Nitrite accumulation and microbial behavior by seeding denitrifying phosphorus removal sludge for partial denitrification (PD): The effect of COD/NO₃^- ratio

Effect of influent COD/NO₃^- ratios (2.0-5.0) on partial denitrification (PD, NO₃^- to NO₂^-) was investigated by seeding denitrifying phosphorus removal (DPR) sludge at ambient temperature (16 ± 2℃). At COD/NO₃^- ratio of 2.5, the optimal NO₂^- effluent and nitrate-to-nitrite transformation ratio (NTR) reached up to 27.18 mg/L and 82.18%, respectively, and nitrate removal efficiency (NRE) (59.34 → 97.98%) showed positive relationship with increasing COD/NO₃^- ratios. The variations were further illustrated by denitrification kinetics, where faster COD degradation (1.55 g COD/(gVSS·d)), more NO₃^- consumption (0.55 gN/(gVSS·d)) and higher NO₂^- production (0.52 gN/(gVSS·d)) were observed although the NO₂^- peaks happened at anoxic 30 min. Microbial analysis showed lower community diversity and more concentrated composition with dominated genera Thauera (14.10%), Terrimonas (9.40%), Saprospiraceae (13.50%) and Flavobacterium (28.23%) at COD/NO₃^- ratio of 2.5. Based on the achievement of PD, the application feasibility of integrated PD-DPR-Anammox in a two-sludge DPR system for deep-level nutrient removal was discussed.