Assessment of characteristics of fecal sludge from different sources and its effect on dried sludge

Abstract

The purpose of this study was to evaluate the characteristics of raw fecal sludge emptied from different types of onsite sanitation facilities, and to assess its effect on the operation of FSTPs. Raw FS samples emptied from different types of containments were collected from Dhaka and Sakhipur (Tangail), and were analyzed for a wide range of parameters, including pH, EC, TS, TDS, TSS, COD, NH4+, NO3- and PO43-. The raw FS samples were classified into four broad categories: (a) Sakhipur Public Septic Tanks; (b) Sakhipur household pits; (c) Dhaka household septic tanks; and (d) Dhaka public septic tank. The mean pH values of the raw fecal sludge samples were in the neutral range, varying from 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 solids in FS. The TSS concentration of FS samples from household pits in Sakhipur were also 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 244 mg/l) and phosphate (mean 170 mg/l) concentrations indicate that FS could be an excellent source of nutrient for use in agriculture. No clear trend was observed between the source of FS and their characteristics. However, the COD values of FS samples collected from public septic tanks and pits were found to be significantly higher than those collected from containments in 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 much more dilute. Because of lower TSS contents of raw FS, the estimated solids loading rates for the unplanted drying beds in Sakhipur FSTP have been found to be significantly lower than 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 was found that significant moisture content reduction could be achieved through the storage of dried fecal sludge. The ammonium nitrogen content (mean 215 mg/kg) and phosphate content (117 mg/kg) suggest that resource recovery from dried FS would be beneficial. Heavy metal (Pb, Cd, Cr, Cu, Ni, Zn) contents of the dried FS samples were significantly lower than the corresponding Bangladesh standards for sludge. The mean calorific values of dried FS samples varied over a relatively narrow range of 8.7 to 13.4 MJ/kg. The dried FS samples from public septic tanks appears to have a slightly higher calorific values than those from household septic 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 through combustion. Considering the properties of FS, co-composting, which is widely used in 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/volume of flush water) and empting methods, more extensive studies should be carried out 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).