Limitations of <i>E. coli</i> Monitoring for Confirmation of Contamination in Distribution Systems due to Intrusion under Low Pressure Conditions in the Presence of Disinfectants

Abstract

Low/negative pressure events that increase the risk of contaminant intrusion may take place in distribution systems and may become more common in ageing infrastructure. Guidance on whether to issue an advisory after loss of pressure is based on the duration and extent of pressure loss and is accompanied by Escherichia coli monitoring obligation. In this paper, the limitations of E. coli monitoring to detect intrusion is demonstrated through hydraulic and water quality modeling using a conservative 5-h pressure loss and considering intrusion of raw sewage. Ingress of contaminated water and fate and transport of E. coli throughout a 30,077-node network are simulated using a realistic pressure-driven hydraulic model coupled to a multispecies water quality model (EPANET-MSX). For a chlorinated distribution system, the limited positive nodes show the challenge of any contamination confirmation even for the studied conservative scenario. In a chloraminated system, a larger number of nodes (2,905 nodes) experienced E. coli over the simulation duration compared to the chlorinated system (166 nodes), increasing the likelihood of detecting contamination. Overall, numerical predictions can guide utilities for redefining more reliable sampling strategies for both confirmation and clearance sampling. Large-volume sampling at at-risk nodes identified by advanced numerical models provide greater credence in negative results to manage boiling advisories.