Potential impact on calcium phosphate recovery by hydrogen in anaerobic digestion of cow manure
The use of secondary source phosphorus is essential to secure future phosphorus supply. Animal manure is the largest secondary phosphorus source and cow manure (CM) is the most abundant animal manure in the European Union. Efficient phosphorus recovery from CM is essential to enable feasible valorization of phosphorus from CM. Calcium phosphate (CaP) granulation has a high potential to recover phosphorus in concentrated form during anaerobic digestion. The presence of inorganic carbon can inhibit CaP granulation, because calcium can precipitate as calcium carbonate in the presence of inorganic carbon instead of forming calcium phosphate [Cax(PO4)y]. In this study, hydrogen gas was added to stimulate hydrogenotrophic methanogenesis (HM) for converting inorganic carbon into methane and possibly inducing CaP granulation. The hydrogen gas addition was compared with a reactor with nitrogen gas addition with similar mixing conditions but no stimulated HM. The addition of hydrogen gas increased the methane production from 74 to 198 L-CH4 kgVS-1 and the overall biogas production from 5 ± 2 to 9 ± 2 L d-1. This resulted in a 10–25% removal of inorganic carbon from the sludge bed. The relative abundance of microorganisms responsible for HM significantly increased in granules > 2.5 mm. The phosphorus removal increased slightly from 30% to 34% with hydrogen gas addition, but no (Cax(PO4)y formed. A PHREEQC simulation indicated that the removal of inorganic carbon would need to be increased 6-fold to induce Cax(PO4)y precipitation. Optimizing the availability of hydrogen gas for HM by increasing the reactor height could increase the inorganic carbon removal and eventually induce CaP granulation.