Estimation of biological kinetic parameters from an analysis of the BOD curve of waste waters : Effects of a chemical preoxidation

Urban waste waters were treated with pure ozone or combinations of ozone, hydrogen peroxide and/or UV radiation to study the course of resulting BOD (biological oxygen demand)-time profiles and to propose a kinetic model. BOD-time profiles of chemically treated waste waters show an initial lag period that first order kinetic models cannot describe. A second order kinetic model is then proposed that satisfactorily fits experimental BOD-time profiles, except when hydrogen peroxide has been used. In these cases, BOD-time profiles present the highest lag periods observed. By applying this model, three parameters are determined: the biokinetic constant (k) which is an index of the biological removal rate; the potential amount of biodegradable matter (BODT), and the measure of the size of inocula and microbial activities of microorganisms (λ). The model was checked with experimental results of BOD-time profiles corresponding to both untreated and chemically ozonated urban waste waters. Ozonated waste waters showed the highest values of k and BODT, which implies an improvement of waste water biodegradability after ozonation. However, values of λ corresponding to ozonated waste waters presented lower values than those of untreated waste waters. This was due to the lag period observed in the BOD-time profile, which was a consequence of a lack of microorganism acclimation to ozonated waste waters. The effect of the ozone dose, pH and carbonates during ozonation on COD (chemical oxygen demand) and the above indicated parameters was also studied. There was an optimum ozone dose which was 138 mg/l for this specific system. This led to the highest biodegradable fraction (φ) and the highest biokinetic constant (39% increase in φ and 4.7-fold increase in the value of k, respectively, compared to untreated waste waters.). Another significant fact was that a higher COD reduction was observed in the absence of carbonate during ozonation at basic pH values. In addition, the percentage of variation in the biodegradable fraction (Δφ) of ozonated waste water increased compared to the untreated waste water at acid pH. The results suggest that ozonolysis, the direct molecular ozone way of reaction, due to its selective character, increases the biodegradability of waste water more than other chemically advanced oxidation processes based on hydroxyl radical reactions.