Advances in wastewater flow measurement – good news but …

February 23rd, 2015

March 2015 Water & Sewerage Journal
Advances in wastewater flow measurement – good news but …
by Andy Godley, SWIG and Senior Consultant – Flow Measurement and Metering, WRc plc
Reliable and accurate measurement of treated and untreated wastewater flows is becoming increasingly important for environmental protection, efficient control of processes and to mitigate risks of pollution incidents. This is in a water industry where costs are being further squeezed in the latest price determination and other priorities, for example prevention of sewer flooding, are also clamouring for investment. So when alternative technologies become available that offer good measurement potential and benefits for reducing installation costs and maintenance, they need to be seriously considered.

Last year, both the SWIG workshop on Advances in Flow Measurement and the WWEM event in Telford saw recent developments in non-contact flow measurement on display in the form of the radar-based Raveneye from Flowline, the Laserflow from Teledyne Isco and the microwave wastewater meter from Dynamic Flow Technologies, the latter currently aimed at sewer and drainage flows rather than wastewater treatment flows.

Non-contact velocity sensors for open channel flow measurement are not new; Flo-dar has been around for several years and has found some niche applications but it has not been widely taken up. Maybe, however, the new instruments will help open up this market and change that. Non-contact sensors seem to be a desirable way to go when measuring wastewater streams as the risk of sensor fouling is minimised. This potentially improves measurement reliability and reduces sensor maintenance. Without the need for civils work to construct flumes or weirs, installation times and costs should also be favourable when compared to conventional gauging structures.

But (isn’t there always a but?) the industry and its regulators need to have confidence in the operation and performance of such equipment. The achievement of MCERTS certification by the Laserflow is an important step in this. WRc’s test engineers carried out much of the testing on the Laserflow to support its certification. This proved quite challenging as facilities had to be developed, modified and adapted to enable them to conduct the robust testing of the key aspects on the new technology that MCERTS requires.
However, WRc’s engineers are used to testing unconventional equipment and our test facilities can be adapted to cope with many types of device so testing was completed satisfactorily. No flow measurement solution is a panacea that will work in every situation – this has been learned the hard way through the various new flow technologies which have been ‘oversold’ over the years, making users cautious of adopting new approaches.
Though MCERTS product certification includes a field test, further site trials, such as those being supported by Oliver Grievson at Anglian Water, will be invaluable in demonstrating the performance and operating envelope of these devices under different operational and hydraulic conditions.
Sharing of this experience through events run by the Sensors for Water Interest Group (SWIG) and social media channels will also help generate confidence and avoid inappropriate installations. However, as these devices do find applications and move from trials, where there is usually another installed device to enable performance to be evaluated, to stand-alone applications, the next challenge will be how to validate performance in situ.
The MCERTS Flow Scheme requires Inspectors to validate the instrumentation during the inspection process. On-site validation of wetted velocity area devices such as the Nivus Pipe Sensor and POA wedge sensor has already been raised with the MCERTS steering committee. These are increasingly being installed in pipe sections not running full and channels where flume construction is not feasible or economic.
Conventional gauging structures can be measured and compared against long-established British and International standards, and the level sensors checked with reference plates or survey techniques, but validation of velocity area instruments, both wetted and non-contact, is more difficult. The devices make two measurements: liquid level from an integrated or separate sensor to give the water depth, and hence the wetted area; and liquid velocity. The overall volumetric flow is obtained by multiplying the calculated area by the mean velocity through the section. From this, the four principal areas of uncertainty are immediately apparent: the uncertainty of the two measurements themselves (stream velocity and water level); the conversion of the measured velocity into the mean velocity; and the conversion of the measured depth to the cross-sectional area. Inspectors will need to be able to either validate the whole flow, for example from tank or wet well level changes if available, or check stream velocities and levels independently. The practicality and feasibility of different methods will vary from site to site and this area requires further consideration by the inspectors and wider flow community. WRc is currently running a discussion thread on its Innovation platform to gather opinion on this.
(See www.wrcplc.co.uk/portfolio)
The two measurements of velocity and level made by the velocity area instruments are also the reason why non-contact devices do not entirely eliminate the need for maintenance as it will still be necessary to clean the flow channel. Silting and biofouling on walls could change the relationships between the water level and the wetted area and also between the measured and the average velocity, leading to rors in the flow measurement. The availability of alternative flow measurement solutions, particularly the kind of non-contact methods discussed here, should be welcomed as a useful and valuable step forward. Not only will they find application on treatment works flows, but they also open new possibilities for smarter wastewater networks where flows can be measured and actively managed. Improved flow management will facilitate balancing loads into works and allow more efficient operation, identifying the onset of blockages, reducing the risk of sewer flooding and reducing CSO spills, thus helping better protect the environment.