The effect of sludge discharges and upflow velocity on the removal of suspended solids in a UASB reactor treating settled sewage at moderate temperatures

The removal of Total and Volatile Suspended Solids (TSS/VSS) was studied in a pilot-scale UASB (Upflow Anaerobic Sludge Bed) reactor treating settled sewage under subtropical conditions (Salta, Argentina). The effect of sludge bed height (hsb) and upflow velocity (Vup) on the removal of TSS and VSS was assessed. TSS and VSS removal efficiencies of up to 95% have been achieved when Vup ≤ 0.43 m/h (HRT ≥ 6 h) and hsb ≥ 0.92 m. Effluent concentration was extremely low (≤ 5 mgTSS/L). TSS and VSS removal efficiencies were inversely proportional to Vup. A safe and efficient operation can be achieved in UASB reactors treating settled sewage with hsb between 1 and 2 m, and Vup around 0.5 m/h. The reactor was operated at 0.85 m/h during periods of up to two days, but no significant sludge wash-out was observed, even after up to 50% of the sludge bed was discharged. Specific Methanogenic Activity (SMA) was not affected by sludge discharges. Excess sludge discharge operations can be performed once a year. TSS/VSS removal efficiency could be a useful criterion to decide the right moment for sludge discharges.