The production of ferrous ions driven by anaerobic degradation of plant biomass and improved phosphorus removal in constructed wetlands

Abstract

A constructed wetland that contains two separate beds and produces ferrous ions (Fe2+) ex-situ by anaerobic iron-reducing between iron rich substrate and plant biomass, named as Fe-CONV-CW, was developed. Three experimental systems for tertiary treatment were constructed for the investigation of production of Fe2+ and phosphorus removal. One bed consisted of a gravel filter for wastewater treatment, and the other was filled with deciduous leaves (DL) and red soil (RS) to generate Fe2+ in the Fe–CONV-CWs. And the mass ratio of DL to RS was 1:2, 1:3 and 1:4 respectively in three systems, referred to as S1:2, S1:3, and S1:4. During 120 days of operation, the beds containing DL and RS continuously produced Fe2+, albeit at fluctuating concentrations. Within the first 60 days, the highest average concentration of Fe2+ was observed in S1:4 (40.3 ± 11.8 mg/L). After 60 days, the S1:3, which was temporarily supplemented with 0.15 kg of deciduous leaves, had the highest average Fe2+ (46.5 ± 11.4 mg/L). Phosphorus removal was improved greatly with entering and subsequent deposition of Fe2+ in gravel filters of systems and influenced by concentration of Fe2+. After 60 days, the increasing concentration of Fe2+ greatly promoted the improvement of phosphorus removal in S1:3. However, phosphorus removal was not stable due to the large fluctuations of Fe2+. Compared with the other two systems, S1:4 had more stable phosphorus removal levels during whole operation period. Both soluble organic compounds and lignocellulose in the leaves supported the production of Fe2+, with the former having a faster rate. Utilizing a high ratio of red soil during construction and temporary supplementation of deciduous leaves during operation could increase the production of ferrous irons and thus phosphorous removal in Fe-CONV-CW.