Struvite precipitation from the liquid fraction of the digestate of a municipal waste biogas plant

Currently, municipal waste biogas plant digestate is treated both as an alternative fertilizer and as a potential source of water. In practice, before advanced purification technologies, the liquid fraction of the digestate is subjected to pretreatment, aiming also at recovering the dissolved nutrients and making them into a concentrated fertilizer, e.g., by struvite precipitation. In this study, the possibility of struvite (MgNH4PO4 center dot 6H2O) precipitation from the liquid fraction of municipal digestate was determined. In the experiments, MgCl2 and NaH2PO4 were added to the treated solution as a complementary source of magnesium and phosphorus. Their doses were chosen to achieve the most favorable conditions for controlling struvite precipitation. The results obtained confirmed the possibility of struvite precipitation from the liquid fraction of municipal digestate. The process realized for pH 9.0, temperature 20-23 degrees C, molar ratio N:Mg:P = 5.13:1:1 and 1:1.1:1.1, reaction time 5 min with a stirring rate of 160 rpm, provides a high efficiency of struvite precipitation from the liquid fraction of digestate. From the viewpoint of process economics (less amount of reactants added with similar process efficiency), a molar ratio of N:Mg:P = 1:1.1:1.1 was found to be optimal for the reaction of precipitation.