TechnologiesfordomesticwastewaterrecyclingB.Jeerson*,A.Laine,S.Parsons,T.Stephenson,S.JuddSchoolofWaterSciences,CraneldUniversity,Craneld,BedfordMK430AL,UKReceived9December1999;receivedinrevisedform5April2000;accepted13April2000AbstractDomesticwastewaterrecyclingisstillinitsinfancyandassuch,thereisapaucityofreliableinformationrelatingtoboththenatureofgreywaterandtherangeofrecyclingtechnologiesavailable. Thelackofwaterqualitystandardsandthepoorunder-standing of the nature of grey water have led to the development of a plethora of technologies for recycling application. The paperdiscusses the relative merits of the dierent options and describes the current situation within the UK. 2000 Elsevier Science Ltd.Allrightsreserved.Keywords:Greywater;Technology;Performance1.IntroductionWastewaterrecyclingisemergingasanintegral partof water demand management, promoting as it does thepreservationofhigh-qualityfreshwatersuppliesaswellas potentially reducing the pollutant in the environmentand reducing overall costs. Water recycling can beconducted both internally, where water is retainedwithina localprocess loop, orexternallywherewaterissourceddirectlyfromasewage treatment works. ThepreferenceintheUKcurrentlyis for internal systemssuch as industrial or domestic reuse, although somewater utility companies are proactive in pursuing marketopportunitiesinbulkreuseoftreatedmunicipal waste-water.Theviabilityof internal recyclingis tosomeextentcontingent uponthere beingsubstantial dierences inwater quality and quantities demanded for dierentoperations. The proportion of water used required to beof the highest qualityis small. This thenimplies thatmost of thedemandswithintheprocessschemeisforlowergradewater, permittingreuseofwaterfromoneapplicationtoanother. Anexample of this is in thedomestic environment where the reuse of greywaterssuchas baths andshowers for toilet ushing canbeachievedwithlittleornotreatment(Sayers,1998).Domestic water recyclingis anattractive optioninthe UKdue toarelativelyhighdomestic water con-sumptioncoupledwithanintensivepopulation. Waterusefordomesticpurposesconstitutes40%of thetotaldemand placed on the water companies. Given that overhalf the population lives in towns of 100,000 populationor above(Boschet, Wahliss, &Lack, 1998), water re-cycling in urban regions may be viable especiallythroughthe use of centralisedstorage andtreatment,therebyoeringeconomyofscale.Althoughithasgainedhigherproleonlywithinthelast20yearsorso,communalwaterrecyclinghasbeenan integral part of society for centuries. One of theearliestreportedengineeringsystemswasinthePalaceof Knossos, built byKingMinos of Crete5000yearsago(Horan, 1998), whererainwater fromadedicatedcollectionsystemwas used, amongst other things, toushtoilets. Bathwaterreuseforpersonal gardenirri-gation has also been practised for centuries. Engineeringsystems for internal grey water reuse, on the other hand,havebeendevelopedonlyrelativelyrecently. Anearlyexample arises inthe mid-1970s inthe eldof spaceexploration. Work conducted by the NationalAeronautics andSpace Administration(NASA) dem-onstratedhowbathandlaundrywastewater couldbereusedfortoiletushingbypassingthewaterthroughdiatomaceousearthlterfollowedbyactivatedcarbon(Hypes,Batten,&Wilkins,1975).Greywatertreatmentandreuseare, infact, compli-catedbyissues basednot onlyontechnical feasibilitybut also on human issues. It is arguably the publicUrbanWater1(1999)285292www.elsevier.com/locate/urbwat*Corresponding author. Tel.: +44-1234-750111 ext. 2902; fax: +44-1234-751671.E-mailaddress:b.jeerson@craneld.ac.uk(B.Jeerson).1462-0758/00/$-seefrontmatter 2000ElsevierScienceLtd.Allrightsreserved.PII:S 1 4 6 2 - 0 7 5 8 ( 0 0 ) 0 0 0 3 0 - 3perception of water reuse and the risk associatedwith itwhichultimatelydictatethestandardsofproductwaterquality required and so the appropriateness of candidatetreatment technologies. Aholistic approach to waterrecyclingisthuscrucial, andtheecacyof individualtreatment technologies or overall treatment schemesmustbepreciselyevaluated.This paper describes the current understanding ofproblemsassociatedwiththetechnologyofwaterrecy-clingwithspecic reference tothe technologies eithercurrentlyinplace of existingrecycling schemes or ofdevelopment of practical recycling operations. Technicaldata presented include those arising from a 3-year studyat CraneldUniversity, along withthose fromotherinvestigationsfoundintheliterature.2.GreywatercharacterisationGrey water arises from domestic washing operations.As such sources include waste from hand basins, kitchensinks and washing machines, but specically excludefoul orblackwatersources(toilet, bidetsandurinals).The cumulative owbalance betweenthe grey watergenerated and toilet ushing requirements shows anatural anity at about 30%of the total water use.However, the dynamics of the situation are not so ideal.Greywater is producedat atime slightlyoset fromtoilet ushingandgeneratedover short time periods,whereas toilet ushing takes place more consistentlythroughthe day. This generallyresults inadecit inwater duringthe afternoonand lateevening asdepictedinFig. 1(Surendran&Wheatley, 1998). This mayberectiedbybueringusingappropriatelysizedstoragetanks,butthis substantiallyincreases theoverall systemsize. Detailed examination of the benets of storagereveal that 1m3tankis suitable for awide range ofoccupancyscales(Dixon, Butler, &Fewkes, 2000). In-creasing storage capacity above this provides onlymarginal increases in water savings and increasesproblems associatedwithgreywater degradationanddisinfectionreliability.Grey water is usually generated by the use of soap orsoapproductsforbodywashingandassuch, variesinquality according to, amongst other things, geographicallocation, demographics andlevel of occupancy(Table1). Grey water has a similar organic strength to domesticwastewaterbutisrelativelylowinsuspendedsolidsandturbidity, indicating that a greater proportionof thecontaminants are dissolved. Moreover, although theconcentrationoforganicsissimilartodomesticwaste-water their chemical nature is quite dierent. TheCOD:BODratiomaybe as highas 4:1verymuchgreater than values reported for sewage. This is coupledwithadeciencyof macro-nutrients suchas nitrogenand phosphorus: the COD:NH3:Pof grey water hasbeenmeasuredat 1030:2.7:1, andthis compares with100:5:1 for domestic wastewater. Both the relatively lowvalues of biodegradable organic matter and the nutrientimbalance limit the eectiveness of biological treatment.Themajordicultypresentedfortreatmentofgreywater is the large variation in its composition. ReportedmeanCODvalues varyfrom40to371mg/l betweensites, with similar variations arising at an individual site.Thisvariationhasbeenattributedtochangesarisinginthequantityandtypeof detergent productsemployedduringwashing.Greywaterqualityisalsosubjectedtodynamic variation: signicant chemical changes maytake place over time periods of only a few hours, thoughlittleappears tohavebeenreportedonthis phenome-non. Oneexample(Fig. 2), whichreferstoa20l greywater sample kept either completely mixed or underFig.1.Typicalgreywaterproductionandtoiletushingrequirementsinacollege(Surendran&Wheatley,1998).286 B.Jeersonetal./UrbanWater1(1999)285292quiescent conditions, reveals rst-order kinetics fordecayinBOD5witharateconstantofaround5h1correspondingtoa50%reductioninBOD5overa4-hperiod.Similardecayratesariseforbothquiescentandmixed systems and for both mechanical mixing andaeration, the exact rate of change beingrelatedmoreto the original grey water composition and systemtemperature than the hydrodynamic conditions ofstorage.3.WaterqualitystandardsNo standards exist in the UK at present and, with theexception of proposed guidelines suggested by theBuilding Services InformationandResearchAssocia-tion (BSRIA), no standards are under development.On the other hand, standards are either in placeor under development in other parts of the world(Table2).Thewaterqualitydeterminandsuseddenebacteri-ological quality, biodegradability, clarity and acidity,thoughthe actual permittedlevels vary considerably.Twogroups of standards canbe identiedgeneratedfromdierentideologies. Therstoftheseisbasedonthequalityofthegreywaterbeingcommensuratewithits application, and in such cases the standards aresimilartothoseforbathingwatersincethelevelofrisktotheuserisaboutthesame.Thealternativeapproachis more conservative and considers grey water treatmentinasimilarmanner tothat of municipal or industrialeuent. The dierencein ideologies is manifested in thestandardforcoliformlevels, whichforthemoreprag-matic approach is in the range of a few thousand colonyformingunitsper100mlascomparedtoanon-detect-ablelevelforthemoreconservativeapproach.Fig.2.Eectofstorageonthecharacteristicsofgreywater.Table1TypicalcompositionofgreywaterfromvarioussourcesBOD5(mg/l) COD(mg/l) Turb(NTU) NH3(mg/l) P(mg/l) TotalcoliformsHandbasin 109 263 9.6a2.58 Combined 121 371 69 1 0.36 Syntheticgreywater181 25 0.9 1.5 106Singlepersonb110 256 14 Singlefamilyc 76.5 0.74 9.3 Blockofatsb33 40 20 10 0.4 1 106Collegeb80 146 59 10 Largecolleged96 168 57 0.8 2.4 5.2 106aTotalnitrogen.bHolden&Ward(1999).cSayers(1998).dSurendran&Wheatley(1998).B.Jeersonetal./UrbanWater1(1999)285292 287Thefocusonbacteriological qualityreectsthepo-tential for human exposure to recovered grey water,such that the major criterion is public health protection.Entericvirusesareknowntobethemostcriticalgroupofpathogens astheycancause illness evenat lowdosesandcannot bedetectedbyroutinemicrobial analysis.Theyalso represent the component that is most diculttoprocess:itcanbeassumedthataprocesseectiveinremoving enteric viruses will be similarly eective for allother pathogens (Asano, 1998). It is normal, however, tobasestandardsonthemorereadilyquantiableindica-tor organisms of faecalor total coliforms. These speciesdemonstrateapotentialfordiseasetransmission,ratherthan an actual risk of illness, but are more familiarbacteriological quality determinands thanviruses andare more easily measured. On the other hand, no provencorrelationexists betweenconcentrations of indicatorspeciesandactualpathogenlevels,andsomepathogensareknowntobemoreresistant totreatment thantheindicatorspecies(Asano,1998).ThishasresultedinthemoreconservativeapproachbeingadoptedinAmerica,JapanandAustraliawheregreywater recyclingis anestablishedoperation. Inthe USAspecicallythe USEPA guideline for water recycling (EPA, 1992) promotesnon-detectable concentrations of faecal coliforms forurban reuse combined with a specication for minimumleveloftreatmentrequired.Theguidelinehasimprovedthe applicability of surrogate measures such as indicatororganismsandiscurrentlybeingadoptedinCaliforniaandFlorida.4.AvailabletechnologiesThe paucity of water recycling standards has con-tributedtotheproposalanddevelopmentofaplethoraof technologies whichvarygreatlybothincomplexityandperformance. Processesdevelopedrangefromsim-plesystemsinsinglehousestoveryadvancedtreatmenttrainsforlarge-scalereuse.4.1.Basictwo-stagesystemsCoarse ltration plus disinfection represents the mostcommontechnologyusedfordomesticreuseintheUKto date with a number of companies supplying productsbasedonthis two-stage process. The generic processemploys a short residence time so that the chemicalnature of the grey water remains unalteredandonlyminimal treatmentisrequired.Thecoarselterusuallycomprisesametal straineranddisinfectionisachievedusingeitherchlorineorbromine, dispensedinslowre-leaseblocksorbydosingaliquidsolution.Systemsre-cently tested (Sayers, 1998) have shown a variety ofwater savinglevels rangingfrom3.4%to33.4%withmainstopupactingasastandbysupplyduringperiodsof lowowor process failure. Suchsystems arecom-mercially available at moderate cost (between 5001000installed), oeringminimumpaybackperiodsofaround8yearsforafour-personhousehold. Althoughlittleornoremovalofthechemicalandbiologicalpol-lutionwas reportedfor sucha system(Table 3), thetreated water was free of all indicator organisms makingit potentiallysafefor reuse. However, thesystemalsosuered periodic failure of the disinfection process, suchthat coliformlevels occasionallyexceededall the cur-rentlyproposedwaterqualitystandards.Thebasictwo-stagesystemis designedtomeet thelessstringentreusestandardsakintothoseforbathingwater, rather than the more conservative standards. Thewaterthusremainshighinorganicloadandturbidity,therebylimitingtheeectivenessofthechemical disin-fection process for two reasons. Firstly, grey watercontains occulent particles above40lmindiameter,andthese are knowntoreduce chemical disinfectioncapability because the disinfectant cannot diuse farenough into the ocs to provide complete killing ofpathogens. Secondly, organic matter inthe water im-parts a disinfectant demand. Inthe case of chlorine,disinfectantbyproductssuchaschloraminesandtriha-lomethanesaregeneratedwhichhavelowerdisinfectantTable2Summaryofwaterqualitystandardsandcriteriasuitablefordomesticwaterrecycling(adaptedfromSurendran&Wheatley,1998)Totalcoliformscount/100mlFaecalcoliformsBOD5(mg/l)Turbidity(NTU)Cl2Residual(mg/l)pHBathingwaterstandardsa10,000(m)500(g)2000(m)100(g) 69USA,NSF