Biological nitrogen removal under low temperature in a membrane separation bioreactor for on-site domestic wastewater treatment was studied. The bioreactor was operated under intermittent aeration of a 180-minute operational cycle to achieve simultaneous nitrification and denitrification for nitrogen removal. During stepwise temperature decrease from 25°C to 5°C at every two weeks duration, nitrogen removal started to deteriorate as temperature dropped to 10°C. It decreased from more than 90% at 25°C to 20% at 5°C as a result of inhibition of nitrification at low temperature. However, increasing oxygen supply, i.e. increasing aeration time in operational cycle, could completely recover nitrification at 10°C. Nitrogen removal could be achieved by introducing non-aeration period after complete nitrification was obtained. Average nitrogen removal was 90 and 85% under 10 and 5°C respectively. The results indicated that sufficient oxygen transfer could be maintained in the membrane separation bioreactor even if the temperature was as low as 5°C. Analysis of respiratory quinone component of sludge suggested the decrease of strict aerobic bacteria percentage in mixed liquor during temperature decrease and increase of their percentage during the recovery of nitrification at 10°C. These changes could be related to the nitrification through the changes of oxygen transfer condition in the system. Insignificant difference of maximum volumetric nitrification rate obtained at 25 and 5°C probably suggests that there was not much difference in oxygen availability for nitrifying bacteria between both the temperatures once high and stable nitrogen removal was achieved.

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