Study of Biological Activity and Process Stability in Submerged Membrane Bioreactors

Synthetic wastewater was treated in a bench scale submerged membrane bioreactor (SMBR). A long‐term experiment was conducted by varying the sludge residence time (SRT) (10–500 d) and BOD loading (1.3–0.25 kg/m³·d). The biological activity was observed in terms of the oxygen utilization rate (OUR) and adenosine triphosphate (ATP) profile; the process stability was analyzed based on the extent of organic degradation and suction pressure. The microbial population in the SMBR was dependent on the SRT and BOD loading, and its biological activity was increased with an increase in the SRT or BOD loading. At a low feed to microorganism (F/M) ratio (0.06 kg BOD/kg MLSS·d), the sludge production of the reactor was reduced to 0.04 kg MLSS/kg BOD, which is much less than in the conventional activated sludge process (0.4–0.6 kg MLSS/kg BOD). The F/M ratio influenced the biological activity (via ATP and the OUR) significantly at a short SRT (≤90 d). However, the effect of the F/M ratio ceased at a low F/M ratio (≤ 0.07 kg BOD/kg MLSS·d). The accumulation of organics in the SMBR was accompanied with an increase in the supernatant TOC, which caused a high suction pressure and an abrupt change in the operating conditions to process instability. However, the process stability of the SMBR increased with an increase in the SRT and a decrease in the BOD loading along with a concomitant decrease in the biological activity and sludge production.