Nitrogen and phosphorus recovery from cow digestate by struvite precipitation: Process optimization to maximize phosphorus recovery

The combination of the anaerobic digestion of waste biomass (e.g., livestock manure, energy crops and organic wastes) and the digestate treatment allows green energy to be obtained (starting from biogas/biomethane) and nutrients to be returned to the soil, satisfying the principles of the circular economy. Digestate has valuable potential as an organic fertilizer and soil amendment, as it can be an important source of nutrients, nitrogen and phosphorus (N and P), essential for crop growth. However, digestate production sometimes exceeds the capacity of nearby lands to use the nutrients. Therefore, the recovery of nutrients from digestate has become an important task for anaerobic digestion plants, to meet current regulations. In this study, a method combining struvite precipitation and ammonia stripping was tested in the laboratory to better understand the nutrient removal/recovery efficiency of the two nutrients. Results indicate that more than 60% of total P was removed as struvite, which can be used in N–P fertilizer, substituting synthetic mineral fertilizers. The recovery of N and P from digestate by struvite crystallization allows the removal of nutrient excess at farm level, reducing the environmental impacts of using slurry/digestate in agriculture. The recovery of struvite allows the production of a fertilizer that can be easily exported from farms, promoting the circular economy.