EFFECTS OF DISSOLVED-OXYGEN AND DIFFUSION RESISTANCES ON NITRIFICATION KINETICS

The effects of the dissolved oxygen concentration on biological nitrification in suspended biomass processes are investigated. A model of nitrification, in which the kinetics is expressed by considering both the intrinsic rate of ammonia oxidation and the diffusion rate of substrates inside the biological floc, is presented. The model is calibrated and validated utilizing experimental data of tests performed in conditions of oxygen limiting kinetics. Excellent agreement is found between experimental results and model predictions. Finally, the model is applied to evaluate the effectiveness factor, eta, i.e. the ratio between the actual substrate removal rate and the intrinsic rate, as a function of the biofloc's diameter at different levels of dissolved oxygen concentration. The results obtained show that in nitrification processes, the effects related to oxygen internal diffusion resistances cannot be neglected in evaluating the overall kinetics. A marked decrease in the eta value is found at a biofloc diameter greater than 100-mu-m, particularly when the dissolved oxygen concentration is less-than-or-equal-to 2 mg l-1.