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Pen Y Bont Wastewater Treatment Works
Enabling > mitigation > capital works
Pen Y Bont Wastewater Treatment works serves 160,000 people in the Bridgend catchment. Located adjacent to a SSSI, it is fed by six Archimedean pumps designed to lift wastewater from the catchment into the inlet works at a maximum flow rate of 4400l/s.
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The screw pumps had been in operation for twice their designed service life which had led to repeated intervention by DCWW for major repairs in order to maintain forward flow to treatment.
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The pre-treatment screening system had also reached the end of its service life and had no standby facility, so MMB was tasked with a scheme to replace all six pumps, construct a new motor control centre, install new ladder screens and replace redundant valve gear and control equipment.
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Enabling works
Feasibility studies identified several challenges in delivering the scheme, the most significant being uncertainty surrounding the structural integrity of an access bridge. This prompted a concrete repair to bring the structure up to standard, together with the installation of a corrosion protection system to future-proof the structure for the next 20 years.
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Our programme depended entirely on the adequacy of this bridge to sustain loading from vehicles (incl. 96T cranes); we established assessment, analysis and live load monitoring to verify vehicle safety, and where a conventional structural assessment would likely have concluded the bridge needed replacing, advanced analytical techniques in conjunction with structural monitoring confirmed its adequacy, engineering out the need for a replacement.
Mitigation
On the lead up to the main scheme, the screws suffered a catastrophic failure resulting in loss of forward flow treatment. Immediate mitiagation was needed.
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Working closely with DCWW, emergency works were installed, including a temporary offline pumping system able to deliver the equivalent flows to that of the screw pumps
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Available space to locate pumps and pipework of suitable size to cope with storm conditions led to a project in its own right. Working closely with pump suppliers, we installed a system that first addressed the emergency and then the subsequently upgrade.
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This allowed us to undertake an interim programme of mitigation work to repair the failed screw pumps, providing the resilience needed to allow the phased installation of the main scheme without jeopardising treatment consent.
Construction
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Key to the success of the scheme was a close working relationship with DCWW Operations, a relationship first established during the mitigation and enabling phases. This helped us to manage many of the safety challenges at Pen Y Bont, challenges linked to the coastal and tidal location, unprecedented winter storms, environmental sensitivity, and further failures of existing assets
The inlet works were adopted in phases to allow each set of screw pump sets to be replaced in a controlled manner, with forward flow maintained throughout the screws installation via the temporary over pumping array, which was monitored 24hrs a day via remote telemetry system to minimise process risk.
Access to the inlet well was not possible during storm conditions due to flood risk. This, coupled with difficult access for emergency rescue, meant access for personnel was minimised. To address this, a robotic Brokk machine was used break out the concrete in each of the screw troughs, minimising confined space entry and eliminating HAVS.
The coastal location meant lifting operations were often impacted heavily with high winds and poor visibility. The large tidal range and
rapid changes in fluvial flow at the River Ogmore meant access to the bridge foundations for monitoring was limited. Detailed weather and tidal forecasting helped overcome these restrictions, together with a collaborative approach to site access between crane operators, installation subcontractors and bridge engineers that facilitated bridge crossings at abnormal times. This avoided additional costs for cranes or leaving HGV loads being stranded at the works.
For the inlet works refurbishment, bespoke double stop log systems were installed at key locations within flow channels allowing a dry working area to be created rapidly during windows of opportunity in foul weather conditions. Phased commissioning of new equipment allowed forward flow to be maintained with reduced equipment availability. Again, process compliance was critical as 1600 l/s forward flow to treatment had to be maintained throughout.
Severe storms led to alternative business continuity plans being developed with regulatory authorities to ensure acceptable risk was maintained whilst achieving project aims.
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Want to learn more? Speak to Rob Dancer or James Croston @ MMB