Changes in wastewater treatment performance and activated sludge properties of a membrane bioreactor at low temperature operation

The membrane bioreactor (MBR) activated sludge process is being applied more and more for wastewater treatment due to its high treatment efficiency and low space requirement. However, the usefulness of the MBR process in low-temperature zones is less studied than that under normal conditions. This study determined the effect of low temperature (similar to 13 degrees C) operation on MBR performance and activated sludge characteristics. When the wastewater temperature decreased from 22 degrees C to 13 degrees C, the average effluent COD concentration increased from (10 + 5) to (25 + 4) mg L-1 and the nitrogen removal efficiency appeared not to be affected. The abundance and diversity of nitrifying bacteria such as Nitrosospira (ammonia-oxidizing bacteria) and Nitrospira (nitrite-oxidizing bacteria) in the activated sludge were reduced under low temperature exposure. The total biomass concentration decreased from about 10 000 mg COD L-1 at room temperature to 8200 mg COD L-1 at 13 degrees C at the same solid retention time. Furthermore, the sludge became bulking at 13 degrees C with a significant increase in the sludge volume index. The resultant sludge bulking was accompanied by accelerated membrane fouling resulting in a two-fold increase in the frequency of membrane cleaning. The results suggest that the performance of the MBR activated sludge process deteriorated at low wastewater temperatures even though the effluent water quality was still good enough for its applications in low temperature zones.