Calorific characterization of faecal sludge and its use as fuel: A case study of Kampala

Abstract

Using faecal sludge (FS) as a fuel to provide energy for industry could help to solve both the sanitation challenge and reliance on unsustainable or expensive fossil fuels. This research aimed at testing the viability of using FS as a solid fuel – an end use that could unlock an environmentally and financially beneficial replacement for costly, disposal-oriented FS management solutions. FS is defined as sludge collected from onsite sanitation systems, and has highly variable characteristics, including the solids concentration. To determine the average calorific value of FS, and how it varies with source and age, samples were collected the different divisions in Kampala, Uganda. Samples were tested for calorific value, total solids, total volatile solids, moisture content, ash content, biochemical oxygen demand (BOD) and chemical oxygen demand (COD).FS samples were collected from unlined, partially lined and fully linedpit latrines,septic tanks, and drying beds fed with FS from these systems. On average, calorific value of FS was 17.2 MJ/kg dry solids (DS), which compares well with other commonly used fuels such as rice husks at 15.6 MJ/kg DS, forest residues at 19.5 MJ/kg DS, coffee husks at 19.8 MJ/kg DS, and sawdust at 20.9 MJ/kg DS. FS age and its source did not affect the calorific value. The total solids content of FS depended on its source. The Total Solids (TS) of FS from unlined pit latrines was 6% of wet weight, which was higher than that in lined pit latrines at 2.7% of wet weight and septic tanks at 1% of wet weight. This is attributed to the water in the unlined pit latrine sludge draining into the soil. For industries to derive net energy from FS at 17.2 MJ/kg DS, the sludge must be dried to ≥ 27% dry solids. Any increase in dry solids above 27% increases the energy requirement to dry the FS, even though it also increases the benefit to the end user. The TS of FS from drying beds in Kampala was above 30% of wet weight after two weeks, indicating that additional energy for drying the FS can be harnessed with minimal recurring costs assuming land is available. More research should be done on cheaper drying technologies for the FS. This case study shows that FS can be sustainably used as a fuel in industrial processes in Kampala.