The effect of available free-energy on nitrification kinetics

Abstract

A rap id rate nitrification filter, called a retained biomass disk (RBD). has been used to examine the response of Nitrosomonas spp. to different levels of nitrite (14- 196 mg-N/L) an d the response of Nitrobacter spp. to different levels of of nitrate (11- 250 mg-N/L). Fresh water at 28.4°C and pH 8.5 was used for the experimental trials. The substrate oxidation rates of both bacteria were measured for various combinations of substrate and end -product concentrations. For both bacteria spp. the responses of the filter to substrate and end-product concentrations were found to be best explained by a mixed Monod inhibition model. The maximum specific oxidation rate of Nitrosomonas was determined to be about double that of Nitrobacter. This large difference in growth response reveals the fact that in a nitrification filter the nitrite generation rate by Nitrosomonas can be twice the nitrite consumption rate by Nitrobacter when ammonia loading suddenly Increases. Thus, a nitrite pulse in reactor effluent is inevitable. By knowing the kinetic parameters for Nitrosomonas. Nitrobacter and anticipated ammonia loading schedules, management strategy can be devised to avoid nitrite peaks. Further, the oxidation rates of both bacteria were found to be linearly related to free-energy changes (AG) for the respective oxidation reactions with correlation coefficients of 0.841 and 0.934. It is shown that the inhibition model that accounts for both substrate and product concentrations can be replaced by a simpler model based on the availability of free-energy. At the same ratio of ammonia /nitrite and nitrite /nitrate over the range of 0.001 to 0.1. ammonia conversion rate was about double the nitrite conversion rate. Management strategies can be devised to maximize the nitrite removal rate. Further, by knowing the thermodynamic models' parameters and cell population, the filter's performance can be maximized during peak periods while minimizing high nitrite pulses