Kinetics of organochlorine removal from bleached kraft pulp mill effluent during biological treatment

Batch biodegradation kinetic studies were conducted with various bleached kraft pulp mill effluents containing organically bound chlorine measured as adsorbable organic halogen (AOX) (6.0 - 35 mg Cl/L). Biomass, generated in both a continuous bench scale reactor and a mill scale activated sludge system, at various concentrations (0.4 - 3.1 g VSS/L) was added to various effluents (about 70% ClO₂ substitution). AOX removal kinetics were best described by a simple recalcitrant first order model: - dS/dt = k_s,1X(S-S_∞), where S and S_∞ are AOX concentrations (mg Cl/L) at time t and t_∞, respectively, k_s,1 is the AOX removal rate constant (L/g VSS.h) and X is the biomass concentration (g VSS/L). The model indicated a constant non-biodegradable fraction of AOX (0.4 mg of AOX per mg of influent AOX) and was able to predict the effect of dilution on AOX removal with a single value of k_s,1 (0.13 L /g VSS.h). The degradation constants (0.16 L /g VSS.h) in the absence of effluent from the neutral sewer (i.e. for a mixture of acidic and alkaline effluent only) were significantly higher than those for whole mill effluent.