The basic concept of chemical reaction engineering, as applied to the
disinfection of water in a continuous flowing system, is summarized, a
nd the effect of flow dispersion (i.e. the spread of residence time) o
n the efficiency of the process is highlighted. It is shown that defic
iencies compared with the ideal batchwise situation can be offset by a
djustments to the residence time or the disinfectant concentration, pr
ovided that the design is reasonable. The term 'compensation factor' i
s offered as a means of characterizing tanks which are intended for a
specific duty. Reference is also made to recently available software w
hich now enables tanks to be evaluated at the design stage without res
orting to specialist computing services.