Modeling the effect of powdered activated carbon addition in activated sludge reactors treating dairy wastewater

FERRO OROZCO A.M., CONTRERAS E., ZARITZKY N.

Viña del Mar, Chile

Congreso; ICEF 10 International Congress on Engineering and Food; 2008

Powdered activated carbon (PAC) addition is a feasible alternative to enhance the performance of activated sludge reactors treating food industry wastewaters with high organic matter contents. In this work, the combined effect of activated sludge and PAC treating a model wastewater of a dairy plant was studied and a mathematical model was developed. Batch experiments were performed to evaluate the effect of PAC concentration on the growth kinetics of activated sludge; in all cases the initial soluble COD was 4.8 g/L. In the experiments with PAC addition, a wood based PAC (Clarimex S.A., type 061) was used. The biomass growth yield (YX/S) and the substrate oxidation coefficient (YO/S) were mconstant; however, the observed maximum specific growth rate (obs) and the specific oxygen uptake rate (qO2) decreased as the initial PAC concentration increased. The obtained results suggested that PAC reduces the substrate bioavailability; thus the adsorption process of the cheese whey onto the PAC was analyzed. The specific adsorption capacity (qC) as a function of the cheese whey equilibrium concentration followed the Langmuir adsorption isotherm with coefficients qCm = 316 ± 20 mgCOD gPAC-1, and b = 1485 ± 289 mgCOD L-1.  In order to develop the mathematical model that represents the effect of PAC on the bioavailability of the cheese whey substrate (S), it was assumed that PAC can only adsorb this substrate following the Langmuir adsorption isotherm. Besides, it was assumed that the substrate uptake by the microorganisms (X) followed the Monod kinetics. Lag-phase and growth-linked microbial soluble products (P) were included in the model. Oxygen uptake rate (OUR) was expressed as the sum of the exogenous and the endogenous respiration rates. The model was fitted to the batch growth data (S, X, and OUR as a function of time) for the different tested PAC concentrations (0 – 8 g L-1); in all cases a good agreement between the model and the experimental data was obtained. Thus, the proposed model could be useful to predict the performance of activated sludge reactors with the addition of PAC for the treatment of dairy food industry wastewater with high organic matter contents.