Effect of temperature on membrane bioreactor performance working with high hydraulic and sludge retention time

Highlight
• Low temperatures affect COD biodegradation increasing supernatant concentration.
• Membrane Rt in MBR increases at temperatures <15 °C due to lower COD biodegradation.
• High VSS/TSS ratio can be expected in MBR working at high SRT due to lower bH value.
• MBR sludge wasting falls if temperature rises due to bH rising faster than Yobs.
• MBR membrane aeration or recirculation affects nitrate removal at low temperature.

This study analysed the effect of temperature on MBR performance. For this purpose, a full-scale pre-denitrification MBR system was used. This system operated at a constant flow of urban wastewater (0.42 m3/h) with continuous monitoring of mixed liquor temperature (9–31 °C). Its sludge retention time (SRT) was 35 days and its hydraulic retention time (HRT) was 35 h. Variations in temperature did not affect the effluent COD concentration. Permeability was reduced both by the increase in membrane flux resistance (Rt) at temperatures <15 °C and by viscosity. A lower temperature produced a decrease both in the endogenous respiration rate (bH) and the observable biomass yield coefficient for heterotrophs. This effect was particularly evident in the bH and led to an upward trend in the concentration of the TSS and the VSS/TSS ratio. The system was found to have a high nitrification and denitrification potential regardless of temperature. However, residual dissolved oxygen due to membrane aeration or recirculation reduced nitrate removal at low temperatures.