Galglass secures prestigious contract for design and construction of AD digester tanks
UK-based anaerobic digester (AD) specialist Galglass has now extended its UK Utility customer
base by securing a project with Northumbrian Water.
The contract is to design and construct three reinforced concrete digesters (RCD),
20m diameter x 20m high, the largest structures of their kind in the UK.
The main difficulty of designing RCD structures of this size is controlling crack
widths that occur in the structure as a result of very high operating temperatures
produced during the AD process.
The industry's standard design of these structures, now seen as somewhat antiquated,
were all suffering from the same problems. There was significant cracking and leaking
when water tested, mainly as a consequence of the high hydrostatic loads combined with
the high temperature demands.
More steel and thicker tank wall sections were seen as the best solutions to avoid
these problems however, in the longer term, these solutions were seen as impractical,
unproductive and very expensive ways of treating what were inherent design flaws.
Galglass has since gone on to design a structure that is leak free and can also
be delivered in a safe and cost-effective manner. This is done using a specialised
post-tensioning system within the cast in-situ wall which puts the wall into
compression, and virtually guarantees a crack free structure, thus resulting
in first time success in the testing phase.
On top of this, the company also employs techniques to ensure a fast-track construction process, which
generally saves 40-50% on build times, with safety at the forefront of the construction techniques.
Galglass also worked for Morgan Est (now Morgan Sindall) on the Afan site in Dwr Cymru, to build
such structures for the flagship AD plant for Welsh Water during 2009/2010.
These digester tanks were originally specified in steel. In the end the main reason for
choosing concrete structures was based on simple value engineering principles.
Steel digesters carry a 25 year design life, concrete digesters carry a design life
of 40-60 years, based on BS8007 and BS8110. Any cost saving obtained in building a
steel tank instead of a concrete tank was then lost by the steel structure having
such a short design life.
Given that realistically steel tanks need to be replaced at least once to achieve the
same lifespan of the concrete tank, not to mention the huge costs associated with having
to de-commission and recommission the plant to deal with the tank replacements, the
choice of a concrete structure turned out to be a very simple one in the end.
Concrete is a more reliable, efficient, safe, value for money tank structure. It is
also a far more robust constructed material that withstands additional loads, such
as access steelwork and mixers far better than any steel structure.
The construction programme at the Afan site ran for approximately 35 weeks. This was for
two bases, two walls, and two gas tight RC roofs along with specialised access steel work.
This brought huge benefits to the contract against a conventional build as the tanks were
ready for their second fit much quicker than expected, and ultimately, the tanks could
have been operational ahead of any expected delivery dates. Galglass is now constructing
the second such project for Dwr Cymru at Five Fords Waste water treatment works for Costain.
Click here to download a copy of the article in PDF format.