Fe(III) reduction-mediated phosphate removal as vivianite (Fe3(PO4)2⋅8H2O) in septic system wastewater

• Onsite wastewater phosphate removal was increased by ferric iron amendment.
• The mineral vivianite was the dominant phosphate form recovered.
• Up to 14 mM phosphate was removed by ferric iron reducing microbes.
• This strategy can be applied to existing onsite wastewater treatment systems.

Phosphate is a water contaminant from fertilizers, soaps, and detergents that enters municipal and onsite wastewater from households, businesses, and other commercial operations. Phosphate is a limiting nutrient for algae, and is one of the molecules that promotes eutrophication of water bodies. Phosphate is especially problematic in onsite wastewater because there are few removal mechanisms under normal operating conditions; a system must be amended specifically with compounds to bond to or adsorb phosphate in the septic tank or within the leach field. Vivianite (Fe3(PO4)2⋅8H2O) is a stable mineral formed from ferrous iron and phosphate, often as the result of Fe(III) reducing microbial activity. What was unknown was the concentration of phosphate that could be removed by this process, and whether it was relevant to mixed microbial systems like septic tank wastewater. Data presented here demonstrate that significant concentrations of phosphate (12–14 mM) were removed as vivianite in growing cultures of Geobacter metallireducens strain GS-15. Vivianite precipitates were identified on the cell surfaces and within multi cell clusters using TEM–EDX; the mineral phases were directly characterized using XRD. Phosphate was also removed in dilute and raw (undiluted) septic wastewater amended with different forms of Fe(III) including solid phase and soluble Fe(III). Vivianite precipitates were recovered and identified using XRD, along with siderite (ferrous carbonate), which was expected given that the systems were likely bicarbonate buffered. These data demonstrate that ferric iron amendments in septic wastewater increase phosphate removal as the mineral vivianite, and this may be a good strategy for phosphate attenuation in the septic tank portion of onsite wastewater systems.