Ammonia removal from anaerobically digested dairy manure by struvite precipitation

Ammonia is one of the most important contaminants impairing the quality of water resources. When this is considered along with the fact that the global demand for nitrogenous fertilizers is in constant rise, the need for recovery as well as removal of nitrogen is well justified. Crystallization of N and P in the form of struvite (MgNH4PO4·6H2O), which is a slow releasing and valuable fertilizer, is one possible technique for this purpose. This study investigated the removal of NH4+ through struvite precipitation from the effluents of one- (R1) and two-phase (R2) anaerobic reactors digesting dairy manure. To force the formation of struvite in the anaerobic reactor effluents, Mg2+ ion was added by using both Mg(OH)2 and MgCl2·6H2O. To prevent the effect of different total phosphorus (TP) concentration in the effluents of R1 and R2, as well as to not limit the formation of struvite, an excess amount of PO43− (0.14 M) was added in the form of Na2HPO4. Different stoichiometric Mg2+:NH4+:PO43− ratios were tested to determine the required Mg2+ concentrations for maximum NH4+ removal by keeping NH4+:PO43− ratio constant for the effluents of reactors R1 and R2. The results revealed that very high NH4+ removal efficiencies (above 95%) were possible by adding Mg2+ ions higher than 0.06 M concentration in the effluents from reactors R1 and R2. It was also observed that the initial pH adjustment to 8.50 using NaOH did not result in any significant increase in the removal of NH4+ and the removal of NH4+ in the reactors treated with MgCl2·6H2O was higher than those treated with Mg(OH)2 for the same Mg2+ concentration.