Over the past decade many utilities have invested significant resources in modifying, upgrading and building new treatment facilities with an expectation of improved water quality being delivered to their customers, only to see an increase in customer complaints.
These problems are often the result of major changes in the chemistry of the water entering and/or within the distribution system. The chemistry changes tend to destabilize the materials attached to the walls of the distribution system piping making this material available for resuspension following any hydraulic disturbance.
Typical problems encountered by new or upgraded systems include:
- Loss of disinfectant residual within the distribution system
- Bacterial regrowth
- Excessive levels of THMs and/or other DBPs
- Inadequate or ineffective corrosion control treatment
- Taste and odor problems
- Discolored water
Of all the available tools, flushing is the single most powerful tool available to systems in their effort to maintain water quality control. Flushing techniques fall into two broad categories: conventional flushing and unidirectional flushing.
Conventional flushing is the most commonly used method and consists of opening hydrants in a specified area of the distribution system. No valve isolation is used, and water to the hydrant often flows from several mains with the resultant velocity in each individual main being low. Although this type of flushing does promote water flow in the pipes and expels some of the poor-quality water from the system, the low velocities generally are not enough to dislodge biofilms and other debris. Moreover, because water used to flush a main may not originate from a segment that has already been flushed, the cleaning efficiency of conventional flushing is not maximized.
Unidirectional flushing consists of isolating a particular pipe section or loop, typically through closing appropriate valves, and exercising the hydrants in an organized, sequential manner to produce velocities of 6 fps or more. These high velocities are sufficient to produce a scouring action, removing any accumulated debris, biofilms and/or corrosion products. The program is designed to flush from the source, such as a treatment plant or storage tank, to the periphery working from the larger mains to the smaller ones and bringing clean water into the dirty area.
When used on a system-wide basis, unidirectional flushing can help prepare a system for any changes in chemistry and minimize the potential for water quality complaints. Once a comprehensive flushing has been completed, this technique can also be used on a spot basis in response to localized water quality complaints.
The benefits of unidirectional flushing tend to last much longer than conventional flushing. In addition, a study that was done in Edmonton, Alberta, demonstrated that unidirectional flushing used 40 percent less water as compared to conventional flushing.
The 1998 AWWA Annual Conference and Exposition in Dallas includes a preconference workshop on Sunday, June 21, entitled “Is your Distribution System at Risk of Serious Water Quality Problems?” This interactive workshop will cover a variety of topics pertaining to water quality control in the distribution system including unidirectional flushing, operation and maintenance of storage, monitoring, etc.
