31st May 2016

Challenges of building anaerobic digestion plants

In all construction and engineering industries, there are always challenges when it comes to building. The anaerobic digestion (AD) industry is no exception to the obstacles that occur throughout the construction process. The challenges that the AD sector faces when building AD plants does not just occur during the design and construction phase but before the process begins and after it has been commissioned.


It is advised that the chosen AD technology provider has considerable experience to be able tailor the plant to the unique requirements of each client. It is important that the company offers to and liaises with banking institutions, local planning authourities, the Environment Agency and local residents on behalf of the client before the process begins. This means that any potential issues that might arise when submitting planning or grid applications can be resolved beforehand to prevent delays.


The first challenge that occurs when building an AD plant is gaining authorisation on grid connections and from the Environment Agency. With the boost in other renewable technologies in recent years, the grid capacity around the country is becoming more saturated in the countryside. It is becoming more difficult to achieve a full connection or the capacity required. The Environment Agency also needs to be informed of works where waste products are in the feedstock (this does not apply to crop only plants) to ensure compliance and to allow for site inspection.


Feedstocks can raise a number of challenges depending on which is used. It is important to not only utilise the available feedstock but to also understand its gas yield potential and impact on the process. If large quantities of feedstock i.e. maize has to be imported in, it starts to negatively affect the income stream. Some manures, i.e. poultry litter in large doses can have a damaging effect on the AD plant, due to containing high levels of ammonia that restricts the biological process of the plant. Hydrogen Sulphide (H2S) also can be hugely problematic to the AD plant as it is not only dangerous to humans but it reacts and breaks down the concrete tanks and the CHP; it is extremely important that it is considered during design and monitored when operating.






Another challenge that occurs when building an AD plant is being able to finance the project. With recent changes in legislation regarding RHI and the deductions in FiT (Feed in Tariff) rates, the industry is being squeezed. Proposals for changes to RHI (presently at consultation stage) have also been put forward to restrict crop only plants and digestate drying. These cuts and changes haven’t by any means made AD uneconomical but it creates more of barrier when financing the plant build. The banking regulations when it comes to funding and loans, poses a potential difficulty to get the project started; especially as the banks won’t lend to an existing failing business. It is vital to obtain key payback and return figures when it comes to gaining investment from banking institutions or renewable funders.


In regards to planning, ground conditions and the visual impact has a key part to play. Ground conditions can be a deciding factor on whether a project can get off the ground on the first place, constructing on soft, permeable ground will not only lead to potential subsidence issues but it will likely be challenged by the Environment Agency and will draw in extra expenditure with bunding the tank. The visual impact and odour management of an AD plant is an area that planning officers are hot on, especially if there are surrounding businesses and residential areas. It is important that local residents are on board with the build, even though AD has been around for a number of years, it is still a fairly misunderstood sector in terms of the impacts that it has on the area.


Physically constructing the AD plant doesn’t tend to raise too many issues as long as the ground conditions and locality is suitable; it is more the feedstock and the durability of the materials used. Due to the potentially reactive digestate that is produced by AD plants, it is important for construction and manufacturing companies to install robust parts; especially the pipework, tank and CHP.


Reliability is also a challenge when it comes to operating AD Plants, as they require biological and mechanical maintenance. The plant should be consistently running, unless there has been scheduled downtime for maintenance. This means that monitoring of the plant’s performance is crucial to ensure the digester continues to function properly. Regular sampling needs to occur to check that the plant’s biology is at its optimum. If the engine is not running then income won’t be generated.

A prime example of an AD plant that had to overcome the challenges that occur when building is Four Barns, a recently been completed AD4Energy project based in Romney Marsh, Kent. Due to the soft surrounding ground, this posed a big issue to the company, especially as the tank was a partially buried, concrete tank. To resolve this issue, the company designed the tank with a wider base to spread out the load weight of the digester.


The 200kWe, semi-plug flow AD plant is now producing around 17,200m³ of biogas a week (above its predicted weekly target), running on its current feedstock of maize. It is consistently reaching between 95 – 100% total possible CHP output. Through combatting the challenges that come with building an AD plant, it has provided the client with a reliable route to generate revenue and energy.


Richard Apps, from Four Barns AD plant stated the following:


“Taking the step to build a small-scale AD plant on our farm was worthwhile as the benefits are vast, especially economically. It gives us a sustainable business model for the future and we can budget on our return to avoid some of the risk from the unpredictable markets, which the agricultural industry faces today.


As our plant runs on 100% energy crop, it has given us two extra crops to our rotation. This is one of the few weapons we have to fight against an ever increasing blackgrass problem… The larger variety of crops in our rotation has allowed us to decrease the blackgrass seed bank in the soil and still gain a good margin per hectare.


There is no doubt that without the AD plant the business would not be able to sustain current levels of employment with such low commodity prices.”



4Barns 3
AD4Energy is a British firm that specialises in the design, build and commissioning of small-scale AD plants for the agricultural sector. As with other AD companies it has experienced the difficulties of building AD plants and the challenges they present. However, through experience it has learnt how to combat the main issues.


The company supplies AD technology either in the form of a partially buried, rectangular, semi-plug flow digester or as an above ground, circular, CSTR digester. The partially buried digesters are particularly favourable with planners, combatting the a potential planning issue as it has a low visual impact and can be designed to fit in with the surrounding farm buildings if necessary.


Both types of digester can be tailored to closely match the client’s feedstock and energy requirements; making it more cost effective and efficient. The feedstock is also sampled by the company to provide key data on what feedstock can be used and the quantities. This allows the farm to dispose of the waste in an environmentally friendly and economical way. Planned preventative maintenance is also provided to by the company to help monitor the plant biologically and mechanically, to reduce the risk of the plant failing or being damaged.





This article was first published the magazine, Bioenergy Insight, in their May/June 2016 edition (Volume 7, Issue 3)

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