Investigation of biochemical methane potential of organic feedstocks, wastewater sludge, and faecal sludge to improve the digestibility of wastewater and faecal sludge through co-digestion

When compared to the Global North, the recovery of renewable energy from wastewater sludge and other organic waste is uncommon in the Global South. Organic waste is frequently regarded as waste rather than a resource and is frequently discarded inappropriately. As a result, the quality of water sources and the environment, in general, has deteriorated. A study was undertaken at the Nakivubo Wastewater Treatment Plant (WWTP), Kampala, Uganda, in which three organic wastes; food waste (FW), market waste, and slaughterhouse waste (SHW), together with wastewater sludge (WWS), and faecal sludge (FS), were characterized, mono digested and co-digested in varying mixing ratios. The biochemical methane potential (BMP) experiment was carried out on a laboratory scale using the manometric method at mesophilic temperatures (35 ± 0.25 ˚C), and the pH was controlled with the optimum range of methanogenic activities (7.0 to 8.5). The main objective was to assess the biodegradability of organic waste such as food waste, market waste, slaughterhouse waste, wastewater, and faecal sludge in Kampala, Uganda, with the aim of improving the anaerobic degradation of wastewater and faecal sludge generated from Nakivubo WWTP, and Lubigi Faecal Sludge Treatment Plant (FSTP), respectively through co-digestion. The result for anaerobic mono digestion of organic wastes for co-digestion with WWS, and FS indicated that food waste (FW) had the highest BMP of 294 ±3.5 mL CH4/g VS, followed by market waste with a BMP of 285±4.8 mL CH4/g VS, and slaughterhouse waste (SHW) had the lowest BMP of 165±2.8 mL CH4/g VS. Whereas mono digestion of wastewater sludge (WWS), and faecal sludge (FS), produced the BMP of 152±0.1, and 8±0.2 mL CH4/g VS, respectively. This implies that the anaerobic biodegradability of WWS and FS is poor as indicated by low methane yield. However, co-digestion of WWS and FS with other organic waste of high methane yields such as FW and MW would substantially improve the biodegradability of WWS, and FS as indicated by results obtained from anaerobic co-digestion of the mixture of organic feedstocks (MOF) with WWS and FS, which suggested that when 15 and 20% of MOF were mixed and co-digested with WWS, the BMP of WWS increased from 152±0.1mL CH4/g VS to 255±0.2 and 149±5.2 mL CH4/g VS, respectively. Whereas when 15% of MOF was co-digested with FS, the specific methane yield of FS increased from 8±0.2 mL CH4/g VS to 100±0.5 mL CH4/g VS. Furthermore, when WWS was co-digested with FS using the mixing ratios of 50/50% (g/g), and 60/40% (g/g) of WWS/FS, the specific methane yield was also very low i.e. 51±7.1, and 88±0.9 mL CH4/g VS, respectively for the mixing ratios of 50/50% (g/g), and 60/40% (g/g) of WWS/FS. The low specific methane yield obtained from mono and co-digestion of FS with other co-feedstocks indicates that FS may not be a good feedstock for anaerobic digestion. The low specific methane yield could be attributed to the low biodegradability of FS resulting from the loss of volatile matter at the pre-treatment stages such as partial anaerobic degradation in the containment systems, and stabilisation in the sedimentation tanks where solid-liquid separation occurs and a fraction of volatile matter is carried in the effluent to the primary and secondary effluent treatment units. Therefore, this research recommends that future research on the anaerobic biodegradation of FS should focus on the use of fresh faecal sludge with reduced loss of volatile matter