Treatment of strong domestic sewage in a 96 m 3 UASB reactor operated at ambient temperatures: two-stage versus single-stage reactor

Abstract

A 96 m 3 UASB reactor was operated for 2.5 years under different conditions to assess the feasibility of treating strong sewage (COD tot = 1531 mg/l) at ambient temperatures with averages of 18 and 25 °C for winter and summer respectively. During the first year, the reactor was operated as a two-stage system at OLRs in the range of 3.6–5.0 kg COD/m 3 d for the first stage and 2.9–4.6 kg COD/m 3 d for the second stage. The results of the first stage showed average removals of 51% and 60% for COD tot and COD ss respectively without significant effect of temperature. The second stage reactor was unstable. The temperature affected sludge stabilization. During the second year, the first stage was operated as a single-stage UASB reactor at half of the previous loading rates. The results showed an average removal efficiency of 62% for COD tot during summer, while it dropped to 51% during wintertime. However, the effluent suspended solids were stabilized with VSS/TSS ratio around 0.50 all over the year. The sludge in the single-stage reactor was well-stabilized and exerted an excellent settlability. During the last three months of research, sludge was discharged regularly from the single-stage UASB reactor. The results showed no significant improvement in the performance in terms of COD tot. Based on the results of the experiment, a single-stage UASB reactor operated at relatively long HRT is preferred above two-stage system at the Jordanian conditions.