Demonstration of anaerobic stabilization of black water in accumulation systems under tropical conditions

The anaerobic digestion of “human waste” was studied at Mlalakuwa residential settlement in Dar-es-Salaam, Tanzania at ambient tropical temperatures (24–31 °C). This settlement experiences a high water table with flooding during the rainy season, resulting in a very costly emptying of the latrines once per month. To improve the situation, two plastic tanks (while one is in use, the other one is on stand-by) of 3000 l capacity each, named as Improved Pit-Latrines Without Urine Separation (IMPLWUS), were used as latrine pits. They received faeces + urine + wash water; basically, an accumulation system. Septic tank seed sludge was used. The dissolved chemical oxygen demand (CODdis) remaining when the reactor was closed after 380 days was about 8 g COD/l, volatile fatty acids were 100 mg COD/l and total ammonium nitrogen was about 2.8 g N/l, implying the possibility of methanogenesis inhibition. Stability results indicated a need for more degradation time after reactor closure. Estimated biogas production from wastewater generated by 10 people was 544 g COD-CH4/day, not enough for cooking purposes.