The effects of COD/P and N/P ratios on denitrifying phosphorus removal with nitrite as electron acceptor

The ratios of COD/TP and NO₂^--N/TP in the influent of wastewater were observed to have substantial effects upon the performance of the A₂/O SBR system. The amount and the releasing velocity of phosphate in the anaerobic zone increased as the COD/TP ratio increased, but when the ratio of COD/P was 28.5, surplus carbon sources went into the anoxic zone, which resulted in reduction of nitrite by external carbon sources, inhibiting phosphate accumulating in the anoxic zone. While the ratio of COD was 21, carbon sources were not enough for being turned into PHB, the removal efficiency of phosphorus in anoxic zone decreased; moreover the limited loading of COD may suppress the development of polyphosphate accumulating organisms(PAOS), leading to the establishment of microbial communities without polyphosphate accumulation. When the ratio of COD/TP was 25, the performance of A₂/O SBR was the best. The appropriate N/P ratio in the influent wastewater was related to the ratio of COD/P. At the COD/P ratio of 25, the optimum ratio of N /P was 3. When the ratio of NO₂^--N /P was 1.8, denitrifying phosphorus removal process ceased, owing to the lack of nitrite. When the ratio of N to TP was 5, the removal efficiency of phosphorus in the system decreased, because of the nitrite left in the anaerobic period of the last cycle.