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.