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-Anammox two units system operated at room temperature (20 ºC) 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 HCO 3 -C. This procedure provoked a decrease of the pH value to 6.0 to regulate the inhibitory effect over ammonia oxidizing bacteria (AOB) caused by free ammonia (FA). Simultaneously the NOB experienced the inhibitory effect of free nitrous acid (FNA) 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 of 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 (NLR) of 0.1 g N·L -1 ·d -1 was efficiently removed. KeywordsAmmonium/inorganic carbon (NH 4 + /IC) ratio, Anammox, free ammonia (FA), free nitrous acid (FNA), partial nitrification.