Effect of Volatile Fatty Acids and Trimethylamine on Nitrification in Activated Sludge

The effect of volatile fatty acids and trimethylamine on the nitrification activity of activated sludge was studied in laboratory batch experiments. The critical concentration of inhibitor IK at which the activity ceases was determined by modelling. IK values for ammonia oxidation were found to be 6 mM for isobutyric, 37 mM for n-valeric, 6 mM for isovaleric 36 mM for n-caproic acid and 3 mM for trimethylamine. IK values for nitrite oxidation were found to be 2 mM for formic, 115 mM for acetic, 68 mM for propionic, 33 mM for n-butyric, 8 mM for isobutyric, 75 mM for n-valeric, 7 mM for isovaleric and 81 mM for n-caproic acid. For propionic acid and trimethylamine, the undissociated compound appears to act as an inhibitor. For formic, isobutyric and isovaleric acids, the degree of inhibition seems to be determined by the total concentration of acid. Experiments with anaerobically treated wastewater stripped of sulphide showed that volatile fatty acids and trimethylamine alone cannot account for the inhibition of the nitrification activity, indicating that other factors are also involved.