Update, 5:55 a.m. Friday: The boil-water advisory has been lifted. The Sewerage and Water Board advises all those affected to flush internal & external plumbing by running water for several minutes.

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For the second time in less than three months, New Orleanians found themselves turning to bottled or boiled water on Thursday as the entire east bank of the city was placed under a boil-water advisory that will not be lifted until Friday morning at the earliest and that left two utilities casting blame on each other for power problems at the Sewerage & Water Board’s Carrollton water treatment plant.

While residents are growing increasingly frustrated with periodic notices that their water may not be safe to drink, the key component in the S&WB’s solution to the problem — a pair of water towers that would keep the system pressurized even if it loses power temporarily — will not be online for at least two years.

The two advisories this year, and the S&WB’s string of similar incidents since Hurricane Katrina, call for a more immediate fix and, likely, an investigation by the City Council, council President Jason Williams said.

“It’s completely unacceptable for a modern city to be having these troubles,” he said.

Power went out at the Carrollton plant about 8:15 p.m. Wednesday and was restored about 20 minutes later, S&WB Executive Director Cedric Grant said. In that time, pressure at 14 of the agency’s 18 pumping stations dropped below 15 pounds per square inch, the threshold at which the state Department of Health and Hospitals requires issuance of a precautionary boil-water notice.

Residents were advised not to ingest tap water — whether for drinking, cooking or brushing their teeth — unless they brought it to a rolling boil for a full minute first. Showering is considered OK for adults who do not have open wounds or a weakened immune system, though the S&WB recommends the use of boiled or bottled water or a sponge bath for babies to keep them from swallowing potentially contaminated water.

The boil-water advisory is necessary because low pressure in the pipes can allow water to seep in that could contain bacteria that would make residents sick. Officials have described the advisory as “precautionary” and said the level of chlorine in the New Orleans system would likely prevent any problems.

New Orleanians will suffer through the advisory at least until Friday morning, when 90 samples collected throughout the water system will be tested. Those tests can be made only after the samples have been incubating for at least 24 hours.

Saying that power went out elsewhere in the Carrollton area Wednesday night, Grant blamed the problem on Entergy New Orleans.

“Something happened, and it wasn’t just in the four walls of this plant,” Grant said during a news conference Thursday.

But while acknowledging the nearby outage, which was caused when a metallic balloon hit the power lines, Entergy spokeswoman Charlotte Cavell said in an emailed statement that the two lines that feed into the water plan “remained in service at all times.”

Entergy is looking at data from its substations to determine whether there may have been a “brief power fluctuation” on those lines, Cavell said.

In the short term, the S&WB doesn’t have an easy solution.

Of the eight pumps that pressurize the water system, four are working at any given time, S&WB General Superintendent Joe Becker said. Two of those are fed by power from Entergy, while the other two use either steam or power from the S&WB’s own plant, he said.

While backup systems are in place, Wednesday night’s problem apparently occurred in the time it took to switch from one power supply to another, Becker said.

The system is supposed to be able to keep pressure about the critical 15 psi threshold, even if only two of the pumps are working, Becker said. While that hasn’t been the case in the incidents this summer, Becker said bringing an additional pump fed by internal power sources online would add stress to those machines and potentially cause problems.

The pumps could be run on internal power, especially after ongoing upgrades to the century-old power station at the plant are completed, but doing so would be significantly more expensive than relying on Entergy, Becker said. However, if reliability continues to be a problem, the utility could decide to make that switch, he said.

Nor will ongoing repairs to the water lines in the city, which leak about 40 percent of their water into the ground before it reaches people’s taps, have a significant impact on keeping the pressure up, Becker said.

Those repairs, as well as upgrades to the filtration system at the plant, are being paid for through rate increases approved several years ago.

That leaves one major project that could prevent future issues: the construction of a so-called “water hammer,” two elevated tanks that would keep water flowing in the system if the pumps go down for whatever reason. Those tanks could keep the pressure in the system up for about 40 minutes, enough time to deal with short-term power outages.

That project will take about a year and a half to complete — and construction won’t begin until drainage work being done outside the water treatment plant is complete.

“I can’t warranty that nothing will ever happen ever again,” Grant said. “But our goal is to make ourselves as self-sufficient and self-reliant as we can.”

Williams said he wants a more immediate solution and has already spoken with experts about what kind of mechanisms can be put in place to prevent future losses in power or in pressure.

“I think two years is a bit much. I understand that the water tower solution is a fairly complicated and fairly costly solution, but I think there are some steps that can be taken by the Sewerage & Water Board that aren’t as extensive as the water tower that we could have done before this year,” Williams said.

The situation this week recalled a July boil-water advisory that also covered the east bank and which S&WB officials said was due to a power surge. Entergy said it found no issues with its lines or equipment in that incident.

At the time, S&WB officials came under fire from the public and the City Council because the agency did not issue the advisory for seven hours after learning about the problem. Earlier in the year, a smaller boil-water advisory on the West Bank caused by a ruptured water main also drew criticism because of the long delay in notifying residents.

The response was much quicker this time: A boil-water advisory went out about a half hour after the power had been restored.

“I’m definitely glad we let people know what’s going on, but there’s definitely frustration out there,” Williams said.

Follow Jeff Adelson on Twitter, @jadelson.