Passive microbial electrochemical system for nitrogen removal in free surface water – LowNitrate

The goal of LowNitrate is to design a passive microbial electrochemical technology applicable in artificial wetlands or other similar ecosystems for nitrogen (NO3-) removal. The system is based on the harvesting of diffuse energy from sediment to induce a low-cost and efficient denitrification. A key idea is to induce local anoxic zones to promote denitrification without affecting the ecosystem. The decrease of nitrate concentration from surface water, especially runoff and drained water from intensive agriculture, is of prime importance. For this purpose, implementation of buffer zones as nitrogen retention ecosystems has to be optimized for denitrification processes at watershed scale in order to reduce land occupation and at the same time increase nitrate removal. IRSTEA develops a restauration program involving an artificial wetland built in 2010 at Rampillon (Seine et Marne) to demonstrate the potential for reducing the environmental impact of agriculture and provide safer/cleaner water resources. Runoff water input in the artificial wetland contains nitrate at concentration that can be higher than 50 mg/L (the threshold in drinking water is 50 mg/L, a lower threshold of 25 mg/L is recommended in surface water). A decrease of NO3- concentration was observed from the inlet to the outlet of the wetland but improved treatment must be proposed. The system we propose is a low-cost 3D-electrode allowing a more active microbial denitrification at the sediment-water interface. Microbial oxidation of organics from sediments will produces a current that is transferred to the upper zones at which microbial O2 and NO3- reduction occurs. This system is based on the principle of a sediment microbial fuel cell, but contrarily to this latter application, the energy taken from sediments will not be recovered but directly used for remediation. In this configuration, the microbial electrochemical system becomes passive and fully integrated in the artificial wetland with minimum upkeep. No additional electrical set-up is needed because no electrical energy is recovered nor provided from/to the system. Nitrate reduction occurs at the sediment water interface and is inhibited in the presence of oxygen. We hypothesize that denitrification will be accelerated by a proper electrode design and implementation in the wetland. The fundamental investigations in LowNitrate will be dedicated to the design of 3D-electrodes inducing a local anoxic area. This property will be used for selective growth of biofilm in a natural environment allowing (1) O2 reduction and (2) NO3- reduction. The optimal microbial electrochemical system will be evaluated in the artificial wetland managed by IRSTEA at Rampillon. The quantitative objective is at least to double the denitrification rate that is typically below 350 mg N-NO3-/m2/day. We have the ambition to produce a detailed study of the ecology associated with denitrification reactions in the wetland of Rampillon. A microbial indicator of the technology will be developed. A quantitative model of the denitrification in the flow conditions of the wetland will be elaborated. The final product developed in the project is a low cost/efficient 3D-electrode for faster denitrification in an artificial wetlands.