Assessment of the Risk Associated with E. coli Bacterial Intrusion in Drinking Water Distribution Networks

Maintaining clean drinking water is a key factor in maintaining sustainable development and a healthy environment. Thus, in many countries, the safety of drinking water is a key goal which is expected to be achieved in water distribution networks (WDNs). This experimental study was performed to assess the risks of E. coli bacterial intrusion into drinking WDNs. Three different scenarios are examined: (a) intrusion of E. coli bacteria during a sudden power shutdown associated with low or negative pressures; (b) planned intrusion through direct injection into the network using a high-pressure dosing pump; (c) premeditated or accidental E. coli contamination of water sources (tanks). While running the three scenarios, it was found that the rate at which bacteria reach customers is very comparable; however, higher risks are found with premeditated bacterial intrusion. The theory that contaminants immediately intrude into water networks through cracks and loose joints after a power shutdown accompanied by low/negative pressure was tested. It was found that there is a slim chance for E. coli and other contaminants to find a path into the system after sudden low/negative pressure events. High pressure within water networks pumps out water from the system, and then, the mixture (water and contaminants) is sucked back into the network if enough time is allowed after the power shuts down. Results show that it takes from 3.5–20 s for contaminants to intrude into the system after a sudden power outage if the network maintained an operating pressure between 1 and 4 bar prior to the shut down of the water pumps. This does not allow contaminants to enter the network unless there is a long-power shutdown that allows the contaminants to be sucked into the system. The minimum time required for contaminants to intrude into the system was found to depend on the water pressure prior to the power shut down. Meanwhile, it was found that bacteria directly injected to the system represent a major risk as it takes only 30–90 s for the contaminants to reach the customer at an operating pressure of 1–3 bar.