Evaluation of the potential of anaerobic co-digestion of scum produced from settling-thickening tanks with faecal sludge

Abstract

Poor hygiene and sanitation is a major public health concern mainly due to lack of proper method of disposal of faecal sludge. However, Lubigi combined wastewater and faecal treatment plant has attempted to provide safe disposal measures of faecal sludge with use of settling thickening tanks of an effective volume of 1250m3 to separate the solids and liquids in the faecal sludge. In the tank, faecal sludge settles at the bottom while the scum accumulates at a rate of 0.033m3/m3.day per faecal sludge inflow at the top. This project attempts to characterise the scum and faecal sludge formed, to evaluate the biogas potential of the scum and faecal sludge and to determine the quality of the slurry produced. In this regard, samples of faecal sludge and scum were picked from Lubigi combined wastewater and faecal treatment plant and analysed for total solids (TS), total volatile solids (TVS), BOD5, COD, total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), lipid content, pH and E coli. Biomethane potential experiments with and without pH adjustment were also run at 5 different volatile solids concentration: faecal sludge and scum ratios of 1:0, 4:1, 3:1, 1.5:1, 0:1. Each bottle was added with 80mL of seeding sludge picked from the Nakivubo biogas reactors. Alkalinity, volatile fatty acids, pH and ammonia were monitored while running the experiments. Scum quality parameters obtained were as follows: TS; 198010-263986.6mg/L, TVS; 42.30 64.68%TS, BOD; 5000mg/L, COD; 100000-148400mg/L, E. coli; 3.16x105- 1,73x107, TN; 0.97%, lipid content; 11.45% of dry matter, TP; 2006mg/L, pH 8.65-8.88. While the faecal sludge quality parameters gave the following results: TS; 117890-210993.33mg/L, TVS; 55.48-62.63 %TS, BOD; 3400-3900mg/L, COD; 88000-122000mg/L, E. coli; 555.56- 1.76x105, TN; 0.61-0.79, TP; 620-1368mg/L, pH 8.37-8.43. Before adjusting the pH, the biogas accumulated per grams of volatile solids were as follows: (inoculum) ISR 4:1 gave the highest accumulated biogas of 134mL/g VS of scum, followed by ISR of 3:1 with 73.51 mL/g VS of scum and lastly ISR of 1,5:1 with 33 mL/gVS of scum. The scum control reactor produced 4 mL/g VS. Biogas yield increased with a decrease in the scum loading but faecal sludge was not able to produce biogas when scum was not added. With pH adjustment, the gas produced increased by 145% for ISR 4:1. For ISR 1.5:1, the gas produced increased by over 200%. For the FS control, the gas produced increased from 0 to 166.5 mL. For the scum control, gas production increased by 54%. However, this trend wasn’t the same for ISR 3:1, which needs to be proven by further experiments.