The objective of this project was to optimize the Bellevue Utilities Department's (BUD) water main flushing program so that it effectively achieves BUD's goals of maintaining drinking water quality in accordance with the State of Washington Department of Health's standards, while minimizing the impacts of distribution system flushing activities on receiving surface waters. BUD is a wholesale customer of the City of Seattle and has little control over the quality of water that enters the drinking water distribution system. However, BUD does have full control over the water distribution system and the operational practices that can have a significant impact on water quality delivered to its customers. Since BUD has the opportunity to focus its attention and resources on the distribution system, BUD staff have developed monitoring and operation and maintenance practices that are designed to minimize deterioration of water quality through the distribution system while meeting other environmental goals of the Utility. One of the important maintenance tools implemented by BUD is a pro-active water main flushing program designed to remove sediment and other materials from the drinking water distribution system. BUD conducts a unidirectional flushing program across the entire distribution system every 4-5 years. During flushing operations BUD field crews have targeted a flushing velocity of 7 feet per second (fps) or greater. BUD staff use the 7 fps velocity with the assumption that a velocity greater than operating conditions (2-5 fps) is needed to remove sediments. Flushing velocity also determines the volume of water for a given flushing period ultimately discharged. This project is a first step in optimizing flushing velocity and minimizing discharged water volume. BUD has a uni-directional flushing program that is state-of-the-art as far as potable water is concerned; but it may be possible to refine the existing program to better meet the requirements of more stringent environmental regulations and community expectations. BUD's primary source water, the Tolt River, was unfiltered until December 2000. The long detention times and low flow conditions possible in the water distribution system can be conducive to settling and accumulation of sediment in the pipes. The new filtration capabilities of the Tolt source, brought on-line in December 2000, may reduce, but will not likely eliminate the need for a flushing program. The primary outcome of this project was the revision of the BUD Zone Flushing Standard Operating Procedures (SOP) with respect to an optimum flushing velocity for BUD's water distribution system. The goal of the revised flushing velocity is to enable BUD water quality and operations personnel to achieve acceptable cleaning with minimum water volume and minimize the impact of flushing activities on receiving surface waters directly attributable to flushing activities. Includes tables, figures.
