We recently received the following Press Release which inspired this article in which we discuss the relative merits of precast concrete vs steel biogas tanks for anaerobic digestion plants (i.e. Glass/ Epoxy Coated Steel Tanks).
Quite possibly when you read the article which follows you will, like us, say. “That's interesting. Why is it that every biogas plant I've seen in the last 10 years has the same looking steel biogas reactor tanks?”
“Why is it that the anaerobic digestion and biogas industry is dominated by steel biogas reactor tanks which are hidden behind aluminium profile sheeting and thick insulation for heat retention when in other industries, such as water treatment, the dominant tank material is concrete?”
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.
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 now providing a very robust storage solution, without the need for in-situ concrete, which can reduce construction time by up to 50%.
Whites Concrete’s pre-cast panels can also be utilised to maximise silage storage, such as a new silage clamp at Sherburn in Elmet, constructed in 3 compartments with a capacity of 4500 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 bulk density 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:
- 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.
- The precast units use conventional reinforcement, with appropriate cover for the exposure conditions, and are neither pre-tensioned or 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.)
- 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.
- 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 remains suitably warm to hot, are allowed for by the provision of the necessary anti-crack steel on the outer surface of the concrete units.
- 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 reason for including the above list is that it shows that the use of precast concrete for tanks of this type is an existing proven technology which could be in much more widespread use for all 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:
- 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?
- 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.
- 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.
- Reinforced Concrete is Inherently a More Sustainable Material than Steel for this Type of Construction. This is a subject which 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
- 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.
- 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.
- None of the problems which can occur to concrete curing conditions for in-situ cast walls constructed out in the open in all weather conditions.
- 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
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 which was to last even 10 years longer that 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“.
Press Release contact Details:
Whaley Road, Barugh Green
Barnsley, South Yorkshire
S75 1HT England
+44 (0)1226 320814