Improvement of nitrogen removal with iron scraps in floating treatment wetlands

Floating treatment wetland (FTW) in restoration of low C/N ratio wastewater was deemed to a frequently used method. However, the nitrate removal performance in floating beds was limited due to insufficient organic carbon sources. Iron scraps as a potential electron donor was beneficial to the NO₃^--N reduction. To research the removal performance and mechanism of denitrification in FTW with iron scraps, FTW with Iris pseudacorus was built, and iron scraps were added as an electron donor to improve nitrogen removal efficiency. The batch experimental results demonstrated that the proper mass ratio of iron scraps to NO₃^--N was 500:1. With iron scraps, the NO₃^--N removal efficiency of FTW and control system increased significantly to 98.04% and 44.42% respectively in 2 weeks, while there was no obvious influence on the removal of NH₄⁺-N. After adding iron scraps, the proportion of bacteria in the systems related to iron cycle and the relative abundance of nitrifying and denitrifying bacteria have increased obviously. By calculating the nitrogen balance, nitrogen reduction via plant uptake accounted for 8.79%, and the microbial denitrification was the main nitrogen removal pathway in FTW. Graphical abstract: [Figure not available: see fulltext.]