Turbidity and beyond

The presence of even quite low levels of particles in drinking water is a matter of some concern, because of the possible presence of Cryptosporidium, which has been responsible for several recent outbreaks of waterborne disease. Turbidity measurements are by far the most common means of monitoring for particles in filtered water, but it is known that turbidity monitoring is rather insensitive for particles in the size range of Cryptosporidium oocysts (4-6 mu m). The physical basis of turbidity by light transmission and light scattering is briefly reviewed, and alternative methods are considered. These include particle counting, which can give detailed information on the number and size of particles, and is highly sensitive for particles larger than about 1 um. It is shown that a simpler technique, based on fluctuations in transmitted light, is very much more sensitive than turbidity for larger particles, and has some advantages over particle counting for routine monitoring of filtered water quality.