Is Flushing Wasteful? Tony Wyatt TAUD West, TN Field Supervisor Routine flushing of water mains can be controversial in many water systems. Operators know that it is a necessity to provide clean safe water to their customers and to meet water quality parameters. Some customers object when water from flushing floods their yards, gardens, etc., while others complain when sediment or air shows up in their water from lack of flushing. Utility officials or mayors may comment that flushing water onto the ground is a waste of water and therefore a waste of money. Occasionally these officials have stopped operators from routine flushing. So, what do the rules say about the kind of flushing program that a utility must have? 1200-5-1.17(10) All community water systems having more than 50 service connections shall establish and maintain an adequate flushing program. The flushing program established shall help ensure that dead end and low usage mains are flushed periodically, drinking water standards are met, sediment and air removal and the free chlorine residual specified under Rule 1200-5-1.17(4) is maintained. Records of each flushing are to be maintained by the water system. These records shall include date, time, location, persons responsible and length of flushing. In addition to the above information, the free chlorine residual will have to be measured and recorded on the end of dead end mains after being flushed. The rule above indicates that the purpose of the flushing program is to provide a safe high quality water supply to the utility’s customers. Air and debris can enter and accumulate in a distribution system. Disinfectant residuals can deplete due to low usage and chlorine demand in the water lines. Disinfectants may also combine with materials in the system to form undesirable byproducts such as trihalomethanes. An adequate flushing program can correct each of these situations. But what constitutes an adequate flushing program? Is once a year sufficient to maintain a desirable water supply or do you need to flush more often? This depends on several factors such as the
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layout of your distribution system. A system with long mains with few customers may require more frequent flushing to maintain the required chlorine residual or prevent the buildup of disinfection byproducts. The type of pipe in the distribution system and the water quality can also be factors that affect the need for increased flushing. Many distribution systems are well served by a systematic flushing of the entire distribution system once or twice per year. A systematic flushing consists of starting at the source, whether it is the treatment plant, master meter or a tank, and flushing hydrants and blow-offs along the main until the end of the main is reached. In this type of flushing, debris or air that is not completely removed at one flush point will be drawn along the main until it is removed at the end. Systems that have loops may require that valves be closed to create dead ends to flush toward. This type of flushing must be conducted at adequate velocities to scour the inside of the main, breaking debris free to be flushed out of the main. This cannot be accomplished with a 3/4� tap on a large line or simply cracking a hydrant open and leaving it running for several hours. The Design Criteria for water systems has this to say about blow-offs and flushing velocities: 9.0.3 Dead Ends b. Where dead-end mains occur they should be provided with a fire hydrant, when fire flows are available, or blow-off for flushing purposes. The blow-off shall be at least 2 inches in diameter, but should provide flushing velocities of 2 feet per second or greater. In addition to the routine flushing mentioned above, most systems require more frequent flushing of problem areas such as dead-ends where debris collects more often or low usage areas where the chlorine residual is difficult to maintain. Some systems have areas that may require
TENNESSEE UTILITY NEWS