Ecological Engineering 2011

Ecological Engineering 37 (2011) 99112

Contents lists available at ScienceDirect

Ecological Engineering

journa l homepage: www.e lsev ier .com

Review

Cloggin ds:contrib

Paul Kno cae

a Sustainable Eb School of Appc arhus Unive , Denmd Helmholtz Ce ), Perme Environmenta , TechMdul D-1, 080

a r t i c l

Article history:Received 8 October 2009Received in revised form 13 August 2010Accepted 23 August 2010Available online 8 October 2010

Keywords:Clog matterConstructed wDesignHorizontal owVertical owLoading ratesReview

intenfor wastewater treatment, and can ultimately limit the lifetime of the system. This review considers overtwo decades of accumulated knowledge regarding clogging in both vertical and horizontal subsurfaceow treatment wetlands. The various physical, chemical and biological factors responsible for cloggingare identied and discussed. The occurrence of clogging is placed into the context of various design andoperational parameters such as wastewater characteristics, upstream treatment processes, intermittent

1. Introdu

Subsurfaremoving pical simplirequiremenment techn(TWs) consmacrophyteimproveme(HSSF) or vmanently scycles of satem interm

In SSF trand physicathe porous

CorresponE-mail add

0925-8574/$ doi:10.1016/j.etland

or continuous operation, inuent distribution, and media type. This information is then used to describehow clogging developswithin, and subsequently impacts, common variants of subsurface ow treatmentwetland typically used in the U.S., U.K., France and Germany. Comparison of these systems emphasizedthat both hydraulic loading rate and solids loading rate need to be considered when designing systems tooperate robustly, i.e. hydraulic overloading makes horizontal-ow tertiary treatment systems in the U.K.more susceptible to clogging problems than vertical-ow primary treatment systems in France. Futureresearch should focus on elucidating the underlying mechanisms of clogging as they relate to the design,operation, and maintenance of subsurface ow treatment wetlands.

2010 Elsevier B.V. All rights reserved.

ction

ce ow treatment wetlands are used worldwide forollutants from wastewaters due to their mechan-

city and low operations and maintenance (O&M)ts in comparison to conventional wastewater treat-ologies. Subsurface Flow (SSF) Treatment Wetlandsist of a bed of porous media planted with emergents, through which wastewater is passed for qualitynt. The ow through SSF TWs can be either horizontalertical (VF). Typically, the media in HSSF TWs is per-aturated, whereas the media in VF TWs goes throughturation because wastewater is dosed through the sys-ittently (Vymazal and Krpfelov, 2008).eatment wetlands, the cumulative biological, chemical,l treatment processes may cause gradual clogging ofmedia. Clogging may be accompanied by a decrease in

ding author. Tel.: +44 758 864 5822.ress: [email protected] (G. Dotro).

treatment performance or hydraulic malfunctions such as pondingofwastewater on the surface of the system and bypass of untreatedwastewater. These issues will require intervention if satisfactoryoperation is to be maintained. Advanced clogging may eventuallynecessitate remediation of the clogged media; thus limiting theasset lifetime of the system (Cooper et al., 2005; Caselles-Osorioand Garca, 2007; Kadlec and Wallace, 2009; Nivala and Rousseau,2009).

The realization that clogging is a widespread operational prob-lem in SSF TWs has become apparent through frequent reportingover the past two decades (as summarized in this article), in whichtime thousandsof systemshavebeen constructedworldwide.Orig-inal predictions of HSSF TW system longevity were on the order of50100 years (Conley et al., 1991; Bavor and Schulz, 1993)whereasthis estimate has been progressively shortened to 15 years (Cooperet al., 1996), 10 years (Wallace andKnight, 2006) andmost recently,to eight years in the U.K. (Grifn et al., 2008). Admittedly, the long-term viability and maintenance requirements of these systems arestill unknown (Kadlec and Wallace, 2009). As such, there is moti-vation to understand and control clogging. The desire to mitigateclogging has directly inuenced the design evolution of SSF TWs

see front matter 2010 Elsevier B.V. All rights reserved.ecoleng.2010.08.005g in subsurface-ow treatment wetlanuting factors

wlesa, Gabriela Dotrob,, Jaime Nivalac,d, Joan Garnvironment Research Group, Aston University, Birmingham B4 7ET, United Kingdomlied Sciences, Craneld University, Craneld, Bedfordshire MK43 0AL, United Kingdomrsity, Department of Biological Sciences, Ole Worms All, Building 1135, 8000 arhus Cnter for Environmental Research (UFZ), Environmental and Biotechnology Center (UBZl Engineering Division, Hydraulics, Costal and Environmental Engineering Department34 Barcelona, Spain

e i n f o a b s t r a c t

Clogging is amajoroperational andma/ locate /eco leng

Occurrence and

arkoserstrae 15, 04318 Leipzig, Germany

nical University of Catalonia c/Jordi Girona 1-3,

ance issueassociatedwith theuseof subsurfaceowwetlands

100 P. Knowles et al. / Ecological Engineering 37 (2011) 99112

Table 1Non-hydrous components of clog matter categorized into whether the accumulation is an intentional part of the wastewater treatment process, or an incidental result ofthe wastewater treatment process. Incidental accumulations include accidental operations, which are italicized.

Component

Organic solid

Inorganic so

(Brix and Sthe factorsproblem prprevention

This revion the factodesign andhow cloggiof the guidBased on thging manifopinions onothers. Thisanism of clcharacterizRather, it isence for we

2. Factors

Cloggingwith treatmational factcompositiowill vary deter typicallythan70%, v/The typical

The comits constituegranularmesubsurfacehydraulic ction of surfThe net accuexternal andecompositnot the cloglevel.

This secin Table 1ltration osolids contcontributiobiodegradadiscussed in

2.1. Solids e

Suspendtreatment wment (Yaoresponsiblecollisions b

partiWasns. Asiblebala

al efexpnismlacebservat arall toge to971

y, a l(Levi8) obundartict remPartiablee porubbeticleal eftions. Theiclesmenioniche ate relydrolly ae ass gentionas oith

olm

nspodescanssoIntentional accumulation (external loads)

s Wastewater solids

lids Wastewater solids Chemical precipitates

chierup, 1989; Murphy and Cooper, 2010); however,that lead to clogging are still not well understood. Thisecludes wetland design documents from addressingor long-term management strategies.ew summarizes the current knowledge and experiencers that cause clogging (Section 2), and the inuence ofoperational parameters (Section 3). Section 4 exploresng inuences SSF TW hydraulics and conveys someelines that are recommended for clogging mitigation.e reviewed content, Section 5 differentiates how clog-ests in different SSF TW design variants and offerswhy some systems are more prone to clogging thanpaper does not claim to explain the dynamic mech-

ogging, as the necessary detailed scientic studies toe this are currently lacking in the wetland literature.intended that this document will form a useful refer-tland researchers, designers and practitioners.

attributing to clogging

occurs due to accumulation of materials associatedent (e.g., intentional or external loads) and other oper-ors (e.g., incidental or internal loads). The quantity andn of this material, hereafter referred to as clog matter,pending on these internal and external loads. Clogmat-consists of highly hydrated gels and sludge (oftenmorev,water)with inorganic andorganic solids (IWA, 2000).components of clog matter are categorized in Table 1.position of clog matter is often a lower density thannts such that it can effectively reduce pore space in thedium(Baveyeet al., 1998). It canaccumulateboth in theand on the surface of the SSF TW, reducing subsurfaceonductivity in the former case, and preventing inltra-ace ow through to the subsurface in the latter case.mulation rate of this material is a balance between thed internal loads (Table 1), and loss due to export andion (Tanner et al., 1998). This determines whether ormatter will create hydraulic issues on the macroscopic

tion elucidates the manner in which the factors listedcontribute to clogging. The discussion will considerf particulate matter by the porous media, additionalributions from microorganisms (biolms), vegetationns and chemical precipitation. The composition andbility of typical clog matter accumulations will also bethis section.

these1964).positiorespon

Theremov(2009)mechatakes pbeen ocles thtoo smtoo laret al., 1quentlrangeal. (200most a

As psequenspace.are capinto thtion (H

Parchemicinteracforces)of partDetachin thecome tthey artion). Hespeciaincreaseffect igravitamon, afaced w

2.2. Bi

Tracan be(Hermntrapment

ed solids are ltered and retained by subsurface owetlands via the mechanisms of transport and attach-

et al., 1971). These mechanisms are the same as thosefor occulation,whereby transportmechanisms createetween particles, and attachment mechanisms cause

forms as bimatter susp1999). Furtnized medidevelopmeorganic maGarca et al(2006) founIncidental accumulation (internal loads)

Biomass growth Plant roots Biolm and plant detritus Solids introduced during construction Solids from chemical erosion of gravel Solids introduced during construction

cles to adhere upon impact (Swift and Friedlander,tewater contains solids with a variety of sizes and com-range of different physico-chemical mechanisms arefor overall TSS reduction (Table 2).nce of the forces listed in Table 2 leads to a range ofciencies for different particle sizes. Kadlec and Wallacelain that sedimentation of large particles is the majorof solids removal in HSSF TWs, which predominantlyin the rst few meters of the wetland. It has frequentlyed that the nadir in removal efciency occurs for parti-

e approximately 12m in diameter, because they arebe removed by inertial and other physical effects, butbe affected by electrostatic and Brownian forces (Yao

; Logan et al., 1995; Zamani and Maini, 2009). Conse-arge number of wastewater particles fall into this sizene et al., 1991; Tchobanoglous, 1993) and Puigagut etbserved that particles in the 0.72m range were theant in the inuent and efuent of a HSSF TW in Spain.les accumulate within the media, the efcacy of sub-oval is often enhanced due to the reduction in pore

cles which are electrostatically attracted to each otherof stacking and often form dendrites which protrudee space and increase the likelihood of particle intercep-et al., 2009).retention on media surfaces is due to the electro-fect of adsorption (summationof electrical double-layerand the dipole interactions known as van der Waalsstrength of attachment depends on the relative charge, media surfaces and bulk uid (Hermansson, 1999).t of particles can occur for several reasons. A changestrength of the wastewater may neutralize or over-

tractive force between particles and surfaces such thateased back into solution (an effect known as peptiza-dynamic shear forces can lead to sloughing of particles,s pores become constricted and interstitial velocitiesa consequence (Zamani andMaini, 2009). However, thiserally conned to closed, pressurized reactors and inl ow systems such as SSF TWs this effect is uncom-

w will nd an alterative path (such as overland) oncehydraulic resistance (Maloszewski et al., 2006).

clogging

rt and attachment of microbes through porous mediaribed using the same set of principles outlined for solidsn, 1999; Tufenkji, 2007). In SSF TWs, most biomass

olms on the surfaces of the media, with very littleended in the wastewater (Khatiwada and Polprasert,

her biomass can proliferate once microbes have colo-a surfaces. Several studies conclude that greater biolmnt occurs at the inlet region where the concentration oftter in the wastewater is greatest (Ragusa et al., 2004;., 2007; Tietz et al., 2007). Caselles-Osorio and Garcad that biolm clogging in an experimental HSSF TW

P. Knowles et al. / Ecological Engineering 37 (2011) 99112 101

Table 2The various physico-chemical mechanisms that are responsible for solids removal in SSF TWs.

Large particle removal mechanisms

Sedimentati sity to

Hydrodynam f a paInertial dive eamlinInterception e radiStraining an ed an

ularly

Small partic

Brownian m s resp

Electrostatic pensi

Bridging pores

Coagulation s prom

reduced inlductivity.

The inutheir heterooften formditions (Van1999; Dupican trap pa(Mays andlation compconductivitmeric slime(Rittman anStoodley (1networks wing them reand Baveyeother inorga2010). If biplugging wporous medbiolm (Wa

2.3. Vegeta

The roleOne of thethat root grKnemann,vide amacrdata in thethat root exthe depth oyears, whicexperiencethat root groverland o1994b). Acchave 5005rhizomes, wand occludezone. Indeeto have hydal., 2004), wthan the hySSF TWs (U

en wetritplanet alateriutesthatn anle, Kcumuf opealf orelaationile sealany pl000

be dibloc

ver, dorgaplan000f hu100

geneThe

ons fc maon and buoyancy According to Stokes Law, particles with different deneffect of gravity, until they impact a surface

ic effects Non-uniform hydrodynamics forces across the body orgence Particles with signicant inertia may deviate from str

If the streamline conveying a particle is closer than thd trapping Particles that are larger than pore spaces will be strain

irregularities. Filamentous/brous particles are partic

le removal mechanisms

otion Colloidal particles are inuenced by the thermal forcethrough the ow eld

forces Repulsive or attractive forces between particles in sustrajectorySmall particles can be removed within relatively largebridgingThe coagulation of smaller colloids into larger particle

et hydraulic conductivity to 64%of outlet hydraulic con-

ence of biolms on hydraulics is highly variable due togeneous nature (Thullner, 2010). For instance, biolmslamentous colonies or aggregates in saturated con-devivere and Baveye, 1992a,b,c; Dupin and McCarty,

n et al., 2001), developing webs across pore spaces thatrticles more prociently than uniform biolm coatingsHunt, 2005). Biomass density will vary with accumu-osition and community, which impacts its hydraulicy. The majority of biolms secrete extracellular poly-(Madigan et al., 2006) that are typically 99% water

dMcCarty, 1980; Tanner and Sukias, 1995). deBeer and995) expound that these slimes are analogous to gelith pore diameters on the nanometric scale, thus mak-latively impermeable (Taylor et al., 1990; Vandevivere, 1992c) and procient at forming associations withnic andorganicmaterials (Baveyeet al., 1998; Thullner,olms on separate gravel particles bridge, then poreill occur and the hydraulic conductivity of the bulkia will tend towards the hydraulic conductivity of thellace and Knight, 2006).

tion contributions

of plants in SSF TW clogging is an evolving debate.original assumptions of the Root Zone Method wasowth would counteract media clogging (Kickuth and1988), and the tubular structure of the rootswouldpro-o-porous network for ow.However, there are very few

Whplant dtion ofAsaedaRoot mcontribstatingpositioexampface acyears oup to hremaindegrad

WhNew Zging b13003couldof poreMoreoparedand un12002tions othe topwhileoutlet.tributiorganiliterature to support this claim. Fisher (1990) observedpansion in an Australian eld-scale HSSF TW increasedf the reactor at the inlet zone by 60mm over threeh would serve to counteract clogging. However, earlywith soil-based HSSF TWs in Northern Europe provedowth did not preclude hydraulic malfunctions such asw (Netter and Bischofsberger, 1990; Pauly, 1990; Brix,ording to IWA (2000), a dense macrophyte stand will000g dry weight (DW) per m2 of subsurface roots andhich provides additional surface area for accumulations one quarter to one third of pore volume in the root-d, the rootmat of Phragmites australishas been reportedraulic conductivity on the order of 120m/d (Baird ethich is approximately two orders ofmagnitude smallerdraulic conductivity of clean gravels typically used inS EPA, 2000).

system.Several o

ity of root-zrooting med(1990) survof root growthat this vetal effect oresearchersusing waterow along tal., 1984; Band Chick,been postutated upperresistive bowastewater will move vertically across the ow-eld under the

rticle will cause it to drift across the ow-eldes as ow diverges around obstacles, and impact a surface

us of the particle interception of the media surface will occurd particles may also be trapped by media morphologicalsusceptible to these modes of removal

onsible for Brownian motion which induce random trajectory

on, and particles and media surfaces will inuence particle

if numerous particles arrive simultaneously and block the pore by

otes their removal through the previously outlined mechanisms

inter plant die-back occurs, if the accumulation rate ofus exceeds decomposition rates then a net accumula-t detritus will occur over time (Kirschner et al., 2001;., 2002; Rybczyk et al., 2002; ChimneyandPietro, 2006).al contributes to subsurface clogging and leaf litter-fallto surface clogging, with Kadlec and Wallace (2009)typically 515%of plant detritus is recalcitrant. Decom-d accumulation rates will vary geographically but, foradlec and Watson (1993) measured an average sur-lation of 2410gdrymatter perm2 leaf litter after threeration in a HSSF TW in Kentucky (U.S.). Aboveground,f the standing plant biomass (stems) may be dead, buttively stable due to esters in the plant tissue that resist(Tanner et al., 1998).

tudying dairy wastewater treatment HSSF TWs ind, Tanner (1994) illustrated the potential for clog-

ant detritus. Organic matter accumulation rates ofg/m2yr were recorded but only 4001600g/m2 yrrectly attributed to wastewater loading, and only 4%kage was due to live root penetration (Tanner, 1994).uring the same study Tanner and Sukias (1995) com-

nic matter accumulation rates in equivalent plantedted beds and found that accumulation rates wereg/m2 yr higher in the planted systems. Accumula-mic substances were two to eight times higher inmm of substrate than they were in the lower strata,

rally only 50% greater in the inlet region than at therefore, the authors concluded that the humic con-rom plants, rather than accumulation of wastewatertter, were the controlling factor in the clogging of thisther references have also indicated that the vastmajor-one establishment is concentrated in the top layer ofia (US EPA, 1993; Reed et al., 1995). For example, Parreyed 12 HSSF TWs in the U.K. and found that 70100%th occurred in the top 20 cm of themedia. It is believedrtical gradient in plant establishment has a detrimen-n the hydraulic efciency of HSSF TWs. Numeroushave performed internal hydrodynamic visualizationstracers and report similar observations of preferentialhe bottom of the bed (Spangler et al., 1976; Gersberg etowmer, 1987; Fisher, 1990; Waters et al., 1993; Breen1995; Garca et al., 2003; Knowles et al., 2010). It haslated that the higher hydraulic resistance in the vege-gravel layer causes ow to short-circuit along the lessttom layer.


Some authors have found that water retention in the litter layerand contribution of plant evapotranspiration counteracts any dis-cernible loss of hydraulic retention time due to plant-mediatedclogging. FoTW mesocoplanted syssystems. Ongating the sdue to the wdisturb theto the subslarly beneal., 2006).

2.4. Chemic

Chemicacipitation,adsorptionand syntherous (Wallafor HSSF TWAggregatesphorous redo not becoadsorption

Howevephides (Sheal., 1999) aform lm-ltions withibiological inindustrial wbiological a(Kadlec andNivala et alin an aerateet al. (2007the highly ofaces and aporosity wa

2.5. Clog m

The biodimpact onter would dwould contganic solidspredicted ta HSSF TWclogging solestimationsand SchulzHowever, taccumulati

Regardin(1998) stateTW were v6396% oftory (Nguycomposednocellulosicplant detrittion rates w

residues and morphologically unstructured humic compoundswith a range of molecular weights and packing densities. Fur-thermore, humic compounds are highly colloidal and amorphous

reatensemattr angreaardinandil-baatsond inpositly de2009umuary tprethen eaccalcnorgithw-dy (Kampodlec1kg/d 77te thnd a2009ur ti240cclusaulic) (Pe

ign a

disthichfactoteristemened wce o

astew

wledcontg cloydracreasceedentgrab6) es

y as. Systpoo

s lowr example, when studying planted and unplanted HSSFsms in Canada, Chazarenc et al. (2007) found thattems had consistently longer HRTs than unplantede possible benet of plants to SSF TWhydrology ismiti-urfaceowassociatedwith surface clogging. Thisoccursind-induced sway of emergent shoots and stems thatsurface sludge layer and create macro-pores throughurface. It is perceived that this mechanism is particu-cial to the operation of VF TWs (Brix, 1994a; Molle et

al effects

l treatment processes, such as adsorption and pre-can also contribute to clogging. Physico-chemicalis associated with the removal of metals, petroleum,tic hydrocarbons, ammonical-nitrogen, and phospho-ceandKnight, 2006). For instance, it is commonpractices in Norway to incorporate Lightweight Expanded Claywith high sorption capacity, for the purposes of phos-moval (Zhu et al., 1997). In general, adsorbate lmsme thick enough to create clogging problems as theis limited by the sorption capacity of the media.r, chemical precipitation of metal hydroxides and sul-oran andSheoran, 2006), calciumcarbonate (Fleming etnd elemental sulphur (Kadlec and Wallace, 2009) mayike coatings on media surfaces. Chemical transforma-n a wetland environment can be encouraged throughteractions (Dotro et al., 2010), and systems that treatastewaters may clog in this manner if both the rate ofctivity and concentration of these compounds is highWallace, 2009). Such a phenomenon was described by

. (2007), who observed severe iron fouling of the mediad HSSF TW for the treatment of landll leachate. Nivala) attributed this to ferric hydroxide precipitation underxidizing conditions, which then sorbed to media sur-ssociated with interstitial biological matter such thats heavily reduced and media became welded together.

atter composition

egradability of accumulated clog matter dictates itsclogging. Initially it was assumed that organic mat-ecompose sufciently such that only inorganic solidsribute to SSF TW clogging. Based on an inuent inor-loading of 8 kg/m3 over 18 months, US EPA (1993)

hat only 1% of interstitial volume would be lost from. Similar calculations based on the assumption ofely by inorganicmatterwere responsible for early over-regarding system longevity (Conley et al., 1991; Bavor, 1993; Blazejewski and Murat-Blazejewska, 1997).he literature conrms that clogging can be caused byon of both organic and inorganic matter.g predominantly organic clog matter: Tanner et al.that 80% of accumulations in a dairy wastewater HSSF

olatile although subsequent investigation found thatthese organic matter fractions were relatively refrac-en, 2000, 2001). Accumulations were predominantlyof humic, humin and fulvic acids, derived from lig-and humic compounds in the dairy wastewater and

us. Nguyen (2000) discusses that variable decomposi-ould lead to a mixture of macro-morphological plant

with g(Christof clog(Tannematter

RegPlatzerin a soandWmulatein comfactoriet al. (ter accsecondmation(1993)solids iearlieron an iation wform locapacitThe coand Kaand3727% anpostulacium aet al. (age foversus(66% oof hydr45m/d

3. Des

Thewith wtionalcharacarrangoperatinuen

3.1. W

KnosolidsstandinTWs, hThis inwill exto prevLangeral. (200reliablloadedto haverates ahydrophilic potential and physical binding propertiesn et al., 1998; Nguyen, 2000). Indeed, the accumulationer in this site was approximately 80% water by volumed Sukias, 1995). These properties give humic organict potential for pore blockage.g predominantly inorganic clogmatter accumulations:Mauch (1997) reported that less than 3% of the depositssed VF TW in Merzdorf, Germany were organic. Kadlec(1993) described that over 80% of the clogmatter accu-a HSSF TW for lagoon efuent polishing was inorganicion, explaining that biological matter was being satis-graded. Similarly, Caselles-Osorio et al. (2007), Llorens) and Pedescoll et al. (2009) also reported clog mat-lations with inorganic content greater than 75% in sixreatment HSSF TWs in Catalonia, Spain. From the infor-sented by Llorens et al. (2009) and Kadlec and Watsonporosity loss attributable to accumulation of inorganich of these studies is very small, and in agreement with

ulations mentioned above (13% pore occlusion basedanic solids density of 2650kg/m3). However, complex-the small quantities of biological material present canensity gelatinous sludgewith very highwater retentiondlec andWatson, 1993; Tanner et al., 1998; IWA, 2000).site clog matter in the studies of Llorens et al. (2009)and Watson (1993) had resulting densities between 33m3 which increasedeffectiveporeocclusion tobetween%. In the case of Kadlec and Watson (1993) the authorsat this occurred due to silica algae combining with cal-luminium at low redox values. In the case of Llorens), the sludge density in the inlet region was on aver-mes lower than that at the outlet region (60kg/m3

kg/m3) but was doubly effective at reducing porosityion versus 28%). This corresponded with lower valuesconductivity at the front end of the bed (20m/d versusdescoll et al., 2009).

nd operational inuences

ribution of clogging through the system and the speedit develops depends on numerous design and opera-rs, such as the wastewater loading rate and pollutantics, the physical properties of the porous media, inlett and system dimensions, and whether the system isith periods of resting. This section elaborates on thef these parameters.

ater characteristics

ge of wastewater characteristics, such as ow rates,ent and pollutant characteristics, are vital for under-gging processes in SSF TWs. In both HSSF TWs and VFulic overloading may lead to periods of overland ow.es the likelihood that surface layer accumulation ratesmineralization rates and intervention may be requiredcomplete hydraulic failure (Platzer and Mauch, 1997;er et al., 2003; Zhao et al., 2004). For example, Molle ettablished that French VF treatmentwetlandswill worklong as they are not consistently hydraulically over-ems that had been consistently overloadedwere foundrlymineralized surface sludge depositswith inltrationas 2.5m/d, thus providing a bottleneck to ow.


Fig. 1. Correlathree VF TWs i(Caselles-Osor

RegardinbiodegradaLow TSS doby Caselles-tests on exporganic mathe systemconductivitthem to condue to its re

In genersystem clogSukias, 199and COD coof 21 VF TWent in systeTSS.

Buildingcompares tfor six HSSFLlorens et aFrance (Chaondary HSSCOD loadingmulation raChazarenc(67g/m2 d osolids accumFrench systevidence ofHSSF TWs.

3.2. Interm

Intermitner such thaconditions aple is fundaloading to rate withoutfor a particucold and wthose in hotto weeks aand Mauchwith differebe uninterr

Variants of VF TWs such as ll-and-drain wetlands (Zoellerand Byers, 1999; Lahav et al., 2001), tidal ow wetlands (Zhao et

4; Austin, 2005) and reciprocating wetlands (Behrends et1) infaceittensopefacetemg offractcumdepg in toperdiesr rev8). Bw ford in

edia

hydia si-CarightwereHabewer

medirfacetry;due

pronbridwskalla

sequF TWers bstemed efzenyhyd

ubstto a

nd to(Co, 20tion between areal COD loading rate and solids accumulation rate forn France (Chazarenc and Merlin, 2005) and six HSSF TWs in Cataloniaio et al., 2007; Llorens et al., 2009; Pedescoll et al., 2009).

g pollutant characteristics; both the physical form andbility of wastewater constituents will affect clogging.es not necessarily preclude clogging, as demonstratedOsorio and Garca (2006) who performed side-by-sideerimental HSSF TWs system fedwith different forms oftter (glucose versus starch). Their results suggest thatfed with dissolved glucose had lower inlet hydraulicies than a system fed with particulate starch, leadingclude that glucose stimulated greater biolm cloggingadily biodegradable nature.al, most authors report a positive correlation betweenging and both TSS and COD loading rates (Tanner and5). Winter and Goetz (2003) found loading rates of TSSrrelated positively with severity of clogging in a survey

s in Germany. Clogging problems were not appar-ms that received less than 20g/m2 d COD and 5g/m2 d

on the work of Caselles-Osorio et al. (2007), Fig. 1he COD loading rate versus solids accumulation ratesTWs in Catalonia, Spain (Caselles-Osorio et al., 2007;l., 2009; Pedescoll et al., 2009), and three VF TWs inzarenc and Merlin, 2005). Over two years, the sec-F TW systems in Catalonia received average inuentrates of 12g/m2 dwith a resulting average solids accu-te of 11kg/m2 yr. Accordingly, the primary VF TW ofand Merlin (2005) received a larger COD loading raten average) corresponding to a proportionally greaterulation rate (51kg/m2 yr on average). Despite this the

ems purportedly operated without clogging whereassurface ponding existed in several of the Catalonian

al., 200al., 200subsurintermsystemthe surthe syscloggingravelthat acreactorstill cloand Co

Stucial foal., 200to allorestore

3.3. M

Theto medKozenyand Knsands,1990;as theylargercic suchemisceptionmediaand theBlazeje2003; W

Sub25 HSSdiametthe syachievthe Koa clean6mmshas ledTWs agravelsNORMittent operation

tent operation involves feeding wastewater in a man-t subsurface aerationwill occur,which restores aerobicnd accelerates clog matter mineralization. This princi-mental to the operation of VF TWs. The periodicity ofesting will determine the ability of the system to oper-clogging (Cooper, 2005). The required recovery periodlar cell will depend on climatic conditions. Systems in

et climates will require a longer recovery period thanand arid climates. Resting periods on the order of days

re suggested for VF TWs in Northern Europe (Platzer, 1997; Langergraber et al., 2003; Green et al., 2006),nt cells rested in rotation so that plant operation willupted.

which summover the ye2009).

Particle shydraulic cavailable tomediahydris relativelyit reduces pable for bio2006). The smedia specannouncedbuilt after 2a narrow dicorporate operating strategies that promote increasedoxygen availability, such as water level uctuation,t dosing, and alternate operation. Additionally, theseratewithout the typical applicationofwastewater overof the bed, which in principle reduces the tendency ofto clog on the surface. Zhao et al. (2004) showed thata tidal ow wetland could be mitigated by arrangingions with increasing diameter from top to bottom, soulated solids were better distributed throughout theth. However, studies indicate that these systems mayhe upper surface despite their operationmode (Cooper, 2005; Sun et al., 2007).also suggest that intermittent operation may be bene-ersing clogging in HSSF TWs (Nguyen, 2000; Corzo etatchelor and Loots (1997) rested a eld-scale HSSF TWsurface sludge layer mineralization, and successfullyltration to the subsurface.

characteristics

raulic conductivity of a porous media is very sensitiveze, as emphasized by the squared relationships in themen equation for ow through porous media (Kadlec, 1996). Despite this, smaller media, such as soil andoriginally used (Brix and Schierup, 1989; Coombes,

rl and Perer, 1990; Netter and Bischofsberger, 1990)e believed to offer superior treatment performance toa. Indeed, the smaller the particle size, the higher spe-area available for biolm establishment, and surface

and the greater the likelihood of suspended solids inter-to narrower pore diameters. However, this makes nee to rapid clogging by pore occlusion from ltrationging of surface accumulations (Blazejewski andMurat-a, 1997; Platzer and Mauch, 1997; Langergraber et al.,ce and Knight, 2006).ent work by Grifn et al. (2008) on the performance ofs in the U.K. showed that larger media, with medianetween 6 and 11mm, did not impair the ability ofto achieve treatment requirements. All media sizes

uent BOD levels below 5mg/L, although according toCarmen equation, the 11mm substrate would haveraulic conductivity over three times greater than therate. Resultantly, the desire tomaximize asset longevitygradual increase in the media sizes employed in HSSFwards the implementation of coarser media, such asoper et al., 1996; US EPA, 2000; Iwema et al., 2005;05; Garca and Corzo, 2008) as reected in Table 3arizes media specications that have been published

ars (Wallace and Knight, 2006; Kadlec and Wallace,

ize distribution andparticle shape also inuencemediaonductivity. Adaptations of the Ergun equation aredescribe the inuence of grain size distribution on bulkaulic conductivity (KadlecandKnight, 1996).Media thatnon-spherical or angular will exacerbate clogging asorosity and increases the specic surface area avail-

lm growth (Kadlec and Knight, 1996; Hynkov et al.,ignicantbenet toasset lifetimeofferedbycontrollingication was emphasized by Grifn et al. (2008), whothat Severn TrentWater (a U.K. water utility) HSSF TWs008would only incorporatewashed round gravelswithstribution of particle sizes between of 1012mm.


Table 3Examplesofmedia sizedistributions recommendedbyvariousnational and interna-tional design guidelines and publications. Adapted from Wallace and Knight (2006)and Kadlec an

Country

Austria

Czech RepubGermanyUnited King

United State

European DeGuidelines

InternationaAssociatio

3.4. Upstrea

Accidenment proceTWs (Coopeandmainteof this occuor clarierstominimize

Alternatfor use in csolids loadOsorio andprimary-seextend thehowever, thusage and sfrom beingments inclu(Green et aHydrolyticUCorzo et al.2008); all oin COD andsome of thesuited to w

3.5. System

As previlead to prefinlet, wherpreferentiawastewatertrolled byarrangemenform arealratios withgood widthing of wastextensive csystemspro

width exceeded the length (Brix and Schierup, 1989). This designreportedly suffered less overland ow issues and hence becamerecommended by most HSSF TW design guidelines.

ardins neof t

cur.f perg cot al.,SSFwiturfas shinedowclog

utorgen

le pounifinletsurvace ber bean thll susors be e

djustn andallyUS Eisedsurfaropey haamb, 199pipehe uize tove

ulatid Wallace (2009).

Size distribution Source

04mm (gray water) NORM (2005)14mm (tertiary treatment)48mm (primary treatment)

lic d4mm Wallace and Knight (2006)

sign 36mm EC/EWPCA (1990)612mm

l Watern

816mm IWA (2000)

m treatment processes

tal spillover of solids and sludge from upstream treat-sses can create shock loads of solids to downstream SSFr et al., 2005; Caselles-Osorio et al., 2007). Good designnanceof upstreamprocesses canmitigate the likelihoodrrence; however, it is recommended that efuent ltersare used between the upstreamprocess and the SSF TWthe impact of solids carry-over (Tchobanoglous, 2003).

ive pretreatment technologies are under investigationonjunction with SSF TWs, with the aim of minimizinging and thus, maximizing system lifetime. Caselles-Garca (2007) estimate that when compared against

ttled efuent, physicochemical pre-treatment couldlife of a HSSF TW by approximately 10 years. They note,at the cost andO&M requirements of coagulant, energyludge handling responsibilities will prevent this optionsuitable in every situation. Other possible pretreat-de Upow Anaerobic Sludge blanket (UASB) reactorsl., 2006; Barros et al., 2008; Dornelas et al., 2008),powSludgeBed reactors (HUSB) (lvarez et al., 2008;

RegVF TWsurfacewill ocwork oby usinMolle e

In Hface oror subsU.K. hamaintaof theunevendistribas theymultiploadedfoundfrom aof surfachievthat spare stioperat

Somwith aWatsoeventu1993;comprgravelland Eubut thetion chOgdensimplepipe. Tmaximof owaccum, 2008) and oating treatment wetlands (Grifn et al.,f which can produce a primary efuent relatively lowTSS in comparison to conventional settling. However,aforementioned technologies, such as UASB, are best

arm climates.

aspect ratio and inlet distribution

ously explained, the factors responsible for cloggingerential clog matter accumulation in the vicinity of thee the wastewater is most concentrated. The extent ofl clogging can be mitigated by uniformly loading theover the greatest area possible, and is hence con-

the system dimensions and the inuent distributiont. In VF TWs this is achieved by designing for uni-distribution. In HSSF TWs, early designs had aspectgreater lengths than widths, which helped to achievedistribution but concentrated the cross-sectional load-ewater (Watson et al., 1990). Resulting experience oflogging and overland ow in the inlet region of thesempted theDanish tomodify the aspect ratio, so that the

loadings, sotributors thincludedistcell (Munoz

Anotherbeen to incsubsurfacereducing ac1993; King

4. The inhydraulics

Problemfunctions,surface of tarise for difwith impuldevelops, thwastewaterg inuent distributor design, the operation of typicalcessitates distributors that are located on or near thehe bed so that vertical percolation through the mediaUniform areal distribution is approached using a net-forated pipes that cover the majority of the surface, orncrete splash pads (GFA, 1998; Brix and Arias, 2005;2005; NORM, 2005).TWs, whether clogging initially develops on the sur-hin the subsurface corresponds to whether a surfacece-based inuent distributor is used. Experience in theown that poorly designed, incorrectly installed or ill-inlet distributors will encourage uneven distribution(Rousseau et al., 2005; Grifn et al., 2008) and henceging of the bedmedia (Knowles et al., 2010). Early inletdesigns in U.K. HSSF TWs exacerbated this situation,erally comprised discrete point sources, distributed atints along the width of the bed, such that ow was notormly (Murphy and Cooper, 2010). Cooper et al. (2008)distribution problems that required intervention at 34ey of 255 HSSF TWs in the U.K. In the U.K., the designased distributors for HSSF TWs has evolved to try andtter width distribution, for instance by using troughsewidth of the bed (Murphy and Cooper, 2010). Troughsceptible to solids accumulation but are preferred byecause of their ease ofmaintenance (Grifn et al., 2008).arly U.S. HSSF TWs also utilized surface manifoldsable multiple outlet ports (Steiner and Freeman, 1989;Hobson, 1989). However, the design of these systems

shifted towards the use of subsurface manifolds (TVA,PA, 1993). Initial subsurface distributors for HSSF TWsa pipe distributor, located a few centimeters below thece. Designs of this type are still prevalent in main-(Vymazal et al., 1998; Vymazal and Krpfelov, 2008),ve been superseded in the U.S. by subsurface inltra-ers perpendicular to the ow direction (Campbell and9; Wallace and Knight, 2006), due to the opinion thatdistributors resulted in clogging in the vicinity of these of subsurface inltration chambers is intended tohe cross-sectional area perpendicular to the directionr which the solids are loaded and reduce preferentialon of solids. To furtherminimize cross-sectional organicme U.S. authors have utilized extended inuent dis-at load wastewater along the width of the bed, but alsoributionmanifolds thatprotrude longitudinally into theet al., 2006; Wallace and Knight, 2006).method for achieving uniform width distribution hasorporate an open trench at the inlet that precedesow. This also serves as a solids retention basin, thuscumulation of clog matter within the gravel (US EPA,et al., 1997; Murphy and Cooper, 2010).

uence of clogging on SSF treatment wetland

s associated with clogging are usually hydraulic mal-such as undesirable ponding of wastewater on thehe system. However, the motivation to intervene mayferent reasons. For example, VF TWs typically operatese dosing of wastewater onto the surface. As clogginge inltration time for each does will increase such thatbegins to pond on the surface, until the ponded water


Table 4Reported values for media hydraulic conductivities in the inlet and outlet regions of 21 eld-scale HSSF TWs. Information is also included regarding the design and operationof these systems. Adapted from Knowles et al. (2010).

Reference Ageof st(mo

Caselles-Oso 484848483636

Kadlec and W 41

Watson and 72727272

Fisher (1990 333333

Sanford et al 26Knowles et a 177Knowles and 12

Drury and M 3030

Pedescoll et 177218

a denotes av

no longer coSun et al., 2that this caunecessitate

Contrastprompted bof pondingoperator exU.S. regulatater to proTWs exhibitate restoradegree of pproblem istypically siDeclining trtion for restet al. (2005)U.K. all manexhibiting s

Surfacewhere cloggIn HSSF TWconductivitues at the iat the outleTable 4 whters, themelower than

Numerodesigners ators maxim

or to5), aSystemname

Treatmenttype

Hydraulicloadingrate(mm/d)

DimensionsL:W:D (m)

rio et al. (2007) Verd 1 Secondary 190 30:31:0.5Verd 2 Tertiary 409 27:16:0.4Alfs Secondary 49 32:38:0.5Corbins Primary 178 35:35:0.5Almatret N Secondary 54 23:20:0.5Almatret S Secondary 54 28:18:0.5

atson (1993) Benton Cell 3 Secondary 17 333:44:0.8

Choate (2001) Jones Secondary 10 13:3.1:0.5Gray Secondary 19 12:3:0.5Terrell Secondary 21 2.7:1.8:0.3Snelling Secondary 58 4.3:2.4:0.3

) Scirpus Secondary 50 100:4:0.5Typha Secondary 48 100:4:0.5Control Secondary 38 100:4:0.5

. (1995) Bed 4 Landll leachate 14 33:3:0.6l. (2010) Moreton Morrell Tertiary 467 15:15:0.6Davies (2009) Fenny Compton Tertiary 171 12:40:0.6

ainzhausen (2000) Cell 1 Acid drainage 41 33:33:0.7Cell 2 Acid drainage 100 16:28:1.2

al. (2009) Verd 1 Secondary 77 30:31:0.6Corbins Secondary 59 35:35:0.5

erage value across the width of the bed.

mpletely inltrates between doses (Molle et al., 2006;007). However, it is the associated treatment problems

rates f(Tableses, rather than the hydraulic issues themselves,whichintervention.ingly, intervention into clogged HSSF TWs is usuallyy the need to correct the hydraulic problem. The extentthat qualies as undesirable varies depending onpectations and regulatory requirements. For example,ory agencies prohibit the surface exposure of wastew-tect public health (US EPA, 2002) and therefore HSSFting even a small degree of surface ow will necessi-tive action (Kadlec and Wallace, 2009). A much greateronding is often tolerated in U.K. HSSF TWs before theaddressed (Cooper et al., 2005), as these systems aretuated within enclosed wastewater treatment plants.eatment performance has rarely been cited as motiva-oration in cloggedHSSF TWs, as supported by Rousseauwho found that twelve treatmentHSSFwetlands in theaged to consistently meet discharge standards despiteymptoms of heavy clogging.ow most often occurs in the inlet region of SSF TWs,ing is greatest due to the previously discussed factors.s, a corresponding longitudinal gradient in hydraulicy has been measured by numerous authors, with val-nlet at least an order of magnitude lower than valuest (Table 4). For example, for the 21 systems listed inich all have varying design and operational parame-asured inlet hydraulic conductivitywas on average 60%the outlet hydraulic conductivity.us guidelines have been proposed to try and helpccount for clogging in system sizing, and to help opera-ize longevity. These often take the form of areal loading

sectional arloading ratearly guidelresents thebest describTWs. Theseclogged hylibrium detclog matterguidelines dtivity and aconductivitunsuccessfuclogging. Fcity of thehydraulic cothat varied

5. Clogging

The conclogging ishorizontalConsideringentiate howbased on thtures, andbetween Htion, and semedia.at timeudynths)

Substratesize range(mm)

Inletconductivity(m/d)a

Outletconductivity(m/d)a

Method ofmeasurement

612 2 12 Falling head612 25 61 Falling head612 7 2 Falling head612 2 200 Falling head612 1 87 Falling head612 1 82 Falling head

1423 2500 27,500 Survey

36 1000 5400 Survey36 10,200 8100 Survey36 4,900 4700 Survey36 85 325 Survey

310 1800 25,000 Survey310 2500 25,000 Survey310 2500 25,000 Survey

5 4150 3370 Survey39 2 26,000 Constant head612 1065 84,000 Constant head

620 6 3500 Survey620 6 3500 Survey

612 20 45 Falling head612 3 55 Falling head

tal solids, biochemical and chemical oxygen demandlthough Wallace and Knight (2006) use the cross-

ea in the direction of ow to recommend a maximume of 250g/m2 d BOD for HSSF TWs. Table 5 also listsines for equilibriumhydraulic conductivity, which rep-equivalent bulk reactor hydraulic conductivities thate the total head loss observed across eld scale HSSFguidelines were prescribed on the assumption that

draulic conductivity would eventually reach an equi-ermined by the balance between plant growth andaccumulation (Cooper et al., 1996). However, theseo not consider spatial variations in hydraulic conduc-re generally unable to describe the low inlet hydraulicies reported in Table 4. Resultantly, they have beenl at producing HSSF TW designs that operate without

urther, Knowles et al. (2010) highlighted the speci-se guidelines by comparing the suggested equilibriumnductivity guidelines for 5mm gravels, nding valuesbetween 86 and 2600m/d.

in common variants of SSF treatment wetlands

solidated literature conrms that the development ofhighly specic to system design and operation (e.g.,or vertical ow; surface or subsurface inlet loading).the material presented so far it is possible to differ-clogging develops in common SSF TW design variants,e relationship between ow conguration, design fea-operational parameters. Here we make a distinctionSSF TWs with surface or subsurface inuent distribu-parate discussion is given to VF TWswith sand or gravel


Table 5Published guidelines that have been suggested for the design consideration and prevention of clogging in various SSF treatment wetland designs. These include equilibriumhydraulic conductivity of media in SST treatment wetlands that will result from clogging, and loading limits for hydraulic loading rate (HLR), total suspended solids (TSS)and biological

Type Tre BODloadlimi(g/m

HSSF All HSSF Sec HSSF Sec HSSF All HSSF Pri HSSF Sec HSSF Sec HSSF Sec 250HSSF Sec 6VF Sec VF Pri 30VF Sec VF Sec VF Sec 10

a Refers to c real-b

5.1. Horizonloading

Systemswidely use(Cooper, 20have also(Caselles-O2009) and Sthe upstreaor tricklingsloughing stinuously fthe width opipes with mweirs. Histoof 612mmwell describ

The natupreferentiaand on theite of wastestages of defrom adequexample, st(1998) idena 50mm-deof media (NU.K., Coopeface sludge40mm at thtions in thethe U.K., repsludge builalso speculainfestationRousseau e

Overlandwhere cumof the 255 swas presenthese systeto the inten

ivenent ineventheance

rizon

temsinenary tuouse locguidthosm.D(200subs inwas

ions,surfa006oxygen demand (BOD).

atment Location Media type and size Equilibriumhydraulicconductivity(m/d)

United Kingdom Gravel (36mm) 86ondary United States Gravel (36mm) 2600ondary Germany Soil/sand (0.21.0mm) 78

Europe Gravel (816mm) 1782mary Austria Gravel (48mm) 9173ondary Australia Gravel (510mm) ondary South Africa Gravel (35mm) ondary United States Gravel (d10 > 4mm) 260ondary Unites States Gravel (2030mm) 1000ondary Austria Sand (0.064mm) mary France Graded gravels (070mm) ondary Germany Sand (d10 0.10.3mm) ondary Germany Sand (d10 0.10.2mm) ondary United States Gravel (d>4mm)

ross-sectional loading (perpendicular to direction of ow). All other loadings are a

tal subsurface ow treatment wetlands with surface

of this nature are popular in the U.K. where they ared for the tertiary treatment of municipal wastewater07); however, reports of clogging in similar systemsemanated from France (Linard et al., 1990), Spainsorio et al., 2007; Llorens et al., 2009; Pedescoll et al.,outhAfrica (Batchelor and Loots, 1997). InU.K. systems,m processes are typically rotating biological contactorslters (Grifn and Pamplin, 1998), which are prone toolids to the downstream TW. The wastewater is con-ed through surface-based inlet distributors that spanf the bed (Fig. 2). Distributor designs vary and includeultiple risers and troughs with numerous distribution

rically, the beds have employed gravels on the order. Other details regarding the design of this system areed in Cooper et al. (1996).re of wastewater loading onto these systems results inl clogging at the inlet, within the upper layers of gravelsurface. Clog matter formations are usually a compos-water solids and patchy masses of leaf litter in variouscomposition (Tanner et al., 1998), which are preventedately mineralizing by overland ow of wastewater. For

pervasfrequeThis prwhereconvey

5.2. Ho

Sysof contsecondcontinthat arDesignwhilstas 30min IWA

Theof solidtreatedconditgravelet al., 22008).udies of the dairy wastewater systems of Tanner et al.tied that the accumulated clog matter manifested asep sludge layer and penetrated into the top 100mmguyen, 2001). In their survey of 255 HSSF TWs in the

r et al. (2008) frequently encountered systemswith sur-accumulations in excess of 150mm at the inlet ande outlet. Rousseau et al. (2005) made similar observa-

ir survey of 12 HSSF stormwater treatment wetlands inorting that the vast majority of them had experienced

d-up over the entire surface of the bed. These authorste that symptoms such as poor reed growth and weed

maybe connected to clogging (Cooper et al., 2005, 2008;t al., 2005).

ow results across the surface layer unto the pointulative inltration sufciently meets the HLR. In 30%ystems surveyed by Cooper et al. (2008), overland owt over themajority of theHSSF TWsurfaces. Resultantly,ms may begin operating with vertical ow in additionded horizontal ow regime (Knowles et al., 2010). The

In advaninlet forcesit is able to sinto the subcome fromUS EPA, 199Canada (ChandPolandging dynamTWs may d

5.3. Vertica

The origsand or soiular in couthey are cowastewateris shown iningt2 d)

CODloadinglimit(g/m2 d)

TSSloadinglimit(g/m2 d)

Reference

Cooper et al. (1996) TVA (1993) GFA (1998) IWA (2000) NORM (2005) 40 Bavor and Schulz (1993) 42 Batchelor and Loots (1997)

a Wallace and Knight (2006) US EPA (2000) 20 Langergraber et al. (2003)

35 Linard et al. (1990)20 5 Winter and Goetz (2003)25 Platzer and Mauch (1997)

40 Crites and Tchobanoglous (1998)

ased.

ss of these problems in U.K. systems is highlighted bycorporation of an overow pipe during construction.ts overspill incidents in extensively clogged systemshydraulic loading rate would exceed the diminishedcapacity of the subsurface (Fig. 2).

tal subsurface ow with subsurface loading

of this nature are popular in theU.S., Australia andpartstal Europe where they are predominantly used for thereatment of domestic wastewater. The wastewater isly fed directly into the subsurface through slotted pipesated a few centimeters below the gravel surface (Fig. 3).elines in Europe recommend gravel sizes of 316mme in the U.S. have suggested inclusion of media as largeesign guidelines relevant to these systems canbe found0), US EPA (2000), and Wallace and Knight (2006).surface loading results in a preferential accumulationthe proximity of the inuent distributor. The primary-tewaters received at the inlet often result in anaerobicas typied by the development of black coatings on theces (Bowmer, 1987; Kadlec andWatson, 1993; Suliman; Wallace and Knight, 2006; Vymazal and Krpfelov,ced stages of clogging, the loss of conductivity at thethe ow to surface and take an overland ow path untilufciently inltrate through the surface layer and backsurface. Reports describing clogging of this nature havethe U.S. (Watson et al., 1990; Kadlec and Watson, 1993;3; Zachritz and Fuller, 1993), Australia (Fisher, 1990),azarenc et al., 2007), Czech Republic (Vymazal, 1996)(Maloszewski et al., 2006). Onceowhas surfaced, clog-ics similar to those described for surface loaded HSSFevelop.

l ow with sand matrix

inal vertical ow systems of Seidel (1973) incorporatedl substrates, after which these systems became pop-ntries such as Germany, Austria, and Denmark wheremmonly used for the secondary treatment of domestic. A schematic of a typical VF TW with a sand matrixFig. 4. The most common inuent distribution sys-


Fig. 2. Clogging prole for a typical HSSF wetland with above-surface inuent distribution. Design details are adapted from Grifn et al. (2008); clogging prole is based oninformation from Knowles et al. (2010).

tem consists of perforated piping, equally distributed to cover thesurface area of the bed. The rst layer of media is generally con-sists of 816mm gravel to enhance ow percolation throughoutthe treatmeand ne grBelow the mlayer of incrent collectiallows greamotes aerobspecicatioBrix and Joh

The usearehighlyp

and Green, 1995). For example, Winter and Goetz (2003) reportedthat six from a survey of 21 VF lters in Germany had cloggedand found that this was mainly linked to overloading of wastew-

loggihern thstewffereop 1gingot aminthefor

Fig. 3. Clogginfrom Kadlec annt bed. The main treatment media is a mixture of sandavel with an average grain size smaller than 4mm.ain treatment layer, a transition layer and a drainageeasingly coarser gravels is used to facilitate nal efu-on. Wastewater dosing is typically intermittent as thister oxygen transfer within the wetland bed and pro-ic degradation processes. Further details regarding the

n of these systems are given in ATV-A 262 (GFA, 1998),ansen (2004) and NORM (2005).of sand as opposed to gravel means that these systemsrone to clogging if theyareoperated incorrectly (Cooper

ater. Clayer, wbetweeentwafour diin the t

Clogdoes nmattering ofoxygeng prole for a typical HSSF wetland with subsurface inuent distribution. Design detailsd Wallace (2009).ng preferentially develops in the top part of the maine the greatest biological growth occurs due to contacte ne media, nutrients and organic matter in the inu-ater. Indeed, Tietz et al. (2007) found through the use ofnt methods that microbial biomass was most abundant0% of a VF TW lter.can be extremely rapid if the period between doses

llow sufcient wastewater percolation and organiceralization (Langergraber et al., 2003), because seal-surface by wastewater inhibits further transfer ofmineralization (Platzer and Mauch, 1997). Kayserare adapted from Vymazal et al. (1998); clogging prole is adapted


Fig. 4. Clogging prole for a typical VF treatment wetland with sand substrate. Design details are adapted from NORM (2005); clogging prole is based on information inLangergraber e

and Kunsthealthy of 21% betwcloggedltecycles.

5.4. Vertica

Thegravto treat screof a multiptreatment wat numerou

at aisize dlayernd 2s canFrena sowa

5); i.er, th

Fig. 5. Clogginbed in the seriinformation frt al. (2003).

(2005) observed that the oxygen content of air in alter consistently returned to the atmospheric leveleen doses. However, the oxygen content of air in arwas below5%andwas relatively unaffected bydosing

l ow with gravel matrix

elmatrixVF TWscommonly employed in France is usedened municipal wastewaters, and form the rst stage

pads thwith asition320 asystem

Themulateappliedal., 200Howevle-cell wetland network. A schematic of the French VFetland system is shown in Fig. 5. Wastewater is loadeds points over the surface of the bedonto concrete splash

the surfacebecoming dMerlin, 200

g prole for a typical VF (French-type) treatment wetland with gravel substrate. Thesees (shown) is constructed with larger gravel and retains most of the solids. Design detailom Molle et al. (2005).d areal distribution. The main lter layer is small gravelistribution of 28mm. This layer is followed by a tran-and a drainage layer with respective gravel sizes of

040mm. Further details regarding the design of thesebe found in Iwema et al. (2005).

ch type of VF treatment wetlands are designed to accu-lids layer on the surface of the bed, through which thestewater must percolate (Iwema et al., 2005; Molle ete. they are designed to clog (Kadlec and Wallace, 2009).e French wetlands are operated in a manner such that

layer is benecial to treatment performance withoutetrimental to hydraulic performance (Chazarenc and5). The wastewater is dosed intermittently, which pro-

systems are generally designed with several beds in series; the rsts are adapted from Lienard et al. (1998); clogging prole is based on


Fig. 6. Box-an(bottom) loadindicates theWetland Assoadapted from

vides time fhydraulic co

It is belinicant ponthe reasonvarying up(2005) furtreporting threquired inratios of 60other system

5.5. Why do

From thTW variantthe primaryclogging thcounterintudatasets forg/m2 d) to esection. The(CWA) Wet(POR) weresurface inFederationfor 24 diffeent distribuwhich PORmatrix; andwere takenare comparsents 50% oand the whthe dataset

What is evident from Fig. 6 is that U.K. HSSF TWs receive a com-binationof relativelyhighHLRandTSS load (medianHLR=0.12m/d

edian TSS load=7g/m2 d) in comparison to the other data-hereSF TWn HLF TWids lothe

.06miscuesiraonds inatterhowtingr TSSplaiobler tha/PE vbin

utioystemmilagingGerHLRg mydrathate cl

o the005)posugulacomd-whisker plots showing the relative inuent HLR (top) and TSSings derived from four national treatment wetland databases. PORentire period of record for one system. Data source: Constructedciation Wetland Database (2006), WERF Database (2006), dataWinter and Goetz (2003), and data adapted from Boutin et al. (1997).

or the surface layer to dewater and mineralize so thatnductivity is maintained.eved this mode of operation effectively prevents sig-ding, with Chazarenc and Merlin (2005) citing this as

that several gravel-based VF systems in France of agesto eight years showed no signs of clogging. Molle et al.her support the effectiveness of the French design byat only one out of 71 VF systems reviewed had ever

and msets, wU.S. HS(mediaman VlowsolreceiveHLR (0

As dby undcorresptems. Aclog mSpain;presensmallemay exging prsmalle(0.7m2

The comcontribthese s

A siof clogi.e., thehigherthat clocause hmeansprecludment tet al., 2face exlocal re

This

tervention due to clogging. Generally, mineralization% have prevented this phenomenon from occurring ins (Boutin et al., 1997).

es clogging impact some systems worse than others?

e literature reviewed thus far it appears that some SSFs are more prone to clogging than others. For example,

VF TWs in France appear less prone to problematican tertiary HSSF TWs in the U.K., although this seemsitive. To elucidate this phenomenon, Fig. 6 compareshydraulic loading rates (in m/d) and TSS loadings (inach of the SSF TW variants discussed in the previousse datasets are: the Constructed Wetland Associationland Database (2006), from which Periods of Recordtaken for 71 different U.K. HSSF Tertiary TWs with

uent distribution; the Water Environment Research(WERF) Database (2006), from which POR were takenrent U.S. HSSF Secondary TWs with subsurface inu-tion; data adapted from Winter and Goetz (2003), fromwere taken for 21 different German VF TWs with sanddata adapted fromBoutin et al. (1997), fromwhich PORfor 53 different French VF TWswith gravelmatrix. Dataed using a box and whisker chart, where the box repre-f each dataset with the internal line at themedian valueiskers extend to the minimum and maximum value in, excluding any outliers.

HLR as wellfor longevitproblems wcesses respunderstated

6. Conclus

Two decclogging cathewidesprdent in thisof clogginging four com(HSSF TWsTWs with sdate how dthe literatuclogging prFrench VFaccumulatiomineralizatings. Conveas a resulttion and cathat both theither one or both measures is relatively lower. Thes receive the lowest solids and hydraulic loading rates

R=0.02m/d and median TSS load=2g/m2 d). The Ger-s receive a high HLR (median=0.3m/d) but a relativelyad (median=3g/m2 d). Statistically, the FrenchVF TWshighest TSS load (17g/m2 d) but a comparatively low/d).ssed, problematic clogging is most typically qualiedble surface ponding. However, this does not alwaysto the extent of solids accumulation within the sys-dicated by Fig. 1, French primary VF TWs accumulatealmost ve times faster than secondary HSSF TWs in

ever, the low HLR of French systems prevents this froma problem. In contrast, U.K. tertiary systems receive aload thanFrench systemsbut amuchhigherHLR,whichn the propensity of U.K. systems to experience clog-ms. Tertiary U.K. HSSF TWs are sized to be seven timesn secondary U.K. HSSF TWs that receive equivalent HLRersus 5m2/PE, respectively) (Green and Upton, 1995).ationof hydraulic overloading and relativelyhigh solidsns fromupstreamprocesses diminishes the longevity of

s.r discussion can be used to explain why more reportsproblems emanate from Germany than from France;man secondary VF TWs operate at lower TSS loads butthan the French systems. This increases the likelihoodatter accumulation in German systems will eventuallyulic problems, whereas in French systems the low HLRorganic solids are able to mineralize well enough to

ogging in most cases. This comparison serves as testa-claims of clog-free operation in French VF TWs (Molle. However, French VF TWs generally operate with sur-re of screened sewage andwould not be suitablewheretions prohibit this.parison has emphasized the importance of consideringas wastewater pollutant loadings when sizing systemsy.However, long-termattempts to circumvent cloggingill require a better understanding of the biological pro-onsible for clog matter reduction, and this cannot be.

ions

ades of treatment wetland literature have proven thatn limit the asset lifetime of SSF TWs and may threatenead feasibility of the technology.What has become evi-review, however, is that the development and impactis specic to system design and operation. By compar-mon variants of subsurface ow treatment wetlands

with surface or subsurface inuent distributor, and VFand or gravel matrix), it has been possible to eluci-esign and operation inuence clogging. According tore, tertiary HSSF TWs in the U.K. are more prone tooblems than primary VF TWs in France. This is becauseTWs are hydraulically operated such that high solidsn rates do not cause operational issues, and adequateion of surface clog matter can occur between load-rsely, U.K. systems typically operatewith overland owof high HLR, which prevents clog matter mineraliza-uses progressive clogging. Consequently, it appearse pollutant mass loading and hydraulic loading rates


are imperative considerations when designing SSF TWs for robustoperation.

What is still not known, and is a priority for further research,are the actand operatilated clog morganic solicontributioical properconstituentwill not nechydraulic codevelopmeand requirecan elucidaconductivit

Generaloperate wit

1. Useof anater over

2. Intermittwhich w

3. Reductiocesses.

4. Incorporof upstre

It is impintention orent trend ocontinue. Tguidelinesequilibriumthe hydraul

Acknowled

Paul Knoport and myears to fukindly acknlace for thekindly provdentship grGabriela DoHerff PostdJaime Nivalthrough Hemental Reseand ResearGarca acknEducation aNEWWET2very much tand colleagprivate and

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ds from wastewater treatment processes, and biomassn from internal wetland functions; and often has phys-ties that are more detrimental to hydraulics than itsparts. As a result, the volume or mass of clog matteressarily correlate well with corresponding changes innductivity. This lack of understanding is a barrier to the

nt of simulation tools for SSF TWs (Langergraber, 2008)s new experimental techniques to be developed thatte how the nature of clog matter inuences hydraulicy.points to consider when designing future SSF TWs toh reduced clogging problems include:

inuentdistributorwhichuniformly loads thewastew-the maximum area possible.ent dosing of the inuent in surface-loaded systems,ill encourage surface layer mineralization.n of unintentional solids spillover from upstream pro-

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gements

wles and Jaime Nivala gratefully acknowledge the sup-entoring provided by Scott Wallace over the past 10rther understanding of wetland clogging. All authorsowledge Pascal Molle, Clodagh Murphy, and Scott Wal-ir input. Paul Knowles acknowledges the joint fundingided by Severn Trent Water Plc. (U.K.) and a CASE stu-anted by the EPSRC U.K. (ref. 1121 CASE/CNA/06/28).tro kindly acknowledges the nancial support from aoctoral Fellowship at the University of Memphis (USA).a thanks the Helmholtz Impulse and Networking Fundlmholtz Interdisciplinary Graduate School for Environ-arch (HIGRADE) and the FederalMinistry for Educationch (BMBF) of Germany for funding and support. Joanowledges the support from the Spanish Ministry ofnd Science; projects NEWWET (CTM2005-06457) and008 (CTM2008-06676). Joan Garca also appreciateshe cooperation of undergraduate students, techniciansues involved in these projects. The support of otherpublic institutions is also greatly acknowledged.

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