Use of Distribution System Modeling in Designing Microbial Monitoring Programs

Ongoing concerns with potential microbial contamination of distribution systems and the present process of revising the Total Coliform Rule have focused attention on the need for improved methods for designing distribution system microbial monitoring programs. A quantitative method for assessing alternative microbial sampling programs is described in this paper. A hydraulic/water quality model of a distribution system is used within a Monte Carlo simulation framework to evaluate the effectiveness of a sampling program considering both the variability in the operation/behavior of a distribution system and the uncertainty in the understanding of the microbial entry and behavior in the distribution system. Different paths for entry of the contamination into the distribution system are considered within the probabilistic framework including cross-connections, pipe breaks, intrusion through transient negative pressures, treatment failure, tank contaminations, etc. The resulting contamination is tracked through the network using EPANET and the effectiveness of alternative sampling programs is evaluated in terms of a fitness measure. The method is used to evaluate alternative sampling programs (number, type and location of sampling stations and sampling frequency) in a hypothetical distribution system.