Human, System Errors Cause NC Water Shutoff

Friday, February 10, 2017 @ 05:02 PM gHale

Human and system errors caused last week’s fluoride overfeed at Orange Water and Sewer Authority’s (OWASA) water treatment plant and a pipe main may have burst because of internal and external pressure.

Reports on the incidents released Friday, a day after a public meeting at which the OWASA apologized for the “worst-case scenario” that left 80,000 customers in the Chapel Hill-Carrboro, NC, area without water for over 24 hours.

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The incident began Feb. 2 when water at the Jones Ferry Road Water Treatment Plant became over-fluoridated, forcing workers to shut it down.

Investigators with CH2M HILL North Carolina examined systems data, inspected and tested the plant’s equipment, and interviewed OWASA staff in reaching their conclusions this week.

While the plant typically adds fluoride to drinking water at 0.7 parts per million, a worker accidentally hit an extra key at 11:43 a.m. Feb. 2, increasing the speed at which fluoride adds into the water to 80 percent, instead of 8 percent.

A normal operating range is between 8 percent and 12 percent, officials said.

The increase ended up reversed within seconds, CH2M reported, but the pump did not respond and continued to operate at an increased rate for more than three hours. They noted the lead operator saw the abnormally high fluoride level a little over an hour later but did not correct the issue until 2:40 p.m.

The elevated flouride levels ended up confirmed just after 3 p.m., and the treatment plant shut down 12 minutes later.

The abnormally high fluoride levels did not reach the distribution system, which a state certified laboratory found was at 0.68 parts per million, just under the normal threshhold, they said.

While OWASA was dealing with that issue, one of the worst water main breaks in the utility’s history hit at 10:10 a.m. Friday, Feb. 3, on the northeastern side of Chapel Hill. The utility lost 1.2 million gallons of water, at a time when water levels were already low, forcing OWASA to enact a water ban, they said.

AECOM Technical Services of North Carolina investigated the break in a 12-inch pipe at the intersection of Summerfield Crossing Road and Foxcroft Drive and found the break may have been due to increased water pressure or an increase in external forces that caused the pipe to bend. The pipe’s structural integrity did not play a significant role, they reported.

The 44-year-old pipe was within its reasonable range of service life, they said, noting all the pipes they have studied in regional water systems survived for 40 to 60 years. However, OWASA’s pipe was only 3 feet deep and just 6 to 8 inches above a storm sewer pipe.

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