It will be at least Saturday or Sunday before residents of Donaldsonville can be sure their tap water is safe to drink, Louisiana's health officer said Thursday.

Dr. Jimmy Guidry said the state has approved a plan by the privately run Peoples Water Service Co. to begin disinfecting with a different chemical — chlorine instead of chlorine dioxide. And was expected to take two to three days to implement the plan and do needed testing.

The system serves about 10,000 customers in the town southeast of Baton Rouge. Residents there were advised Tuesday not to drink the water after an inspection showed the possibility that it contained chlorine dioxide levels four to five times Environmental Protection Agency guidelines.

The state said exposure to elevated levels of the chemical can cause serious health effects to the nervous systems of some infants, young children and the fetuses of pregnant women.

Guidry said the "do not drink" advisory was issued out of an "abundance of caution." He said an initial test by the company showing an elevated chlorine dioxide level was not properly followed up with more tests.

"If they had done those tests, then we'd have some answers," Guidry said in an interview.

Mayor Leroy Sullivan declared a state of emergency Tuesday night.

Company officials did not respond to requests for comment submitted Thursday afternoon by telephone and through its website. On its website, the company said operators will be retrained on sampling, testing and reporting procedures and that supervisors have put in place a new method to monitor operator testing results.