• Estimates show that biogas could produce up to 60% of current coal generation output and reduce global greenhouse gas emissions by 18 – 20%.
• Anaerobic digestion (AD) plants that produce biogas should be designed to prevent instability, which could result in significant recovery costs
• The Anaerobic Digestion and Bioresources Association (ADBA) has produced a comprehensive checklist for new AD operators to achieve maximum operational performance

The Anaerobic Digestion and Bioresources Association (ADBA), as part of its Best Practice Scheme, has produced a comprehensive checklist for new AD operators to achieve maximum operational performance. The association’s advice is based on the latest research and industry thinking on the factors that impact the AD process. It includes technical and practical recommendations on how to achieve the best biogas output and quality of digestate, the nutrient-rich material produced by AD.

“AD operators need to keep in mind a series of important duties in their everyday operations,” says Chris Noyce, strategic adviser at ADBA.

First and foremost, the AD plant should be designed and built to maintain biological stability, since instability contributes to foaming, which could result in severe plant failure and significant recovery costs.

The AD process works best with consistent inputs of feedstock composition and feed rate, digester temperature and intervals of agitation. Once in operation, the monitoring of inputs, processes and outputs is essential.

“Monitoring is important throughout the production process and vital to making improvements,” says Dr Jonathan Scurlock, chief adviser, renewable energy and climate change, for the National Farmers’ Union (NFU).

At plants taking in purpose-grown energy crops, whose composition is known with a high degree of certainty, sampling and monitoring before digestion may required less frequently than in other plants that need to do more sampling and monitoring to adhere to standards or satisfy regulations and contracts.

Where a batch of feedstock is received from a new source, it’s best to take random samples and test for composition and quality. Some plants include one or more buffer tanks to allow feedstock to be sampled before being fed to the digestion tank. Depending on the type of feedstock, it is likely that pre-treatment will be necessary.

As with any process, training is important for smooth and productive operations. “We believe the AD industry in Britain fully recognises the need for ongoing training, both at the construction stage of developing an installation and in operations and maintenance,” says Scurlock.

Indeed, the reputation of the AD sector depends on having a good record for compliance with regulations, whether on health and safety or environmental standards, and this will be reflected in the technical and financial performance of AD plants.

The ADBA recommends maintenance is only carried out by a suitably trained and qualified person in order to avoid invalidating any warranties or, even worse, putting people at risk (read more about the ADBA’s policy on best working practices here).

Scheduled maintenance of AD plants may require temporary shutdowns and therefore reduce biogas output and efficiency. One inevitable job is the removal of grit or other sinkables from inside tanks. Fortunately, plants need only be shut down and cleaned every two to five years, depending on the type of plant and amount of accumulated grit. Taking effective measures to prevent or reduce the rate of grit accumulation helps maximise plant availability and biogas yield.

One key industry innovation, announced at the seventh annual ADBA Research & Innovation (R&I) Forum in the spring was the plan to establish a Centre for Anaerobic Biotechnology and Bioresources Research. This will coordinate the findings of often-disparate researchers and disciplines in order to lower costs and demonstrate anaerobic biotechnology’s worldwide relevance and applicability. Estimates show that biogas could produce up to 60% of current coal generation output and reduce global greenhouse gas emissions by 18 – 20%.

At present, there are no standardised methods for the measurement of siloxanes (volatile compounds) in biogas or biomethane. One scientific research project conducted jointly by the UK’s National Physical Laboratory (NPL) and UK-based company Protea demonstrated the advantages of traceable real-time measurements of siloxanes in industrial biogas over traditional measuring methods.

Breaking up feedstock before it enters the digester is important to reduce viscosity and increase contact surfaces for bacteria, which results in a higher gas yield. It is this stage of the process that a new mechanical disintegration system from Vogelsang called the DisRuptor is designed to help. The DisRuptor, winner of a silver award in the EnergyDecental Innovation fair in 2016, has an inbuilt motor of only 15 kilowatts (kW) and reduces the size of the solids within the bio-suspension at the rate of up to 80 cubic metres per hour.

In addition, Yara’s Biogas Production Optimizer helps to accelerate the biogas process by increasing the methane concentration that has a higher energy yield.

The pre-treatment stage is improved if the waste is properly sorted, which is where the Rothenburg automated food-waste management system comes in. Currently in use at Imperial College, London, it provides a new, efficient way of sorting food waste collected from different locations while meeting various biosecurity standards.

Finally, PlanET Biogas Group’s gas-management system received the DLV Novelties Award 2017, one of the most distinguished German innovation awards in agriculture, for its practical impact in ensuring efficient and profitable operations of AD plants.

AD can treat diverse materials, including agricultural slurries and other farm wastes; purpose-grown crops; food processing and household food waste; sewage sludge; garden waste; and industrial effluents.

“The industry is looking at a wider range of feedstocks, including use of agricultural residues, such as used straw and sugar beets as source materials,” says Scurlock. On current evidence, sugar-beet roots and tops have a better output than the likes of maize. Because of their high digestibility and low structural complexity, they require no special pre-treatment (hydrolysis) and are fully digested in 15 days, compared to roughly 90 days in the case of maize.

The ADBA Practical Guide to AD lists feedstocks and their merits. The best manure is from chickens, while the best crops are rapeseed and molasses.