When DeKalb County lifted its two-day ban on drinking tap water this week, it capped two days of irritation and anxiety for people in most of the county and untold losses for local businesses.

People boiled their water or bought bottled. They had second thoughts while showering or brushing their teeth using the taps (both, apparently, no-nos). And those who drank water Wednesday morning — before learning of the warning issued late the night before — worried they'd get sick.

Since last month, this is the second failure of the new $165 million Scott Candler Water Treatment Facility near Doraville. Officials blamed both events on weather-related power outages and an ongoing problem with the plant's backup power system. Officials say a boil-water warning was unnecessary last time.

The county says the power failure caused pumps to stop working, which in turn caused a loss of pressure in the water pipes. Normally, high water pressure prevents contamination by forcing clean water out of any cracks in water pipes. But when the pressure drops too low, dirty ground water can seep in through the cracks.

Untreated water can contain bacteria that sickens people and even kills those with weakened immune systems.

The plant lost power sometime before 7 p.m. Tuesday. That night and over the next two days officials told people in 18 ZIP codes — roughly two-thirds of the county — to boil water for three minutes before drinking because there was a chance the water lines had become contaminated.

Testing later showed the water was clean all along, but by the time the boil warning was lifted after 3 p.m. Thursday, people had gotten a taste of life in the Third World.

The water warning affected most of DeKalb inside I-285 and a swath beyond, from Doraville in the north to beyond Stone Mountain in the south.

Employers bought bottled water for their workers. Restaurants turned off soda fountains and ice machines and bought bagged ice and bottled beverages. And grocery stores chucked produce from displays with automatic misters and tossed meat touched by utensils washed in tap water. A spokesman for Kroger, Glynn Jenkins, said it's unclear how much the precautionary steps cost the company's 13 stores that threw away food.

At the Crescent Moon restaurant in Decatur, server Polly Davidson said she made $15 in tips Wednesday instead of the usual $50 to $60.

"All the customers are angry because they can't get ice," Davidson said Thursday. Workers there hustled to a drugstore across the street to buy bottled soda, but they couldn't find ice.

"Everyone's tips are going down because the customers are angry," Davidson said.

Meanwhile, several tests for contamination came up negative, county spokeswoman Kristie Swink said in a written statement. She wrote that people do not need to flush their water lines.

The Scott Candler plant keeps failing because the emergency generators fail to automatically transfer power to the plant when the electricity blinks out, wrote Swink, who would only take and respond to questions in writing.

Asked twice about the reason for the plant's failure this week, Swink responded twice with the same language: "Severe weather generates power fluctuations which can have an effect on computer control systems. The department is addressing the plant's computer control systems and establishing redundancies to minimize/eliminate the effects of future disruptions."

The plant began operating last year, replacing the original plant built in 1942. County officials touted it as one of the largest fully automated and computerized treatment facilities in the Southeast. It was designed to "meet high standards for reliability and risk based redundancy," one recent county news release said.

Swink wrote that the previous loss of pressure, on June 22, didn't trigger a similar boil-water advisory because it occurred on a Sunday morning, during a "relatively lower demand period." Nearly half the county's 740,000 water customers experienced low, or no, water pressure that time. The county consulted with the Environmental Protection Division and determined no advisory was required, she wrote.

Boil-water advisories are a federally mandated precaution. Officials must warn the public when pressure drops below 16 pounds per square inch for at least 20 minutes, said Janet Ward, spokeswoman for the city's Watershed Management Department. That's about one-sixth of the average pressure, she added. She said the same rule applies for other utilities.

Main breaks in recent years have reduced pressure enough to trigger boil warnings in Atlanta, but the city's water system hasn't experienced a loss of pressure because of power failure for probably five years, Ward said — ever since the city's Northside pumping station was retrofitted.

"We have reliable backup generators now," Ward said.

Ward said that low pressure incidents trigger requirements to test for contaminants such as fecal coliform, which comes from human and animal excrement. Ingesting the bacteria can cause intestinal illness, including diarrhea and vomiting.

Though DeKalb issued warnings to the news media, some people still drank the water. News outlets, including at least one TV station and The Atlanta Journal-Constitution's Web site, carried warnings late Tuesday after the county's 10 p.m. news releases about the plant failure.

Nonetheless, residents such as Cathy Farquharson didn't hear about it until late Wednesday morning.

By then, the Chamblee-area woman had gulped iced tea made from tap water at home. She learned about the plant failure when she got to work and was concerned about her health until she did some research online and realized the odds of contamination were low. Still, she had to go without salad after her local Kroger tossed its produce. She brushed her teeth with bottled water, and she subsisted on Diet Cokes in the fridge for the duration of the 41-hour alert.

"I was just inconvenienced," Farquharson said, adding that she was mostly concerned about her share of the cost for the new water plant. Everyone who pays a county water bill is financing the $152 million construction price tag plus the $13 million in design and engineering costs.

"Why in the world didn't the backup kick in immediately so there wasn't a loss of pressure?" Farquharson asked. "How could this happen?"