Three's a crowd

Neil Grant of Severn Trent Services reports on his company's development of the dual validation unit


Ensuring clean and safe drinking water is one of the most important issues facing any water company. The addition of small measured amounts of chlorine to drinking water at the WTW acts as a disinfectant by killing harmful bacteria. As a result, water is kept clean throughout the distribution system as it travels to the customer's tap.

Chlorine levels are continually monitored by analysers at the WTW and in the distribution system itself. As a supplement to this monitoring process, precise laboratory sampling within the system and at the customer's taps is also undertaken. Since chlorine control is a key factor to producing good-quality water, many companies use three instruments (triple-validated units or TVUs) to measure chlorine levels. The three-chlorine analysers must all read chlorine levels within a pre-set limit or an alarm is generated. Chlorine monitoring is traditionally undertaken using amperometric cells, where the chlorine passing through the cell generates a small DC current between two electrodes. This current is proportional to the amount of chlorine present. Until recently, chlorine residual analysers have required the addition of an acetate or phosphate buffer to reduce the pH to less than five, to ensure all chlorine present is in a form which can be measured.

Hitting the buffer
Although effective as an aid in achieving accurate measurement, buffer is far from ideal. It is not cheap, and since it is discarded with the sample it can cause problems, especially at remote sites where drainage is not readily available. In addition to the unpleasant smell and handling issues, it can act as a nutrient source (buffer often has COD of over 500,000mg/l), and the associated growth can cause fouling and blockages in drainage pipework and soakaway systems.

There is a further problem; while the analysers measure the water continuously and automatically, the buffer needs to be replaced periodically. This often requires an operator to travel out to visit a remote site specifically to replace the buffer.

Severn Trent Services embarked on a collaborative research project to investigate a way of replacing the buffer with a less expensive one - and with less environmental impact. It was shown that, with modifications, the amperometric measurement of chlorine could be performed accurately and reliably without the need for any buffer at all. The development of the cell relied on the principle that proton donors (such as those found in natural water hardness) could be used rather than needing to use the protons formed by lowering the pH with buffer. Harnessing the naturally present proton donors made the pH buffer unnecessary.

Severn Trent Services used this principle to develop the CL1000B bufferless chlorine analyser. In summary the key developments involved: the development of specific wet-end and hydraulics, additional electronic filtering, and the addition of a stable third electrode, resulting in zero consumables. Severn Trent Water's Technology and Development Department worked closely with Severn Trent Services to evaluate the new CL1000B at a number of locations. Each trial site was chosen specifically to test the analysers across a range of extreme conditions; for example water with high iron content, low hardness, and variable pH. The new bufferless analyser displayed an excellent correlation to the existing triple-validation buffered chlorine units previously installed and in use at these Severn Trent Water sites.

Technological developments over the last ten years have resulted in vastly improved instrument reliability, meaning that the need for three instruments is now unnecessary. Based on the success of these trials, Severn Trent Water initiated a wide-ranging replacement programme of triple validation units with CL1000Bs. However, in certain locations with very challenging water quality, and in those instances where water undertakers felt the move from a TVU to single analyser was not appropriate, a dual validation unit (DVU) was required.

Trial and vindication
The DVU was developed with two independent CL1000B chlorine analysers and a small control unit - one analyser measuring the chlorine level and controlling the dose (control analyser), with the second analyser (policing analyser) ensuring the result is within pre-set limit between the two analysers. If the results are deemed to be significantly different an alarm is raised. All parameters are operator-configurable.

Two years of trials have been completed, testing the DVU at many locations with challenging water quality throughout the UK and Europe. The operational benefits of the DVU proved to be improved accuracy and stability, lower operational costs and lower associated maintenance costs when compared to a TVU. As a result, the industry drive towards 'doing more with less' is being delivered.

The CL1000B and DVU version are now being trialled by the majority of water companies in the UK and Europe - and, even in the most challenging of applications, they are consistently exceeding expectations.

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