Explaining the occurrence of coliforms in distribution systems

According to the Total Coliform Rule, coliform bacteria constitute the main indicator used to detect microbial contamination in distribution systems. A major goal for water utilities is to prevent and control coliform occurrences and noncompliance (with respect to regulations). However, the applied solutions are sometimes successful only to a limited extent because of the variety of factors that may give rise to these occurrences. It is therefore important to identify those factors—from the structure and operation of the distribution system to the quality of the distributed water itself—that can influence the occurrence of coliform bacteria in a distribution system. The use of the identified factors makes the modeling of coliform occurrences attractive, and a number of approaches for doing so have recently been proposed. This article provides a review that includes the mechanisms of how coliform bacteria are introduced into treated and distributed drinking waters, the major factors controlling the survival and regrowth of coliforms once introduced into the system, and the modeling efforts carried out to explain or predict their occurrence.