Interactions between wastewater quality and phosphorus release in the anaerobic reactor of the EBPR process
Abstract
Studies on enhanced biological phosphorus removal (EBPR) operation in activated sludge processes have been conducted on a pilot plant at the Sjölunda wastewater treatment plant in Malmö, Sweden, since 1986. The plant receives wastewater from a combined sewer system. The aim of this study was to investigate interactions between variations in the wastewater quality and the effect on the EBPR operation. The paper focuses on the overall role of the concentration of volatile fatty acids (VFA) in the influent wastewater on the EBPR process. Firstly, the relation between VFA, COD and phosphorus in the influent wastewater at the plant is examined. Secondly, the effect of pH on phosphate release under anaerobic conditions is investigated. Finally, the observed relation between phosphate release in the anaerobic zone in relation to the concentration of VFA in the influent wastewater is discussed. The pilot plant consists of two parallel, activated sludge systems with EBPR, one system with a solids retention time (SRT) of 4 days, operated without nitrification-denitrification and one system with a SRT of 21 days, operated with nitrification-denitrification. Results derived from daily, composite samples taken over 3 yr have formed the basis of a general description of the water quality and the performance of the process. In addition, intensive field studies and laboratory studies have been used as ways of investigating certain phenomena in more detail. The results of the pilot plant study showed that the concentration of total phosphorus on average was low in the effluent, below 0.5 mg P/l. Increased phosphorus concentrations were, however, observed on a number of occasions. High concentrations of phosphorus in the effluent were observed every year in July, which is the holiday period at industrial plants in Sweden. Other instances of increased phosphorus concentrations in the secondary effluent were recorded after prolonged periods of rain. Increasing flow rates due to rain lead to a dilution and a change in the composition of the COD in the influent wastewater. A laboratory study showed that the ratio of phosphorus release/HAc uptake depends on pH in the range of 6–8. At neutral pH a ratio of 0.35-0.4 mg P/mg COD was found. The results of an intensive field study on the pilot plant suggested a relationship of 0.5 mg P/mg COD. Hydrolysis and/or fermentation processes have to be taken into account when describing phosphorus release in the anaerobic reactor. However, it was clearly demonstrated that it is primarily the VFA in the influent wastewater that affect the release of phosphorus in the systems studied.
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