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
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
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
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.
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