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Thursday September 9 2021

Biogas Feedstock & Anaerobic Digestor Raw Materials Explored

Anaerobic digesters convent organic material into biogas, which can then be used to generate heat and electricity or upgraded to make biomethane. The organic material used in the AD plant is called the feedstock and this comes in a variety of forms and from a variety of sources.

In this article, we look at the types of feedstocks available and how they differ in terms of biogas quality and yield.

How the feedstock generates biogas

AD plants utilise microorganisms, without the presence of oxygen, to convert and organic feedstock into biogas. This is done over four separate processes, including:

  • Hydrolysis – complex organic matter such as proteins, carbohydrates and fats are broken down by bacterial enzymes into their constituent parts such as sugars, fatty acids, and amino acids.
  • Acidogenesis – various fermentation reactions convert larger molecules into organic acids, alcohols, ammonia, carbon dioxide, hydrogen, and hydrogen sulphide.
  • Acetogenesis – the fermented products are then oxidised into simpler forms such as acetate and carbon dioxide.
  • Methanogenesis – finally, Archaea (single cell organisms) convert hydrogen and acetic acid into methane and carbon dioxide.

Biogas is the combination of methane and carbon dioxide that is produced by the final stage in the process. It also contains water vapour and traces of other compounds such as Hydrogen Sulphide (H2S), silicones, ammonia and hydrocarbons. However, these can be removed by cleaning to create biomethane, which can then be used in the national gas network.

Types of feedstocks

AD plants are capable of utilising a large range of feedstocks to generate biogas, as long as they are organic, biodegradable and non-woody. This is because the microorganisms in a biogas plant are not capable of breaking down lignin, the substance that gives plants their strength, which means materials such as wood and paper cannot be used in a digester plant.

Common anaerobic digester feedstocks fall into several categories, including:

  • Food waste
  • Sewage and wastewater treatment sludges
  • Animal manures
  • Food production, manufacturing and processing residues
  • Purpose grown crops.

These feedstocks vary widely in the yield and quality of biogas they can generate, which is based on a number of factors including the degradability of the material, the dry matter content of the feedstock, the energy it has retained, the length of time in the digester, the type of AD plant being used, the conditions within the digester itself, and the purity of the feedstock.

Food waste

According to WRAP, a charity that promotes the sustainable use of resource through product design, the UK throws away around 9.5m tons of food each year. This comprises of household waste and waste created from food manufacturing processes. Around 70 per cent of this is edible food and 30 per cent comprises of the inedible parts of raw ingredients. It costs UK households on average more than £500 per year and causes disposal issues as much of this waste has traditional gone to landfill where it breaks down, releasing methane into the atmosphere.

Food waste is therefore a valued feedstock for anaerobic digestion. Not only does it take an energy-rich source of organic material and convert it into renewable energy, it also significantly reduces the amount of food waste going to landfill and the uncontrolled production and release of a major Greenhouse Gas.

Food waste is typically processed in a wet anaerobic digestion system. Wet systems process biodegradable feedstocks into a digestate slurry with usually less than 5 per cent total solids. In wet AD, the feedstocks are pumped into the digester, heated, and stirred. This continuous process produces a larger volume of biogas than dry anaerobic digestion.

Any anaerobic digestion plant used to process food waste will need a reliable source of feedstock, preferably subject to long term contracts. Such contracts are typically with local authorities to take household food waste, food manufacturing businesses to take by-products of the manufacturing processes, and with retailers such as supermarkets to process food that for various reasons cannot be sold to consumers.  

Sewage sludge

Sewage sludge is a semi-solid, residual product of the municipal wastewater treatment processes.

In a municipal wastewater treatment plant, sludge is typically generated through primary settlement of solids from the incoming wastewater and during the aerobic treatment of the clarified wastewater stream.

After thickening to improve digester performance, sludge from both these sources is treated through the anaerobic digestion process as it provides opportunities to reduce sludge volumes, generate biogas for beneficial use and stabilise and sanitise the sludge making it suitable for recycling. 

Approximately two thirds of sewage sludge in the UK is treated in this way by water utility companies.

As with food waste, sewage sludge is processed using a wet anaerobic digestion system.

Animal manures

Animal manures, otherwise known as slurries or agricultural residues, are unsurprisingly a common form of feedstock for on-farm AD plants. Anaerobic digestion is an effective way of capturing stored energy in these materials, as well as stabilising them to enable them to be used as digestate fertilisers.

Anaerobic digestion can form part of an effective slurry management system, utilising manure that isn’t required for use as fertiliser. This is particularly the case for farms with land in Nitrate Vulnerable Zones, which further limits the use of natural manures and inorganic, nitrate fertilisers.

Manufacturing and processing residues

Although manufacturing and processing residues essentially come under food waste, there is a fundamental difference. Residues from food manufacturers tend to be of greater consistency, meaning predicting the quality and yield of the biogas produced by them is much easier.

Food waste, by contrast, can be highly variable with significant amounts of organic and inorganic contamination. This makes it a challenging feedstock to treat in an AD plant and one that requires pre-treatment prior to digestion.

Energy crops

The feedstocks we’ve looked at so far are either waste materials or by-products of some other process. Energy crops, however, are grown specifically to be used in biodigesters.

Crops such as maize, grass silage, sugar beet, rye, and wholecrop cereals can be used as high-yielding feedstocks in their own right, or they can supplement or stabilise more variable materials such as low yielding food waste or slurries.

In 2019, 96,000 hectares of arable land was used to grow crops for bioenergy in the UK, generating the oil equivalent of 6.7m tons of plant biomass. This was used to heat and power UK homes, as well as producing biodiesel for the road transport market.

Energy crops fit well into the standard food crop rotation on most farms, meaning they offer farmers a convenient feedstock that produces higher yields than slurries alone.

Are all anaerobic digester feedstocks created equally?

Unsurprisingly, the quality and yield of biogas generated varies significantly with the different types of feedstocks. In fact, feedstock is the biggest influencing factor when it comes to how much biogas is produced, and how clean it is.

Of all the feedstocks available for anaerobic digestion, animal manures tend to offer the lowest yields. This is because they contain relatively low concentrations of dry matter and have been pre-digested by the animals, meaning much of the energy the material did contain has been removed before reaching the digester. Typically, cattle and pig slurry generate just 15 – 25m3 of biogas per wet ton of feedstock.

One exception to this is chicken manure. Whereas this is not as high yielding as other forms of feedstock, under the right conditions it can generate up to 200m3 per ton of feedstock.

There is huge variation in the yield of different types of food waste, depending on the energy density of the particular material. Waste cooking oils and fats, for example, can yield up to 1,200m3 of biogas per ton of feedstock, whereas potatoes yield between 250 and 300m3. Depending on the water concentration, fruits and vegetables tend to sit around the same levels.

However, the actual yield and quality of biogas produced from food waste will depend on the mixture of the different materials in the feedstock, as well as other factors such as digester efficiency.

The exception to this is residue from the food manufacturing and processing industries. Waste streams such as fat or sugar from bakeries provide feedstock with a consistent energy level, meaning biogas yield and quality are easier to predict and measure.

The same holds true for energy crops. Energy crops including maize and sugar beet consistently generate biogas at between 300 and 500m3 per ton of feedstock, with the precise feedstock used generally being based on local availability. Grass silages and maize silages tend to yield less biogas, more around the 200m3 per ton mark, but still provide a consistent and available AD feedstock.

Feedstock cost and availability

Feedstock cost and availability are two of the most important things to consider when planning a commercial AD plant. The simple fact is, without a consistent supply of cost effective feedstock, in sufficient amounts to keep the plant running, it will never generate a return.

For many plant owners, this isn’t a problem. Farmers operating small, farm-based AD plants can produce their own feedstock from energy crops and can supplement these with manures.  

Food manufacturing businesses often invest in on-site biogas plants to manage their wastewater and generate heat and power for their site, or to sell to the national gas network.

Water companies operate their own AD plants as an essential part of the wastewater and sewage sludge treatment process, so do not have to source feedstock externally.

Commercial AD plant operators, however, must secure long term contracts to source feedstocks in the planning phase of the facility. Without these, such an investment would not be viable.

Contracts with food manufacturers without their own AD facilities, local authorities to manage household food waste, retailers such as supermarkets, large, private waste disposal companies, and farmers growing large amounts of energy crops are essential to make a plant sustainable and profitable.

How feedstocks will be sourced, and at what cost, must be written into the business plan for a commercial AD plant if it is to secure funding.

Industry regulations

Unsurprisingly, there are a number of regulations that apply to operating an AD plant, and some of these relate directly to the type of feedstock being used.

The first consideration is Environmental Permitting, which is a scheme to regulate businesses in England and Wales that could have an impact on the environment and human health.

There are three levels of permitting within the scheme, which are:

  • Exemption

If you run a small scale AD plant that utilises non-waste feedstocks, you fall under the exemption. Exempt plants still have to register with the Environment Agency and provide technical information, but no charges apply.

Also exempt are agricultural AD plants that are on farm and utilise slurries as a feedstock (as long as the total amount of waste stored or treated does not exceed 1,250m3), and plants utilising food and other biodegradable wastes to produce a digestate (excluding animal by-product wastes). Under the exemption, up to 50m3 or waste can be stored at anyone time.

  • Standard permit

The standard permit enables the operation of a biogas plant utilising wastes as a feedstock, and for the combustion of the resultant biogas in gas engines. Wastes include those controlled by the Animal By-Products Regulations but not hazardous wastes.

Under a standard permit, AD plants are restricted to processing no more than 100 tons per day. Fixed charges apply.

  • Bespoke permit

A bespoke permit is issued where an AD plant does not fit the criteria of either the exempt or the standard permit. This is subject to variable costs depending on the precise requirements of the plant and can be time consuming to achieve.

Plants also require a permit to spread any digestate fertiliser on agricultural or non-agricultural land and are subject to the Animal By-Products (ABP) Regulations. These regulations permit low-risk ABPs and catering waste which contains meat, or which comes from a premises handling meat, to be used as anaerobic digester feedstock.

However, high risk ABPs cannot be used as feedstock in biogas plants unless they have been rendered to the 133°C/3 bar/20-minute EU pressure-rendering standards.

Manure and digestive tract content are classified as a high risk ABP, but they can be used without processing as biogas plant feedstock. However, where this content is sent to a biogas plant for treatment with other ABPs (including catering waste) the plant must be approved, and the mixture treated to approved standards.

Other regulations that govern the operation of a biogas plant include Duty of Care and Health and Safety, which are in place to protect staff and anyone else who comes into contact with the plant from harm or injury of any kind.

Expert advice

Utilising the correct feedstock in your plant can be the difference between a highly efficient plant and one that doesn’t generate an ROI. To achieve this, comprehensive feedstock planning and analysis should be carried out by experienced biologists with access to the necessary laboratory facilities and combined with ongoing performance monitoring to assess the operational efficiency.

At Birch Solutions, we employ a team of experts who can provide in depth advice on feedstock planning and analysis, AD plant efficiency, service, maintenance, and operations. For more advice, get in touch.