Nitrite accumulation in the treatment of wastewaters with high ammonia concentration

Different operational parameters of the nitritation process were investigated in both jar tests and pilot scale Sequencing Batch Reactors (SBRs). In the laboratory study, 100-1,200 mg N l^-1 of ammonia was used. The pH and temperature were varied. Batch experiments were done on municipal sludge, pectin industrial sludge and sludge from a reject water treatment unit. Ammonia oxidation was observed with relative nitrite accumulations from 2% to 100% and ammonia oxidation rates from 0.01 to 0.58 g N g VSS^-1 d^-1. The nitritation process and relative nitrite accumulation were highly affected by pH, temperature and the sludge type. pH 8.0-8.5 and temperature 30°C were found favourable for the nitritation. Pilot SBR systems for treating reject water achieved 100% of nitrite accumulation under the operational conditions of pH 7.5-8.0, temperature 30°C and dissolved oxygen (DO) 1.0 mg l^-1. Six months of operation revealed that pH regulations were essential to avoid the inhibitions by either free ammonia or nitrous acid. At an unionized ammonia concentration of approximately 20 mg NH₃-N l^-1, half of the normal nitritation ability still remained. Total inhibition occurred when the concentration of nitrous acid reached 3.0 mg HNO₂-N l^-1. However, both types of inhibitions were reversible in the SBR with a proper operation control. Stable and controllable nitritation could be achieved in pilot scale.