A UK on-farm anaerobic digestion (AD) plant, also known as a biodigester, is simply an airtight sealed and heated container on the farm that breaks down organic waste materials to generate biogas, wastewater, and solid remains.
Around the world, these facilities most commonly have the form of covered silos, troughs, or basins and can be found underground or on the surface. In the UK most on-farm AD plants are single-stage reactors (CSTRs).
The biogas produced by the AD process contains around 60% methane and the remainder is mostly CO2.
What exactly is Anaerobic Digestion?
Anaerobic digestion, like composting, is a biological process. It, like composting, uses microorganisms to break down agricultural waste like manure into effluent and biogas. Anaerobic bacteria are among the most ancient forms of life on the planet.
Renewable energy generation has been an increasingly important part of the multifunctionality of farming operations and the diversification of on-farm revenue streams in recent years. However, the adoption of on-farm anaerobic digestion (AD) continues to lag behind that of other renewable energy operations.
The major agricultural kinds that benefit from biogas digesters are dairy, swine, and poultry.
Commonly Used AD Systems on Farms
AD systems used in agriculture are either batch or continuous. An anaerobic digester on a farm is easier to install and maintain than an aerobic treatment plant. They are better suited to most on-farm settings, as continuous process plants, to which organic material is continually or routinely supplied.
The creatures that conduct the “digesting” (breaking down organic material into simpler molecules) developed before green plants' photosynthesis delivered huge amounts of oxygen into the atmosphere. In the absence of oxygen, anaerobic bacteria break down or “digest” organic material, producing “biogas” as a waste product.
On-farm AD is not only a source of renewable energy, but it is also a method of agricultural waste management, therefore improving environmental quality.
In the UK the government policy constraints that have been causing the slow-down in new biogas plants since 2017, will soon be a thing of the past. This is due to improvements in AD plant efficiency, a rising biomethane fuel price, and finance becoming more readily available for green technologies.
The Issue of Farm Waste
Farming activities generate wastes such as manure, animal bedding, feed waste, and runoff, which can pollute rivers, attract mice and insects, and constitute a public health concern. Anyone who has ever been near a farm is familiar with the odour.
The animals in most animal farming enterprises are crammed into tight spaces, and their faeces and urine are channelled into huge waste lagoons. Animal waste slurry lagoons may and frequently do rupture, leak, or overflow, releasing hazardous microorganisms such as Salmonella, E. coli, Cryptosporidium, and others, as well as nitrate, into rivers and aquifers.
They also release poisonous chemicals into the atmosphere, such as ammonia, hydrogen sulphide, and methane, contaminating the environment and contributing to global warming. One of the current techniques is to sprinkle manure as fertiliser onto land. This practise introduces even more of these hazardous chemicals into our air and water systems.
This is not an example of sustainable agriculture, and it is highly harmful to the environment as well as a waste of energy and fertiliser.
Population Pressure and Housing Proximity Lead to Manure Odour Complaints
One explanation for the current surge in interest in digesters is the encroachment of housing projects on farmland. These newcomers complain about smells emanating from farms and agriculture fields following manure treatment. Unfortunately, spreading in some form is required since manure provides critical nutrients that aid crop development. Farmers have been seeking methods to be good land stewards while also combating odour concerns.
They discover that the anaerobic digesting process produces less odour and has less potential to pollute than the manure that enters these facilities. At the same time, the output (known as digestate) maintains all of the nutrients found in the manure that enters the digester tanks.
Current farming techniques squander energy and nutrients, pollute the environment, and contribute to climate change. Currently, many farming enterprises' byproducts are underutilised or just partially used. Instead, it is one of the causes of air and water pollution. Furthermore, the scope of this contamination is enormous. A single large-scale farm may produce the trash of a small town.
Manure Management with the Advantages of Heating, Electricity Generation, and Even Cooking Gas
In certain situations, farmers may be required to implement costly manure management procedures in order to meet environmental regulations about odour and/or other pollution concerns. In such instances, an anaerobic digester might be utilised to successfully solve these issues. The monetary worth of this advantage is difficult to calculate. However, in other instances, it may be of significant economic value.
Biogas, which may be utilised for heating, lighting, power generation, and cooking, is the most desired byproduct of anaerobic digestion. Biogas typically includes 60–70% methane and 30–40% carbon dioxide (Beck). Biogas has an energy content of around 600 Btu/ft3 with 60% methane content, whereas natural gas has an energy content of 1,000 Btu/ft3 (Balsam).
Another important byproduct of anaerobic digestion is biogas digester effluent slurry. The effluent may be composted and reused as bedding material, as well as utilised as soil additives and liquid fertilisers.
How Does an Anaerobic Digester on a Farm Work?
The continuous-type digester is fed with new substrate at regular intervals (one or more times daily) in amounts proportional to the retention time.
For a 10-day retention duration, for example, one-tenth of the digester liquid volume should be supplied every day. The use of modest amounts at a time eliminates the risk of loading shock, which happens when cold input lowers digester temperature or the composition of the input material changes. The estimated retention time is not always the same as the actual treatment time for every individual particle in the slurry, but it does reflect the average treatment duration.
Factors Affecting Biogas Reactor Performance
Temperature, retention duration, loading rate, and agitation are all interrelated environmental variables that influence methane-producing microbial activity. All of these issues must be controlled by provisions in the structural and equipment components of a successful production system.
Stand-alone farm digesters can be run as organics recycling enterprises, community-based organisations, or municipally designed to handle household food waste.
Manure is the major feedstock used in on-farm biogas facilities, followed by food waste. The food waste is sourced locally, such as kerbside collected domestic kitchen trash. However, manure digesters may frequently co-digest other organic compounds, such as food waste.
On-farm digesters may be extremely beneficial to farmers and the local community. Digesters can assist farmers in managing nutrients, reducing smells, and increasing agricultural profitability.
3 Examples of UK On-farm Anaerobic Digestion Community Pushing Government to Maintain Growth Surge
Back in March 2016, when we originally posted this page, farmers in the UK were keen to develop their own biogas plants. So, rather than archiving our original news article, we have retained it below for its historic relevance. Read on to find out about the issues surrounding on-farm biogas in 2016 in the UK:
The UK's On-farm Anaerobic Digestion Community has been working in the last few days to publicize the many benefits of their process which produces biogas. Government seed funding of biogas plant projects can help in a big way, to reduce greenhouse gas emissions from fuel use, in provides jobs, and reduces waste sent to landfill. All these are benefits that it has been publicizing to the government ahead of the annual budget statement later today. The continued adoption of anaerobic digestion, and in particular on-farm anaerobic digestion at small farms at the rate that these plants were being commissioned last year, would maintain a growth surge which is important continues so that the UK can meet its commitment to reducing its Carbon Dioxide emissions.
The urgency for reducing Carbon Dioxide emission reductions globally, could not be more urgent. The rise in average global temperatures this winter has been faster than ever before, in over 100 years of collecting climate data in the UK. As if that was not bad enough, the month of February saw the highest monthly rise in global temperatures ever recorded, pointing out the need for vastly more of the green sustainable energy, which the anaerobic digestion process provides. Read on and read our 3 example articles below, which show how The UK's On-farm Anaerobic Digestion Community has been publicizing their industry, and how very welcome research funding is already in place for the next 5 years, some of which will be spent on new biogas technology research:
Farmers Rally to Support On-farm Anaerobic Digestion at UK Parliament
Government to invest record £26 billion in science by 2021
The Government has announced that it will be investing £26.3 billion in science up to the year 2021. This is approximately £6bn per year.
This funding is key to the long-term success of the Anaerobic Digestion [AD] sector, as research funding to Anaerobic Digestion researchers via bodies like the Biotechnology and Biological Sciences Research Council (BBSRC), the Natural Environment Research Council (NERC) and the Engineering and Physical Sciences Research Council (EPSRC) is secured. Funding to Innovate UK, the body intended to fund innovation to move research into the commercial world, has already been largely secured for this Parliament.
Universities and Science Minister Jo Johnson said:
“From the invention of the lightbulb to the creation of the world wide web, UK scientists have been instrumental in many of the world’s most significant discoveries, and we are determined to continue this legacy on a global scale. In a time of tight control over public spending, we have guaranteed record investment for UK scientists so they can help us tackle climate change, produce disease-resistant crops and cure rare diseases. The government is delivering a decade of sustained investment in science. As long as scientists continue to discover, innovate, and drive economic growth, we will continue to stand right behind them.”
There would clearly never have been an Anaerobic Digestion industry without scientific understanding, particularly in the biological and engineering fields. So, this news must be welcome as funding streams supporting AD [and on-farm anaerobic digestion] can be continued. The industry is in need of further research breakthroughs as we grapple with a fast-changing policy environment. via Government to invest record £26 billion in science by 2021
Calls for 25m GBP research funding from UK government to boost anaerobic digestion industry
[First posted in March 2016. Updated August 2021.]