The city said the loss of water pressure happened after a power outage at Houston's East Water Purification Plant, seen here early Monday.
CNN  — 

The city of Houston on Tuesday lifted the boil water notice issued two days earlier after water quality testing confirmed that tap water now “meets all regulatory standards and is safe to drink,” the city said.

Residents no longer need to boil water before drinking, cooking or making ice, Houston Public Works said in a news release. The boil water notice was issued Sunday “out of an abundance of caution” after a drop in pressure during a power outage at a purification plant, the release said.

“Houston Water has taken the necessary corrective actions to restore the quality of the water distributed by this public water system used for drinking water or human consumption purposes and has provided (Texas Commission on Environmental Quality) with laboratory test results that indicate that the water no longer requires boiling prior to use as of Tuesday, November 29th,” the city wrote.

The boil water notice had been in place for two days and caused Houston schools to close Monday and Tuesday, the district said.

The power outage at Houston’s East Water Purification Plant Sunday was caused by a failed transformer and failed backup transformer, Mayor Sylvester Turner said Monday. Turner said two transformers at the power plant went offline around 10:30 a.m., and another followed shortly after.

When the plant went offline, sensors indicated the PSI, or pounds per square inch, dropped below 20 in 16 of 21 sensors, Turner added.

Carol Haddock, the director of Houston Public Works, said PSI lower than 20 increases the potential for contaminants to be introduced and noted PSI lower than 20 could also affect plants treating and pumping water.

Under a boil water notice, water for drinking, cooking, washing hands and faces and brushing teeth should be boiled for at least two minutes before using to destroy all potentially harmful bacteria and other microbes, the city said.

Houston’s drinking and wastewater utility serves about 2.2 million customers per day, according to the utility’s website.

Haddock said there was no indication of contamination and the boil advisory was issued out of an abundance of caution.

Turner said he is calling for an overall review of the water plan systems to prevent future transformer failures. He anticipates the boil advisory will be lifted once sampling is approved by the Texas Commission on Environmental Quality.

“Earlier today, the water pressure dropped below the Texas Commission on Environmental Quality’s required minimum of 20 PSI during a power outage at the East Water Purification Plant,” a Sunday news release from the city said.

Water pressure has now been restored.

“We believe the water is safe but based on regulatory requirements when pressure drops below 20 psi we are obligated to issue a boil water notice,” Turner’s office said on Twitter Sunday night.

CNN’s Hannah Sarisohn and Zoe Sottile contributed to this report.