Phosphorus precipitation in septic systems induced by iron reduction: a process for phosphorus removal under anaerobic conditions.

Phosphorus in the effluents from decentralized septic tanks in the northeastern US imposes the threat of eutrophication on the sensitive receiving water bodies. Dissimilar to the popular nutrient-removal processes used in the wastewater-treatment plants, a lab-scale septic system retrofitted by iron amendment was developed in this study to stimulate phosphate precipitation in septic tanks. These results demonstrate that iron supplementation can effectively remove aqueous phosphorus in the septic system. In a stabilized anaerobic sequencing batch reactor (ASBR), more than 96% of the phosphorus was removed when iron was added at an Fe/P (molar ratio of Fe(III) addition to aqueous P) of 3. The iron demand for phosphorus precipitation in batch assays was considerably higher (an Fe/P ratio of 10 to achieve a phosphorus removal of 94.5%), indicating the presence of additional pathways for Fe(II) consumption, which was most likely due to the accumulation of extra Fe(II) consumers in the septic materials sampled. The removal of the soluble chemical oxygen demand in the ASBR was improved by 21–44% when the Fe/P ratio was increased from 1.5 to 3, suggesting that organic carbon was used as an electron donor for iron reduction; therefore, carbon mineralization was enhanced. Analysis of X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy show that vivianite was the main secondary mineral resulting from the iron reduction in the septic system. [ABSTRACT FROM PUBLISHER]