Phosphorus recovery from liquid digestate by chemical precipitation using low-cost ion sources

BACKGROUND

Phosphorus (P) recovered and recycled from agricultural waste, such precipitation of struvite (MgNH₄PO₄.6H₂O) and calcium phosphates from the effluent after anaerobic digestion of wastes, is an important approach to developing environmentally sustainable fertilizers. However, for controlled phosphorus precipitation/recovery, it is usually necessary to supplement Ca²⁺ or Mg²⁺ ions; this represents more than 75% of the total operational costs and, as a result, hinders a more extensive application of this process. Therefore, there is a need to investigate alternative and low-cost cation sources to recover phosphorus by precipitation. This study investigated limestone powder as calcium source and seawater as a magnesium source for phosphorus recovery by precipitation from the liquid fraction of digestate. The main parameters affecting the efficiency of the precipitation process were assessed under (i) four different pH values and (ii) three different molar ratios (Ca or Mg: P).

RESULTS

The maximum phosphate removal rate was 44% for limestone powder at molar ratio 2:1 and pH 7. Higher phosphate removal using limestone powder was possibly inhibited by calcite precipitation. For seawater, the phosphate removal rate was 65% at molar ratio 2:1 and pH 10. Results of experiments using seawater at molar ratio 2:1 showed a decrease in K⁺ concentration and phosphate, suggesting that K-struvite precipitation may have occurred.

CONCLUSION

Limestone powder and seawater have a great potential to be used partially or totally as ion source for industrial recovery of P, mainly due to its very low cost. The recovered product could potentially be used as soil amendment and/or as phosphate rock substitute for fertilizer production. © 2021 Society of Chemical Industry (SCI).