The solubilisation of N and P during primary sludge acid fermentation and precipitation of the resultant P

Abstract

Phosphorus (P) and ammonia (N) release, during anaerobic digestion of primary sludge for volatile fatty acid (VFA) production, was studied in laboratory batch experiments, at room temperature (daily maximum 18 to 28°C). Nutrient levels in the digester supernatant liquor averaged 0.03 mgP/mgVFA (as acetic acid) and 0.10 mgN/ mgVFA (as acetic acid), after 6 d fermentation time. The degree of solubilisation, from a fixed amount of sludge solids, was dependent on anaerobic storage time and sludge composition. Lime dosages of 60 to 400 g CaO/kg sludge solids were required to reduce the soluble phosphate levels in the sludge liquor to below 5 mg P/l. No re-release of phosphate occurred in the fermented sludge liquors after 2 d anaerobic storage time after lime dosing. Furthermore, the addition of settled sewage elutriant to fermented sludge to recover the soluble VFA, did not affect the lime dosage requirements (per kg sludge) for phosphate removal. * To whom all correspondence should be addressed.

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