Effect of fruit and vegetable waste on anaerobic digestion using faecal sludge as primary feedstock

Working, basic sanitation in humanitarian settings is still a challenge that is only getting worse with increasing humanitarian migrant population and ongoing threats of war, conflicts and global warming. Robust and cost-effective solutions to prevent sickness and death due to poor human waste management are needed. A treatment line for pit latrine sludge consisting of anaerobic digestion in combination with a pasteurization of the digestate has been suggested. This report evaluates the usage of fruit and vegetable waste o increase and stabilize biogas production during the anaerobic digestion of faecal sludge in Kisumu, Kenya. A tubular digester with a working volume of 22.5 m3 was operated at an hydraulic retention time of approximately 30 d. The faecal sludge (FS) was screened for trash and diluted to an approximate total solids (TS) concentration of 5 %. On average and before dilution, the FS used had a TS of 8.54 %, a volatile solids (VS) content of 5.89 % and a chemical oxygen demand (COD) concentration of 14.54 kg/m3. After 3 months of operations, the team started to include fruit and vegetable waste (FVW) in the daily feeding. The amount of FVW to feed was calculated based on COD calculations for the FS available on that day. The goal was to reach a stable COD input to evaluate if that would produce a steady biogas production. The COD in the sludge was the response variable in a linear regression with R2 = 0.33 using the sludge’s TS as explanatory variable. In a spreadsheet accessible through the operator’s smartphone, the operators would have to input the amount of FS used and their guess of the FS solids content by looking at the texture and color of the sludge. The output of the spreadsheet was the amount of FVW to be added. The biogas production before the addition of FVW was always under 3 m3/d and after the addition of FVW it was on average 4.8 m3/d. The biogas production per kg of COD added into the treatment line was 0.31 m3biogas/kgCOD, but the production did not stabilize with the addition of FVW. Thisreport evaluates the anaerobic digestion performance and makes suggestions for future experiments and implementation of the technology in humanitarian settings.