A laboratory study of struvite precipitation after anaerobic digestion of piggery wastes

Abstract

A dilute (0·3% TS) but relatively strong (COD 10 000 mg litre −1) piggery wastewater was used as feed in 1·6-litre laboratory anaerobic digesters. During batch digestion (56 days), NH 3-N increased from 500 to 800 mg litre −1 and PO 4-P from 21 to 30 mg litre −1 while Mg concentrations decreased from 32 to 20 mg litre −1. A continuously fed digester (mean residence time of 64 days) produced similar increases in NH 3-N concentrations and reductions in concentrations of PO 4-P (22 to 10 mg litre −1) and Mg from 32 to 20 mg litre −1. X-ray diffraction of the digester sludge indicated that langbeinite (K 2Mg(SO 4) 3) was precipitated whilst it appeared that very fine crystals of Ca, Mg and PO 4 compounds remained in suspension. The addition of MgSO 4 to the batch supernate raised to a pH of 9 led to a reduction in PO 4-P concentrations from 33 to 7 mg litre −1 and the supernate from the continuous digester at a pH of 9 from 10 to 0·1 mg litre −1. The precipitate formed by this reaction was comprised of struvite (MgNH 4PO 4.6H 2O), apthitatite (K 3Na(SO 4) 2) and thermarite (Na 3SO 4). The reduction of PO 4-P concentrations to less than 1 mg litre −1, a significant water quality criterion in the management of eutrophication, is of importance. The production of struvite as a valuable fertilizer may offset much of the cost of this form of wastewater treatment.