Simultaneous partial nitrification and denitrifying phosphorus removal (PNDPR) in a sequencing batch reactor process operated at low DO and high SRT for carbon and energy reduction

Abstract

This study reports on a novel partial nitrification-denitrifying phosphorus sequencing batch reactor (SBR) system enriched with non-conventional denitrifying phosphorus accumulating organisms (DPAOs), treating carbon limited synthetic wastewater over 175 days. The non-conventional operating conditions, such as low DO (0.25 mg/L–0.35 mg/L) and relatively long solids retention time (SRT) of 15 d, favored the enrichment of a wide variety of denitrifying phosphorus accumulating organisms (DPAOs), such as Rhodocyclus, Dechloromonas, and Cytophaga. In contrast to the Accumulibacter, these microorganisms can sustain a very low DO environment and simultaneously perform denitrification and enhanced biological phosphorus removal (EBPR) using oxygen, nitrite, and nitrate as electron acceptors. The low DO also favored the washout of nitrite oxidizing bacteria (NOB), leading to simultaneous partial nitrification-denitrifying phosphorus removal (PNDPR). Partial nitrification at a high nitrite accumulation ratio (NAR ~ 80%) was also unfavourable for the growth of denitrifying glycogen accumulating organisms (DGAOs) in the PNDPR system. Simultaneous nitritation-denitritation (SND), nitrogen, and phosphorus removal efficiencies were maintained above 90%. Online cyclic tests and batch studies showed that nitrite shunt was achieved, and denitrification was predominantly controlled by DPAOs rather than ordinary heterotrophs (OHO). Mass balances showed that the COD/NOX−-N ratio for denitrification was 3.8. Compared to the conventional enhanced biological phosphorus removal (EBPR) process, the low DO-PNDPR process achieves reductions in carbon and aeration energy demands by approximately 47% relative to conventional biological nutrient removal (BNR) plants, and 49%, relative to conventional nitrification/denitrification systems, respectively.