Heterotrophic bacteria in water distribution systems. II. Sampling design for monitoring.

In this paper, which is a continuation of the work presented in Part I in this issue, previous information on the spatial and temporal variability of bacteriological data from a water distribution system is used to develop a sampling design for use in future water quality monitoring. The water distribution system is considered to be composed of several zones where the variation of bacterial counts in each zone is modelled by the negative binomial distribution. Under the assumption that the objective of monitoring is to determine whether or not the mean bacterial density of the water exceeds a specific standard, a criterion is given which determines the optimal number of sampling stations allocated to each zone. These stations are determined by assuming that either the risk of sampling (i.e. making the wrong decision) is prespecified or that the total number of stations to be sampled is predetermined. Sequential sampling to evaluate the compliance of the water with the standard is also discussed.