small scale wastewater treatment for single houses
TRANSCRIPT
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KUNGLIGA TEKNISKA HGSKOLAN
Small scale wastewatertreatment in single houses
ValerieSeh,PhilipSunesson,LindeSedell,EmmanouilMilathianakis
20150228
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Tableofcontent
Introduction ..................................................................................................................................... 2
Purpose & methodology ................................................................................................................. 2
Background ..................................................................................................................................... 2
Domestic wastewater .................................................................................................................. 2
Wastewater treatment ................................................................................................................. 3
Parameters for consideration ...................................................................................................... 3
Methods .......................................................................................................................................... 4
Septic Tank & drain field System ................................................................................................ 4
Background .............................................................................................................................. 4
Construction ............................................................................................................................. 4
Treatment and operation ......................................................................................................... 5
Cost .......................................................................................................................................... 6
Maintenance ............................................................................................................................ 6
Miniature water treatment plant .................................................................................................. 6
Background .............................................................................................................................. 6
Construction ............................................................................................................................. 7
Treatment and operation ......................................................................................................... 8
Cost .......................................................................................................................................... 9
Maintenance ............................................................................................................................ 9
Constructed Wetlands ................................................................................................................. 9
Background .............................................................................................................................. 9
Construction ........................................................................................................................... 10
Treatment and operation ....................................................................................................... 10
Cost ........................................................................................................................................ 10
Maintenance .......................................................................................................................... 10
Case study .................................................................................................................................... 11
Location ..................................................................................................................................... 11
Geology ..................................................................................................................................... 11
Protection level .......................................................................................................................... 12
Discussion and conclusion ........................................................................................................... 13
References .................................................................................................................................... 14
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IntroductionWaterisanessentialelementthathumansuseintheirdailylivesandthegenerationofwastewateris
inevitable.Wastewatercontainspathogens,nutrientsandevenchemicalswhichcanbeahazardtobothenvironmentandhumanhealthifitisnottreatedproperlybeforeitisbeingdisposedof.Inmost
developedcountries,wastewatertreatmentplantsandtechnologiesarereadilyavailabletotreat
wastewaterwithinthecities.However,inthecaseofruralareas,theremaybedifficultiestothe
accessibilityandavailabilityofwastewatertreatmenttechnologies.Thisisthecasewhenruralareasare
sparselypopulated,locatedgeographicallyfurtherawayandmayfaceproblemsinaffordability.Assuch,
itisimportanttolookforalternativetechnologiesthatenabletheonsitetreatmentofwastewater
beforeitisdisposedbacktothenature.Thetechnologiesshouldbeinbeneficialtothehealthandthe
environmentofthoselivingintheruralareas.
Purpose & methodologyThescopeofthisprojectistoidentifyappropriatewastewatertreatmenttechnologiesforsingle
householdsthatdonthavetheabilityorforotherreasonschoosetonotgetconnectedtoacentralora
municipalsewagesystem.Theaimistousetheidentifiedmethodstofindagoodsolutionfora
householdsituatedintheareaofstudypresentedinthereport.
Firstathoroughliteraturestudywasconductedtogetagoodoverviewofthedifferentwastewater
treatmenttechniquesthatareusedtoday.Aftertheliteraturereview,fourwastewatertreatment
methodswereidentifiedforfurtherresearch.Eachmethodwasindependentlyexaminedandanalyzed
regardingthebackgroundofthemethod,constructioncharacteristics,treatmentandoperation
proceduresandfinallycostsandmaintenance.
Afterthatthebestmethodfortheareaofthecasestudywaschosendependingontheefficiencyofthe
methodandtherequirementsforthearea.
BackgroundWastewateriswaterthathasbeenaffectedinqualitybyanthropogenicuse. Thewaterproducedin
householdsiscalleddomestic,mostwastewaterishoweverproducedbyindustries.
Domestic wastewaterDomesticwastewateriswastewaterproducedbyasinglehouseholdanddividedintotwocategories.
Firstly,thegraywaterwhichcomesfromshowers,baths,washingmachines,dishwashersandsecondly
theblackwaterwhichcomesfromtoiletsandkitchensinks.Domesticwastewaterismainlycomprised
ofwater(morethan95%)andsmallconcentrationsofsuspendedanddissolvedorganicsolidsas
carbohydrates,lignin,fats,soaps,syntheticdetergents,proteinsandtheirdecompositionproductsas
alsoofinorganicsolids,pathogenssuchasbacteriaandvirusesandfinallynutrientsandgases.
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Table1,thechemicalcontentofdomesticwastewater(USEPA,2002).
TypicalRaw
Residential
Wastewater
Quality
Constituent MassLoadingGrams/person/day Concentration
5daybiologicaloxygendemand(BOD5) 3565 155286mg/l
Totalsuspendedsolids(TSS) 3575 155330mg/l
Chemicaloxygendemand(COD) 115150 150660mg/l
Totalnitrogen(TN) 617 2675mg/l
Ammonia(NH4N) 13 413mg/l
Nitritesandnitrates(NO3N)
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Currentwaterqualitystandardsofthearea,andwhatshouldbethestandardstoachieveafterwater
treatment. Secondly,wewillevaluatetheparametersthatweneedtotakeintoconsiderationwhen
lookingintothevariouswastewatertreatmentsolutionstoensureitsfeasibility.Thetotalcostrequired
inbuilding,operatingandmaintainingthewastewatertreatmentsolutionisaparameterfor
consideration.Itshouldbeaffordableforittobereadilyavailable.Stepsrequiredtooperateand
maintaintheplantifany.Arethehouseownerseducatedenoughtoservicetheirwaterplantorwould
theyhavetobringinatechnician.Theextenttowhichwaterqualitystandardscanbeattainedwiththe
implementedchoiceofsolutioncanbeanimportantfactor.
Methods
Septic Tank & drain field System
Background
Aseptictanksystemcomprisesofaseptictankthatitisconnectedtoaeitherasoiltreatmentsystem,
drainagesystem,oradrainfieldattheend(SPe,2015).Inthisstudy,wewillfocusonthemostcommon
typeofseptictanksystemwhichincludesaseptictankandadrainfield.Aseptictankisaconventional
wastewatertreatmentsystemthatishighlyefficientandselfcontained.IntheUnitedStatesalone,
approximatelyoneinfourhouseholdsutilizestheindividualseptictanksystemtotreattheirwaste
water(EPA,2014).
ConstructionTheseptictankisdesignedasawatertightboxthatisusuallymadeupofbricks,plastics,mortar,or
concretetopreventleakageofwaterintothesurroundingenvironment.Theapproximatecapacityof
theseptictankisabout4000to7500litres.Italsocomprisesofaninletpipe,wherebywastewaterwill
flowfromhomeintothetank,andaoutletpipe.Theshapeofthetankcanbeeithercylindricalor
rectangularandseparatedintotwochamberstoimprovetheremovalofsolids(UNEP,2015).
Figure1,aseptictanksystem(NewTechBio,2015).
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Asforthedrainfield,thedrainsshouldbeputdownhillfromthehousesothesystemcanworkwith
gravity,butifitsnecessaryandtheinfiltrationmustbeuphillitsalsopossibletouseapump.The
drainsshouldlayabout15cmdownfromsurfaceanditshouldalsobeabout1 2metersbetweeneach
pipe.(Naturvrdsverket,1987)Thisistoenabletheblackwaterfromtheseptictanktobedistributedto
thedifferentdrainsinanappropriatepace.Thedrainfieldconnectedattheendoftheseptictankisa
robustsolutionthatdontrequirealotofmaintenance.Italsodoesntrequirethatmuchelectricitywhy
itsagoodmethodinareaswherepowerfailureiscommon.Buttherightconditionsarenecessaryfor
thesystemtowork.Itsimportantthatthesoilisintherightfraction(likegravelorsand),itsnecessary
tohaveatleast50m2ofspacefortheinfiltrationdrains,anditsimportantthatitsasatisfyingdistance
downtothegroundwater.Thedrainfieldsystemcanbeusedincountriesallaroundtheworldandcan
beusedtotreatmixed,urine,blackwaterand/or
greywater.(Avloppsguiden,2015c)Disadvantageswiththesystemisthattheresariskofgroundwaterpollution,anditshardtotestthefunctionoftheinfiltrationandthequalityofthetreatedwatersince
itsallunderground.(Avloppsguiden,2015d)
Treatment and operation
ThemainfunctionaluseoftheSeptictankistoactasastoragetanktoholdthewastewaterlong
enoughforbothsolidsandliquidstobeseparatedbygravity.Inatypicalseptictank,thewastewater
canbestoredforabout24days(UNEP,2015).Thisallowsthewastewatertobeseparatedintothree
mainlayersaccordingtotheirdensity.Firstly,thetopmostlayeristhescumlayerwhichcomprisesof
solidssuchasoilsthatarelessdensethanwater.Secondly,thebottommostlayeristhesludgelayer
madeupfromsolidsthataredenserthanwater.Lastly,theremainingmiddlelayerwouldbethe
partiallyclarifiedfluid(NESC,2014).
Thesettledsolidswillundergoanaerobicdigestion bythebacteriainthewastewaterwhichcanreduce
thevolumeofsolids.Inthisoxygendeficientenvironmentcoupledwithwarmtemperatures,odor
producinggasessuchasmethanemaybeemittedandresuspendsettledsolids,increasingtheamount
oftotalsuspendedsolidsinthemiddlelayer(UniversityofMinnesota,2011).Assuch,additionaleffluent
screensmaybeaddedtoreducethedischargeofthesesolids.Theuseoftheseptictanktotreat
wastewaterenablesabout50%ofBODandsuspendedsolidstobesettledandremoved(UNEP,2015)
Theeventualeffluentwillbedischargedtoadrainfieldoraseepagefieldwherebytheremaining
impuritiesintheeffluentaretrappedandeliminatedinthesoilandtheexcesswateriseliminated
throughpercolationinthesoil,evaporation,plantuptakeorenteringthegroundorsurfacewater.
Inthedrainfield,thewaterisinfiltratedinthesoilwherebiological,physicalandchemicalprocesses
purifiesthewater.Thedrainsshouldbeputdowninalayerofbigfractionmateriallikebrokenstone,
macadam,orgravel.Butitsimportantthatthefractionofallthesoilisnttoobigsothewaterhavethe
timetopurifybeforegettingdowntothegroundwater.Itsalsoimportantthatthedistancefromthe
drainstothegroundwaterlevelislongenoughi.e+2m.Allwells,wetlandsandstreambedshouldnotbe
tooneartoaninfiltrationplant.(Avloppsguiden,2015c)
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Cost
Oneadvantageofthesmallscaleaerobicseptictanksystemisloweconomicalcostassociatedwithit.
Septictankscanbeeasilybuiltwithbasicmaterialsandnoadditionalenergyisneededtomaintainthe
tank.Furthermore,ithastheabilitytotreatsewerageonsite.Thisiswhyseptictanksareusually
situatedinruralareas,wherebythehousesarerelativelylargerandspacedfurtherapartastheoverall
costismoreeconomicalthanbuildingacentralisedsewersystem(NESC,2014).Inaddition,
transportationcostcanbesavedassewerageneednotbetransportedtoanearbytreatmentsystem.
Thedrainfieldsintheseptictanksystemhasalonglifetime,oftenmorethan20years,andsincethe
costofthemaintenanceisratherlowitsoftenconsideredaveryaffordablemethod.Themaincost
whenalltheworkisdoneinstallingthesystemisforemptyingoftheseptictank.
Thecostwilldifferalotdependingonthedifferentsoiltypesandhowbigtheinfiltrationplantshallbe.
Buttheaveragecostforthiskindofsystemwithgoodsoilconditionsareabout70000SEK.These70
000aredividedas25000fortheseptictank,about20000forthepipesandgravel,i.e.theinfiltration
plant,andthelast25000areforthework.(Infiltrationsanlggning,2015)
Maintenance
Septictanksrequirefrequentmaintenancewhichisoftentheresponsibilityoftheownerwhichincludes
theremovalofwastethatisnotdecomposedbyanaerobicdigestion.Theseptictankwillnotfunction
properlyifitisnotmaintainedandcanresultinserioushealthandenvironmentalconsequences.The
pumping frequencyoftheseptictankisdependantonboththetanksizeandthehouseholdsize(nesc,2004).Assuch,therehavebeengovernmentallegislationsuchasTheWaterServicesActin2012by
Ireland'sEnvironmentalProtectionAgencytoregisterallwatertreatmentsystemsandcarryout
inspectiontoensurewaterqualitystandards(EPA,2014).Forthedrainfield,itdoesntrequirealotof
maintenance.Ifitexistsanaerationtubeitshouldbecheckedoutaboutonceortwiceayearsoitsno
waterinside.Theseptictankmustbeemptiedregularlysotherestofthesystemdoesntgetclogged,
thisalsoshouldhappenaboutonceayear.(Avloppsguiden,2015d)
TheseptictankhasaBOD5removalof97%,aNH3removalof60%,removesnitrogento22%,Coliform
bacteriasto99%andphosphorousto12%(UNEP,1998).
Miniature water treatment plant
Background
Aminiaturewatertreatmentplant(MWTP)isasthenameimpliesasmallscaletreatmentplantthatcan
beusedforasinglehousehold.Themainadvantageswithminiaturetreatmentplantsarethattheydo
notrequireaninterventionintheproperty,aresmallandeasytocontrolandconsistofproven
technology(Avloppsguiden,2015b).MWTPsareusuallyusedinverysensitiveareaswhereahighdegree
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ofphosphorusandnitrogenremovalisneeded.IncertainareasaMWTPasthewastewatersolutionisa
requirementinordertogetabuildingpermissionforasinglehouse.
Thedegreeofpurificationfromaminiaturewatertreatmentplantdiffersbetweenmanufacturersbut
shouldreducephosphorousby7090%,nitrogen2050%coliformbacteria60%andBOD78095%
(Naturvrdsverket,2007b). Itsusuallyonlyrecommendedtouseaminiwatertreatmentplantfor
permanenthousings,duetoitbeingconstructedforacontinuousflowofsewagewaterandsinceit
requiresweeklysupervision.
Therearehoweverwaterplants,theyoftenusemembranetechnology,thatcanreachatreatment
efficiencyforCODat96.6%andBOD5at99.7%.Moreovertheyreachnitrificationandnitrogenremoval
efficiency61.0%.Regardingtheoverallsuspendedsolidsandphosphorusremovalefficiencytheyreach
at
99.5%
and
98%
respectively
and
faecal
coliforms
are
removed
at
99.99%.
Construction
Theminiaturetreatmentplantconsistsofallthepartsanormalwastewaterplantwould,justina
smallerscale.Theplantisburiedinthegroundoutsidethehouseanddoesnormallynotleaveanodour.
TheexactdesigndiffersbetweenthedifferentmanufacturersbutaMWTPfromBioKubeispresented
inthefigurebelow.Wherepoint13isthesludgeseparation,5isthedegradationoforganicmaterial,6
isthenitrogenpurificationand7isthenitrogenandphosphorusremoval(IFwater,2015).
Figure2,aminiaturetreatmentplantfromBioKube(IFwater,2015)
Anothertypeofminiaturewaterplantismembranebioreactor.TheMBRtechnologyintegratesthe
biologicaldegradationofwastewaterpollutantswithmembranefiltration,ensuringtheeffective
removaloforganicandinorganiccontaminantsandbiologicalmaterialfromdomesticwastewater
(Pikorova,2012).
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Thesetypesofsmallscalewastewatertreatmentplantsusuallyusethreetanksandarecomprisedof
threetreatmentsteps;thepretreatment,theaerationandthefiltrationwiththeuseofmicrofiltration
membranes.Thesystemisconsistedofthreemaintanksandoptionaltheyarecombinedwithtwo
smallertanksforphosphoruselimination.Thesystemcanbeinstalledinbasementsusingthe
gravitationalforcetoreceivethewaterorinsurfaceroomsinwhichthewaterisledbyapump.Excess
sludgeisrecirculated.Thefilteredwatercanbereusedforirrigationorflushingtoilets.Inthiswaythe
consumptionofdrinkingwaterinahouseholdcanbereducedbyatleastonethird.Ifthiswaterisnot
intendedtobereused,itcanalsobedischargedinsensitiveareassinceitisharmless.
Treatment and operation
Thewastewaterisledtoaminiaturetreatmentplantburiedinthegroundoutsidethebuilding
(Naturvrdsverket,2007a).Thewastewaterisfirstsubjectedtoamechanicalpurificationintheformof
sludgeseparation.Biologicallydegradablecoarsematerialsuchasfecalmatterandtoiletpaperare
dissolvedandasieveisusedtoseparatethenondissolvingcomponentsfromthewastewater.
Then,apumppumpsthewaterfromwhichthecoarsematerialhasbeenseparatedandleadsittothe
aerationsection.Inthissectiontheorganicmatterinthewastewaterisdegradedbiologicallyby
microorganismsandatmosphericoxygenwhichisblownintothetankbycompressors.
Itsafterwardstreatedchemicallyand/orbymembranefiltration.Duringthechemicaltreatmenta
precipitantisaddedtothewastewatermakingphosphorousflockandsinktothebottom.Thetreated
waterisafterwardsledtoforexampleatrenchoranothercompartmentwithintheplant,for
remediationwherenitrogenandothercontagioussubstancesareremoved(Avloppsguiden,2015a).Its
notuncommonforasamplingwell,monitoringthewaterquality,tobeplacedintheoutflowofthe
plant.Someplantstreatthesewagewatercontinuouslywhileothersdoitinbatches,calledSBR
technology.ThetreatmentcycleinaSBRplantisdescribedinthefigurebelow.
IfthewateristreatedthroughMBR,itwillpassthroughamembranefilterinsteadofundergoa
chemicaltreatment.Usuallyultrafinefilterswitha0.4mporesizeareusedtoeliminatesuspended
materialaswellasbacteriaandgermsandfinallytotallyclear,odourless,hygienicallyharmlesswater
leavesthesystem.Hydraulicretentiontimesareabout420hourswhichisenoughtimeforthe
oxidationoforganicmaterialandnitrificationtotakeplace(Bernaletal,2002).
Thephosphoruseliminationisaccomplishedbyadsorptionongranulesinsidethetwomodules.Since
thistechnologyconsistsofbiologicaltreatmentandmembranefiltrationinoneprocess,noadditional
treatmentprocesseslikesandfiltersorotherclarifiersareneeded.Moreover,duetooldersludgeage
andhighermixedliquorsuspendedsolids,MBRsproducelesssludgeandalsocantreatahighercapacity
ofwastewaterinthesamefootprintasacommonwastewatertreatmentplant(Bernaletal,2002).In
conventionalsystems1kgCODwillresultinabout0,3 0,4kgofbiomass.WithMBRsystems1kgCODis
convertedto0 0,2kgbiomass.
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Cost
Miniaturetreatmentplantsarerelativelyexpensivetooperate(about30006500SEK/year)duetothe
consumptionofelectricityandchemicalcomponents.Componentsforthebiologicalandchemical
purificationhavetobeaddedafewtimesduringtheyear.Thecostforatreatmentplantwith
installationisabout100000SEK.
MBRwastewatertreatmentplantsforsinglehousesarecurrentlyavailableforapproximately25000
77000SEKperunit.Moreovertheinstallationcost dependingonunitsizeandshape isabout25000
SEK.Powercostsareabout300SEKperyearbecausethetransmembranepressurehastobemaintained
ataspecificlevelandalsothecostsforaerationarehigh,especiallyathighersludgeconcentrations
(Mulleretal.).heannualcostofdesludgeandmaintenanceisaroundat10000SEK.Thisrangeofcosts
maketheMBRtechnologysystemsoneofthemoreexpensiveforsinglehouseholduse.
Maintenance
Theplantsrequiresupervisionbytheusersonceaweekwherethechemicalcomponentshavetobe
refilledandthemechanicalfilterhastobecheckedandperhapscleaned.Itrequiresservicebya
technician14times/yearandsludgeremoval12times/year(Avloppsguiden,2015b).
ForaMBRmaintenancealsoincludescleaningofmembraneelementswhenthetransmembrane
pressurerisestoexcess(Bernaletal,2002).Suchapressureincreasecanbecausedwhencontaminants
clogtheporesofthemembranesurface.Almostallcleaningproceduresareperformedautomatically.In
combinationwiththeaerationforthebiomass,aircanbeinjectedatthemembranebottominorderto
preventfoulingandcloggingofthemembranessurfaces.Acombinationofbackflushing,relaxationand
chemicalcleaningisalsoemployedtomaintainthepermeabilityofthemembranes.Manysystemsare
designedwithapressurizedbackflushsystemthatremovesthebiofilmfromthemembranesurfaceby
reversingthepermeateflowandpumpingitbackintothemembranes.Chemicalcleaningofthe
membranesismoreeffectiveandimprovesthefiltrationperformanceoftheelements.Chemical
cleaningconsistsofreverseflowofachemicalsolutionthroughthemembranesforovertwohours
(Bernaletal,2002).Chemicalslikesodiumhypochlorite,oxalicacid,citricacidandmorecanbeused.
Constructed Wetlands
Background
Constructedwetlandsarenaturalwastewatertreatmentsystemsandcanbeusedasasecondary
treatmentfacilitywhichcanbeusedinmediumsizedcommunitiesespeciallyindevelopingcountries
(wsp,2008).Thesewetlandscanbeusedtotreathouseholdwastewatersuchasblackwaterorgrey
water.Avarietyofcomplexbiological,physicalandchemicalmechanismsareusedintreatingthewater
andimprovingitsquality.Theremovalefficiencyoftheconstructedwetlandsismainlydependenton
thefiltermaterialandthehydraulicsurfaceloadingrate.
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Construction
Constructedwetlandsareconstructedbyexcavation,backfilling,gradingandinstallingwatercontrol
structurestoensuredesiredhydraulicflowpatterns(wsp,2008).Therearethreemaintypesof
constructedwetlands.Theyare:freesurfaceconstructedwetlands,horizontalflowconstructed
wetlandsandverticalflowconstructedwetlands.Amongthesethreedifferenttypesofconstructed
wetlands,theverticalflowconstructedwetlandisthemostefficientandrequirestheleastamountof
areaforconstruction,almost1to3squaremetersperperson.Usuallythereisaseptictank(anaerobic
digestion)oranaerobictreatmentunit(aerobicdigestion)forpretreatmentinordertoreduceorganic
matterandsuspendedsolids.Eachfiltershouldhaveanimpermeablelinerandacollectionsystemfor
theeffluent.
Treatment and operation
Therearethreemaincomponentsintheconstructedwetlands.Firstly,theimpermeablelayerthatis
usuallymadeupofclay(EPA,2015).Thispreventstheinfiltrationofwastewaterintothelowerground
aquifers.Secondly,thegravellayerwhichprovidesnutrientsandsupportfortherootzone.Thegravel
layerandrootzoneiswherewaterflowsandbioremediationanddenitrificationtakeplace.Constructed
wetlandstakeupnitrate/nitriteintheplantsorchangeittogaseousformsofnitrogen.Removalof
phosphorousislimitedbyhowmuchcanbetakenupbysoilandthesmallamountthattheplantsand
livingorganismsuseforgrowth.Themorephosphorousneededtoberemoved,thelargerareais
needed.Thefinalvegetativelayerthatisabovegroundcontainsplantmaterial.Inthewetland,both
aerobicandanaerobicprocessesexistatthesametime.Theanaerobiccellutilisesnaturalanaerobes
andplantstodegradethecontaminants.Atthesametime,theaerobiccellimproveswaterqualitywhen
plantsareexposedtothemovementofwaterbetweencellcompartments.Fortheremovalofmetals,
straw,manureorcompostcanbeusedinthesewetlands.Phytoremediationisthemainprocessusedin
constructedwetlandsasplantsareusedtobreakdownthecontaminants.
Cost
Thecapitalcostoftheconstructedwetlandisusuallylowandislessexpensivetobuildthantraditional
watertreatmentplants(wsp,2008).InthecaseofCentralAmerica,capitalcostofasubsurface
horizontalflowconstructedwetlandusuallyrangesfromUS$50to$100perpersonservedbythe
systemthatincludespretreatment.
Maintenance
Firstlytheseptictankhastobepumpedeverythreetofiveyearsforsludgeremoval(wsp,2008).During
thefirstdaysofoperationoftheplant,weedremovalisimportantasitcompeteswiththeplanted
vegetation.Distributionpipeshavealsotobecleanedonceayearforsludgeandbiofilmremoval.
Restingintervalswillalsohelpgravelnottoclog,otherwiseifthisisnoteffective,thecloggedpartsof
thefiltermaterialhavetobereplaced.Maintenanceactivitieshavealsogottoensurethatprimary
treatmentiseffectiveatreducingsolidsconcentrationofthewastewaterbeforeitentersthewetland.
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Table2,theremovalcapacityofconstructedwetlands(wsp,2008).
Parameter Influent Effluent Removal(%)
mg/l g/(m2 d) mg/l g/(m2 d)Suspended
solids
98.6 5.22 13.6 1.06 86
BOD5 97 4.8 13.1 0.89 86Total
nitrogen
28.5 1.15 18 0.78 37
Totalphosphorus
8.6 0.33 6.3 0.26 27
Case study
Location
ThelocationchosenforthecasestudyisafarmcalledKarleboatthecountrysideinstergtlands
CountyinthesoutheastofSweden,about170kmsouthofStockholm.Theagricultureatthefarm
consistsbothofbigfieldsaswellasanimalfarming.
Figure3,locationofthehouseinthecasestudy(eniro,2015).
Itsapermanentresidenceofabigfamilyandthenearestneighborlivesapproximate200metersaway.
ThemainhouseislocateduphillfromalakecalledSvinstadssjnwithahorizontaldistanceofabout200
m. Thelakeisabout1.85km2andtherearealargenumberofsummercottagesandvacationhomesin
theareaaroundthislake(Linkping,2004). Theresidencesinthesearetohigherdegreestartingtolive
inthemonapermanentbasis,somethingthattheirsewagesystemsarentreallydimensionedfor. The
lakeSvinstadsjnsuffers,likemanyotherlakesinthearea,fromproblemswitheutrophicationwhich
makethequestionofagoodwastewatertreatmentprocessevenmoreimportant.Thetotal
phosphorouscontenthavebeenupto210g/lwhichisconsideredveryhigh,andtheSecchidepthis
oftenlessthan0,5m.
Geology
Thegeologyofthelocationconsistsmostlyofclay(YellowinfigureX).Butrightatthelocationofthe
houseisaratherlargeareaoftill(blue)andalsosomebedrock(red)
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Figure4,geologyattheplaceofinterest.Itsalsopossibletoapproximatethedistancetothelake(upperright
corner).Yellow=Clay,Red=Bedrock,Blue=Moraine(sgu,2015)
ThegroundwaterlevelattheareaofthefarmcanbeobtainedfromtheSwedishdirectoryofwells
(brunnsarkivet).Attheareaofthehousethereare3differentwellsandthegroundwatertableisabout
4to5metersundergroundlevel.
Protection level
TheCountyofLinkpingdecideswhatkindofprotectionlevelthatisappliedforeachhousehold
(Linkping,2014).Therequirementsforhighprotectionlevelareanyofthefollowing:
Ifthehouseiswithin500metersfromopenwater.
Ifthehouseisinawaterprotectionarea.
Ifthehouseiswithinanareathatneedtobeprotectedofanotherreason.
Ifthehouseholdmeetstherequirementsofhighprotectionleveltherearespecialrulesabouthow
muchpollutionthewastewatertreatmentplantcanrelease(Naturvrdsverket,2006).
.Theselimitationsare:
Thewastewatertreatmentshouldachieveatleast90percentreductionoforganicmaterial(measured
asBOD7)
Thewastewatertreatmentshouldachieveatleast90percentreductionofphosphorus(totP)and 50
percentreductionofnitrogen(totN)
Thewastewatertreatmentenablesrecyclingofnutrients
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Discussion and conclusion
Themainfactortoconsiderwhenchoosingatreatmentmethodintheareaofourcasestudyisthatthenitrogenandphosphorousremovalishighanddependable.Thecost,maintenanceandeverythingelse
areallsecondaryfactorsinthiscase.Outofthefourmethodspresentedwewouldthereforeonlybe
abletouseaminiaturewaterplantoramembranebioreactor.
ThetoughregulationsinLinkpingscountymakeitimpossibleoratleastverydifficulttouseanyother
methodthanthewastewaterminiatureplant.InSwedeningeneralweseethattheregulationson
phosphorousandnitrogendischargearegettingharderandharder,andofcoursethisreducethe
numberofmethodssuitableforthearea.Inthespecificstudyareathereisalakelessthan500meter
awaythatalreadyhaveproblemwitheutrophication.Tonotgetthelakeinevenworseconditionits
importantthattheseregulationsarefollowedandtheformerwastewatertreatmentisprobablyoneof
thebiggestsourcesfortheproblem.Inalesssensitiveenvironmenttheseptictankofcoursewouldbea
preferablemethodfortheusersinceitrequiresbothlessworkwiththemaintenanceandalsoless
yearlycostfortheuser.Inthiscasethough,theenvironmentaladvantagesofmoreremovalofnitrogen
andphosphorousiswinning.
Theseptictankishighlyadaptableanditpresentvariousadvantagessuchasloweconomicalcostasthe
constructionandmaintenancecostassociatedwithitislower.Also,theremovalefficiencyofBODand
fecalcoliformsisveryhigh.However,theseptictankhasalimitationandthatisthelowpercentage
removalofnutrients,withalow12percentremovalofphosphorusand22percentremovalofnitrogen.
Inthecontextofourcasestudy,thecountryofLinkopinghasverystrictregulationtowardstheeffluent
anditscontents,wherebytheeffluenttreatwatershouldhaveachieveatleasta90percentreductionin
phosphorusand50percentreductioninnitrogen.Thisistominimisetheimpactsofeutrophicationdue
tothedischarge.Astheeffluentoftheseptictankdonotmeetthelegislativerequirementsofourcase
study,itisnotthemostoptimalsolution.
Similarly,bytakingtheeffluentqualityandthenitrogenandphosphoruscontentintoconsideration,
constructedwetlandsmightnotbethemostoptimalsolutiontoo.Constructedwetlandsarerelatively
inexpensivetobuildandeasytomaintain.However,theeffluentquality,thoughhighandBODandCOD
removal,thenutrientslevelstilldonotmeettherequirementsinthecountryofLinkping.Fromhere,
wecananalysethatthesesolutionsareeffectiveintreatingwaterqualityandcostfriendly.However,
whenweneedtoconsiderthecontextofthelocalmunicipalitywhichincludesthetypesofregulations,
costeffectivemethodsmaynotbethebestafterall.
Afterevaluatingthedifferentwastewatertreatmentmethodsandthecasestudy,wehavechosenthe
miniaturewatertreatmentplantasourchoiceofsolutiontosmallscalewastewatertreatmentinsingle
houses.
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