Optimization of aeration strategies for intensified nitrogen and phosphorus removal in an aerated electrolysis-assisted submerged fixed bed reactor treating recirculating mariculture effluent

Nutrients such as nitrogen (N) and phosphorus (P) discharged from mariculture wastewater pose a serious threat to the environment. In this study, an electrochemically assisted submerged fixed bed bioreactor (E-SFBBR) with intermittent aeration was used to explore the effect of aeration strategies (aeration time, aeration rate and aeration position) on the removal of N and P. Phosphate (PO43--P) removal was maintained above 95% under different aeration strategies. Although PO43--P removal predominantly occurred in the cathode areas, the aeration position affected the main functional area for PO43--P removal by influencing the pattern of DO distribution within E-SFBBR; and chemical precipitation with iron ions was the main removal pathway as revealed by XPS analysis. E-SFBBR with upper aeration (i.e., UA-E-SFBBR), aerated for 6 h d-1 at 0.8 L min-1 showed the best N removal performance, i.e., 83.78 +/- 4.67%. The cathode area mainly contributed to N removal through mixotrophic denitrification. Aeration position plays a greater role in affecting N removal compared to aeration time and rate. Upper aeration favored the growth of heterotrophic denitrification bacteria (f_Rhodobacteraceae and Marinicella) in the cathode through providing more anaerobic environment, resulting in a better N removal performance. Besides, results of N transformation pathway predicted by PICRUSt 2.0 revealed that upper aeration promoted the ammonia assimilation but inhibited ammonia oxidation and NO3--N reduction processes in the cathode area. This study provided a theoretical basis for the application of aerated E-SFBBR in mariculture wastewater treatment.