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Flushing, Purging, and the Water Used for Each

Tucson Water has occasionally received inquiries like the following:
"I understand the need to flush fire hydrants periodically. What I don't understand is why the water can't be put into a tanker truck for reuse instead of letting it run down the street and into storm sewers. In Tucson, where water is precious, why is Tucson Water seemingly wasting it? Please educate me. Thanks."

                    

On the surface, it might appear that flushing and purging methods are contrary to our messages about using water efficiently. Flushing water mains and lines, and purging wells are necessary components of any water utility operation. The definitions for each are:

flushing—(1) a process that removes sands, silts, rocks, and dirt that accumulate from pumping groundwater over time or that may have entered a pipeline during a water main break. Flushing is done through a fire hydrant or drain valve to achieve maximum pressure. Flushing is also performed after any repairs to a fire hydrant to ensure its operability.
purging—(1) a process that removes concentrations of sand and other sediments from a well; (2) the preparation to disinfect a well that has been out of service for more than a month or has undergone major work.

Both procedures also allow Tucson Water maintenance staff to assess the internal condition of our water infrastructure using advanced technology. We are also committed to ensuring water quality for our customers as well as compliance with all environmental regulations.

How much water is considered wasted? Tucson Water customers use about 30 billion gallons a year. Flushes and purges account for about 19.15 million gallons of water annually. This represents 1 gallon for each 1,560 gallons that our customers purchase, which translates to a "wastage rate" of 0.064%.

There are cost factors too. Using the following as a baseline, we can calculate the expenses and the carbon footprint to flush/purge with the intent of ‘saving’ the water.

Equipment (4,000 gallon tanker truck): $60.00/hr
Employee (wages & benefits): $33.00/hr
Fuel ($3/gal, 4.5 mpg): $3.00/gal
Carbon footprint: 11.91 kg/gal (one way)
Water retail value: $1.50/1,000 gal (national average)

                             

To perform one flush of 4,000 gallons, the cost of the water itself would be $6.00 while the expense to carry out the operation would be $99.00 (one hour minimum time, round trip, plus the carbon footprint). Recovering higher amounts of water used in flushes and purges (much more common) means more hours, more equipment, more drivers, more fuel, etc.

In main line flushing, the term “scour rate” is used to represent the velocity (speed) at which water moves through the pipe. A higher velocity is often necessary to flush out sediments from the water main. This higher velocity uses a higher gallons-per-minute rate. If a water main has to be shut off during the flushing process to change water trucks or wait until they return from their delivery point, this would increase the flushing time, use up more water, and never completely clear the line or well.

In closing, it should also be noted that water that is flushed or purged will find its way back into the aquifer or our reclaimed water system. To those who have inquired about these procedures, thank you.