Integrating approach to maintaining quality

Bob Borrill, water supply process manager at West Midlands Region of Severn Trent Water, explains how the collection and interpretation of data, often provided by computer control systems, is vital in the drive to improve the standard of service provided to its customers.

Severn Trent Water has recognised for some time that Œacting on facts' via the collection and interpretation of relevant data is vital in the drive to continually improve the standard of service provided to its customers.

In addition to controlling processes, the MWCS gather data from a suite of continuous, on-line water quality monitors, monitoring concentrations or values of parameters such as chlorine, turbidity, pH and coagulant residual at various stages through the treatment process. Data from these monitors is collected and displayed in real time as trends, allowing the works operator to detect and correct deviations from normal values quickly.

It isn¹t enough however just to collect the data. Should an adverse trend start to appear in any particular data set it is vital that the operator responds quickly with corrective action. This is achieved in Severn Trent by the use of discrete data collection, data verification and data interpretation stages as follows:

- collection ­ continuous, on-line water quality monitors feed data to the SCADA system;

- verification ­ on site analytical tests verify monitor accuracy; and

- interpretation ­ procedures provide advice to operators should adverse data trends start to develop.

Ultimately Severn Trent Water's performance with respect to drinking water quality is determined by analytical results from statutory samples analysed at Severn Trent Laboratories, an accredited laboratory using sophisticated analytical techniques. This data however is not immediately available to the water treatment plant operator who needs real-time data from water quality monitors and on-site tests to optimise treatment processes.

In order to standardise on-site water quality monitoring across the company Severn Trent Water has developed an ŒOn-Site Water Quality Testing manual' which provides a standardised approach to real-time water quality monitoring within a flexible framework.

The primary objectives of the manual are to ensure that appropriate parameters are monitored at the correct frequency using established best practice for sampling, analysis and record keeping. Water quality data is derived from a mixture of continuous on-line monitors and on-site tests and the On-Site Water Quality Testing manual provides standard instructions on the way in which the two monitoring methods should be combined.

Water quality parameters are separated into two categories; ŒRegion 1' and ŒRegion 2':

- Region 1 parameters are the core testing suite which are critical to process plant control and performance. These parameters are usually monitored continuously. Should a continuous monitor fail for any reason the frequency of on-site tests is increased to compensate until the monitor is repaired. The manual prescribes the minimum testing frequency to be used.

- Region 2 parameters such as conductivity, manganese and ammonia are viewed as additional core tests appropriate to the treatment process in question and they are more usually measured by on-site tests rather than continuous monitors. A procedure for determining the testing frequency of Region 2 parameters is included in the On-Site Water Quality Testing manual. The procedure assigns scores to elements such as Œcriticality', Œspeed of impact' and Œwhether the monitor controls dosing of a chemical' in order to determine the frequency of on-site testing.

The manual covers the whole range of tests required to operate a modern water treatment plant effectively including tests to ensure that treated water has an acceptable taste and odour and tests on waterworks sludge to check sludge treatment plant efficiency.

Continuous on-line monitors are maintained and calibrated within a pre-planned maintenance programme but it is prudent to check monitor accuracy regularly in between maintenance visits by checking the monitor against a simple on-site analytical test.

Severn Trent Water's water quality procedures play a vital part in the management of water treatment processes. Their power lies in the way in which they encapsulate the knowledge and experience gained at a water treatment works over many years to help operators provide an instant, informed response to problems 24 hrs/day. The procedures start by stating the alarm level for the parameter in question after which operators are prompted to read a short section containing important data such as who needs to be informed and how time critical the problem is.

The main body of the procedure contains sections on checks to be carried out and possible remedies and this is where historical expert knowledge of the plant is encapsulated.

Severn Trent Water now has a standardised approach to on-site water quality testing at its water treatment works thanks to sophisticated data collection and trending systems coupled with the standardised approach provided by the codified On-Site Water Quality Testing manual. Feeding this data into water quality procedures provides advice 24 hours a day through encapsulated expert knowledge, which helps to ensure that Severn Trent's customers continue to receive very high quality drinking water.


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