The effects of different concentrations of several volatile organic compounds (VOC) such as ethanol, acetate, propionate, and butyrate ranging from 0 to 2000 mg/L as well as a mixture of volatile fatty acids (MVFA) at a 4:1:1 (acetate:propionate:butyrate) ratio on the nitrification rate in batch cultures were studied. The results showed that ethanol and acetate were consumed in a mixotrophic way by the nitrifying sludge. At a concentration of 500 mg/L, the nitrification rate of inhibition was different for each compound in which propionate and butyrate were the most inhibitory. At 2000 mg/L the inhibition was 80% with ethanol and 100% with acetate, propionate, and butyrate. With similar concentrations of MVFA, the inhibition was also similar to that in acetate. The effect of the addition of pulses of MVFA at a ratio of 4:1:1 during 14 h on the performance of the continuous nitrifying process with a hydraulic retention time of 3 d was also studied. No inhibition of the nitrification process was observed with pulses of 750–3000 mg of MVFA/L in the input of the reactor. The results in batch cultures suggest that the different degrees of inhibition of the nitrification process were related to the type of organic matter added. The noninhibitory effect of the organic matter in the continuous cultures on the nitrification efficiency of the nitrifying sludge might be related to the feeding pattern.
