Effect of carbon source on nitrite accumulation and pH value of effluent during denitrification process

Effects of different carbon sources(including methanol, ethanol and sodium acetate) as electron donors on nitrite accumulation and effluent pH value during denitrification was investigated in a lab scale sequencing batch reactor(SBR). The results showed that nitrite accumulation was correlated with the types of carbon sources and sludge loading employed. Only small amount of nitrite accumulated during denitrification process when nitrate addition was 20 mg/L with low sludge loading. The specific denitrification rate(rDN) and effluent pH value in different systems increased by the order of sodium acetate, ethanol and methanol. Under the condition of high sludge loading, large amount of nitrite accumulation was observed except for methanol system due to the incomplete denitrification. Nitrite accumulation and effluent pH value were higher than those under low sludge loading. It was also observed that nitrite accumulated in mixed electron acceptor systems with different addition ratio using sodium acetate as carbon source. The specific denitrification rate increased with the amount of nitrate added, implying the reduction of nitrite was inhibited by nitrate. The increment of pH during denitrification was also found to increase with COD/N ratio in addition to carbon source and sludge loading through monitoring pH variation. As a result, appropriate addition of sodium acetate combined with other carbon sources was recommended in denitrification to eliminate the adverse effects of high effluent pH value and low denitrification rate caused by single input.