Enhanced nutrients removal resulting from energy metabolism improvement in the anoxic-anaerobic-oxic process (reversed AAO)

Comparison of energy metabolism between the conventional anaerobic-anoxic-oxic (AAO) process and the anoxic-anaerobic-oxic process (reversed AAO) was carried out. Although in terms of biological nutrient removal, both processes offered excellent removal efficiencies for organic matter and total phosphorus, there was a significant difference in regards to nitrogen removal. It was found that the ammonia nitrogen in the effluent in the AAO and the reversed AAO processes was 3.45 ± 0.64 and 1.53 ± 0.48 mg L–1, respectively, with an influent ammonia nitrogen of 23.22 ± 1.77 and 23.07 ± 1.57 mg L–1, respectively (Mixed liquor suspended solids (MLSS): 3,000 ± 500 mg L–1 and HRT: 12 h). This study investigated whether reversing the anaerobic and anoxic compartments could change the microbial activity and eventually promote efficient microorganism removal ability. Several key enzymatic indicators such as, 2,3,5-triphenyl tetrazolium chloride dehydrogenase activity (TTC-DHA), 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride electron transport system (INT-ETS), and adenosine triphosphate were measured and confirmed the improvement in energy metabolism in the reversed AAO process. Owing to the increased microbial activity, both processes showed identical phosphorus removal efficiency while less carbon source was consumed in reversed AAO process. Therefore, more carbon sources can be used for denitrification because of the highly efficient phosphorus removal mechanism. These findings highlight the improved removal capacity of reversed AAO process, demonstrating its real applicability.