Recent studies have suggested that addition of electrically conductive biochar particles is an effective strategy to improve the methanogenic conversion of waste organic substrates, by promoting syntrophic associations between acetogenic and methanogenic organisms based on interspecies electron transfer processes. However, the underlying fundamentals of the process are still largely speculative and, therefore, a priori identification, screening, and even design of suitable biochar materials for a given biotechnological process are not yet possible. Here, three charcoal-like products (i.e., biochars) obtained from the pyrolysis of different lignocellulosic materials, (i.e., wheat bran pellets, coppiced woodlands, and orchard pruning) were tested for their capacity to enhance methane production from a food waste fermentate. In all biochar-supplemented (25 g/L) batch experiments, the complete methanogenic conversion of fermentate volatile fatty acids proceeded at a rate that was up to 5 times higher than that observed in the unamended (or sand-supplemented) controls. Fluorescent in situ hybridization analysis coupled with confocal laser scanning microscopy revealed an intimate association between archaea and bacteria around the biochar particles and provided a clear indication that biochar also shaped the composition of the microbial consortium. Based on the application of a suite of physico-chemical and electrochemical characterization techniques, we demonstrated that the positive effect of biochar is directly related to the electron-donating capacity (EDC) of the material, but is independent of its bulk electrical conductivity and specific surface area. The latter properties were all previously hypothesized to play a major role in the biochar-mediated interspecies electron transfer process in methanogenic consortia. Collectively, these results of this study suggest that for biochar addition in anaerobic digester operation, the screening and identification of the most suitable biochar material should be based on EDC determination, via simple electrochemical tests.

In order to make anaerobic digestion-based processes for short chain organic acid (SCOA) production attractive, the key performance variables, i.e., concentration, yield, and productivity of the produced SCOAs need to be maximised. This study analysed recent literature, looking for the effect of process operating parameters (feed concentration, pH, temperature, and residence time) on the performance variables. Data from 551 experiments were analysed. Mean values of the SCOA concentration, yield, and productivity were 10 g l⁻¹, 32% (chemical oxygen demand (COD) COD⁻¹), and 1.9 g l⁻¹ day⁻¹, respectively. Feed concentration and residence time had the most important effect. Higher feed concentration corresponded to higher product concentration and productivity, but to lower yield. The mean feed concentration was 109 gCOD l⁻¹ and 19 gCOD l⁻¹ in the experiments with the highest product concentrations and in the experiments with the highest yields, respectively. Shorter residence times corresponded to higher productivity. The mean HRT (hydraulic residence time) in the experiments with the highest productivities was 2.5 days. Sequencing batch reactors gave higher values of the performance variables (mean values 29 g l⁻¹, 41% COD COD⁻¹, and 12 g l⁻¹ day⁻¹ for product concentration, yield, and productivity, respectively) than processes without phase separation.

As an alternative to conventional anaerobic digestion for methane production, anaerobic fermentation (AF) of organic matter can produce short chain organic acids (SCOAs) in a sustainable way. This study investigated the effect of microwave (MW) pre-treatment on the AF of model food waste to SCOAs and ethanol. The MW pre-treatment was investigated at three temperatures (120, 150 and 180 °C) and residence times (2, 5 and 8 min). The MW treatment gave a significant reduction in the pH and volatile suspended solids (VSS). The largest reduction in the VSS was 20%, indicating solubilisation of the organic matter. The latter was also confirmed by the increase, although it was not statistically significant, in the soluble chemical oxygen demand (COD) and soluble carbohydrates. In the fermentation batch tests, the total product yield was higher (17.5% COD COD⁻¹) than for the untreated substrate (11.1% COD COD⁻¹). An electricity price of GBP 0.06 kWh⁻¹ would correspond to the market value of the additional SCOAs produced with the pre-treated substrate. Although this price is lower than the current business price of electricity in the UK, the MW pre-treatment could become economically feasible with scale-up effects and by using free excess electricity coming from renewable resources.