Effect of Free Ammonia, Free Nitrous Acid, and Alkalinity on the Partial Nitrification of Pretreated Pig Slurry, Using an Alternating Oxic/Anoxic SBR

The effect of free ammonia (NH(3) or FA), free nitrous acid (HNO(2) or FNA), and total alkalinity (TA) on the performance of a partial nitrification (PN) sequencing batch reactor (SBR) treating anaerobically pretreated pig slurry was studied. The SBR was operated under alternating oxic/anoxic (O/A) conditions and was fed during anoxic phases. This strategy allowed using organic matter to partially remove nitrite (NO(2)(−)) and nitrate (NO(3)(−)) generated during oxic phases. The desired NH(4)(+) to NO(2)(−) ratio of 1.3 g N/g N was obtained when an Ammonium Loading Rate (ALR) of 0.09 g NH(4)(+)-N/L·d was applied. The system was operated at a solid retention time (SRT) of 15–20 d and dissolved oxygen (DO) levels higher than 3 mg O(2)/L during the whole operational period. PN mainly occurred caused by the inhibitory effect of FNA on nitrite oxidizing bacteria (NOB). Once HNO(2) concentration was negligible, NH(4)(+) was fully oxidized to NO(3)(−) in spite of the presence of FA. The use of biomass acclimated to ammonium as inoculum avoided a possible effect of FA on NOB activity.