Nutrient removal from anaerobically digested cattle manure by struvite precipitation

Abstract

This paper studied the removal of nutrients including phosphate, ammonium, and potassium from anaerobically digested and centrifuged manure effluents by struvite precipitation. Due to coexistence of high levels of NH4+ and K+ ions in manure effluents, struvite precipitation should include formation of MgNH4PO4·6H2O (struvite) and MgKPO4·6H2O (K-struvite). Nutrient removal experiments with MgCl2 as the magnesium source were conducted with and without phosphate addition to evaluate the influences of the Mg/PO43– ratio, pH, temperature, PO43–/NH4+ ratio, and reaction time. It was found that the required Mg/PO43– molar ratio was more than 5 times higher than the stoichiometric value of 1 when no phosphate was added. Meanwhile, the ammonium removal efficiency increased with the increase of phosphate addition. However, the ammonium removal from digested manure effluents by struvite precipitation is not highly efficient, as the removal efficiency reached only 56% even at a Mg/PO43–/NH4+ molar ratio of 1.5:1.25:1. Furthermore, the potassium removal efficiency was significantly lower than that of ammonium, and the molar percentage of the removed K+ over the total removed NH4+ and K+ was less than 10%. The composition analysis in the precipitated solids indicated that some phosphate in digested manure effluents be removed as magnesium phosphates other than struvite. In addition, different magnesium salts were tested for struvite precipitation. Their superiority for phosphate removal followed an order of MgCl2 > MgSO4 > MgO > Mg(OH)2 > MgCO3. Key words: phosphate removal, ammonia removal, potassium, struvite precipitation, digested cattle manure, magnesium.