Enhanced ammonia removal at room temperature by pH controlled partial nitrification and subsequent anaerobic ammonium oxidation

The Anammox-based processes are suitable for the treatment of wastewaters characterized by a low carbon to nitrogen (C/N) ratio. The application of the Anammox process requires the availability of an effluent with a NO 2^- N/NH 4⁺ N ratio composition around 1 g g^-1, which involves the necessity of a previous step where the partial nitrification is performed. In this step, the inhibition of the nitrite-oxidizing bacteria (NOB) is crucial. In the present work, a combined partial nitrification-ANaerobic AMmonia OXidation (Anammox) two-units system operated at room temperature (20°C) has been tested for the nitrogen removal of pre-treated pig slurry. To achieve the successful partial nitrification and inhibit the NOB activity, different ammonium/inorganic carbon (NH 4⁺/IC) ratios were assayed from 1.19 to 0.82 g NH 4⁺-N g^-1 HCO₃ C. This procedure provoked a decrease of the pH value to 6.0 to regulate the inhibitory effect over ammonia-oxidizing bacteria caused by free ammonia. Simultaneously, the NOB experienced the inhibitory effect of free nitrous acid which avoided the presence of nitrate in the effluent. The NH 4⁺/IC ratio which allowed the obtaining of the desired effluent composition (50% of both ammonium and nitrite) was 0.82±0.02 g NH 4⁺ N g^-1 HCO 3 ^- C. The Anammox reactor was fed with the effluent of the partial nitrification unit containing a NO 2^- N/NH 4⁺ N ratio of 1 g g^-1 where a nitrogen loading rate of 0.1 g N L^-1 d ^-1 was efficiently removed.