Mechanisms and Model Process Parameters in Bioelectrochemical Wet Phosphate Recovery from Iron Phosphate Sewage Sludge

Phosphate recovery from sewage sludge is possible with a bioelectrochemical system (BES) also referred to as microbial fuel/ electrolysis cell (MFC, MEC). The investigated process is based on phosphate removal with iron salts, which is extensively used in wastewater treatment. The mechanisms and reaction parameters of the bioelectrochemical phosphate recovery process was examined by modeling and model reactions for future scale up works. The mechanistic analyses concerned the electron reduction process, the role of the pH as well as the observed metal removal capacity. Iron oxidation state analyses showed that the iron reduction mechanism was of negligible importance under microbial electrolysis cell conditions. The cathodic iron reduction was outperformed by fast iron precipitation and phosphate remobilization process depended largely on chemical base (OH-). Fluid particle kinetics and shrinking core modeling determined the relevancy of the reaction parameters in order to accelerate phosphate remobilisation. Rate enhancements were possible at higher pH, increased temperature and faster stirring. With the elucidated mechanisms and reaction kinetics parameters, the scale-up of bioelectrochemical system based phosphate recovery was given a foundation for scale-up works.