Acetate limitation and nitrite accumulation during denitrification

Nitrite accumulated in denitrifying activated sludge mixed liquor when the carbon and electron source, acetate, was limited. If acetate was added to o btain a carbon-to-nitrogen (C:N) ratio in the range of 2:1 to 3:1, nitrate was completely consumed at the same rate with no nitrite accumulation, indi cating that nitrate concentration controlled the respiration rate as long a s sufficient substrate was present, However, when acetate was reduced to a C:N ratio of 1:1, while nitrate continued to be consumed, >50% of the initi al nitrate-nitrogen accumulated as nitrite and 29% persisted as nitrite thr oughout an endogenous denitrification period of 8-9 h. While nitrite accumu lated during acetate-limited denitrification, the specific nitrate reductio n rate increased significantly compared with the rate when excess acetate w as provided as follows: 0.034 mg-NO3-N/mg-mixed liquid volatile suspended s olids/h versus 0.023 mg-NO3-N/mg-mixed liquid volatile suspended solids/h, respectively. This may be explained by nitrate respiration out-competing ni trite respiration for limited acetate electrons. Complete restoration of ba lanced denitrification and elimination of nitrite accumulation during denit rification required several weeks after the C:N ratio was increased back to 2:1.