The electrochemical microbial tree: A new concept for wastewater treatment

•An EMT is a conductive porous 3D structure whose upper section is exposed to air.•The EMT supports a biofilm whose aerial upper section is humidified by capillarity.•Organic matter is sucked up from the wastewater and oxidised on the aerial section.•pH evolution shows that a snorkel phenomenon occurred due to support conductivity.•COD removal rates were increased by a factor of 2.6 without any energy input. Most wastewater treatment plants involve activated sludge units, in which the organic matter to be removed is oxidised by aerobic microorganisms. These units are highly energy intensive because of the power consumed to force oxygen transfer to the wastewater by aeration. An innovative device is presented here; it is called the “electrochemical microbial tree (EMT)” and is based on the opposite strategy: the organic matter is drawn up from the wastewater towards the air phase by capillary action along a porous structure, which hosts the microorganisms that oxidise the organic matter. The EMTs were made of carbon felt, with the bottom immersed in wastewater (14 cm), while the top emerged into the air at different heights (4, 8 or 12 cm). COD removal increased linearly with the height of the aerial section. The greatest height led to COD removal rates of 807 ± 62 mg O2/L/h, i.e. 2.6 times those of the control experiments. During COD removal, the pH decreased from 7.6 to 7.4 ± 0.2 with EMTs, while it increased to 7.9 ± 0.1 in the controls. The biofilm on the immersed section developed as the height of the aerial section increased. Many electroactive species were identified in the microbial populations, belonging to the Bacteroidia, Gammaproteobacteria and Clostridia classes. These observations revealed that electron transfer along the conductive felt contributed to organic matter oxidation, in parallel with mass transfer by capillarity. These pioneering results present the EMT as a promising new wastewater treatment disposal system that does not require any energy input.