CIRCULATION OF PHOSPHORUS IN A SYSTEM WITH BIOLOGICAL P-REMOVAL AND SLUDGE-DIGESTION

Pilot plant studies of enhanced biological phosphorus removal (EBPR) b ased on an activated sludge process have been conducted at the Sjolund a plant since 1986. One of the main questions with respect to this pro cess alternative is how the sludge treatment system is to be operated. In order to study the release of phosphorus from an EBPR sludge durin g anaerobic digestion, a study was performed with manually operated la boratory reactors. Fractionation procedures were used in order to char acterize the sludge with respect to phosphorus. The influent wastewate r quality determines the release and precipitation of phosphorus durin g anaerobic digestion. The potential for precipitation is in principle determined by the amount of metals in the influent wastewater which a re removed with the wasted sludge. At the Sjolunda plant, a process sc heme based on an EBPR process and anaerobic digestion requires that th e circulation of phosphorus is prevented by means of addition of chemi cals in order to acquire a phosphorus concentration of around 0.5 - 1 mg P/l in the secondary effluent.