Biodegradation Kinetics by Activated Sludge of the Soluble Organic Matter of a Synthetic Medium
The conventional activated sludge process used for wastewater treatment removes from 80 to 95% of the total organic matter. However, a quantity of "not well identified" (particular, colloidal and soluble) organic matter is always present in the treated effluent. Reducing this residual (and improving the treatment efficiency) requires knowledge of the origin of that organic matter and especially to determine the fraction originating from the influent and the fraction generated by the biomass. This research has been conducted in batch conditions and studies the soluble COD (CODs) removal kinetics of a synthetic effluent (casein + starch + acetate + mineral salts), in contact with different activated sludge originating from six different wastewater treatment plants (loads varying from 0.06 to 1.14 kg BOD[inf]5/kg VSS. d). Experiments have been conducted with different So/Xo values (ratio between CODs initial concentration and VSS initial concentration) in order that these values correspond to the CODs and VSS values found in the plants. In accordance with GRAU et al. (1975), CECH and CHUDOBA (1983), PITTER and CHUDOBA (1990), CHUDOBA et al (1992), the So/Xo ratio is a fundamental parameter governing the kinetics reactions. Experiments have been conducted under continuous aeration at 20°C where the synthetic wastewater (500 ml) is in contact with activated sludge (200 ml) collected 24 h before and stored at 4°C until the batch is started. In this manner, the initial So is due to the CODs of the synthetic effluent (So eff=197 mg/l) and to the CODs originating from the sludge (5 to 70% of the initial CODs). The initial VSS concentration (Xo) is between 0.6 and 2.5 g/l. Kinetics of CODs removal are simulated by two functions: the first order function, where the initial rate is the maximal, and the sigmoidal function where the maximal rate is reached after a lag time (3 to 8 h). Concerning the first order functions, the degradation rate is faster when the ratio is low (So/Xo lower than 0.44). This is not the case for the sigmoidal functions. In the results of this study, the residual of COD is always lower when the degradation kinetic follows the exponential model. Our experiments show that the type of degradation kinetics (first order or sigmoidal) is not only controlled by the So/Xo parameter but also by the proportion of CODs brought by the sludge and that parameter can play a determinant role. When the proportion of CODs brought by the sludge is very large (between 41% to 46%) the degradation reactions follow the sigmoidal type. These results can possibly be explained by the low biodegradability of the polymers or molecules originating from the inoculum which has been stored during 24 h, or by the low activity of the biomass after 24 h of storage on the biodegradation of the soluble organic matter