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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