Wastewater sensor goes non-contact

WRc has developed an instrument to measure organic load by optical non-contact methods, says senior consultant Steve Russell

The search for reliable wastewater instrumentation data has gone down many roads: compressed air blasts, water jets, wipers, multiple beams, biocide washes, dual/triple sensors, even frequent manual cleaning as a last resort. We have learned, largely by trial and error, which methods work best with which sensors, and where in the process they are effective.
There is now a much greater need for highly reliable instrumentation, driven by the installation of new control systems. Companies are investing £50-150K per plant on advanced process control to achieve energy savings, but the promised returns are much less attractive if the system has only 60% or 70% up time due to instrument
fouling. The best returns require reliable instrumentation with realistic maintenance requirements.
This changed landscape has generated the driver to develop WRc's non-contact optical technology. WRc co-ordinated and led an EC project 'Loadmon' from 1998-2002, which developed an instrument to measure organic load by optical non-contact methods. The project was targeted at the real time control of sewers, but was not taken forward to manufacture.
The measuring principle is to interrogate the wastewater sample using light at different wavelengths. From the light detected by the instrument it is possible to deduce some of the wastewater properties such as suspended solids. The great advantage is that this can be done at a range of over a metre, so that it is truly non-contact, like an ultrasonic level transducer. Little maintenance is required and is restricted to occasional cleaning of the window and a calibration check.
The main application for the new project is to provide data input to advanced process control systems, particularly to measure organic load on settled and crude sewage, where contacting sensors usually have the greatest difficulties. It may also be used on treated wastewaters.
The intention is that the instrument can be mounted over a flowing channel of wastewater and that it will operate reliably in the presence of some foam, scum and debris and will accommodate changes in sample level. The technology does not require large amounts of power, so a battery or solar-powered option is feasible. To achieve this at low power levels requires very careful optical and electronic design, and sophisticated signal processing. The current design avoids high-powered lasers entirely and would be 100% eye safe.
Work at WRc began in October 2010 with modelling of the system to understand the theory of operation. It will follow through with development of the method in the laboratory, then the building and site testing of a pre-production prototype with a manufacturing partner.
The instrument development is being supported by United Utilities, Severn Trent Water, Northumbrian Water and Northern Ireland Water, but WRc also gained government support through the Knowledge Transfer Partnership (KTP) scheme. Oxford University's Department of Engineering Science will bring their knowledge and experience of optics and electronics to the project and a full-time KTP Associate will be appointed to work at WRc.
The original idea of using the technology further upstream in the sewer network remains an important application, but will be tackled as a further development.
The history of the technology shows how easy it is to get the timing wrong in developing an idea.
The current project would not be viable if energy prices were at their year 2000 level. The sewer application will probably need to wait until there is a demand for control systems which include parts or all of a catchment. Preparations for such schemes are being considered during the AMP period, but are unlikely to roll out until AMP6.
Optical non-contact techniques are widely used in other contexts such as atmospheric measurements, satellite remote sensing, and at shorter range for precision dimension measurement.
It is new to the water industry for process measurement and represents a significant step in the direction of using more sophisticated technology. This development is one of a number of new technology applications which will be needed to meet the industry's strategic aspirations. In this case, it is part of the drive to give improved value to customers, to reduce carbon footprint and to reduce the risk of pollution to the environment. WRc is very pleased to be at the forefront of these developments.

www.wrcplc.co.uk

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