G. V. R. Marais

A hypothesis for the cause of low F/M filament bulking in nutrient removal activated sludge systems

Abstract Laboratory research has indicated that a possible cause of low F/M filament bulking in ND (nitrification-denitrification) and NDBEPR (nitrification-denitrification biological excess phosphorus removal) systems occurs as a result of competition for substrate between filamentous and floc-forming organisms which have different denitrification pathways. In ND and NDBEPR systems alternating anoxic-aerobic conditions prevail and continuous utilization of particulate slowly biodegradable COD (SBCOD) by floc-forming organisms in these systems leads to accumulation of the denitrification intermediates nitrite (NO2−) and nitric oxide (NO) under anoxic conditions. It is proposed that a cause for low F/M filament bulking is that the intermediate NO inhibits the utilization of SBCOD by floc-formers under subsequent aerobic conditions, with high concentrations of NO2− exacerbating this effect, thereby allowing filamentous organisms, which do not accumulate NO, to dominate. Some experimental evidence to support this proposal is presented.

The activated sludge process—IV: Application of the general kinetic model to anoxic-aerobic digestion of waste activated sludge

Abstract 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.

pH CONTROL AND COST SAVINGS IN AEROBIC DIGESTION

Abstract In aerobic digestion of waste activated sludge reduction in pH due to nitrification is commonly observed. pH reduction in turn affects nitrification efficiency, settleability and other properties of the digested sludge. Consequently for optimal performance it is necessary to monitor the pH and to control its value to near 7 by lime addition. An alternative control procedure, that eliminates both the need for pH monitoring and its control by lime addition, is to operate the process under alternating aerated/unaerated periods with 50 percent aerated, 50 percent unaerated. With this ratio all the nitrate produced during the aerobic period is denitrified during the unaerated (anoxic) period; the Alkalinity loss during nitrification is completely regained by denitrification and ammonification arising from protein degradation during both periods. A further advantage in anoxic-aerobic digestion is that by incorporating denitrification the oxygen requirement is approximately 20 percent less than that for purely aerobic digestion. Solids destruction is not affected.