INTRODUCTION

As water passes through a treatment plant, the various processes remove or inactivate many of the organisms present in the raw water. The final treatment in a water-treatment plant is disinfection of the treated water. The correct meaning of disinfection is under threat and the word is often incorrectly used. Disinfection is not sterilization, which implies the inactivation of all organisms; rather it is the killing of pathogenic organisms, those which cause disease. Disinfection is normally the most critical process from the point of view of supplying safe water. There are two aspects to disinfection: the first is the disinfection of the water to kill all pathogens that have passed through the various treatment stages, and the second is to apply a residual disinfectant so that the water leaving the treatment works remains safe as it passes through the distribution system to the point of use.

There are three sorts of pathogenic micro-organisms that are of greatest concern in water treatment: viruses, bacteria, and protozoa. It is not straightforward to classify micro-organisms, which is complicated by there being a continuum of organisms, rather than a number of distinct types, but it is useful to understand some of the key differences. Viruses are the smallest. They are simple organisms, consisting of a core containing nucleic acid (either RNA or DNA) surrounded by an envelope of protein. They are obligate parasites able to multiply only in other living cells. It is quite possible to argue that viruses are not living organisms...