Energy recovery from solid wastes in pit latrine faecal sludge

Abstract

Pit latrines in slums have faecal sludge mixed with solid waste. These solid wastes in pit latrines are incompatible with faecal sludge treatment as they can cause blockage at treatment plants, and neither can they be disposed off at landfills as they are hazardous and leachate from such waste can contaminate water sources. This creates a problem of limited options available to manage solid wastes from pit latrines, which results in a need to co-manage the faecal sludge together with the solid waste in pit latrines. This study was aimed at investigating the effect of solid wastes in pit latrines on the method of energy recovery from faecal sludge. Faecal sludge and solid wastes from pit latrines were considered for this study. The effect of solid waste on the method of energy recover was assessed by the determination of the solid waste composition and characteristics of faecal sludge mixed with the solid waste. The choice of incineration method was also determined with regard to the characteristics obtained. A total of 7 pit latrines were emptied and sampled. The waste from the 7 pit latrines was used to determine the solid waste composition and characteristics of the fresh faecal sludge.

The study revealed that the solid waste composition in pit latrines consisted of ; 5.1 ± 3.8% organics, 9.1 ± 7.5% polyethene, 11.2 ± 8.3% textile, 1.6 ± 0.4% plastic, 1.1 ± 1.6% glass, 9.2 ± 8% sanitary towels, 0.2 ± 0.2% rubber, 0.3 ± 0.4% metals, 15.9 ± 16.8% paper, 24.9 ± 25.8% rubble and 21.3 ± 21.3% others.The solid waste composition showed that the average percentage of total combustible solid waste was 52.3% of the total solid waste which indicated that about half of the solid waste from the pit latrines was combustible. The characteristics of the fresh faecal sludge were; 87.6 ± 3.2% Moisture Content, 35.7 ± 13.5%TS Ash Content, 64.3 ± 13.5 % TS Total Volatile Solids, 11508 ± 709 mg/L COD and 0.17 ± 0.04 COD/TVS ratio.The energy characteristics of dried faecal sludge alone and faecal sludge mixed with solid waste were determined based on samples from 3 pit latrines. The mean results were; 35.8% Moisture Content, 59.4 %TS Total Volatile Solids, 2720 J/KgoC Heat capacity and 15.0 MJ/Kg Calorific value for dried faecal sludge alone while for the mixture of solid waste and faecal sludge, the mean results were; 46.4% Moisture Content, 59.3% TS Total Volatile Solids, 4333 J/KgoC Heat capacity and 29.1 MJ/kg Calorific value.

There were correlations between the energy characteristics (calorific value and heat capacity) of the faecal sludge and the total mass of solid waste in the pit. The study showed that solid wastes from the pit latrine had an effect on the energy characteristics of the faecal sludge and hence influenced the choice of incineration option for the different mixtures of faecal sludge and solid waste.