In anaerobic digestion of excess sludge originating from enhanced biological phosphorus removal (EBPR) a complete hydrolysis of polyphosphate occurs. Varying information exists about the resulting return flow of phosphorus to the activated sludge plant. Magnesium, calcium and zeolites 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 quantified in experiments. The precipitation of struvite (MgNH4PO4·6H2O) evolves according to the equilibrium, whereas the fixation by calcium occurs kinetically controlled in the form of brushite (CaHPO4·2H2O) or hydroxyapatite (Ca5(PO4)3OH). Zeolites in digester sludge originate mainly from detergents. They represent around 7% of total solids entering the digester and their impact on digestion is given by 4 modes: The control of free cation concentrations as an exchanger, as a source for calcium ions, as generator of aluminum phosphate precipitation after being partly hydrolyzed and as a nucleus for crystallization processes. With the current sludge composition phosphate fixation processes in anaerobic digesters avert serious recycle load problems.

This content is only available as a PDF.