Co-current nitrification/denitrification and biological p-removal in cyclic activated sludge plants by redox controlled cycle operation

Several variations of SBR processes are reported in the literature; one of these uses the acronym CAST, Cyclic Activated Sludge Technology, and has been developed specifically for large-scale domestic wastewater treatment application (20,000 to 400,000 P.E.). This process was used to relieve existing overload conditions at the Groβarl WWTP which treats sewage from a ski resort using a conventional nitrifying/denitrifying (N/DN) process. The efficacy of the conventional and cyclic processes were compared through parallel operation over a one year period. Through examination and comparison of the nitrification rates, the stability of co-current nitrification/denitrification and the biological phosphorus removal it was found that the cyclic system was demonstrably superior in treatment efficiency when operated at the same loading conditions as the conventional process. Nitrification rates were about 35 percent higher in the cyclic system. Phosphorus elimination was about 83 per cent in the cyclic system and 55 per cent in the conventional system at the same loading conditions. Applying the results of the design criteria used for the cyclic system, a volumetric saving of about 30 per cent compared to the use of conventional treatment technology is obtainable for the proposed full-scale amplification. The optimization strategy of selecting set-point dissolved oxygen concentration in the basins according to the on-line measured oxidation reduction potential (ORP) and its rate of change throughout a cycle proved to be a valuable tool for optimizing nitrogen and phosphorus removal.