Carbon source priority and availability limit bidirectional electron transfer in freshwater mixed culture electrochemically active bacterial biofilms.

This study investigated, if a mixed electroactive bacterial (EAB) culture cultivated heterotrophically at a positive applied potential could be adapted from oxidative to reductive or bidirectional extracellular electron transfer (EET). To this end, a periodic potential reversal regime between - 0.5 and 0.2 V vs. Ag/AgCl was applied. This yielded biofilm detachment and mediated electroautotrophic EET in combination with carbonate, i.e., dissolved CO2, as the sole carbon source, whereby the emerged mixed culture (S1) contained previously unknown EAB. Using acetate (S2) as well as a mixture of acetate and carbonate (S3) as the main carbon sources yielded primarily alternating electrogenic organoheterotropic metabolism with the higher maximum oxidation current densities recorded for mixed carbon media, exceeding on average 1 mA cm-2. More frequent periodic polarization reversal resulted in the increase of maximum oxidative current densities by about 50% for S2-BES and 80% for S3-BES, in comparison to half-batch polarization. The EAB mixed cultures developed accordingly, with S1 represented by mostly aerobes (84.8%) and being very different in composition to S2 and S3, dominated by anaerobes (96.9 and 96.5%, respectively). S2 and S3 biofilms remained attached to the electrodes. There was only minor evidence of fully reversible bidirectional EET. In conclusion the three triplicates fed with organic and/or inorganic carbon sources demonstrated two forms of diauxie: Firstly, S1-BES showed a preference for the electrode as the electron donor via mediated EET. Secondly, S2-BES and S3-BES showed a preference for acetate as electron donor and c-source, as long as this was available, switching to CO2 reduction, when acetate was depleted.

Examining sludge production in bioelectrochemical systems treating domestic wastewater.

Sludge production in microbial bioelectrochemical systems (BES) was assessed in conjunction with anaerobic and aerobic control reactors. Effluent after primary settling tank (EAPS) and depleted EAPS spiked with acetate were treated. The reactors were loaded with total suspended solids (TSS) and chemical oxygen demand (COD) at average loading rates of 22 mg TSS d(-1)L(-1) and 86 mg COD d(-1)L(-1), respectively. Carbon cloth anode equipped BES reactors delivered the highest performance. They achieved on average a COD removal of 80%, a Coulomb efficiency of 77% for EAPS, a maximum current density of 39 µA cm(-)(3)/175 µA cm(-)(2) for EAPS and a TSS removal of 59%, yielding a sludge production of only 80 mg TSS per g ΔCOD. This study provides further evidence that BES can improve the economics of wastewater treatment via lower sludge production as well as providing a framework for understanding sludge production in BES.

Evaluating the effects of scaling up on the performance of bioelectrochemical systems using a technical scale microbial electrolysis cell.

This study focuses on the challenges of the scaling up process of bioelectrochemical systems on the example of a technical scale microbial electrolysis cell referred to as the "prototype". Anodically treating real wastewater and operated in continuous mode at a hydraulic retention time of 1.23 d with an average chemical oxygen demand (COD)-loading rate of 0.5 g O2 d(-1) L Reactor(-1) the prototype on average showed COD removal efficiency of 67% with effluent concentrations of 210 mg O2 L(-1) and an ammonium elimination rate of 17.8 ± 3.9 mg Nd(-1) L Reactor(-1) resulting in effluent concentrations of 30.7 ± 3.7 mg NL(-1) with a removal efficiency of 40% at a current generation of 72 µA cm(-2) and Coulomb efficiency of 11%. A model is described as a method for comparing conventional and BES based technology using the above mentioned criteria and balancing them against the respective loading rates.