Effect of fluoride on nitrification of a concentrated industrial waste

The potential for biological nitrification of an industrial waste containing 4,000 mg of ammonia N (NH(4)-N) and 10,000 mg of fluoride per liter was investigated. Ammonium sulfate and sodium fluoride were tested in various combinations of 100 to 2,000 mg of NH(4)-N per liter and 0 to 5,000 mg of F per liter in suspended-growth stirred-tank reactors containing enriched cultures of nitrifying bacteria from a municipal sewage treatment plant. The stirred-tank reactors were fed once per day at a constant hydraulic retention period and cell retention time of 10 days. Temperature was 23 degrees C, and pH was 7.0 to 7.5. Clarified secondary effluent was used to make up feeds and to provide minor nutrients. Steady-state data, confirmed by mass balances, were obtained after five to six retention periods. In the absence of fluoride, nitrification efficiency was near 100% for up to 500 mg of NH(4)-N per liter. The influence of fluoride was studied at a low ammonia concentration (100 mg/liter) and exerted no significant effect on nitrification at concentrations of up to 200 mg/liter. Maximum effect of fluoride was reached at 800 mg of F per liter, and no greater inhibition was observed for up to 5,000 mg of F per liter. At the highest concentrations studied, ion pairing of ammonium and fluoride may exert a significant effect on kinetic coefficients. Kinetic analyses showed maximum specific substrate removal rates (q(max)) of NH(4)-N to be about 2.3 mg of N per mg of volatile suspended solids per day in the absence of fluoride and 0.85 mg of N per mg of volatile suspended solids per day in the presence of fluoride. The form of inhibition due to the presence of fluoride was shown to be not competitive, conforming to a mixed inhibition model.