Pilot plant studies of enhanced biological phosphorus removal (EBPR) based on an activated sludge process have been conducted at the Sjölunda plant since 1986. One of the main questions with respect to this process alternative is how the sludge treatment system is to be operated. In order to study the release of phosphorus from an EBPR sludge during anaerobic digestion, a study was performed with manually operated laboratory reactors. Fractionation procedures were used in order to characterize the sludge with respect to phosphorus. The influent wastewater quality determines the release and precipitation of phosphorus during anaerobic digestion. The potential for precipitation is in principle determined by the amount of metals in the influent wastewater which are removed with the wasted sludge. At the Sjölunda plant, a process scheme based on an EBPR process and anaerobic digestion requires that the circulation of phosphorus is prevented by means of addition of chemicals in order to acquire a phosphorus concentration of around 0.5 – 1 mg P/l in the secondary effluent.

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