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Anaerobic Digestion – The Digester

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Tank design plays an important role in optimising the biogas output from anaerobic digestion. Sam Hinton, technical support manager at the Anaerobic Digestion and Bioresources Association (ADBA) identifies some key considerations for owners and operators.

AD is a simple, centuries-old technology and hugely complex beast. In principle it is simple; you put organic matter into a reception vessel with bacteria and it will produce gas. That is why it is often, not always correctly, likened to a concrete cow. The more you seek to optimise this living structure and its gas production, however, the more complex and sophisticated the system becomes. While the feedstock delivers the gas, it needs the right environment to maximise results and that means paying attention to tank design. As with all things practical, form reflects function.

 

As organic feedstock varies widely between facilities and can include agricultural wastes, crops, sewage sludge, garden waste and commercial, manufacturing and household food waste, or a combination of these, flexibility is the most important aspect of any design.”


 

The digester facilitates the series of biochemical processes when anaerobic microorganisms biodegrade material feedstocks and convert them into biogas. Ideally, the process could be split into four separate tanks; hydrolysis, acidogenesis, acetogenesis and methanogenesis, but due to space, cost and symbiosis, this is not the case.

However, the process is not as neatly staged, as the biochemical processes often occur in parallel not series, so the digester and buffer tanks need to facilitate all stages. As organic feedstock varies widely between facilities and can include agricultural wastes, crops, sewage sludge, garden waste and commercial, manufacturing and household food waste, or a combination of these, flexibility is the most important aspect of any design. This will allow operators to increase or decrease or change feedstocks throughout the plant’s lifetime and it is highly unlikely that the feedstock will remain standardised for this period of operation.



 

How many or how big?


After determining feedstocks and tonnages, the questions regarding tank design are primarily: how many tanks are there? What is their configuration and use? What is the capacity? The answers will indicate the plant’s hydraulic retention times (HRT), organic loading rates, digestate storage capacity and ability to operate during maintenance or biological collapse.

Designing a plant with multiple digesters means that any maintenance requirements to one tank can be carried out without shutting the plant down but operating at a reduced capacity. It also gives the ability to re-seed one tank from the other if the biology becomes unstable. Even if two digesters are fed the same feedstock, they can behave differently. One can often see two digesters meters apart operating with significantly different levels of ammonia whilst being fed the same feedstock mix from the same clamp.

Tank size indicates the plant’s HRT – capacity of the plant that is not only used but mixed – which is the time the substrate spends in the digester. HRT acknowledges the difference between design and actual capacity, as a tank with 8m walls will not be filled to 8m due to viewing windows, membrane, sensors or gas pipes that will be blocked or damaged by digestate. If feedstock throughput is too fast then feedstock will be removed from the tank prior to full gas extraction.

Crop plants tend to require a larger capacity, with the HRT of energy crops - even after a good harvest delivering feedstock of low fibre and high digestibility - requiring around 80 days to optimise their gas yield compared to food waste plants at around 30-50 days with a buffer/hydrolysis tank. Plant size, however, can mean the facility has the ability to process larger amounts of feedstock. For example, a Nature Energy plant consisting of seven digesters each with a capacity of 9,500m3 can process 1,050,000 tpa.





Size is not always determined by process design but planning, says Jonathan Smith of Balmoral Tanks. Site footprint and load bearing capability of the ground can both affect the height of the digester and reduce the need for piling, which would increase costs.

Whatever the size, safety measures need to be designed to suit the plant; burst plates for foaming, high pressure, sensors and alarms for over-filling/temperature/foaming etc. and pressure relief valves (PRVs). Roof design is integral to delivering these. It must be able to withstand the maximum operating pressure. Jonathan emphasises that as tank suppliers, they need to be involved in technology choices when it comes to buying PRVs to ensure everyone is clear on the maximum release pressure the tank should be working to. Operating pressure is also a critical factor in the prevention of explosive atmospheres. To avoid this, the plant should operate at positive pressure. Juergen Kube, head of technology at O&M specialists Future Biogas, states that gas domes are a good design consideration. By using ‘blowers’, you can manage your gas distribution between tanks and reduce the need to use the PRVs and the release of methane emissions to atmosphere in periods of high gas production.



 

What mixing and material?


There are multiple types of pumps that need to be considered for design, but here we solely consider entry and exit location choices. Juergen Kube says feedstock should be augured or pumped in near a mixer, to ensure its distribution throughout the tank. The exit point should be positioned away from the entry point or undigested feedstock may exit the digester and release emissions during separation.

This process design needs to be practical to allow efficient operational access for maintenance requirements, which means virtually all pump connection points are at low level around the tank. Kube acknowledges that pumps should be positioned next to the tank and ideally have one task each to reduce single points of failure and mechanical strain on a single pump, which allows you to carry out multiple jobs simultaneously. A good pump design gives the desired flexibility to pump between tanks for emptying partial operation during down-time or re-seeding following the acidification of a tank.

Mixing type and positioning is an article in itself, which will be covered in the autumn edition of AD&Bioresources News. The positioning and type of mixers are crucial and can be simply categorised as  wall, roof or externally mounted if it is gas or jet mixing. The importance of mixing design is highlighted by UK firm AcrEnergy, which says it “may not be evident for the first 3-5 years but becomes a point of failure. Further to this any maintenance can cause large periods of downtime and lost revenue”.

Thomas Minter, director at Malaby Biogas, ensured gas mixing was part of the design at the company’s award-winning facility, which was the first plant in England to achieve ADCS certification. “Having direct control over design and being able to make the most of opportunities to optimise and adapt are important factors in success,” said Thomas Minter. “Operators need to be continually involved in the design and build process to understand the plant which will help it to operate with greater efficiencies.”

As all working parts are external from the tanks, routine maintenance is far quicker and easier. It also controls grit accumulation and allows monitoring of potential tank dead spots whilst improving gas quality with a reduced parasitic load compared to mechanical mixing. A design issue with many submersible mixers, according to Juergen Kube of Future Biogas, is the need to remove the membrane, increasing CH4 emissions and decreasing the plant’s sustainability credentials during maintenance. He opts for mixers that can be removed from the tank without removing the membrane and blinding the mixer hatches to reduce fugitive emissions.

This is in line with Danish company Landia’s approach to retrofitting gas mixing to the exterior of digesters to ensure complete mixing and reduced maintenance. To mitigate the labour-intensive task of grit removal, some digesters have also installed entrances on their digesters to make the process quicker and less onerous. The ability to modify and install additional entrances to the digester leads us to question and common discussion of steel versus concrete.

 

Steel vs concrete


Jonathan Smith of Balmoral Tanks acknowledges the benefits of steel; lower CAPEX implications, faster installation times, extendibility and localised thickening of steel to improve structural capabilities and, particularly for exports or international projects, much easier logistical deliveries. Concrete, however, has the design benefits of increased structural capabilities and smaller insulation requirements due to a lower heat loss co-efficient, meaning water and gas tightness testing can be less time consuming. How long the site needs to be operational for is often the deciding factor - concrete has a design life of 60 years compared to steel’s 25.

In high dry matter agricultural plants, the expense of a concrete roof was deemed beneficial due to the increased mixing ability through vertical stirrers in the centre of the tank, removing the risk of dead spots in the digester. Noticing that the concrete roof can aid mixing design but hinder tank height - due to increased wall loads - is how tank design choices are made, finding what is best and most important for certain plants, capacity or mixing, longevity or cost and so on.

Aside from the issues addressed, other design considerations such as heating, insulation, sample points and PPM also have a bearing. From site footprint to feedstock, there is a lot to digest when it comes to tank design and it is clear there is no one design that fits all. As AcrEnergy MD Daniel Scheven said: “Tall and thin to wide and small, made of concrete or steel - and whatever the dimensions, material and feedstock mix - they have all proven to work fine as long as the construction has been done diligently.” Amaya Arias-Garcia from waste management firm Suez agrees with Daniel and adds that choosing a competent tank contractor and ensuring that the contract is detailed and clear is probably as important as the design. A disorganised contractor can significantly affect the program and the quality of the installation.