The Effect of Volatile Fatty Acids on the Nitrification of a Saline Effluent

Abstract

Anaerobically produced volatile fatty acids (VFA) may affect the nitrification yield. The effect of the type of VFA (acetic, propionic and butyric acid) on nitrification of a saline (24 g NaCl l-1) medium was studied. Nitritation (40 mg N-NH4 +l-1) and nitratation (100 mg N-NO2 -l-1) were assessed in batch cultures fed with different VFA (32 mg TOC l-1). The effect of increasing VFA concentrations on nitrification was studied in batch reactors fed with an initial concentration of 40 mg N-NH4 +l-1 and C/N ratios of 0, 2, 4, 8 and 16 as well as in a continuous mixed flow reactor operated at 30 °C and pH 7.5 and fed with 500 mg N-NH4 +l-1 and 500, 1000, 2000, and 4000 mg TOC l-1. Nitritation and nitratation rates were decreased by organic matter; inhibition increased with the VFA size. A non-competitive inhibition model fitted the experimental data on nitrification rate reduction at increasing acetic acid concentrations; inhibition constants were 685 mg acetic acid l-1 for ammonia oxidation and 74.3 mg acetic acid l-1 for nitrite oxidation. The continuous reactor's nitrifying ability decreased from 82% to 40% at C/N ratios 1 and 4, respectively. Loss of nitrification, but a 50 % ammonia removal was found at a C/N of 8. It was concluded that the nitrification rate reduction is proportional to the VFA molecular weight and that an increase in VFA concentration diminishes the nitrifying ability due to kinetic limitations.