• Energy Research

AbstractGrowing scientific interest in mixed microbial culture‐based anaerobic biotechnologies for the production of value‐added chemicals and fuels from organic waste residues requires a parallel focus on the development and implementation of strategies to control the distribution of products. This study examined the feasibility of an electrofermentation approach, based on the introduction of a polarized (−700 mV vs. the standard hydrogen electrode) graphite electrode in the fermentation medium, to steer the product distribution during the conversion of organic substrates (glucose, ethanol, and acetate supplied as single compounds or in mixtures) by undefined mixed microbial cultures. In batch experiments, the polarized electrode triggered a nearly 20‐fold increase (relative to open circuit controls) in the yield of isobutyrate production (0.43±0.01 vs. 0.02±0.02 mol mol−1 glucose) during the anaerobic fermentation of the ternary mixture of substrates, without adversely affecting the rate of substrate bioconversion. The observed change in the fermentative metabolism was most likely triggered by the (potentiostatic) regulation of the oxidation–reduction potential of the reaction medium rather than by the electrode serving as an electron donor.

Here, a 12-liter tubular microbial electrolysis cell (MEC) was developed as a post treatment unit for simultaneous biogas upgrading and ammonium recovery from the liquid effluent of an anaerobic digestion process. The MEC configuration adopted a cation exchange membrane to separate the inner anodic chamber and the external cathodic chamber, which were filled with graphite granules. The cathodic chamber performed the CO2 removal through the bioelectromethanogenesis reaction and alkalinity generation while the anodic oxidation of a synthetic fermentate partially sustained the energy demand of the process. Three different nitrogen load rates (73, 365, and 2229 mg N/Ld) were applied to the inner anodic chamber to test the performances of the whole process in terms of COD (Chemical Oxygen Demand) removal, CO2 removal, and nitrogen recovery. By maintaining the organic load rate at 2.55 g COD/Ld and the anodic chamber polarization at +0.2 V vs. SHE (Standard Hydrogen Electrode), the increase of the nitrogen load rate promoted the ammonium migration and recovery, i.e., the percentage of current counterbalanced by the ammonium migration increased from 1% to 100% by increasing the nitrogen load rate by 30-fold. The CO2 removal slightly increased during the three periods, and permitted the removal of 65% of the influent CO2, which corresponded to an average removal of 2.2 g CO2/Ld. During the operation with the higher nitrogen load rate, the MEC energy consumption, which was simultaneously used for the different operations, was lower than the selected benchmark technologies, i.e., 0.47 kW/N·m3 for CO2 removal and 0.88 kW·h/kg COD for COD oxidation were consumed by the MEC while the ammonium nitrogen recovery consumed 2.3 kW·h/kg N.

A methane-producing microbial electrolysis cell (MEC) was continuously fed at the anode with a synthetic solution of soluble organic compounds simulating the composition of the soluble fraction of a municipal wastewater. The MEC performance was assessed at different anode potentials in terms of chemical oxygen demand (COD) removal efficiency, methane production, and energy efficiency. As a main result, about 72-80% of the removed substrate was converted into current at the anode, and about 84-86% of the current was converted into methane at the cathode. Moreover, even though both COD removed and methane production slightly decreased as the applied anode potential decreased, the energy efficiency (i.e., the energy recovered as methane with respect to the energy input into the system) increased from 54 to 63%. Denaturing gradient gel electrophoresis (DGGE) analyses revealed a high diversity in the anodic bacterial community with the presence of both fermentative (Proteiniphilum acetatigenes and Petrimonas sulphurifila) and aerobic (Rhodococcus qingshengii) microorganisms, whereas only two microorganisms (Methanobrevibacter arboriphilus and Methanosarcina mazei), both assignable to methanogens, were observed in the cathodic community.

We investigated the effect of a starch/acetate mixture on the formation of intracellular storage biopolymers compared with system behaviour where these compounds served as a single organic substrate. Three laboratory-scale sequencing batch reactors (SBRs) were operated at steady state with a sludge age of 8 days, one fed with acetate, another with starch and the third with a starch/acetate mixture. The SBR operation involved six cycles per day and continuous feeding during each cycle. Both acetate and starch generated storage biopolymers under continuous feeding. A poly-hydroxybutyrate (PHB) or glycogen pool was formed depending on the selected single substrate. In addition, around 18% of the acetate fed within each cycle was converted to PHB, while the remaining 82% was directly utilized for microbial growth. A higher glycogen formation of 44% was observed for starch. Substrate storage as PHB and glycogen continued with the feeding of the acetate/starch mixture. This observation, supported by microbiological analyses, indicated that the acclimated biomass in the corresponding SBR system sustained microbial fractions capable of performing metabolic functions associated with the formation of the two storage biopolymers. PHB accumulation was reduced as acetate could be more readily used for direct microbial growth in the presence of starch.

Agricultural waste is a huge pool of untapped biomass resources that may even represent economic and environmental burdens. They can be converted into bioenergy and bio-based products by cascading conversion processes, within circular economy, and should be considered residual resources. Major challenges are discussed from a transdisciplinary perspective, focused on Europe situation. Environmental and economic consequences of agricultural residue management chains are difficult to assess due to their complexity, seasonality and regionality. Designing multi-criteria decision support tools, applicable at an early-stage of research, is discussed. Improvement of Anaerobic Digestion (AD), one of the most mature conversion technologies, is discussed from a technological point of view and waste feedstock geographical and seasonal variations. Using agricultural residual resources for producing high-value chemicals is a considerable challenge analysed here, taking into account innovative eco-efficient and cost-effective cascading conversion processes (bio-refinery concept). Moreover, the promotion of agricultural residues-based business is discussed through industrial ecology, to promote synergy, on a local basis, between different agricultural and industrial value chains. Finally, to facilitate a holistic approach and optimise materials and knowledge flows management, the connection of stakeholders is discussed to promote cross-sectorial collaboration and resource exchange at appropriate geographic scales.

The production of polyhydroxyalkanoates (PHAs) by activated sludge selected in a sequencing batch reactor (SBR) has been investigated. Several SBR runs were performed at the same applied organic load rate (OLR), hydraulic retention time (HRT) and feed concentration (8.5 g COD L(-1) of volatile fatty acids, VFAs) under aerobic conditions. The effect of the feeding time was only evaluated with a cycle length of 8h; for this particular cycle length, an increase in the storage response was observed by increasing the rate at which the substrate was fed into the reactor (at a fixed feeding frequency). Furthermore, a significantly stronger effect was observed by decreasing the cycle length from 8h to 6h and then to 2h, changing the feed frequency or changing the organic load given per cycle (all of the other conditions remained the same): the length of the feast phase decreased from 26 to 20.0 and then to 19.7% of the overall cycle length, respectively, due to an increase in the substrate removal rate. This removal rate was high and similar for the runs with cycle lengths of 2h and 6h in the SBR. This result was due to an increase in the selective pressure and the specific storage properties of the selected biomass. The highest polymer productivity after long-term accumulation batch tests was 1.7 g PHA L(-1)d(-1), with PHA content in the biomass of approximately 50% on a COD basis under nitrogen limitation. The DGGE profiles showed that the good storage performance correlated to the development of Lampropedia hyalina, which was only observed in the SBR runs characterized by a shorter cycle length.