Iron-doped biocarriers enhanced nitrogen removal from saline wastewater in an unplanted constructed wetland-microbial electrolytic cell system

Enhanced nitrogen removal by an iron-doped biocarrier (IDBC) in an unplanted constructed wetland-microbial electrolytic cell (UCW-MEC) system was investigated. The results showed that the UCW-MEC with IDBC achieved efficient nitrogen removal from saline wastewater. IDBC had excellent physicochemical properties and had a significant effect on UCW-MEC for nitrogen removal (p < 0.05). The system with IDBC achieved optimal performance at 1.4 V (Current density 0.154 A/m²) for efficient nitrogen removal, with removal efficiencies of 70.9 %, 84.2 % and 87.2 % for ammonia (NH₄⁺-N), total nitrogen (TN), and total organic carbon (TOC), respectively. The nitrate (NO₃^--N) and nitrite (NO₂^--N) concentrations were reduced from 14 mg/L and 1 mg/L in the influent to 0.78 mg/L and 0.06 mg/L in the effluent, respectively. The relative abundances of denitrifiers such as Xanthomarina, Sedimenticola, Marinicella, and Marinifilum significantly increased in the system (p < 0.05). The results indicated that iron promoted the enrichments of denitrifiers such as Sedimenticola on the IDBC and cathode, and Marinifilum and Xanthomarina on the IDBC. The voltage of 1.4 V increased the abundances of the denitrifiers, Marinicella and Oceanospirillum. Additionally, Iron-redox microbes colonized both biocarriers and activated carbon cathodes. A mechanism framework for the iron-enhanced nitrogen removal in the UCW-MEC with IDBC was proposed. This study provides insights into saline wastewater treatment using IDBC-enhanced UCW-MEC technology.