P-FIXATION BY MG, CA AND ZEOLITE A DURING STABILIZATION OF EXCESS SLUDGE FROM ENHANCED BIOLOGICAL P-REMOVAL

In anaerobic digestion of excess sludge originating from enhanced biol ogical phosphorus removal (EBPR) a complete hydrolysis of polyphosphat e occurs. Varying information exists about the resulting return flow o f phosphorus to the activated sludge plant. Magnesium, calcium and zeo lites from detergents are the main species involved in the refixation of dissolved phosphate in the digester supernatant. Their interactions and mechanisms of phosphate removal have been characterized and quant ified in experiments. The precipitation of struvite (MgNH4PO4 . 6H(2)O ) evolves according to the equilibrium, whereas the fixation by calciu m occurs kinetically controlled in the form of brushite (CaHPO4 . 2H(2 )O) or hydroxyapatite (Ca-5(PO4)(3)OH). Zeolites in digester sludge or iginate mainly from detergents. They represent around 7% of total soli ds entering the digester and their impact on digestion is given by 4 m odes: The control of free cation concentrations as an exchanger, as a source for calcium ions, as generator of aluminum phosphate precipitat ion after being partly hydrolyzed and as a nucleus for crystallization processes. With the current sludge composition phosphate fixation pro cesses in anaerobic digesters avert serious recycle load problems. Cop yright (C) 1996 IAWQ.