Originally Published: 27 NOV 22 23:13 ET

Updated: 28 NOV 22 11:25 ET

By Chris Boyette and Rosa Flores, CNN

(CNN) -- All public schools in Houston were closed Monday after the city issued a boil water notice Sunday evening due to a loss of water pressure at a purification plant, officials said.

The Houston Independent School District said on Twitter it was monitoring the situation and would provide additional updates Monday.

"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 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.

Water pressure has been restored but the boil water notice will likely remain in place until at least Tuesday morning so proper testing can be performed, Houston Public Works and the mayor's office said.

"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," the office of Houston Mayor Sylvester Turner said on Twitter Sunday night.

A water sample plan has been sent to the Texas Commission on Environmental Quality (TCEQ), according to Erin Jones, Houston Public Works Public information officer. That water sample plan could be approved by TCEQ Sunday night or Monday morning, she said.

Once the water sample plan is approved, the water samples have to be collected by the City of Houston Public Works who will then submit the water samples to TCEQ and the samples have to sit in a TCEQ lab for 18 hours to see if anything grows on them, Jones said.

If nothing grows on the water samples, the state will give the city the green light to rescind the boil water order. The city has to wait for the state and cannot rescind the boil order independently, according to Jones.

If something does grow on those water samples, then the rescinding of the order will obviously take longer, Jones added.

"We are working closely with the City of Houston and stand ready to review the city's water sample results and offer any technical assistance that may be needed," TCEQ said in a statement.