Concrete vs Steel Biogas Tanks for Anaerobic Digestion Plants

Concrete vs steel biogas tanks and which is the best material for Anaerobic Digestion Plants is the subject of this page, based on a Press Release provided by the precast concrete manufacturer Whites Concrete.

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We refer to precast concrete units made and cured in the factory versus steel biogas tanks fabricated from mild steel panels with fused on glass enamel, or epoxy coated.

Quite possibly when you read the article which follows you will, like us, say. “That's interesting. So. why is it that every biogas plant I've seen in the last 10 years has the same looking steel biogas reactor tanks?”

We reply that design and build turnkey AD plant designers/ installers tend to repeat their steel tank designs.

“Why is it that the anaerobic digestion and biogas industry is dominated by steel biogas reactor tanks which are hidden behind aluminium profile sheeting. They have thick insulation for heat retention when in other industries, such as water treatment, the dominant tank material is uninsulated concrete?”

We would like to know!

The thought-provoking Press Release which follows seemed to us to be unique. Read it and our discussion below, and see if you agree… YOUR COMMENTS ARE APPRECIATED.

Press Release:

The Oval AD Tank Solution by Whites Concrete

A special elliptical tank (rectangular in shape with the corners formed into a radius) made from precast concrete panels supplied by Whites Concrete (part of the Naylor Group) has recently been installed for a new Anaerobic Digestion plant in Lincolnshire.

This AD plant is one of an increasing number of successful biogas plants operated by a leading multi-disciplinary construction, engineering and operating group, Whites Concrete.

The company has also provided a circular Sealwall™ tank for the farm, which uses cattle manure as its main feedstock.

The elliptical tank, approximately 30m by 10m and 4m high, comprises 68 precast concrete panels, whilst the circular Sealwall™ tank is constructed with 30 units.

Both tanks are now providing a very robust storage solution, without the need for in-situ concrete.

Using precast concrete panels can reduce construction time by up to 50% when compared with in-situ cast concrete tanks of similar capacity.

Whites Concrete’s pre-cast panels can also be utilised to maximise silage storage. Their products have been used in this application on many occasions. An example is a new silage clamp at Sherburn in Elmet, constructed in 3 compartments with a capacity of 4,500 tonnes.

Whites Concrete were called upon to create a design that would use the space to full effect, keeping silage dry and clean whilst ensuring that load demands from the heavy bulk-density of the stored materials could be safely resisted by the units. via Whites Concrete-Naylor

A telephone call to Michael Wright, the Director responsible for Whites Concrete, revealed the following additional information about the precast concrete units used:

1. Each precast concrete wall unit stands alone with external counterforts to enable the wall unit to act as a vertical cantilever requiring minimum temporary propping during erection.
2. The precast units use conventional reinforcement, with appropriate cover for the exposure conditions, and are neither pre-tensioned nor post-tensioned, so there are no concerns about the tendons becoming corroded. (Some alternative precast concrete tank designs have been criticised because, in the event of tendon corrosion, failure can occur suddenly and without warning.)
3. Each Sealwall™ unit is jointed using a tried and tested compressible hydrophilic jointing method. Stainless steel fixings hold the precast units together, making a visibly robust system.
4. The wall units are factory cast to very high-quality standards and designed to the exacting requirements of the Water Retaining Concrete Code of Practice BS8007, and Eurocode EC1992. This ensures that the high-temperature gradient stresses across the walls during winter weather, while the contents of the biogas reactor remain suitably warm to hot, are allowed for by the provision of the necessary anti-crack steel on the outer surface of the concrete units.
5. Similar precast concrete designs have been in use for many years in the sewage and industrial effluent treatment industry and have proved their super longevity.

The above 5 points show that the use of precast concrete for tanks of this type is existing proven technology. One that could be in much more widespread use for large biogas reactor tanks.

The Reasons Why the Option of Precast Concrete vs Steel Biogas Tanks is Worth Considering by AD Plant Designers

Technical Advantages of Concrete Tanks versus Glass Coated Mild Steel Tanks

Reinforced concrete has the following advantages over steel as the material for these anaerobic digestion/ digester tanks, (and tanks in general) as follows:

1. Concrete has better inherent anti-corrosion characteristics than steel. The length of life of a steel tank depends upon the longevity of the protective coatings. Once the protective layers of a steel tank are penetrated, even at small points of damage, a corrosion cell develops and the risk of a rapid onset of leaks then occurs. Steel tank manufacturers seldom warrant the life of their products beyond 10 years, and that period is often conditional on active maintenance to identify any points of corrosion and take remedial action on any exposed metal or areas of chipped coatings. Concrete tank designs are carried out to the buyer's specified lifetime, and “design life” can exceed 100 years. By comparison, is a steel tank really a “permanent” structure at all?
2. Concrete is a Better Insulator than Glass Coated (and Epoxy Coated) Steel (although additional insulation may be needed for some AD plants). The normal practice for precast concrete reactor tank walls is to leave them exposed on the surface, and readily available for inspection for the start of any leaks. This is unlike insulated GCS (steel) tanks, where the structural tank walls are hidden behind insulation materials and aluminium profiled sheeting, and unseen damage may have occurred during installation.
3. Concrete Can be Drilled at any Time to Make Openings (for example for replacement Digestor Digestate Mixers to be retrofitted if needed. The same cannot be said for steel tanks due to concerns about creating bare steel and corrosion points at any new openings.
4. Reinforced Concrete is Inherently a More Sustainable Material than Steel for this Type of Construction. This is a subject that can be debated long and hard.

In different applications of concrete versus steel construction, the sustainability case may go either way. However, when used in biogas digester tanks, the author considers that the extended life of a concrete structure, as opposed to the much shorter life of a steel tank, renders the concrete option the most sustainable.

So, there are many advantages to concrete as a material for biogas plant tanks, and yet the advantages go further. There are also additional advantages to precasting the concrete, as discussed next:

Advantages of Precasting Concrete vs In-situ Cast Concrete

1. Casting in factory conditions permits high-quality standards from a highly trained specialist workforce, and economies of scale are present where a continuous production line exists for many clients.
2. No need to transport shuttering the often large distances to the site, and reduces the need to work at height when fixing reinforcement up to 4m off the ground.
3. None of the problems can occur to concrete curing conditions for in-situ cast walls constructed out in the open in all weather conditions.
4. Precast construction is far more rapid than in-situ construction of concrete, saving money both on construction site staff overheads and on lead-in times for the AD plant operator.

The above all tend to reduce the cost of precast concrete, leading us to our final section in which we discuss the relative costs of concrete vs steel biogas tanks for anaerobic digestion plants:

Cost of Concrete vs Steel Biogas Tanks for Anaerobic Digestion Plants

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At this point, we would be expecting our readers to be asking. OK. But what about the relative costs of Concrete vs Steel Biogas Tanks…

For biogas reactor tanks up to about 4 m deep it is considered by the precast concrete manufacturers that their tank would be cheaper to build overall.

They also point out that to compare build-cost alone would not be reasonable because a precast product that was to last even 10 years longer than an alternative steel tank would be of much greater value to the client. Experts point out that there are very many examples of similar concrete structures still in use after 60 years or more of service.

For taller tanks above 4 m depth, which are not yet common in the UK and Europe, Glass/ Epoxy Coated (GC) Steel Tanks (which take their loading in tension around the circumference and can be built 10 m or more high) would naturally win on cost and construct-ability.

According to Tracy Taylor, Product Manager, Whites Concrete:

“Precast can be applied to meet numerous design criteria and, in addition to tanks, can be used in a wide variety of applications: vertical walls, horizontal panels in King-post Walls, roofs, and spillway walls to name but a few. The modular  nature of precast means installation is faster and there is no waiting for it to gain its design strength. As with any construction, early involvement is always an advantage, but precast will always offer more options – and will reduce the overall cost of a project.”

The last word on the pre-cast concrete to GC Steel cost comparison comes from Michael Wright who said:

“Our recent successful projects show that precast concrete can compete and win as the design material for AD plant tanks. As the AD plant market matures with more of these tanks being built we expect to see many more precast tanks used as biogas reactor tanks in the future“.

Whites Concrete | www.naylor.co.uk | concrete@naylor.co.uk

Whaley Road, Barugh Green
Barnsley, South Yorkshire
S75 1HT England
+44 (0)1226 320814

[Article first published February 2017. Updated November 2021.]

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Exothermic refers to the release of heat. But actually the anaerobic digester process is a microbiologic process. You are literally raising livestock... Methonogens.
They require the right environment and foodstock, like any livestock.
The exothermic takes place when utilizing the methane produced. ... Which can vary in volume and quality produced according to the specific make up of the food waste.
You also have to maintain the correct C/N ratio (carbon/nitrogen 30:1) ... mixing in straw, paper, etc.
25kgs of food waste will produce 1.3 - 2 cubic meters of gas.
Methenagens are most productive at the thermophilic levels which is 120° to about 140° f.
The mesophilic level is about 80° f to 100° f. It's not as productive but it has a smaller parasitic load on the system. Most waste treatment plants run on the mesophilic levels. But here's the rub. They don't try to turn a profit from that system. They are concerned with health and decontamination issues not microenergy production. Running at thermophilic levels is more productive, but very hard to accurately engineer the thermodynamics of the generators and the boilers and the radiation pipes in the tanks. Typically you see these types of systems run in Europe where there is financial support for the systems, and they run with amazing micro circuitry and instrumentation.

Any slide rule monkey can design a system that works in the laboratory or in the theoretical world,...
Anaerobic digestion thrives in Asia, and parts of Africa, where the temperatures remain warm throughout the year. But I'm trying to design a system that serves a common community good, a broad geographic region, and for as much of an entire year's seasons as possible.
Relying on the gas production from an anaerobic digester to support its self-heating needs, AND utilize the gas to generate value through electricity or heating barns or greenhouses is just not possible over a great deal of North American communities.
This project I'm doing is a big step down from the 800 cubic meter system I previously built. It's much easier to identify critical data as valid on the system that big as opposed to a small system whose critical variables can be too small to record.

If you look at the video I sent you it's a an extremely complicated thermodynamic calculation for all the variables I've introduced.
So the forum feedback I got was really helpful and breaking down the problem to priorities, and making sure those specific functions were met at the priority level, and then follow behind with the remaining design assignments.

So yes in order to create a home scale digester, that is financially feasible, and will operate in the colder weather, over a broader geographic region, then in my opinion you need solar. I play with hydrogen production too as a supplemental heat resource (interesting but a long story!)

I think readers should keep in mind that with anaerobic digestion there are three options, either the goal is for waste treatment, or for energy production, and in rare cases both. But when we're talking about a home scale digester, you're talking about just enough methane production to save up and use to heat the greenhouse and help out a little bit with the digester itself. They're still will not be enough gas production to completely support the digester during the cooler months. But even if it could support the digester with its own
gas production, The main goal would be met which would be to neutralize the food waste's harmful microorganisms and transitions into a odorless environmentally compatible fertilizer.

Sincere apologies for this long note, I just realized how long it got!

Thanks so much for this forum, it's pretty amazing ,
Steven

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