Phosphorus recovery from pig manure : Dissolution of struvite and formation of calcium phosphate granules during anaerobic digestion with calcium addition

Summary

Phosphorus is an essential but finite and scarce element, which is used extensively in pig farming. For recovering phosphorus from pig manure, we hypothesize that calcium addition during anaerobic treatment can trigger calcium phosphate granulation and enable efficient phosphorus recovery. In this study, we tested the recovery of phosphorus from pig manure by adding calcium during anaerobic treatment. Size–separated (<200 µm) pig manure was treated in two identical up-flow anaerobic sludge blanket reactors for 456 days. Calcium was supplied to one of the reactors to study calcium phosphate granulation. Calcium addition showed a positive effect on treatment stability and efficiency, increasing chemical oxygen demand and phosphorus removal. Phosphorus removal increased from 60% without calcium addition to 74% with calcium addition. The addition of calcium increased the phosphorus concentration in significantly larger particles, calcium phosphate granules, (79% >0.4 mm compared with 11% without calcium addition) in the reactor with calcium addition, which enables separation and a transport reduction of 85% for the recovered phosphorus. The main phosphorus phase in the reactor sludge bed changed from struvite to calcium phosphate with calcium addition. The Mg:P molar ratio was 0.09 with calcium addition and 0.88 without calcium addition in particles >2.5 mm. Thus, calcium addition enhances the treatment of pig manure and enables phosphate recovery as calcium phosphate granules.