Treatment process finely tuned

Water particle counters are being installed at ten Southern Water sites. Paul Hamilton and Dr Simon Parsons of Cranfield University, examine the case for particle counting devices


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Although most groundwater supplies in the UK are free from significant contamination,

those that are influenced by surface waters carry more of a risk.

Particle counters have been imported from other fields, such as the pharmaceuticals

and semiconductor industries. They were first used on-line in a US water treatment

works in 1982. Southern Water, which supplies areas in Hampshire, Sussex, Kent

and the Isle of Wight, first used a particle counter at one of its works in

1992. It is currently working through a programme of site-based trials with

Cranfield University to assess potential uses of these monitors at its groundwater

pumping stations.

clouding the issue

Turbidity is a simple measure of water ‘cloudiness’ that has been used in water

treatment for most of the last century. Modern, on-line turbidimeters usually

measure the amount of light scattered at 90°, by particles in a sample cell

relative to a known standard. Readings are expressed in nephelometric turbidity

units (NTU) and are heavily influenced by the number of tiny, submicron particles

in the sample.

In contrast, most particle counters measure the amount of light blocked by

larger particles as they pass through a laser beam. In this way, particles can

be individually counted and sized within different, discrete bands, usually

from 1 or 2µm upwards, depending on the type of sensor used.

Despite the higher level of information provided by particle counters, some

doubts still remain as to their real value. Turbidity is a fairly reliable,

cheap, accurate measure of water quality and remains the first choice particle

monitor. Particle counters will only be useful if they tell the user something

about the water that turbidimeters do not. Although both instruments respond

to different sizes of particle, in most instances of groundwater monitoring

their readings have similar profiles, effectively making one of the monitors

redundant.

The value of counting particles across many different size ranges is a subject

of on-going research and arguably, particle counters still have much to prove

in this respect.

However, particle counters do show one clear benefit over turbidimeters. In

very low-turbidity water, they are more sensitive to changes in quality. Other

researchers have seen this extra sensitivity in other water treatment applications.

Turbidimeters can be relatively insensitive to changes in water quality below

0.1 NTU and are prone to ‘flat-lining’. Particle counters can therefore be used

to ‘fine-tune’ treatment processes.

site specific

The work conducted so far, suggests that particle counters can be a useful addition

to turbidimeters in monitoring groundwater quality, but only at certain, low-turbidity

sites. The decision whether to install these instruments should also be taken

in consideration of perceived pathogen risk, since ironically, low turbidity

waters are usually among the safest to drink. However, where there is a significant

risk attached to such a source, companies may want to consider using particle

counters to fine tune the treatment process.

As a result of preliminary investigation work, Southern Water is permanently

installing particle counters to monitor treated water at ten of its surface

and groundwater sites. These will be used primarily for process research and

optimisation purposes.

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