Assessment of the effectiveness of process-induced struvite crystallization process on the recovery of nutrients from high-strength liquid digestate of an anaerobic biogas plant

Abstract

This study aimed to recover magnesium (Mg2+), ammonium (NH4+) and phosphate (PO43-) as struvite from high-strength liquid digestate (LD) of an anaerobic biogas plant, where poultry manure was processed for biogas production. Effluent quality after the struvite crystallization process was investigated in terms of the change in Mg2+, NH4+ and PO43- concentrations. The change in chemical oxygen demand (COD) concentrations was also investigated to determine whether the struvite formation could help to decrease extremely high initial concentrations in the LD. Results showed that a certain level of Mg2+, NH4+, PO43-, and COD removal occurred under the optimized experimental conditions, leaving an effluent with relatively high residual concentrations. To further reduce the residual concentrations, an attempt was made by performing a secondary struvite crystallization process as a process-induced additional treatment step, which has never been investigated in the literature. The results revealed that an improved quality of effluent was achieved after the process-induced struvite crystallization process as compared to the single-step process. The morphology and the elemental composition of the produced precipitates confirmed by XRD and EDS analysis showed that the structure was typical of struvite crystals. This study clearly demonstrates that the process-induced struvite crystallization process could effectively lessen the adverse environmental impacts arising from the problematic wastes of biogas plants. The cost analysis reveals that low-cost alternative sources of Mg and PO4, such as wastes rich in Mg and/or P content, should be used instead of high purity reagents to improve the economic sustainability of the process.