Anaerobic digestion (AD) is a widely adopted process for converting organic waste into biogas, a renewable energy source. Traditionally, single-stage AD has been the norm. However, multistage anaerobic digestion is now gaining attention for its commercial advantages and increased yield potential. This article explores the benefits of multistage AD, highlighting the improvements in yield compared to single-stage processes, with a focus on pre-treatment and hydrolysis.

Enhanced Feedstock Utilisation

Multistage AD offers enhanced feedstock utilisation compared to single-stage processes. In a single-stage system, complex organic compounds may not be completely broken down, leading to reduced gas production. Multistage AD incorporates pre-treatment and hydrolysis stages that break down complex organic matter into simpler compounds, improving overall feedstock conversion. The additional stages allow for better utilisation of recalcitrant materials, such as lignocellulosic biomass, agricultural residues, and wastewater sludge, resulting in higher biogas yields.

Increased Biomethane Production

By implementing pre-treatment and hydrolysis stages, multistage AD enhances the breakdown of complex substrates and increases the availability of easily digestible compounds for microbial activity. Pre-treatment methods, such as mechanical, thermal, chemical, or biological processes, help disrupt the structural integrity of feedstock, facilitating subsequent hydrolysis. Hydrolysis further breaks down complex polymers into soluble compounds that can be readily converted by methanogenic bacteria. This optimised breakdown of organic matter leads to increased methane production, maximising overall energy generation potential.

Improved Process Stability and Efficiency

Multistage AD systems improve process stability and efficiency compared to single-stage processes. The addition of pre-treatment and hydrolysis stages reduces the accumulation of inhibitory compounds and allows for better control over process parameters. By breaking down complex substrates into easily digestible components, multistage AD promotes balanced microbial activity, reducing the risk of process failure due to acidification or volatile fatty acid accumulation. The improved stability translates into higher biogas production rates, consistent performance, and a more robust and reliable operation.

Flexibility in Feedstock Selection

Multistage AD provides flexibility in feedstock selection, enabling the utilisation of a wider range of organic materials. Pre-treatment and hydrolysis stages improve the breakdown of diverse feedstocks, including lignocellulosic biomass, crop residues, food waste, and industrial by-products. The ability to process varied feedstocks enhances operational flexibility and mitigates feedstock availability and inflation risks. Additionally, the optimised breakdown of complex substrates widens the scope for feedstock blending, allowing for better nutrient balance and efficient biogas production.

Enhanced Nutrient Recovery

Multistage AD systems facilitate improved nutrient recovery. Pre-treatment and hydrolysis stages enhance the release of nutrients, such as nitrogen and phosphorus, from organic matter. The soluble nutrients can be captured during the process and utilised as valuable fertilisers for agriculture or other bio-refinery applications. The ability to recover nutrients from organic waste stream not only adds value to the biogas plant operation but also supports sustainable nutrient management practices.

Potential for Value-Added Products

Multistage AD opens opportunities to produce value-added green products alongside biomethane, examples include green hydrogen, ammonia and organic compounds. The controlled breakdown of organic matter in the pre-treatment and hydrolysis stages generates intermediate compounds that can be further processed to obtain additional products such as organic acids, platform chemicals, and bio-based materials. The diversification of revenue streams through value-added product generation enhances the commercial viability of the biogas plant. Positioning it within the bio-refining value chain required by the post fossil fuel economy.

Conclusions about multistage anaerobic digestion

Multistage anaerobic digestion offers significant commercial advantages over traditional single-stage processes. With 20-30% increases in gas yield typically achieved in conjunction with reduced hydraulic retention time (HRT). The incorporation of pre-treatment and hydrolysis stages optimises feedstock utilisation, increases biomethane production, improves process stability and efficiency, allows for feedstock flexibility, enhances nutrient recovery, and opens avenues for value-added product generation. By embracing multistage AD, biogas plant operators can unlock higher yields, maximise energy generation potential, and create a more sustainable and economically viable operation. The multi-stage AD is by definition modular. Enabling progressive upgrading of infrastructure to increase returns from existing assets. There are over 5,000 biogas plants across Europe, suited to a multi-stage AD upgrade. With co-ordinated effort a new industry operating model could emerge to significantly increase biogas output from existing assets and feedstock. Good for energy security, sustainability and plant operators.

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