Occurrence and Source of Phosphite (HPO32–) during Municipal Wastewater Treatments

The occurrence, fate, and source of phosphite (HPO₃^2–), a highly soluble, reduced phosphorus (P) compound that is often overlooked in the P biogeochemical cycle, were investigated in municipal wastewater treatment systems. Wastewater samples were collected from influent wastewater, preprimary and postprimary clarifiers, postaeration basin, postsecondary clarifier, return activated sludge (RAS), final discharge, anaerobic digester influents, and effluents. HPO₃^2– was present in wastewater samples but varied depending on the source. Highest concentrations were measured in anaerobic digester influents and effluents (50–60 μg L^–1). Based on the studies to evaluate the source of HPO₃^2–, it appears that the decomposition of organic matter, particularly solids rich in microbial biomass (secondary solids versus primary), is responsible for the elevated concentrations observed and that elevated temperatures increase its production and may prevent its use by dissimilatory phosphite oxidizers. It also helps to explain the previous reports of dissimilatory HPO₃^2–-oxidizing bacteria in wastewater treatment systems, particularly digesters, and suggests organic matter decay as a possible source for previously observed HPO₃^2– concentrations in sediments. The knowledge of the occurrence and source of HPO₃^2– may shed light on its important and hitherto unrecognized role in the global P cycle.