Impact of feedwater protein contents on calcium phosphate mineralization in anaerobic digesters

Calcium phosphate (CaP) mineralization and accumulation in anaerobic digesters represent an attractive approach to recovery phosphorus (P) from wastewater. Previous studies demonstrated that under certain anaerobic digestion conditions, favorable localized environment can be developed to facilitate CaP granule growth. However, factors that trigger the CaP nucleation in anaerobic reactors have not been elucidated. In this study, to examine the driving forces in the microenvironment for the CaP nucleation, two laboratory upflow anaerobic sludge blanket (UASB) reactors fed with two types of synthetic wastewater were operated under mesophilic conditions. The feed of one reactor (G reactor) had glucose as the sole carbon source, and the feed of the other reactor (G+B reactor) had glucose (60% chemical oxygen demand (COD)) and bovine serum albumin (BSA, 40% COD) as the combined carbon sources. The results showed that P and Ca removals were only observed in the G+B reactor. The main difference between the two reactors was the elevated pH in the G+B reactor, which may be attributed to the degradation of amino acids. The elevated pH caused the deprotonation of the negatively charged functional groups in the sludge, creating available active surfaces for Ca2+ complexation. The high availability of OH - and the enriched Ca in the G+B reactor built a favorable microenvironment to overcome the activation energy barriers hindering the CaP nucleation. The stabilized CaP mineralization largely depended on the well-established microbial community, where the efficient hydrogenotrophic methanogens and syntrophic acetogens may maintain a stabilized pH environment to prevent the dissociation of CaP minerals.

Automated summary

This study demonstrated that increasing the pH in anaerobic reactors can promote the nucleation and mineralization of calcium phosphate, facilitating phosphorus removal. The presence of amino acids in wastewater, along with microbial communities including acetogens and methanogens, plays a crucial role in maintaining pH stability and protecting the stability of calcium phosphate minerals.