Upgrading a trickling filter wastewater treatment plant to biological nutrient removal standard

The Selfs Point Wastewater Treatment Plant in Hobart was upgraded to biological nutrient removal standard in early 1997. The upgrade incorporated the existing trickling filter and anaerobic digesters. The new nutrient removal capability is provided by the BioDenipho process, and is supported by prefermentation and alum dosing of the digested sludge sidestream. This study reviews the development of wastewater treatment processes and the history of sewage treatment and collection in Hobart leading to this upgrade. All of the major processes at the Plant are reviewed as to how each impacts and is impacted by nutrient removal processes. Results from the first 18 months of operation of the plant following commissioning of nutrient removal are discussed, along with some of the factors affecting performance. Prefermentation has been found to be critical in achieving good biological phosphorus removal. A solids residence time of 5 days in a combined activated primary tank and sidestream prefermenter system has provided good performance. Use of existing rock trickling filters in a combined carbonaceous removal and nitrification role has permitted a smaller BNR reactor volume and a plant with greater flexibility for treating high influent loads or flows. The anaerobic digestion of phosphorus rich activated sludge led to a solubilisation of about 30% of the phosphorus entering the digesters. Alum dosing of the digested, sludge prior to the belt filter press at 1 to 1 1:1 molar (Al:P) ratio has been effective at reducing return of phosphorus to the main process to less than 1%.