few have focused on fungi78 and protozoa79. Molecularmethods [16S rRNA library technique and PCR-

Table 3—Studies on pathogen removal by constructed wetlands (CWs)

Types of wastewater Pathogens Plant spp

Storm water60 Faecal bacteria Schoenoplectus mucronatus,Eleocharis acuta, Baumea

rubiginosa, Typha domingensis

Secondary Escherichia coli, faecal Phragmites australis, Typhamunicipal streptococci , total orientalis, Scirpus validus,Lemna

wastewater40,61-63 coliforms , Faecal minor coliforms, Salmonella sp .Clostridium perfringens

C. perfringens spores and Miscanthus sinensis giganteus,Pseudomonas aeruginosa Phragmites australis

Faecal coliforms, Eichhornia crassipes , duckweedS.typhimurium, Giardia

cryptosporidium, PRD-1Faecal coliforms Typha latifolia L.) or fescue

(Festuca arundinacea Schreb.)

Domestic E. coli and MS-2 Cyperus alternifolius, Cyperuswastewater38,41,56 Coliphage isocladus, Typha latifolia and Iris sp .

Faecal coliform Lemnagibba L.Canna ̄ accida,, Cyperus

alternifolius,Faecal coliforms Sagittaria lancifolia, Scirpus sp .,

T. latifolia, Collocasia esculenta,Gladiolus sp., Iris sp . and Thalia sp .

Mix of domestic Total coliforms, Faecal Typha sp. and Phragmites sp .and 1.5-3% coliforms and Faecalindustrial streptococci

wastewater43

Mix of domestic Faecal coliform P. australisand agriculturalwastewater44

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denaturing gradient gel electrophoresis (DGGE)] arebeing used to reveal bacterial diversity in CWs and tocharacterize communities present. In one study80, 166bacterial sequences were verified, 80% of which wereaffiliated with Proteobacteria and includedrepresentatives of all five classes within that group. Inanother wetland system used for treating swine waste,bacterial communities were dominated by well-known

soil bacteria81

(Pseudomonas, Arthrobacter and Bacillus). However, patterns of microbial diversity andfunction are currently difficult to determine. Zhang et al82 reported that microbial biomass, but not activity, iscorrelated with hydrophyte species richness in CWs. InVF CWs, one study83 revealed that bacterial biomass issignificantly higher in surface regions than in subsurface,whereas in another study84 bacterial populations showedlittle variation with depth.

Organic Matter (OM) Degradation

A major objective of wastewater treatment isreduction of organic content (carbonaceous BOD).Microbially-mediated removal of OM may be carriedout either aerobically or anaerobically. Aerobicdegradation of soluble organic chemicals is governed bytwo groups of microorganisms: aerobicchemoheterotrophs, which oxidise organics and releaseammonia; and chemoautotrophs, which oxidiseammoniacal nitrogen to nitrite and nitrate (nitrification).Both groups consume organics but faster metabolic rateof heterotrophs means that they are mainly responsiblefor reduction in system BOD. Insufficient supply of oxygen will greatly reduce performance of aerobicbiological oxidation in CW. However, if oxygen supply isnot limited, aerobic degradation will be governed by the

Types of Heavy metals Plant namewastewater

Secondary domestic wastewater Cd, Cu, Pb, Zn, Cr, Ni, Phragmites australistreatment but receives Al, Fe, and Mnstormwater as well67

Water from domestic wastes, Cd, Mn and Pb Potamogeton pectinatus L.food & metallurgical factories, & Potamogeton malaianus

printing factory, Zn smelter, Mn Miq.smelting plant68

Sewage was obtained from a BOD, COD, TN, TP and R. carnea, A. gramineus, A.drain of some factories in Jinhua, Mn, Fe, Cu, Cr, Pb, Cd orientale, A. calamus, I.

Zhejiang, China69 pseudacorus, L. salicaria

Wastewater from the industrial Cr, Ni & Zn Eichhornia crassipes (waterprocesses and sewage from hyacinth), Typha

factory were treated together70 domingensis (cattail) andPanicum elephantipes(elephant panicgrass)

Stream carrying secondary Mn, Cu, Cd, Co, Zn, Pb, T. latifolia L.

effluent71 Ni and Cr

Municipal wastewater72 Al, As, B, Ba, Cd, Co, Phragmites australisCr, Cu, Fe, Mn, Mo, Ni,Pb, Se, Sn, V, U and Zn

Municipal wastewater73 Sb, U, Cu, Pb, Cd, Ba, P. PhalarisMo, Cr, gallium Sn, Be,Fe, Ni, thallium, Se, Hg

& vanadium,Ag, Ru, B, Li, Sr, Co,

Mn, As, Pa

Municipal sewage74 As, Ba, Co, Cr, Cu, Fe, Phalaris arundinaceaGa, Hg, Mn, Ni, Pb, Sb

and U

Six different locations on sewage Pb, Cu, Cr, Zn, Fe, Mg,carrying canals including an and Mn

industrial effluent-fed fish pond75

Table 4—Heavy metal removal by constructed wetlands (CWs)

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amount of available OM17. Anaerobic degradation occurswithin CWs in absence of oxygen and is carried out byanaerobic heterotrophic bacteria. It is slower than aerobicdegradation. When oxygen is limiting at high organicloadings, anaerobic degradation will predominate17.

Nitrogen RemovalMajor removal mechanisms of N in CWs are

nitrification and denitrification. Nitrification is achemoautotrophic process in which ammonia is oxidisedto nitrate by nitrifying bacteria ( Nitrosomonas and

Nitrobacter ) in aerobic zones. Nitrifying bacteria aresensitive organisms and are inhibited by a range of environmental conditions17 including high concentrationsof NH4N, pH (7.5-8.6), temperatures below 4 or 5°Cand oxygen concentrations below 1 mg/l. Denitrificationis conversion of nitrate to N2 gas and can be achievedbiologically under anoxic conditions. Several genera of

heterotrophic bacteria ( Achromobacter, Aerobacter, Alcaligenes, Bacuillus, Brevibacterium,Flavobacterium, Lactobacillus, Micrococcus,Proteus, Pseudomonas and Spirillum) are capable of denitrification. Denitrification may be inhibited by severalfactors including presence of dissolved oxygen,insufficient carbon (OM) and temperatures below 5°C17.Recently, alternative microbial pathways for N depletionin CWs have been reported including heterotrophicnitrification, anaerobic ammonium oxidation(ANAMMOX), anaerobic methane oxidation andoxygen-limited autotrophic nitrification anddenitrification85-87.

Metals Removal

Microbially-mediated metals removal in CWs canbe carried out aerobically by metal oxidising bacteria(MOB), or anaerobically by sulphate reducing bacteria(SRB). Former include oxidation of metals (iron, nickel,copper, lead and zinc) by MOB (Thiobacillus

ferrooxidans in case of iron) followed by precipitationof metal oxyhydroxides. In anaerobic zones, activity of SRB results in reduction of sulphate ions to producehydrogen sulphide, which is ionised in water to givesulphide ions that react with metal ions to produce metalsulphide which precipitates. In addition to reducing sulphurcompounds, activity of SRBs also generates alkalinity inCW s17, 88 . SRB community in a CW has beencharacterized using a combination of enrichment cultureand molecular methods. This revealed presence of 12genotypes, four of which could be identified withpreviously known taxa suggesting that a novel SRB

diversity exists within CW89. Potential importance of SRBs in CWs has further been highlighted90.

Removal of Pathogens and Parasites

Indicator Organisms

Because of wide range of organisms involved and

difficulty in detecting, it has long been the practice touse indicator organisms to monitor presence of faecalcontamination in general, and of pathogens in particular.Most commonly used indicators in wastewater treatmentsystems are coliform bacteria (especially faecal coliformssuch as E. coli) and faecal streptococci. Otherorganisms that have been used are bacteriophage andcoliphage viruses91. There are, however, no reliableindicators of protozoa or helminth parasites so these aremonitored directly.

Removal Mechanisms

A variety of processes66,92,93 (filtration throughsubstrate and attached biofilm, sedimentation,aggregation, oxidation, exposure to biocides, antiobiosis,predation, attack by lytic bacteria and viruses, naturaldie-off and competition for limiting nutrients or traceelements) are involved in removal of pathogens andparasites from wastewaters in CWs. In free surfacewater systems, temperature, solar radiation and mediafiltration are considered to be the most significant factorsaffecting removal of faecal coliforms94. By contrast, insubsurface flow wetlands, aeration and hydraulic flow 55,95,microbial competition and predation96, 97, natural die-off 95,98 and inactivation99 are reported as being of mostsignificance. Filtration, entrapment and sedimentationhave been cited as likely important processes in removalof relatively large structures [protozoan (oo)cysts andhelminth ova], whereas natural die-off is not thought tobe an important process because of prolonged survivalof many types of cysts and ova in environment100 .

Constructed Wetlands as Mosquito Breeding Sites

A major requirement for mosquitoes is water standingfor some duration, which can be as little as one week,

containing food and protection cover. Mosquitoes arecommon inhabitants of natural wetlands so their invasionof CWs with free water surface should be expected101 .However, such problems do not apply to subsurface flowCWs, which, when managed properly, do not providebreeding sites for mosquitoes. On free water surfacewetland systems, Karpiscak et al102 showed thatmosquito populations in an area of Arizona, USA,increased, sometimes by several orders of magnitude

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for several years, after CWs had become operational.By contrast, Anderson et al103 found that mosquitonumbers were not significantly higher after constructionof a wetland in an area of eastern North Carolina, USA.A number of design and operational parameters, whichhave been demonstrated to reduce mosquito larval

numbers in CWs104-107, include introducing a variety of macrophytes, thinning vegetation to reduce shading,designing deeper ponds with steep sides, conservingnatural populations of macroinvertebrate predators and/ or introducing larvivorous predators, maintaining highlevels of dissolved oxygen, and periodically drainingwetland. Most studies on mosquito populations specificallyrelated to CWs have been carried out in developedcountries, and very few studies have been carried out inless-developed countries with tropical climates such asIndia.

Costs

Basic investment costs for CWs include land, siteinvestigation, system design, earthwork, liners, filtrationmedia (HF and VF CWs) or rooting media (FWS CWs),planting, hydraulic control structures and miscellaneouscosts (fencing and access roads). Vymazal &Kröpfelová15 summarized available data from HF CWsin the USA, Czech Republic, Portugal and Spain andfindings are as follows: excavation cost, 7 - 27.4; gravel,27 - 53; liner, 13 - 33; plants, 2 - 12; plumbing, 6 - 12;control structures, 3.1 - 5.7; miscellaneous, 1.8 - 12%.Individual costs can vary widely in different parts of theworld. Also, larger systems demonstrate greatereconomies of scale 13. The total investment costs varyeven more, and the cost could be as low as 29 USD perm2 in India108 or 33 USD per m2 in Costa Rica109 , or ashigh as 257 EUR per m2 in Belgium110 . Capital costs forFWS CWs are usually less than for subsurface flowsystems mainly because cost for media is limited torooting soil on the bottom of beds. CWs have very lowoperation and maintenance costs. In addition, wetlandshaving a higher rate of biological activity than mostecosystems can transform many of the common

pollutants that occur in conventional wastewaters intoharmless byproducts or essential nutrients that can beused for additional biological productivity. Economicbenefits from CWs are an important considerationespecially in developing countries, where additionalincentives are required to encourage communities tomaintain treatment wetlands. Gains in vegetation biomassin CWs can provide economic returns to communitieswhen harvested for biogas production, animal feed, fiber

for paper making, compost111 and floriculture, therebyproviding employment and further economic justification.

Conclusions

Constructed wetlands (CWs) are being extensivelyused in developed countries to treat domestic,

agricultural, and industrial wastewaters and urban andhighway run off. This review covers current status of application of CWs in wastewater treatment especiallyof domestic wastewater, describing alternative CWdesigns and cost, treatment mechanisms operative, andefficiency of removal of various pollutants.

Acknowledgements

Authors thank financial support through EPSRC, UK(grant reference EP/E044360/1) and RC-UK DST (EP/ G021937/1) India funded project.

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