Owned and operated by Dŵr Cymru Welsh Water, Five Fords WwTW is a sewage works 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 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, indicating the lanes were not efficient.
The uneven distribution of flow between the ASP lanes was caused by a 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.
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 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. A weir-based solution would necessitate the hire of temporary pumps for the duration of the construction, which could be around four months, representing a substantial extra cost. This alternative was 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 represented a 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 ‘magflow meter’ demonstrated a 60% saving in cost and 95% saving in embodied carbon. The magflow option was therefore also disregarded.
THE SOLUTION
“Before making the final decision, a number of internal ‘surgeries’ were conducted involving hydraulic, mechanical and process engineers,” explains Josh Sparks, MMB contracts manager & EICA regional practice lead.
The preferred solution involved the deployment of three actuated penstocks that would be lifted into the first three channels. The 4th channel would be isolated, but could be made available if necessary, in the future. Effective control of the penstocks would depend on the reliable measurement of flow in each channel downstream of the penstocks.
“This was also a major challenge,” Sparks explains. “Most traditional flow measurement methods were ruled out due to the physical constraints of the site, along with the cost of solutions involving significant civil works. As a result, we engaged with Nivus and determined that their cross-correlation flow meters offered the best solution.”
Nivus was contracted to install flow metres in the relief pipe, in the main inlet channel, and in each of the three ASP inlet lanes. All of these instruments are MCERTS and EX-rated and employ the Nivus cross-correlation flow measurement method.
Two Nivus POA wedge sensors were fitted to L-shaped bed-mounted brackets – one on either side of the main inlet channel. These sensors were connected to a (NF750 M9) transmitter, to which the pipe insertion probe (velocity profiler and ultrasonic level) was also connected.
Each of the ASP lanes was fitted with a radar level sensor, working together with POA wedge sensors, also mounted on L-shaped bed-mounted brackets. Each of these sensors was connected to a single input transmitter (NF750).
FLOW MEASUREMENT
The ultrasonic cross-correlation method developed by Nivus measures individual velocities continuously at different levels within the flow. This area velocity flow measurement technique provides a 3-dimensional flow profile that is calculated in real-time to provide reproducible and verifiable flow values in full or partially filled channels or pipes.
The method is based on the principle of ultrasonic reflection, in which particles, minerals or gas bubbles within the water body are scanned using an ultrasonic impulse with a defined angle. The resulting echoes are saved as images or echo patterns, and a further scan is conducted a few milliseconds later.
This creates echo patterns which are compared with previously saved signals, and the positions of unambiguously identifiable reflectors are identified to determine velocity, by using the beam angle. This means the particle velocity, which is equivalent to the wastewater flow velocity, can be calculated from the temporal shift of the reflectors.
The method produces accurate readings without the need to perform additional calibration measurements.A key advantage is that it can operate in the most demanding and heavily polluted applications.
KEY OUTCOMES
The Nivus flowmeters were installed before the penstock solution was implemented, so that flow variation could be observed both before and after the penstock installation. The overall objective was to equalise flow rates within the three ASP lanes so that treatment efficiency and capacity would be improved. Sparks says: “By adopting a scheme based on cross-correlation flow measurement for both the relief pipe and the ASP channels, we were able to avoid substantial civil works, which delivered dramatic financial cost and carbon savings, as well as almost no interruption to the operation of the plant.”
MMB has estimated a conventional build solution involving modification of the channels with a new distribution chamber would have taken around eight months and cost over £4.5m. MMB’s solution was delivered at around £1m.
MMB’s cost savings deliver financial benefits to MMB and Dŵr Cymru Welsh Water, while the water company’s customers also benefit. The Nivus cross-correlation flow measurement technology will be rolled out to future wastewater projects.