Valorisation of digestate through nitrogen and phosphorus recovery via chemical-physical precipitation and microbubble insufflation to reduce COD in the liquid

Abstract The combination of anaerobic digestion of waste biomass (e.g. livestock manure, energy crops and organic waste) and digestate treatment makes it possible to obtain green energy (from biogas/biomethane) and return nutrients to the soil, satisfying the principles of the circular economy. Digestate has valuable potential as an organic fertiliser and soil conditioner, as it can be an important source of nutrients, nitrogen and phosphorus (N and P), which are essential for crop growth. However, digestate production sometimes exceeds the capacity of farmland to utilise the nutrients. Therefore, the recovery of nutrients from digestate and organic matter becomes important for the sustainability of livestock farms and environmental protection. In this study, a method combining struvite precipitation and ammonia stripping was tested in the laboratory, allowing the recovery of N and P, while the presence of oxygen aeration allows for the reduction of the organic load (COD). The results obtained indicate that over 60% of the total P was removed in the form of struvite, which can be used as a fertiliser to replace fossil mineral fertilisers. The recovery of N and P from digestate through struvite crystallisation allows the removal of excess nutrients and COD, reducing the environmental impact of the use of slurry/digestate in agriculture. Finally, the recovery of struvite allows the production of a fertiliser that can be easily exported by farms, promoting the circular economy.