Stable and high-rate nitrogen removal from reject water by partial nitrification and subsequent anammox.

A combined process of partial nitrification (PN) and anaerobic ammonium oxidation (anammox) was carried out to treat reject water with a high concentration of ammonium and a low level of hardly biodegraded organic carbon. Stable treatment performance was obtained under high nitrogen loading rates of 5.7 kg-N/m3/day and 10.5 kg-N/m3/day for the PN and anammox reactors for more than 2 months, respectively. Successful nitrite accumulation was observed in the PN reactor, with an effluent NH4-N/NO2-N ratio of 1:1.1 and marginal nitrate production, which is suitable for the subsequent anammox process. The strict control of DO concentration was adopted as the main manipulating strategy for the stable running of the PN reactor. And results indicated that the value of FA and FNA within a favorable range was essential for the successful operation of PN reactor. The anammox process was carried out in an up-flow fixed-bed reactor. The influent NH4-N/NO2-N ratio played a vital role in obtaining efficient nitrogen removal. The anammox reactor was successfully operated with a nitrogen removal rate of 9.1 kg-N/m3/day for 2 months, indicating high operational stability. Inorganic carbon was shown to have a positive impact on the high nitrogen removal rate during the combined process. In addition, the characteristics of the sludge in both reactors were investigated. The Stover-Kincannon model was used for kinetics studies. KB and Umax were determined as 30.1 g/L/day and 13.9 g/L/day, respectively, for the PN reactor and 42.1 g/L/day and 31.2 g/L/day, respectively, for the anammox reactor.