Understanding the risk of iron and phosphorus release from constructed wetlands treating chemically-dosed wastewater.

Abstract

Constructed wetlands (CW) consistently provide efficient wastewater treatment for the removal of organics and solids but have limitations when removing phosphorus. Recent changes in legislation have led to stricter consents for phosphorus discharge and the adaptation of CW based sites with the retrofitting of upstream dosing of chemical coagulant, such as iron salt. As part of this process, the sludge accumulating on the CW is enriched with phosphorus and iron and, although it was thought to be irreversible, evidence of release of both phosphorus and iron has been reported but the mechanisms remain unclear. This project then aimed to understand the risk, mechanisms, and appropriate management options of iron and phosphorus release from CWs treating chemically dosed wastewater. To meet this aim a variety of methodologies, including data mining for treatment performance, several biological assessment techniques to understand microbial dynamics, and controlled lab-scale trials to demonstrate the potential for release and evaluate remediation measures, were used. It was found that CW adaptation by iron dosing improved the robustness of the sites not only for phosphorus but also for BOD and suspended solids, at both current and potential future demand levels. The bacterial communities of CWs were unimpacted by the introduction of adaptations. The mechanism by which iron and phosphorus can be released from these sites was shown to require the establishment of appropriate environmental conditions that triggered the related microbial pathway, identified as the need for a source of iron, anaerobic conditions and an available carbon source. This triangle of needs gave rise to a mitigation strategy by dosing of nitrate which was demonstrated to be effective in supressing the release. This research showed that CW adaptation is required to robustly meet current and future demands however they must be operated correctly to ensure there is no re-release of pollutants.