On-Site Fecal Sludge Treatment with the Anaerobic Digestion Pasteurization Latrine

The Anaerobic Digestion Pasteurization Latrine (ADPL) is a self-contained and energy neutral on-site sanitation system using anaerobic digestion of fecal sludge to generate biogas and then uses the biogas to pasteurize the digester effluent at 65-75°C to produce a safe effluent that can be reused locally as a fertilizer. Two ADPL systems were installed on residential plots with 17 and 35 residents in a peri-urban area outside of Eldoret, Kenya. Each system comprised three toilets built above a floating dome digester and one heat pasteurization system to sanitize the digested effluent. ADPLs are simple systems, with no moving parts and relying on gravity-induced flows. Adoption at the two sites was successful, and residents reported that the systems had little to no odor or flies. ADPLs were monitored for biogas production and temperatures in the pasteurization system. ADPLs serving 17 and 35 residents produced on average 16 and 11 L_biogas/person/day (maximum of 20 and 15 L_biogas/p/d), respectively. The temperature in the sterilization system was greater than 65°C on 58% and 87% of sampling days during the most stable period of operation. Treated effluent was analyzed periodically for chemical oxygen demand (COD), biochemical oxygen demand (BOD), total ammonia nitrogen (TAN), pH, and fecal coliform (FC). On average, the effluent at the two locations contained 4,540 and 6,450 mg COD/L (an 85% or 89% reduction of the estimated input), 2,050 and 3,970 mg BOD/L, and 2,420 and 4,760 mg NH₃-N, respectively, and greater than 5 log reductions of FC (nondetectable) in the sterilization tank. Results from this field study show that anaerobic digestion of minimally diluted fecal sludge can provide enough energy to pasteurize digester effluent and that the ADPL may be a suitable option for on-site fecal sludge treatment.