Effect of carbon dioxide on nitrification rates

Abstract.

Lab-scale ideal mixed, aerated reactors were employed to test the influence of carbon dioxide (CO₂) on the growth rate of a nitrifier community. The buffer medium used did not contain any carbon sources. Reactors were inoculated alternatively with sludge from a nitrifying membrane assisted bioreactor, reflecting autotrophic material, or with sludge from a plant having denitrification and nitrification steps, which reflects mixed heterotrophic and autotrophic material. CO₂ was added as a gas with the intake air supply. Nitrification rates were related to the CO₂ in the intake air as well as to the total inorganic carbon in the medium. The batch experiments show a relationship between CO₂ concentration and growth rate. The optimum growth rate occurred at 5 mg CO₂/L, corresponding to 0.4% (V/V) CO₂ in the inlet air. Different CO₂ optima for autotrophic and mixed sludges were found. In the case of the autotrophic sludge, the observed optimum growth rate was about 0.47/d and the optimum for the mixed sludge was about 0.75/d. Higher CO₂ concentrations lead to a decreasing growth rate. The first part of the kinetic graph can be described by Monod kinetics. Overall, the resulting graph can be described by Haldane kinetics.