The comparison of alkalinity and ORP as indicators for nitrification and denitrification in a sequencing batch reactor (SBR)

Selection of an easy-to-measure, well-correlated surrogate parameter is critical for ensuring the effective operation of a nitrification/denitrification process on a real-time basis. The study aimed at investigating alkalinity as a reliable indicator for effluent nitrogen concentration under a series of operational conditions (chemical oxygen demand (COD), NH4+, hydraulic retention time) and under allylthiourea (ATU, a chemical inhibited nitrification) shock. The accuracy of alkalinity indication was compared with redox potential (ORP) in a sequencing batch reactor (SBR). Although both ORP and alkalinity exhibited clear variations in a SBR cycle, alkalinity presented a better indication than ORP, especially when there was a progressive decease in nitrification/denitrification efficiency. Effluent alkalinity exhibited a linear reverse correlation with nitrogen concentration (Alk = −4.26[N] + 180, R2 = 0.92), with alkalinity lower than 100 mg/L indicating insufficient denitrification, while alkalinity higher than 200–250 mg/L indicating insufficient nitrification. Furthermore, alkalinity difference between influent and effluent (ΔAlk), which reflected an overall result of alkalinity consumption in nitrification and alkalinity production in denitrification, was studied as another indicator. ΔAlkinf.-eff. decreased with better denitrification (ΔAlk = 6.99[N] + 22, R2 = 0.82) but increased with better nitrification (ΔAlk = −5.54[N] + 126, R2 = 0.76). The strong correlations of alkalinity with effluent nitrogen concentration, and ΔAlkinf.-eff. with nitrification/denitrification efficiency demonstrate that both alkalinity and ΔAlkinf.-eff. can be used as indicators in nitrification/denitrification processes. In addition, the benefits and problems of alkalinity, ORP and pH indications for nitrogen removal were comprehensively compared in the paper.