Performance and fouling behaviors in a membrane-assisted biological nutrient removal process with focus on the effect of influent COD/N ratio

Abstract

A bench-scale University of Cape Town-membrane bioreactor was utilized to treat synthetic municipal wastewater with focus on the effect of chemical oxygen demand (COD)/total nitrogen (TN) (COD/N) ratio on biological nutrient removal and membrane fouling behaviors. The process showed a strong capability of anti-shock organic loading, and an average 89.9% removal efficiency of organic matter was achieved, indicating that COD removal was independent of the COD/N ratio. The average removal efficiencies of TN and total phosphorus (TP) were highest at a COD/N ratio of 7.3 at 90.3% and 92.4%, respectively. The proportions of TN removal via simultaneous nitrification and denitrification (SND) and TP removal via anoxic phosphorus removal increased to 27.9% and 44.91% from 1.6% and 7.94% (COD/N ratio of 3.2), respectively. A higher fouling rate was observed with increasing COD/N ratio, due to the changes in the nitrogen removal pathway. The increase of organic loading and decrease of dissolved oxygen induced SND behavior that affected the physiochemical properties and metabolic productions of the aerobic biosolids. The sludge filterability deteriorated due to the higher bound extracellular polymeric substances and soluble microbial products that were produced under SND condition, which also resulted in a higher modified fouling index both for suspended solids and soluble components. Biofloc sizes decreased slightly with high air shear stresses owing to the decreased dissolved oxygen. The nitrogen reduction across the membrane via denitrification of the biofilm attached on the membrane surface was also assessed.