Treatment process finely tuned

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.