Nitrification denitrification via nitrite in a pilot-scale SBR treating the liquid fraction of co-digested piggery/poultry manure and agro-waste

Nitrogen removal processes from the liquid fraction of digested agro-wastes is becoming more and more important as reduced nitrogen loads can be applied to land due to stringent European regulations. A pilot-scale SBR operating the nitritation/denitritation process was tested for more than 300 days and the process was proven to be technically feasible in spite of high influent variability. Nitritation rates of up to 0.5 gN L^-1 d^-1 (MLVSS = 7 gVSS L^-1, DO = 0.8 mgO2 L^-1, and pH = 8.4) and NO2/NOx > 80% were achieved as well as 95% denitrification efficiencies. Free ammonia and free nitrous acid inhibition on AOB were assessed in batch respirometric tests at increasing ammonium and nitrite concentrations: free ammonia resulted as the main inhibition factor. Effects of DO concentration on the nitritation rate were also assessed by means of respirometric tests. N2O emissions were highly dependent on the C/N ratio during denitrification.