The challenge of non-reactive phosphorus: Mechanisms of treatment and improved recoverability using electrooxidation

Abstract

Recalcitrant phosphorus (P) species, i.e., soluble non-reactive phosphorus (sNRP), are generally not effectively removed or recovered in conventional wastewater treatment processes. This was substantiated in our meta-analysis, which showed that nearly one-third of wastewater facilities’ effluent P was primarily in the non-reactive form. Transformation of sNRP to more readily removable/recoverable soluble reactive phosphorus (sRP) may offer a viable pathway to enhance P removal and recovery. Electrooxidation (EO) may offer one route for sNRP to sRP transformation. During EO, different sNRP transformation pathways may occur, influencing the extent and efficiency of sNRP transformations as a function of water quality. To explore these mechanisms, we conducted oxidant quenching tests as well as cyclic voltammetry and chronoamperometry experiments using a synthetic water matrix spiked with the sNRP compound beta-glycerol phosphate (BGP). We found that direct electron transfer was responsible for BGP transformation. To assess the applicability of EO for wastewater sNRP to sRP transformation and improved recoverability, EO was used to treat municipal wastewater centrate, followed by tests of sNRP recoverability using the P-selective LayneRT™ ion exchanger. Complete transformation of centrate sNRP to sRP was not achieved with EO, but subsequent removal of sNRP using ion exchange increased after 2 hr of EO treatment. Longer periods of EO treatment did not improve sNRP removal. Improved sNRP adsorption after EO was likely due to decreased competing organics in the centrate after EO treatment. Overall, this study showed that EO can improve sNRP removal using subsequent ion exchange and facilitate enhanced P recovery.