Anaerobic digestion (AD) is an important biotechnology for treating biodegradable residues and producing bioenergy, yet its full potential remains untapped. We investigate a two-phase AD system for biorefinery applications, producing valuable bioproducts, such as volatile fatty acids (VFAs) and biogas, from grass feedstock. We introduce a demand-driven operational approach to match market conditions, while minimising water use by reusing the process effluent. The proposed biorefinery model yields ~23 kg of VFAs and 75 kWh of biogas, with a potential gross revenue of €84 per tonne of grass. However, a preliminary economic analysis indicates that this biorefinery model is currently unprofitable. A sensitivity analysis suggests that reducing operating costs through technology advancements and policy support are vital to ensure economic viability. Such biorefineries offer opportunities for the diversification of farmers' incomes and the transition away from fossil resources. Our work exemplifies the role of AD as a key biotechnology in the circular bioeconomy.

Decarbonisation of industry will necessitate innovative technological solutions that allow for both reduced carbon footprint and security of supply. Biogas systems can allow electricity production on-demand. This study evaluated and compared different two-phase anaerobic digestion systems for continuous volatile fatty acid (VFAs) production and demand-driven biogas production at a whiskey distillery using by-products as feedstock. The systems analysed were a two-phase continuously stirred tank reactor (CSTR) and a leach bed-expanded granular sludge bed reactor (LBR-EGSB) system. Both systems exhibited continuous VFA production (up to 10.4 g. L- 1 VFA made up of primarily butyric acid and acetic acid) and rapid biogas production, achieving peak flow rates within 30 min. The peak biogas production from the demand-driven EGSB was 3.1 L. L-1reactor. d-1 compared to the CSTR, which recorded 1.8 L. L-1reactor. d-1. Three kinetic models of both systems were assessed, which were able to describe the cumulative biogas production. A theoretical evaluation of processing butyric acid into biobutanol, to be blended with diesel, could produce a more sustainable transport fuel than diesel, reducing distillery-associated transport emissions by 16%.

Lignin extraction from lignocellulosic biomass can enhance its bioconversion efficiency, whilst the recovered lignin can provide added economic value. This study comprehensively investigated the impacts of short-chain carboxylic acid-based deep eutectic solvents (DESs) pretreatments on lignin extraction from willow and assessed subsequent biomethane production through anaerobic digestion. Process parameters (including the DESs type, molar ratios of DESs components, temperature and reaction time) in the DESs pretreatments of willow for lignin extraction were optimized using the central composite surface response methodology. Results showed that lactic acid-based DES pretreatment outperformed acetic acid and propionic acid-based DES pretreatments in terms of lignin removal efficiency and methane production. Under the optimal conditions (choline chloride:lactic acid with a molar ratio of 1:10 at 160 degrees C for 15 min) lactic acid-based DES pretreatment retained over 94% of the glucan content in the raw willow whilst achieved the highest lignin removal of 80%. The recovered lignin showed a purity of above 81%. Compared with the biomethane production of 89.9 mL/g total solid from raw willow, the biomethane production significantly increased by 36.3% after the lactic acid-based DES pretreat-ment. The optimal condition reduced the digestion time from 22 to 10 days. The overall energy conversion efficiency of 62.7% demonstrated that lactic acid-based DES pretreatment of lignocellulose could be a promising method to co-produce renewable gaseous fuels and lignin in a sustainable approach.

Future energy systems necessitate dispatchable renewable energy to balance electrical grids with high shares of intermittent renewables. Biogas from anaerobic digestion (AD) can generate electricity on-demand. High-rate methanogenic reactors, such as the Upflow Anaerobic Sludge Blanket (UASB), can react quicker to variations in feeding as compared to traditional AD systems. In this study, experimental trials validated the feasibility of operating the UASB in a demand-driven manner. The UASB was operated with leachate produced from a hydrolysis reactor treating grass silage. The UASB demonstrated a high degree of flexibility in responding to variable feeding regimes. The intra-day biogas production rate could be increased by up to 123% under 4 hours in demand-driven operation, without significant deterioration in performance. A model based on kinetic analysis was developed to help align demand-driven operation with the grid. The findings suggest significant opportunities for UASBs to provide positive and negative balance to the power grid.

The valorisation of whiskey by-products was assessed and compared in three anaerobic digestion systems. The systems produced similar methane yields, which could satisfy up to 44% of the thermal energy demand at a distillery. Using methane generated from by-products would displace natural gas and reduce the distillery's carbon footprint. Two-phase systems had higher methane content (ca. 75 %vol) than the traditional system (54 % vol) and furthermore, unlocked opportunities for volatile fatty acid production. The potential value that could be generated from the extraction of butyric acid and caproic acid was approximately euro6.76 million for a 50 million litre alcohol facility (0.14 euro per litre of whiskey). All three anaerobic digestion systems showed the potential to valorise whiskey by-products and convert current linear distillery production processes into circular repurpose and reuse production processes.