With the increasing demand for renewable energy and environmental protection, biogas technology has attracted considerable attention around the world. Fecal sludge (FS) is rich in organic matter, and it contains high concentrations of excreted pathogens that cause gastro-intestinal infection. In Tanzania, fecal sludge management from on-site sanitation systems poses a threat on environmental safety. This study aimed to assess the feasibility of the use of anaerobic digestion (AD) for the treatment of FS and the production of biogas as renewable energy to achieve multiple benefits in Tanzania. For the experiments, FS and food waste (FW) were used as feedstock, and rice straw-derived biochar (RSB) was added as an additive to improve biogas production. The mesophilic anaerobic digestion resulted in a methane yield of 287.5 ml/g VS for FS + FW co-digestion and 396 ml/g VS for FS + FW + RSB co-digestion. At ambient temperature (20–26°C), the system produced a methane yield of 234 ml/g VS for FS + FW co-digestion and 275 ml/g VS for FS + FW + RSB co-digestion. Three different scenarios (digester with volumes of 4, 100, and 400 m3, respectively) and strategies for FS treatment by AD in Tanzania were proposed and analyzed. These treatments can produce methane volumes of 1.95, 49.5, and 199.5 m3 with pay-back periods of 3, 5, and 15 years and net present values of + 28, +1,337, and +52,351 USD, respectively. The calculations also showed that the heat value from the produced biogas and energy needed to heat the digester at 26–37°C resulted in energy balance values of + 0.012, + 0.53, and + 2.22 GJ/day for the 4, 100, and 400 m3 digester volumes, respectively.

The biochar addition to enhance anaerobic co-digestion (AD) of fecal sludge and food waste for methane production was investigated with a focus on biochar dosage and particle size of biochar. Optimal dosage range of biochar was determined as 5 - 20 g/l working volume based on lab-scale batch AD. Effects of biochar with different particle sizes at optimal dosage of 15 g/l were evaluated in a semi-continuous AD experiment, results of which showed that all the examined biochar’s with different particle sizes 0.075mm, 0.15mm and 0.45mm substantially enhanced the average methane yields (270ml/gVS, 241 ml/gVS and 222ml/gVS) compared to control digesters less than 200ml/gVS. The core reason for this phenomenon was that the biochar’s with different particle sizes had different properties i.e density, surface area and pore size and that the floating of large particle size of biochar was not conducive to microbial growth. Methane (CH4) in the biochar and control treatments were significantly different in all experiments. The results show that biochar added to digesters can significantly reduce H2S production with rising CH4 production.