Effect of substrate variability on activated sludge kinetics during the treatment of bleached kraft mill effluent

Solids retention time (SRT) is the primary control parameter for activated sludge units. However, treatment performance in full-scale systems at pulp mills does not appear to be a direct function of SRT. To determine the extent to which substrate variability masks the effects of SRT, the effect of SRT on activated sludge treatment kinetics and stoichiometry during bleached kraft mill effluent (BKME) treatment were investigated during a three year laboratory study. The treatment performance was monitored by biochemical oxygen demand (BOD), chemical oxygen demand (COD), and respirometric assays using BKME, methanol, formate, and acetate as substrates to monitor the activity of different populations of biomass. BOD removal was unaffected by SRT, however COD removal increased with increasing SRT and increasing influent BOD concentration. The BKME respirometric kinetics were constant with respect to SRT. The methanol and acetate degradation rates were variable, but were not a function of SRT. The maximum formate degradation rate increased with SRT, as did the yield on formate. Changing wastewater characteristics produced larger changes in treatment kinetics than those produced by changing the SRT (over a range from 6 to 30 days) or by the addition of an aerobic selector. The proportion of the formate metabolic rate relative to the methanol metabolic rate varied over the course of the project with the wastewater composition. The time required to adapt to different BKME batches varied from 5 to 30 days, and did not seem to depend on any of the measured variables (BOD, COD, or SRT).Key words: activated sludge, respirometry, kinetics, BKME, SRT, selector.