Abstract:
Thepurposeofthisstudywastoevaluatethecharacteristicsofrawfecalsludgeemptiedfrom differenttypesofonsitesanitationfacilities,andtoassessitseffectontheoperationofFSTPs. RawFSsamplesemptiedfromdifferenttypesofcontainmentswerecollectedfromDhakaand Sakhipur (Tangail), andwere analyzed for a wide range ofparameters, including pH, EC, TS, TDS, TSS, COD, NH4+, NO3-and PO43-. The raw FS samples were classified into four broad categories:(a)SakhipurPublicSepticTanks;(b)Sakhipurhouseholdpits;(c)Dhakahousehold septic tanks; and (d) Dhaka public septic tank.
ThemeanpHvaluesoftherawfecalsludgesampleswereintheneutralrange,varyingfrom
6.84 to 7.46. The EC values of the FS samples ranged from 2.68 mS/cm to 4.27 mS/cm, with no clear correlation with TDS values, possibly due to the presence of non-ionized dissolved solidsinFS.TheTSSconcentrationofFSsamplesfromhouseholdpitsinSakhipurwerealso found to be significantly higher, possibly due to the leaching of water from pits. The mean COD and TSS values were 2,405 mg/l and 4,220 mg/l, respectively. The ammonium (mean 244mg/l)andphosphate(mean170mg/l)concentrationsindicatethatFScouldbeanexcellent sourceofnutrientforuseinagriculture.NocleartrendwasobservedbetweenthesourceofFS andtheircharacteristics.However,theCODvaluesofFSsamplescollectedfrompublicseptic tanksandpitswerefoundtobesignificantlyhigherthanthosecollectedfromcontainmentsin Dhaka. Compared to the reported raw FS characteristics from African regions (where dry toilets and use of toilet papers are common), the raw FS samples collected in this study are muchmoredilute.BecauseoflowerTSScontentsofrawFS,theestimatedsolidsloadingrates fortheunplanteddryingbedsinSakhipurFSTPhavebeenfoundtobesignificantlylowerthan those typically used in the design of unplanted drying beds. It appears that introduction of settling-thickening tanks, as a pre-treatment unit, could significantly improve treatment efficiency and reduce the land requirements for FSTPs employing drying beds.
Dried FS samples produced in unplanted drying beds were collected from Sakhipur and Faridpur FSTP, and were analyzed for moisture content, nutrient and heavy metal contents. It wasfoundthatsignificantmoisturecontentreductioncouldbeachievedthroughthestorageof driedfecalsludge.Theammoniumnitrogencontent(mean215mg/kg)andphosphatecontent (117 mg/kg) suggest that resource recovery from dried FS would be beneficial. Heavy metal (Pb,Cd,Cr,Cu,Ni,Zn)contentsofthedriedFSsamplesweresignificantlylowerthanthe
correspondingBangladeshstandardsforsludge.ThemeancalorificvaluesofdriedFSsamples varied over a relatively narrow range of 8.7 to 13.4 MJ/kg. The dried FS samples from public septictanksappearstohaveaslightlyhighercalorificvaluesthanthosefromhouseholdseptic tanks and pits. Calorific values of dried FS calculated from elemental composition using the modified Dulong formula appear to provide results closer to the experimental values. The relatively lower calorific values of dried FS appear to be a constraint for energy recovery throughcombustion.ConsideringthepropertiesofFS,co-composting,whichiswidelyusedin Bangladesh and worldwide, is an excellent option for the recovery of resources from FS.
Since the characteristics of FS varies significantly due to a range of factors, including type of containment, environmental conditions (e.g., infiltration capacity of soil), usage (e.g., use/volumeofflushwater)andemptingmethods,moreextensivestudiesshouldbecarriedout to assess the characteristics of raw FS and its implication on the design/ operation of FSTPs and quality of end products (e.g., compost or solid fuel).