Phosphate Fixation in Sludges from Enhanced Biological P-Removal During Stabilization
During the start-up phase of an enhanced biological phosphorus removal (EBPR) plant, the fixation mechanisms of phosphorus in the excess sludge were investigated. The major part of the eliminated phosphorus was bound as polyphosphate (poly-P) and was estimated to be some 50 to 70 % of total P. During anaerobic stabilization the poly-P was completely hydrolysed but only a part of the released phosphate remains in solution whereas as much as 60 to 70 % of total P was fixed by physico-chemical mechanisms.
Among the possible counterions for P-precipitation, aluminium, magnesium, and calcium are discussed in detail. In addition to the sludge from the pilot plant, stabilised sludge samples from large EBPR and conventional treatment plants were also investigated. It was found that a major part of the phosphate was associated with aluminium which was demonstrated by means of acidimetric titration. In the pH range of 1 to 3.5, aluminium and phosphate release were strongly correlated with a molar release ratio of 1.45 M A1/M P. The amount of phosphate which was due to aluminium phosphate fixation was calculated as some 35 to 68 % of total P. Besides a precipitation as AIPO4, adsorption to SiO2 and/or aluminosilicates and complexation with mono- or polynuclear aluminium are thought to be mainly responsible for the fixation of PO4.
Also, phosphate is bound as magnesium ammonium phosphate. With X-ray diffraction the crystallization of a solid phase was identified as struvite. The solubility product of struvite was determined to log Ks = — 12.53 ± 0.05. It seems that the sequential dilution test is an appropriate method for determining the amount of struvite precipitation. In EBPR sludges from plants with the main stream process, 25 to 37 % of P was eliminated as struvite whereas in the sludge from a plant with the side stream process, the amount of struvite precipitation accounted for only 14 % which is mainly due to the reduced amounts of magnesium as a result of the stripping process.
Although in some sludge samples exceptionally high calcium levels of up to 8.5 % were reached, no indications of calcium phosphate interactions were found.
KeywordsCrystallization Phosphorus Magnesium Silicate Titration
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