What is the Water Main (Hydrant) Flushing Program?

To improve water quality and minimize discoloration, City of Somerville water mains are comprehensively flushed by a technique known as unidirectional flushing. The procedure is performed overnight (usually Mondays) in warm-weather months and involves the systematic opening and closing of hydrants, one section of main at a time, to force the water through the pipes at high velocity, removing accumulated mineral sediment until the water is clear. The operation can take from a few minutes to over an hour, although most last about 30 minutes.

Unidirectional flushing differs from conventional flushing in that it involves the closing of adjacent valves and the use of a single hydrant per flush so as to isolate individual sections of main. Conventional flushing also removes accumulated sediment from pipes, but it involves the flushing of multiple hydrants at a time and does not include the valving off of individual sections of main, thereby diminishing the “scouring” efficiency. Conventional flushing is used in areas of the city where fewer minerals build up in the mains.

Is It Safe to Drink or Cook With Tap Water During Flushing?

Your tap water may appear discolored during hydrant flushing but it does not pose a health hazard.

How Will Flushing Affect Water Service?

When flushing crews are working close to your residence or business, you may experience periods of very low pressure or even a complete stoppage of service. We attempt to notify customers in advance if flushing is expected to result in a complete water outage, but such outages are rare.

Flushing operations may also lead to discolored water, which can be drawn into homes and businesses if the water is being used during or immediately following the flushing. Such events should affect customers for a few hours at most. The discoloration is caused by iron (red color) or manganese (black color) particles being dislodged from the water main which can stain porcelain and laundry. If discoloration occurs, open the cold tap nearest the water meter—usually a basement sink—to full flow until the water runs clear. In some situations, this may take 5 to 10 minutes. It is also advised that you make sure your water is clear before doing laundry or other projects for which discolored water could cause problems.

We ask for your patience and understanding during this critical main cleaning process.