Anaerobic Digesters

We are all familiar with processes such as the fermentation of hops and fruits to make alcoholic drinks, something we’ve been doing for thousands of years, which involves anaerobic digestion. It is only in the last 40 years, however, that we have been seriously looking at ways to produce valuable fuel in the form of biogas from our waste. It is a process that occurs naturally around our planet, particularly in old landfills, and can produce by-products such as methane gas. In fact, the first discovery of a flammable gas being created from waste matter goes as far back as the 17^th Century but it wasn’t until recently that research and development made it possible to utilise this potential on a much larger scale.

As a species we produce a large amount of organic waste that in the past might have been sent direct to a sewage farm or thrown onto landfill. More and more we are attempting to find ways to better use this waste and produce valuable resources that can help us reduce our carbon footprint. Anaerobic means in the absence of oxygen. In this kind of process, bacteria in a sealed vessel begin to consume waste matter producing two things that are of great use. The first is bio-fertiliser which is high in nutrients and highly valued by the agricultural industry. The second, and perhaps the most important where renewables are concerned, is biogas which can be used to power homes and industry if produced in large enough and clean enough quantities. Biogas from anaerobic digestion is composed mostly of methane and carbon dioxide. One of the largest plants in the world was opened in Lahti, in Finland and produces 150 GWh of biogas a year. In the UK we’ve lagged behind but there is now a growing movement that believes anaerobic digestion and biogas production can have a huge impact on our energy production. 

There is some evidence that biogas was used to heat water in Assyria as far back as 600BC but it wasn’t until 1776 that the connection between the amount of waste and the amount of gas produced was discovered by Count Alessandro Volta. Scientists including Robert Boyle and Stephen Hales were some of the first to notice that disturbing sediment in streams and rivers sometimes caused a release of gas. The fact that decaying matter could produce methane was again highlighted by Sir Humphry Davey in 1808 when he studied cow manure but it was in an Indian leper colony in the mid-nineteenth century that the first anaerobic digester was originally built. In the UK, the first one wasn’t constructed until the end of the century, in Exeter. In 1907 Karl Imhoff patented the first anaerobic digester which was originally designed to handle sewage. Initially it was countries such as India and China that developed anaerobic digestion certainly more than the West where it was primarily used in sewage treatment if at all. China were thought to have as many as 6 million anaerobic digesters by the beginning of the 1970s. With growing fuel prices and the need to find solutions that made countries like the UK more energy independent, greater resources were put into developing this method of producing biogas. Whilst the greatest growth was in farm-based anaerobic digesters, primarily because of the fertiliser by-product, a number of large scale units have been built across Europe and now the UK. By 2012, there were 78 biogas installations in the UK (outside of water companies) and hundreds more are being planned with encouragement from Government incentives.

Anaerobic digestion takes place in an air tight container and involves a variety of micro-organisms including those that help acids form and those that create methane. A great deal depends on creating the right conditions for the process to work and it can often take a period of time for the micro-organisms to establish themselves. Essentially, anaerobic digestion has four distinct stages that break down the waste matter into smaller and smaller components:

• Hydrolysis: Waste is made up of complex matter such as carbohydrates and proteins. The bacteria breaks all this waste down into simpler molecules, for instance, changing complex carbs into simple sugars.
• Acidogenesis: The waste is further broken down producing alcohols and fatty acids as well as by-products such as carbon dioxide and ammonia.
• Acetogenesis: The alcohols and fatty acids are converted a bit more and produce hydrogen, carbon dioxide and acetic acid.
• Methanogenesis: The hydrogen and acetic acid, with the introduction of methanogenic archaea, are turned into methane and carbon dioxide.

The process can take weeks or months to complete and at the end we are left with just a few products: methane, carbon dioxide, water and digestate or organic material. The biogas produced can be used for a number of purposes including heating and the production of electricity. The digestate is rich in nutrients and can be used in fertiliser or, more recently, also transformed into building materials. AD depends on the temperature and there are generally two different processes used to create biogas:

• Mesophilic Digestion normally takes place between 20 and 40°C and is a slow process which better promotes the growth of bacteria.
• Thermophilic Digestion occurs at a higher temperature – between 50 and 65°C – which works quicker but is not so kind to bacteria.

Whilst we are still in the early days of full development of anaerobic digesters, their potential is considered to be important for the energy future of the UK. According to the Anaerobic Digestion and Bioresources Association(ADBA):

“Instead of burning it, or sending it to landfill, AD plants could potentially turn that waste…into 10% of the UK’s domestic gas, which means less landfill, stable energy prices, fewer carbon dioxide emissions, and even a financial saving for homes, businesses, farms  and councils across the country.”

Some of the figures regarding the benefits of AD might well surprise you:

• We produce a large amount of methane in the UK and nearly 16% comes from manure via our livestock. A simple change using AD can mean that up to a third of this release into the atmosphere can be combatted, reducing our carbon footprint significantly.
• Food waste is now regularly being sent to anaerobic digesters by local councils rather than being thrown into landfill, reducing CO2 emissions. In fact, we could save over £300 million a year by avoiding landfill altogether for food and other biodegradable waste.
• Growing crops specifically for the purpose of AD is, in some cases, actually more efficient and better for the land.
• AD is helping to deliver a significant amount of electricity with a lower carbon footprint.
• If we took the approach of Oslo, who converted their entire bus fleet, using biomethane to run our vehicles could cut emissions by up to 60%.
• The nutrients in the digestate produced by AD could well boost the farming industry to the tune of £200 million.

Whilst there may still be some way to go, getting anaerobic digesters to handle our daily waste may not only create a large number of jobs but could also have a highly beneficial effect on the future of renewables in the UK.

The type of anaerobic digester will depend on the heat system being used. Mesophilic digesters normally operate around 35°C, which is considered the optimum temperature for the bacteria, making them more stable. Thermophilic digesters work at their best around 55°C and because of the higher temperature the bacteria are much more sensitive to changes. As a rule thermophilic digesters are more expensive than mesophilic despite providing a faster reaction.

Anaerobic digesters have come a long way in the last few years and the technology is still being developed. There are two main types currently on the market, divided into single and multi-stage. In single stage digesters all the processes are carried out in the same place which means they are generally cheaper and easier to maintain than multi-stage. There are many types of single stage digester including Continuously Stirred Tank Reactors and Plug-Flow Digesters. Multi-stage digesters have the advantage that you can provide optimal conditions for each stage of the digestion process, most importantly methanogenesis. There are normally just two stages in these installations – one that handles the initial hydrolysis and the second that gets the most out of methane production. The advantage of multi-stage digesters is that they can optimise the output and provide greater stability of the biological matter. The downside is that they incur a higher cost and are more complicated to operate.

Anaerobic digestion actually needs a large amount of waste to produce enough biogas to make it worthwhile which is why you will not be seeing private installations as you do with other renewable technology such as solar or wind power. The majority of ADs in the UK are commercial and many are operated by farms who have access to large amounts of waste or councils who run waste collection in towns and cities. London’s first commercial AD plant opened in 2014 after funding from the Green Investment Bank. Situated in Dagenham, it has the potential to handle 50,000 tonnes of waste a year and produces an electrical output of 1.5 MW electricity and provides heating of 1.15 MW to a nearby recycling business. 

The cost of commercial anaerobic digesters is larger compared to other renewable technologies but the payback time, because the waste is free, is often fairly quick. Projects need to take into account the amount of work that is needed to maintain safe operational procedures when dealing with toxic or flammable materials. This was highlighted when the owner of a digester in Dorset was prosecuted after the death of a maintenance engineer who was overcome by hydrogen sulphide fumes. Copys Green Farm, Norfolk: This 500 acre farm installed an anaerobic digester at a cost of £750,000 which was designed to be paid back within 8 years. The plant produces about 130 kW which is used to heat nearby businesses and benefit for income from the Feed in Tariff. The cost of disposing of waste is also greatly reduced and provides a much greener way for the farm to operate. The one downside to the installation, and one which is shared by many farmers and commercial enterprises, is the bureaucracy involved particularly concerning the management of environmental issues. On a much larger scale is the £10 million AD plant that has been built in Somerset. It is designed to handle some 30,000 tonnes of food waste each year and should supply about 9,500 MW of electricity from the biogas produced, the large proportion of which will be sold to the grid.

Because of the nature of the installation, any anaerobic digester will need planning permission and will certainly require the owner to undertake regular maintenance and adhere to health and safety provisions. This may well also include extensive record keeping and appropriate training of workers to use the facilities.

The Importance of Biogas

For many of its supporters, biogas represents an opportunity to ween ourselves off the fossil fuel gas that we have been dependent on for so long. It has the potential to be used in meeting a variety of energy needs, including:

• Heating a property
• Generating electricity
• Fuelling vehicles
• Refined for the gas network
• Reducing the cost of industry and better management of waste products

The fact that it uses natural waste in production and helps to cut down our collective carbon footprint is something that is often touted by biogas supporters. It’s not just a simple case, however, of replacing our natural gas with that produced in anaerobic digesters. The cost of making sure that the gas is suitable for inclusion in the national supply chain or use in vehicles and machinery is a hurdle that needs addressing. More research and development has to be done on making the process cheaper. Having said that, the use of biomethane in transport is the subject of a growing number of projects across the globe most notably in Sweden where they have embraced the possibilities of anaerobic digestion perhaps more than most of their European neighbours including the UK.

Whilst the initial signs are good, there is still a long way to go before anaerobic digesters and biogas play a bigger role in our energy production. Much is going to depend on reducing the cost of production and the level of maintenance required, not to mention getting the right infrastructure in place. In 2013 there were some 14,500 biogas facilities across Europe but only a few hundred of these were equipped to upgrade their production to feed into the national gas network. One of the areas that will need to change is the ability to refine the biogas produced through AD so that it can help reduce our dependence on fossil fuels. The change is already coming with Scotland developing a biogas to grid plant, the first in the UK.