Effect of anoxic decay process on simultaneous nitrification denitrification in a membrane bioreactor operated without an anoxic tank

This study was focused on evaluating the role and the effect of anoxic decay on the extent of simultaneous nitrification–denitrification (SNdN) process sustained in a single membrane bioreactor. The membrane bioreactor was fed with relatively strong domestic sewage and operated at steady state at a sludge age of 36 days at a corresponding suspended solids level maintained in the range of 17,500–21,000 mg/L. The SNdN could be sustained due to diffusion limitation of oxygen into the flocs. The evaluation identified an MLSS threshold level of around 17,000–18,000 mg/L below which nitrogen removal was essentially controlled by denitrification and above, the rate limiting mechanism shifted to nitrification maintaining total nitrogen removal efficiency of 85–95% for a typical domestic sewage. The contribution of anoxic decay process to the overall denitrification potential was evaluated as 60%, substantially higher than the remaining 40% associated with the anoxic growth during the SNdN process.