An “unintentional [water treatment plant] operator key stroke”—literally, someone hit the wrong button—caused the overfeed of fluoride that forced OWASA to shut down its water plant last week, according to independent reports released Friday.
The extra fluoride dosage, plus a major water main break the next day, prompted a water shortage in Orange County that closed businesses, placed Chapel Hill and Carrboro under states of emergency, and sent residents scrambling to buy bottled water. Customers were unable to use or drink their water for more than twenty-four hours.
According to the fluoride report, by CH2M HILL North Carolina, at 11:43 a.m. on February 2, a plant operator accidentally instructed the plant’s fluoride feed pump to increase its feed rate to 80 percent, compared with its normal operating range of about 8–12 percent. The operator tried to change the command about twelve seconds later, but according to the report, the change didn’t register.
“The pump appears to have operated at a rate significantly greater than the subsequent commanded speed for approximately 3.25 hours, resulting in a fluoride overfeed condition,” the report says.
A lead operator at the plant noticed the extra high levels “but did not take corrective action, leading to the issue not being resolved until after 2:40 p.m.”
A fluoride level of 5.90 mg/L was recorded at the plant that day, exceeding state and federal primary drinking water standards by 1.9 mg/L, the report says. (Fluoride is added to water to prevent tooth decay.)
The water main break, according to a report by AECOM Technical Services, may have been the result of the pipe bending because of pressure and settling. The pipe that broke, near the intersection of Summerfield Crossing Road and Foxcroft Drive, was buried three feet below the street.
“An increase in water pressure or an increase in external forces may have been responsible for the failure event. As the pipe is shallow, it would have been very sensitive to live loads—more so with an increased point load effect over the storm sewer below; so any combination of increased pressure, differential settling, or increased external loading (or increase in bending moment in this case) may
have initiated the failure,” the report says.
The break leaked about 1.2 million gallons of water, causing pressure in the system to plummet. As a result, customers were ordered not to use or drink their water because low flow can allow bacteria to grow in pipes. Tests show the water was not contaminated.
The OWASA board of directors met Thursday night to discuss the water shortage and hear comments from the public and staff. Another meeting will be held February 23 at 7 p.m. in the Chapel Hill Town Hall.
“Last night, the Board agreed that OWASA would develop solutions to prevent the particular failures of last week, assess outage risks more broadly and take action to improve the water system resiliency, and review communication practices of OWASA and its partners during critical events. We may turn again to outside parties for both expertise and objectivity,” John Young, chair of the OWASA Board of Directors, said in a statement. “OWASA apologizes for the significant disruptions and impact experienced by the community and thanks its many partners for their support.”