Sulphur transformations in constructed wetlands for wastewater treatment: A review

The sulphur-cycle processes inside constructed wetlands (CWs) and their role in the complex network of transformation processes and the long-term stability of wetland systems are still not sufficiently understood. A comprehensive review is accordingly necessary for better understanding to optimize the future design and operation of wetland systems. In this paper, the concept of sulphur cycling in CWs in principle, transformations dynamics of sulphur compounds and their interactions with other element cycles (such as carbon, nitrogen and phosphorus) and plants, as well as importance for treatment efficiencies are reviewed. The abiotic and biotic transformations of sulphur are both spatially and temporally dynamic in CWs. Next to physico-chemical processes, e.g. mineral precipitation and dissolution, biologically catalyzed redox reactions such as assimilatory and dissimilatory sulphate reduction, as well as oxidation/reduction, and disproportionation of more reduced sulphur compounds also simultaneously occur in the rooted zones of CWs. Moreover, the interactions between sulphur transformations and other processes including microbial transformations of carbon, nitrogen and phosphorus, as well as the impact of plants are discussed. Briefly, sulphide as a product of dissimilatory sulphate reduction may remove heavy metals from wastewaters via the precipitation of metal sulphide and the use of CWs in treating heavy metal containing waters (e.g. acid mine drainage) is increasing. However, high sulphide concentrations may also negatively affect growth of wetland plants and microbial activities, especially nitrification, but also influence the mobilization of iron-bound phosphorus. This information may be used to offer future design and operational methodologies that might enhance the performance of contaminants removal and drive a sustainable management of CWs for wastewater treatment.