The activated sludge process—IV

This paper discusses the application of the general activated sludge model as set out by Dold et al. ( Prog. Wat. Technol. 12, 47–77, 1980) and extended by Van Haandel et al. ( Wat. Res. 15, 1135–1152, 1981), to anoxic-aerobic digestion of waste activated sludge. The laboratory scale experimental investigation comprised a 6 day sludge age activated sludge process, the waste sludge from which was fed to a number of digesters operated as follows: single reactor flow-through digesters at 4 or 10 days sludge age (retention times) under aerobic or anoxic-aerobic conditions (with 1.5 and 4 h cycle times) and 3-in-series flow-through aerobic digesters each with 4 days sludge age; all digesters were fed draw-and-fill wise once per day. The general kinetic model simulated accurately all the experimental data without the need to change the values of the kinetic constants. Both theoretical simulations and experimental data indicate that (i) the rate of volatile solids destruction is not affected by the incorporation of anoxic cycles and (ii) the specific denitrification rate constant in a digester is about two-thirds of that in the secondary anoxic reactor of the single sludge activated sludge system; this allows definition of a fourth denitrification rate constant K 4 for the anoxic-aerobic digester with K 4 T = 0.046(1.029) ( T -20) mg(NO 3 -N) (mgAVSS d) −1 , a constant independent of sludge age. An important consequence of (i) and (ii) above is that the denitrification can be integrated readily into the steady state digester model of Marais and Ekama ( Wat. SA 2, 163–200, 1976) and used for design purposes.