SPECIAL WORKS
Innovation drives efficiency at Five Fords WwTW growth mitigation scheme
Engineers working on a project in North Wales have employed innovative techniques to improve wastewater treatment efficiency at a fraction of the cost of traditional methods, saving an estimated £3.5 million. Alison Miles from NIVUS, an instrumentation partner in the project, explains why the choice of flow measurement technology unlocked the enormous benefits that have been achieved.
Background
Owned and operated by Dŵr Cymru Welsh Water, Five Fords WwTW is a sewage works near Wrexham in north- east Wales serving over 130,000 people. The site operates three ASP (activated sludge process) lanes.
As a Capital Delivery Partner and part of the Welsh Water Alliance, Mott MacDonald Bentley (MMB) was challenged to increase the treatment capacity of the ASP lanes in response to a growing population and increased demand from new anaerobic digestion plants.
The challenge
The existing layout of the plant delivered wastewater to the ASP lanes at a different flow rate in each lane. This disparity was reflected by dissolved oxygen measurements within the lanes, which indicated that the lanes were not operating efficiently. “One of the lanes, for example, was operating in a super- critical condition, whilst another was sub-critical,” explains MMB’s Jack Jones. “As a consequence, from a process control perspective, the ASP lanes were difficult to manage effectively.”
The uneven distribution of flow between the ASP lanes was caused by a number of physical issues in the flow channels that fed them. In addition, a large 800mm relief pipe discharged a mixture of RAS (returned activated sludge) and overspill directly into the main inflow channel,
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Circumferential strap with 'top hat' level & Nivus insertion probe inset
which also affected the flow profile.
The ASP lanes were originally built with flumes to control flow, but these were replaced with weirs to increase capacity. The requirement for a further increase in capacity therefore meant that a return to flumes would not be practicable, and would not resolve the flow disparity in any case.
A flow split chamber with equal weirs was also not considered to be practicable because this would cause disruption to the operation of the plant, requiring significant temporary works and civil costs. The ASP lanes can receive up to 1200 l/s in full flow, so a weir-based solution would necessitate the hire of temporary pumps for the duration of the construction, which could be around 4 months, representing a substantial extra cost. This alternative was therefore disregarded.
In addition to resolving the flow disparity in the ASP lanes, it would be necessary to monitor flow in the relief pipe. This also
| February 2025 |
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Nivus sensor on sliding steel bracket
represented a very significant potential cost, because traditional techniques such as electromagnetic flowmeters typically require a full pipe for accurate measurements and involve substantial installation costs and process downtime. A recent trial comparing the NIVUS insertion probe with a traditional
Alison Miles
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