Overloading nitrification systems to accumulate nitrite

Nitrogen removal via nitrite, achieved through partial nitrification and denitrification, allows reducing oxygen and organic matter requirements, enhancing process efficiency. In activated sludge systems when dealing with ambient temperature, nitrite accumulation becomes a challenge as nitrate oxidation occurs preferentially. In the present study, the applied nitrogen load rate (NLR) is chosen as a parameter to evaluate the feasibility of promoting nitrite accumulation in two activated sludge reactors (AS1 and AS2) operated at 25 ºC. Ammonium overload was imposed either by applying relatively high ammonium concentrations (AS1, 130 – 570 mg NH4+-N/L) or inflow rates (AS2, hydraulic retention times (HRT) = 3 h). In both cases supplied inorganic carbon (IC) was intended to be limited to 1 g N/g IC, commonly found in areas with soft water. In AS1 nitrite accumulation took place at concentrations of 500 mg NH4+-N/L (0.4 g N/(L·d)) while inhibitory conditions due to free ammonia (FA) and free nitrous acid (FNA) were occurring. In AS2 nitrite concentrations were always higher than nitrate concentrations treating NLRs close to 0.8 g N/(L·d). In this case, inhibitory conditions were only reached for FA during certain operational periods. Thus, in both systems high NLRs seem to be required but insufficient to maintain stable nitrite accumulation and to avoid nitrate limiting IC concentrations and FA or FNA inhibitory conditions for NOB should be imposed.