Technical evaluation of potential drawbacks in direct UASB treatment of raw domestic sewage

The performance and stability of a domestic sewage treatment system consisting of an upflow anaerobic sludge blanket (UASB) reactor were assessed. This was to come up with a technical evaluation of using such a system to treat directly such wastewater containing high amounts of suspended solids (SS), in developing countries like Lesotho. Through time, the evolution of the sludge bed was monitored closely, in terms of accumulation of particulate matter and its general quality. The average total chemical oxygen demand (COD
t
) of the wastewater was 522 mgL
⁻¹
, and the system could remove 80 % of the organic matter. The sludge production in the reactor system was high (up to 70 % of influent chemical oxygen demand (COD) found as suspended solid), necessitating repeated sludge discharges from the reactor, on average every 100 days. Considering the delicate balance of the methanogens in anaerobic reactors, perturbations of the microbiota arose following the sludge extractions, and these promoted reactor imbalance. This further brought about a decrease in reactor performance and hence of the general sustainability of the UASB process when treating domestic sewage. In addition, the system could not remove the macronutrients nitrogen and phosphorus. Additional treatment steps would be necessary to remove the suspended solids and nutrients, and thus to effectively treat such a wastewater.