In January 2016, as part of a routine inspection one of the digester tanks at Eign WWTW was found to have damage to the roof structure.  The tank, which was nearly 30 years old was taken off line and subsequently purged, emptied and decommissioned to allow a full internal and external inspection.

Working closely with Dwr Cymru Welsh Water (DCWW), Marches Biogas undertook the following works to assess and rectify the damage as well as support the operational process within the other vessel:

• Structural tank surveys
• Developed alternative cost effective design solutions
• Provided biological support to enable the second digester to process sludge at maximum capacity
• Principal contractor duties
• Removed digester roofs and associated access platforms, grit blasted existing structure and surface coated to guarantee the integrity of the structures for a further 10 years
• Replaced complete digester roof sections and gas mixing systems
• Provided new digester access platforms and improved access to adjacent tanks
• Fitted new 500m³ gas holder and associated equipment
• Recommissioned and decommissioned tanks using a ‘spill and fill’ method to minimize downtime
• Installation of lightning protection system

Tank Surveys
DCWW commissioned Marches Biogas to manage and undertake a detailed structural survey on the tank in conjunction with Reliant Tank Installations. A full NDT (non-destructive test) survey of panel metal thicknesses was carried out, external cladding was removed and the tank thoroughly inspected. Despite the age of the tank the epoxy coated tank walls were in overall good condition, however the roof structure was found to be beyond economic repair.

Design
Following the inspection an in-depth report was issued which deemed that the tanks structure was solid with no risk of collapse, but the roof structure and top support ring required replacement. Marches Biogas proposed a number of alternative solutions, including replacement of the top wall sections and roof or a more cost effective solution of a replacement top ring and adaptor plate to allow the fitting of a modern metric dimensions roof structure to the original imperial dimensioned tank. Following a cost benefit analysis a contract was agreed and was successfully completed over a 20 month period.  As part of the scope of works a number of process enhancements were included which would enable the process to be brought back on line aiding both operational compliance, a reduction in opex costs and increased gas yield to promote extended operation of the sites CHP units providing surplus power and heat above that required by the process.

The final agreed scope of work for the this project included the replacement of roof structure of both digesters, replacement of the ageing biogas mixing systems with new dual systems utilising our proven technology, new digester roof platforms, high level slurry and biogas pipework, safety systems, instrumentation and replacement of all insulation and cladding. In addition, Marches Biogas also installed a new 500m³ gas holder and managed the installation of a full lightning protection system covering the digester and gas compound areas. 

Biological Support
Due to the reduced operational digester capacity, the operations team was experiencing major additional costs as a result of having to transfer surplus sludge to facilities further afield. Marches Biogas biological support team were able to use their extensive process knowledge and experience to help incrementally increase both feed rate and dry solids content within the thickened sludge to allow the one operational digester to process all site sludges, fundamentally doubling the feedstock through that digester.

Principle Contractor Duties & Site Works
Utilising our experienced SMSTS site managers, Marches Biogas were employed as Principle Contractor and managed all aspects of the project. The decommissioned digester was the first to be refurbished, leaving the second digester operational. Once the first digester had been fully stripped of the existing degraded roof and associated internal and external pipework it could be rebuilt with new. Marches Biogas process and mechanical design engineers developed the detailed design for all the works.  Marches Biogas also undertook the procurement with sub-contractors used to provide scaffolding, craneage together with packages of the mechanical and electrical works.  Marches Biogas multi skilled engineers undertook smaller sections of work interfaced between the contractors.

Spill and Fill
Marches Biogas commissioning team oversaw the commissioning and decommissioning throughout the works including managing the client site operations team with regards to digester operation.  The first digester was recommissioned utilising a spill & fill methodology. This involved continuing to feed the operational digester but diverting overflow into the refurbished digester, filling it at approximately the same feed rate of the operational digester, with the head space of the tank force ventilated throughout the process. Once the digester was filled, heating commenced and the vessel was nitrogen purged enabling the gas mixing system to be recommissioned. Once stabilised, the feed was introduced and following a ramp up and further stabilisation phase, the second digester was then switched off and decommissioned.  This process took a total of 3 weeks from commencing the fill of the refurbished digester to the decommissioning of the second digester, rather than the months it would have taken from cold commissioning.