Microbial fuel cells (MFCs) that remove carbon as well as nitrogen compounds out of wastewater are of special interest for practice. We developed a MFC in which microorganisms in the cathode performed a complete denitrification by using electrons supplied by microorganisms oxidizing acetate in the anode. The MFC with a cation exchange membrane was designed as a tubular reactor with an internal cathode and was able to remove up to 0.146 kg NO(3-)-N m(-3) net cathodic compartment (NCC) d(-1) (0.080 kg NO(3-)-N m(-3) total cathodic compartment d(-1) (TCC)) at a current of 58 A m(-3) NCC (32 A m(-3) TCC) and a cell voltage of 0.075 V. The highest power output in the denitrification system was 8 W m(-3) NCC (4 W m(-3) TCC) with a cell voltage of 0.214 V and a current of 35 A m(-3) NCC. The denitrification rate and the power production was limited bythe cathodic microorganisms, which only denitrified significantly at a cathodic electrode potential below 0 V versus standard hydrogen electrode (SHE). This is, to our knowledge, the first study in which a MFC has both a biological anode and cathode performing simultaneous removal of an organic substrate, power production, and complete denitrification without relying on H2-formation or external added power.

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Conventional anaerobic digestion based bioconversion processes produce biogas and have as such been widely applied for the production of renewable energy so far. An innovative technology, based on the use of microbial fuel cells, is considered as a new pathway for bioconversion processes towards electricity. In comparison with conventional anaerobic digestion, the microbial fuel cell technology holds some specific advantages, such as its applicability for the treatment of low concentration substrates at temperatures below 20 °C, where anaerobic digestion generally fails to function. This provides some specific application niches of the microbial fuel cell technology where it does not compete with but complements the anaerobic digestion technology. However, microbial fuel cells still face important limitations in terms of large‐scale application. The limitations involve the investment costs, upscale technical issues and the factors limiting the performance, both in terms of anodic and cathodic electron transfer. Research to render the microbial fuel cell technology more economically feasible and applicable should focus on reactor configuration, power density and the material costs.

Previous studies revealed the abundance of Pseudomonas sp. in the microbial community of a microbial fuel cell (MFC). These bacteria can transfer electrons to the electrode via self-produced phenazine-based mediators. A MFC fed with acetate where several Pseudomonas sp. were present was found to be rich in a Gram-positive bacterium, identified as Brevibacillus sp. PTH1. Remarkably, MFCs operated with only the Brevibacillus strain in their anodes had poor electricity generation. Upon replacement of the anodic aqueous part of Brevibacillus containing MFCs with the cell-free anodic supernatants of MFCs operated with Pseudomonas sp. CMR12a, a strain producing considerable amounts of phenazine-1-carboxamide (PCN) and biosurfactants, the electricity generation was improved significantly. Supernatants of Pseudomonas sp. CMR12a_Reg, a regulatory mutant lacking the ability to produce PCN, had no similar improvement effect. Purified PCN, together with rhamnolipids as biosurfactants (1 mg L(-1)), could clearly improve electricity generation by Brevibacillus sp. PTH1, as well as enable this bacterium to oxidize acetate with concomitant reduction of ferric iron, supplied as goethite (FeOOH). When added alone, PCN had no observable effects on Brevibacillus' electron transfer. This work demonstrates that metabolites produced by Pseudomonas sp. enable Gram-positive bacteria to achieve extracellular electron transfer. Possibly, this bacterial interaction is a key process in the anodic electron transfer of a MFC, enabling Brevibacillus sp. PTH1 to achieve its dominance.