Influence of Nitrate and COD on Phosphorus and Nitrogen Removals under Batch Methanogenic and Denitrifying Conditions

In this study the effects of chemical oxygen demand (COD)-to-NO3-N ratio on phosphorus and nitrogen removals were investigated through denitrification and methanogenesis in batch reactors seeded with granulated anaerobic sludge. For the COD-to-NO3-N ratios varying between 2 and 3.7 maximum denitrification was observed. The volatile fatty acid (VFA) production decreased to 120 from 2,000 mg/L for the COD-to-NO3-N ratios varying between 2 and 3.7 through methanogenesis. The methane percentages were between 45 and 60% for the reactors exhibiting COD-to-NO3-N ratios of 2 and 3.7 on days 4–6. After days 10–12, the methane gas production ceased while the daily N2 gas production reached a maximum level of 42 mL for the COD to NO3-N ratios of 2 and 3.75. NO3-N was used as the electron acceptor after methanogenesis had been completed. High and low COD concentrations such as 10,000 and 200 mg/L decreased the PO4-P uptake, resulting in 10% PO4-P removal efficiency. COD concentrations of 2,000 and 4,000 mg/L increased the PO4-P uptake, resulting in PO4-P removal efficiencies of 89 and 95, respectively, in anoxic batch reactors. The NO3-N concentrations decreased to 500 from 1,500 mg/L at COD concentrations varying between 2,000 and 4,000 mg/L through PO4-P uptake under anoxic conditions. Suitable COD-to-NO3-N ratios provide effective NO3-N, PO4-P removals and minimize the external carbon requirement through anaerobic/anoxic treatment of nutrient containing wastewaters.