A Comprehensive Review on the Biofilm-Mediated Removal of Nitrogen and Chemical Oxygen Demand From Different Wastewater Sources

Discharging effluents with high chemical oxygen demand (COD) and nitrogen content into the environment threatens human and aquatic life. An increase in nitrogen load results in depletion of dissolved oxygen (DO), eutrophication, ecological stress, and biodiversity loss. Intake of water containing excess nitrate can cause different diseases. Conventional physicochemical nitrogen removal techniques are expensive and also generate secondary pollutants. In contrast, biological methods offer effective and economical outcomes with global acceptance. Biofilm-based techniques have the advantages of low space requirement, resistance toward toxic shocks, and absence of sludge backflow. The carriers used in biofilm reactors allow the growth of heterogeneous microbial consortia, which can simultaneously remove COD, nitrogenous compounds, and phosphates. This review aims to summarize the outcomes of the individual lab-scale research in this area, critically analyze the scientific findings, and understand the research gap. Conventional nitrification–denitrification and anammox have often been replaced by more efficient approaches such as simultaneous nitrification–denitrification, partial nitrification–denitrification, partial nitritation and anammox, and simultaneous partial nitrification, anammox, and denitrification. Multistage moving bed biofilm reactors have been specially designed with step feeding for complete nitrogen removal. Through anammox in a sequencing batch reactor, a high rate of denitrification could be obtained, whereas simultaneous nitrification–denitrification using a membrane bioreactor resulted in almost complete removal of nitrogen. We expect that this review will provide the direction for designing experiments on enhanced removal of nitrogen and COD from different wastewater sources using microbial biofilms.