The roles of nitrogen dissimilation and assimilation in biological nitrogen removal treating low, mid, and high strength wastewater

Nitrogen dissimilation (nitrification and denitrification) and assimilation (uptake by cell growth) under different operational conditions (chemical oxygen demand (COD) and dissolved oxygen (DO)) were evaluated in a sequencing batch reactor (SBR) system. Nitrogen dissimilation played an important role for nitrogen removal at low to mid CODs, while nitrogen assimilation became more significant with biomass concentration steadily increasing at high COD. Specific denitrification rate increased at low to mid COD (C:N < 15), but decreased at high COD (C:N > 20). Both COD and C:N ratio should be kept in proper ranges to obtain sufficient biomass concentration for nitrogen assimilation in treatment systems. With alkalinity being consumed in nitrification and produced in denitrification, effluent alkalinity indicated the corresponding nitrogen concentrations under different COD loadings. ΔAlk between influent and effluent was also well correlated with assimilation and dissimilation. The discrepancy between ΔAlk_Theory and ΔAlk_Experi was less than 15 mg/L when dissimilation played a predominant role in nitrogen removal, while the discrepancy increased to 30 mg/L when assimilation became dominant.