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Water AuthorityCayman SuppliersoftheWorldsMostPopularDrink
technical guide 1: wastewater basics
PO Box 1104 Grand Cayman KY1-1102 Cayman Islands
Phone: 345-949-2837 (94-WATER) Fax: 345-949-0094 E-mail: development email@example.com
The Water Authority was established in 1983 as a statutory body responsible for sup-plying wholesome and affordable drinking water to the people of the Cayman Islands. The Water Authority is also responsible for the proper treatment of wastewater and for the protection of the fresh groundwater lenses that exist throughout our islands.
Wastewater,orsewage, iswater thathas been used, as forwashing, flushing,orinamanufacturingprocess,andsocontainswasteproducts.
Domes c wastewater is generated inhomesthroughac vi es inkitchens,bath-rooms,laundry,foodprepara onandcleanup. Commercialwastewater is generatedinfacili essuchasoffice,retail,restaurant,hotel as well as ins tu ons such as achurch,school,orhospital.Thewastewaterisgenerated from thesame typeofac vi-esasdomes cwastewater,but indiffer-
entpropor on.Due to thewidevariety inflow (volume and rate) and strength ofcommercial wastewater, it can be moredifficult to treat. For these reasons, someare required to pretreat theirwastewaterbeforedischarge into anonsiteor centralwastewater treatment system. Materialsthatareflammable,explosiveor toxicduetotheirnatureorconcentra oncannotbedischarged to a wastewater treatmentsystem.
WASTEWATER CONTENT Wastewater ismorethan99%water,withless than1%dissolvedandsuspendedsol-idsmadeupoforganic(containcarbonandareofanimalorvegetableorigin)andinorganic (do not contain carbon and are ofmineralorigin)compounds.
Total Suspended Solids (TSS): organicorinorganic solids, that either float on thesurfaceoraresuspended inwater.TSSaremeasured in a lab by filtering awastewatersampletodeterminethemass(quan tyofma er)ofTSSperlitreofsam-ple, measured in mg/L (milligrams perLitre,which is equivalent to ppm or partspermillion).TheaverageTSSconcentra onofuntreateddomes cwastewaterinfluent(water or wastewater that flows into atank) is 250 mg/L; Water Authority dis-charge limitfortreatedeffluent (waterorwastewater thatflowsoutofa tank) is30mg/LTSS.
Biochemical Oxygen Demand (BOD5): ameasureof theorganicstrengthofwastesin water. BOD5 is the amount of oxygenthatmicroorganisms consume infivedayswhile decomposing the organic ma ercontained in a sample. The averageBOD5concentra on of untreated domes cwastewater influent is 250mg/L; WaterAuthoritydischarge limit for treatedefflu-entis30mg/LBOD5.
Pathogens: diseasecausing organisms.Pathogens include bacteria, viruses andparasites. Not all bacteria inwastewatercausedisease;manyarebeneficialanddotheworkofstabilisingorganicwastes.
Nutrients: substances required to supportliving plants and organisms. Wastewatercontains nitrogen and phosphorus whichare difficult to remove by conven onaltreatment processes. The average TotalNitrogen (TN) concentra on in untreateddomes cwastewateris20mg/L.andTotalPhosphorus (TP) is5mg/L.TheWaterAu-thority has not yet established dischargelimitsfornutrientsintreatedeffluent.
NEED FOR TREATMENT
It is important to remember thatwater isneverdisposed,itisonlyrecycled.
The goal of wastewater treatment is toreduce or remove organic ma er, solids,disease-causingorganismsandnutrientstolevels that protect public health and theenvironment.Publichealth isprotectedbyensuring that wastewater is collected,treatedanddischarged inahygienicman-ner. Treatment of wastewater should bedesigned to ensure that the volume andstrengthoftreatedeffluentdoesnotnega-vely affect the quality of the receiving
An overview of wastewater: where it comes from, whats in it, why, and how its treated.
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PRIMARY TREATMENTPrimarytreatmentisaphysicaltreatmentprocessthatreliesongravitytoseparatesolidfromliquidma er.
Primary treatment tanksare referred toasa sep c tank ifusedalone,orasapre-tankortrashtankifusedaheadofasecondarytreatmentsystem.Influententersthetank;thesizeand shape of the tank slows thewastewater flow to allow heavier solids to se le to thebo om(sludgelayer)andlightermaterialssuchasgrease,plas candhair,tofloattothetop(scumlayer).Betweenthesetwolayersisthemidwaterthatcontainsdissolvedandsuspend-edsolids.Aprimary treatment tankofadequatesizeandmaintenance (periodicpumpout)canachieveremovalratesof25%ofBOD5and70%ofTSS.Toachieveeffluentlimitsof30mg/LBODand30mg/LTSSrequiresanaddi onalstep:secondarytreatment.
An overview of wastewater treatment. technical guide 1: wastewater basics
SECONDARY TREATMENT Secondarytreatment isanaerobic (requiresoxygen)treatmentprocesswheremicroorganisms (verysmallorganismsthatcanbeseenonlythroughamicroscope;mostlybacteria,alsoprotozoa,flagellatesandro fers)usethedissolvedwastesforfood,andintheprocessformabiomass(amassmadeupofmicroorganismsfeedingonthewastes,deadmicroorganismsandfinesolids).Biomassbuildsupasafilmonfixedmediaand/orclumpsthataresuspendedinthemixingzone.Inthese lingzone,biomassse lesandtreatedeffluentisdischargedfromjustbelowthesurface(topreventanyremainingfloa ngsolidsfromdischarging).
AerobicTreatmentUnits(ATUs):arepre-engineered,cer fiedsecondarytreatmentsystems.Thereareavarietyoftechnologicaldesigns,butallprovidemechanismstocarryoutthethreefunc onsessen altosecondarytreatment:
Aera on:tocirculateairthroughthewastewatertoprovideoxygentokeeptheaerobicmicroorganismsalive. Mixing:toensureadequatecontactbetweenwastewaterandmicroorganisms. Se ling:toallowthedevelopedbiomasstoseparatefromthetreatedeffluent.
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Additional references and resources can be found on Water Authoritys web site: www.waterauthority.ky