Behavior of waste activated sludge from enhanced biological phosphorus removal during sludge treatment

Phosphate release and phosphate fixation during sludge treatment of waste activated sludge (WAS) was investigated with a pilot plant for enhanced biological phosphorus removal (EBPR), complemented hy laboratory investigations of sludge samples from different large EBPR plants. The major part of the eliminated phosphorus in the pilot plant was due to the storage of polyphosphate (poly-P) in the WAS and was accompanied by an uptake of magnesium and potassium. Thickening and stabilizing WAS from the EBPR pilot plant results in a hydrolysis of poly-P which could be modeled with first-order kinetics and the Arrhenius relationship for the temperature dependence of the reaction constant. As a result of poly-P hydrolysis in stabilizing systems, phosphate, magnesium, and potassium are released, but only potassium remains in solution, whereas magnesium and a part of the released phosphate were precipitated as struvite. Another large fraction of the released phosphate was fixed by aluminium.