Assessing anaerobic co-digestion of faecal sludge with sugarcane press mud and water hyacinth for biogas and biofertilizer production

Abstract

In Uganda especially peri-urban areas, faecal sludge (FS) and agro-industry wastes are abundant, and their poor management poses environmental and public health risks. In a bid to find safe disposal and resource recovery, this study evaluated the potential of anaerobic co-digestion of FS with sugarcane press mud (SP) and water hyacinth (WH) for biogas production. It is postulated that co-digestion with carbon rich substrates (water hyacinth and press mud), could enhance digestibility, stabilisation of bio-solids and improve methane yield higher than from faecal sludge alone. The experiment involved characterization of the of feedstock and residual solids, determination of biogas yields, methane content, and eventual pathogen reduction. Moisture content, pH, total solids, volatile solids, ash content, nitrogen, phosphorus, potassium, carbon, sodium, calcium, total coliforms, Escherichiacoli (E. coli), Salmonella, chemical oxygen demand, ammonium-nitrogen, lead and chromium were measured. Five experimental setups were ran for 51 days and included; FS (100%), SP (100%), WH (100%), FS+WH (1:1), and FS+SP (1:1) in 30L bio-digesters each with 20L of feedstock and 5L of inoculum (activated faecal sludge), with 5L left for gas accumulation. Biogas production was monitored using 20L gas collection bags, with cumulative and specific yields calculated based on volatile solids (VS) added and methane content analyzed using the Geotech BIOGAS 5000 gas monitor. By day 50, FS+SP had the highest cumulative biogas volume of 256 L, an average specific biogas yield of 89.3 L/kg VS with methane content of 64.7%. Following this, was SP that generated 218.7 L of biogas with an average specific biogas of 47.9 L/kg VS, methane yield of 50.9-56.2% peaking at 56.2% methane content by day 21. Water hyacinth alone produced 96.3 L of biogas, an average specific biogas of 46.9 L/kg VS and methane content of 51.5%. A combination of FS+WH produced 178.8 L of biogas, yielding an average specific biogas of 80.3 L/kg VS, with methane content beginning at 49.4% on day 7 and decreasing to 44.3% by day 49. Finally, FS alone had the lowest yield, with 15.4 L of biogas and an average specific yield of 3.7 L/kg VS, peaking at 39.3% methane content by day 49. .Escherichia coli were completely removed in all setups. Substrate SP had the highest nitrogen and calcium content among the substrates. Co-digested substrates FS+SP and FS+WH demonstrated significant enhancement in nutrient composition, with enhanced concentration of nitrogen, phosphorus, and potassium, making them suitable for agricultural applications. The assessment of digestate substrates for fertilizer potential revealed a ranking from highest to lowest as follows: SP, FS+SP, FS+WH, FS, and WH. While all substrates demonstrated potential as fertilizers due to their nutrient content, they required further treatment to lower pathogen numbers, particularly total coliforms, before safe application. The study concludes that co-digestion, enhances methane production and nutrient recovery. Recommendations include use of continuous mixing, explore other substrates, optimal substrate ratios, ammonia mitigation strategies, and field trials for digestate utilization.