Nitrification, denitrification and anammox process coupled to iron redox in wetlands for domestic wastewater treatment

Abstract Constructed wetlands (CWs) is widely used in rural areas as low-cost wastewater treatment technology. However, in rural domestic wastewater treatment using subsurface CWs, the lack of oxygen for nitrification and a carbon source for denitrification is often responsible for low removal of nitrogen. As organic carbon addition requires human and financial investment, which is not suitable for long-term utilization in rural areas. Other electron donors become the preferred choice. Compared to granulated iron materials, the iron scraps with fluffy structure could avoid clogging problem and be effective for nitrogen removal simultaneously. In this study, two CWs were constructed using gravel and iron scraps as substrate, respectively. After 6 months of operation, the total nitrogen and ammonia removal in CWs with iron improved to 71.46% and 70.44%, respectively. Enhancement of 24.08% organic nitrogen removal was observed in iron zone of CWs. Autotrophic denitrifying bacteria became the dominant genus in the system. Traditional nitrification-denitrification and anaerobic ammonia oxidation process could be the primary nitrogen removal pathway in CWs with iron. The presence of iron-reducing bacteria (Anaeromyxobacter, Geobacter, and Geothrix) provided the possibility of iron oxide reduction reactions. Electron transfer and products in iron oxidation and reduction processes significantly affected on nitrogen removal to complete the coupling of iron cycle and nitrogen cycle in CWs.