Bioenergy ProductionPushing the limits

Anaerobic digesters are typically operated at low organic loading rates, within the range of 1-3 kg volatile solids /m³/day. The choice of loading rate has a direct impact on energy production. Lower loading rates result in limited energy production, while higher loading rates can lead to increased methane production. But, elevating the organic loading rate can introduce stability issues, ultimately causing catastrophic system acidification.

The process of restarting a failed anaerobic digester and restoring it to its full design productivity is a substantial undertaking, incurring significant costs in terms of time, waste material disposal, and lost production. This recovery process, from initial start-up to achieving full productivity, can extend for a considerable period, exceeding three months. To put this into perspective, a 3 month production loss from a typical 1000 m³ methanogenic digester can result in a shortfall of over £90,000 from energy sales.

Feedstocks used in anaerobic digesters are typically contaminated with a range of pathogens. Generally employed treatments, such as pasteurization, serve the purpose; however, this implies that a significant fraction of the energy generated in the form of biogas is diverted for feedstock/digestate treatment. Overall, this results in less energy available for sale.