Study of the influence of the reaction parameters on nutrients recovering from digestate by struvite crystallisation

Struvite production by crystallisation is one of the most promising methods of phosphorus recovering from wastewater or livestock waste. Lab-scale batch experiments were carried out to study the effect of supersaturation, magnesium and phosphorous concentration, pH value and temperature on struvite crystallisation reaction using pig manure digestate from an anaerobic digestion plant as raw material. Taguchi methodology has been used as method to define the design of experiments and to analyse the results. In the design of experiments, three levels of each parameter have been studied: Mg/P ratio (1.0-2.0), N/P ratio (4.0-12.0), pH (9.0-12.0) and temperature (20-40 °C). The morphology of the crystals obtained was observed by scanning electron microscopy. The results show that the optimal values of Mg/P ratio, N/P ratio, pH and temperature for struvite crystallisation are 1.5; 4.0; 10.5 and 30 °C, respectively. High supersaturation levels should be avoided to obtain high yields in the process.