Nitrogen loss and oxygen paradox in full-scale biofiltration for drinking water treatment

The nitrogen loss and DO paradox in full-scale biofiltration for drinking water treatment and the possible pathway responsible for them were investigated. A highly contaminated source water was treated at Pinghu Surface Water Plant using four biofilters, which resulted in a steady removal of NH4+-N (2.67 mg/L), a great DO consumption (8.86 mg/L) and an increase in the concentration of NO3−-N (1.77 mg/L). The nitrogen and DO balances indicated that about NH4+-N was lost and the actual DO consumption was about 30% lower than the theoretical DO demand if nitrification was regarded as the only pathway to remove NH4+-N. The analysis of correlation coefficients analysis between several factors and the nitrogen loss suggested that “Aerobic deammonification”, the coupling of shortcut nitrification and the anaerobic ammonia oxidation (Anammox) in an aerobic environment, might be the most probable pathways to explain the occurrence of these phenomena. According to this mechanism, about 57% NH4+-N was removed through complete nitrification and about 21.5% NH4+-N was incompletely nitrified into NO2−-N. The latter then involved in Anammox as the electron acceptor with the remaining NH4+-N as the electron donor. Since the Anammox reaction is anaerobic, the nitrogen loss and DO paradox can be justified.