By Eric M. Johnson

SEATTLE (Reuters) - A Seattle suburb was making progress cleansing its E. coli-contaminated water supply through system-wide flushes and chlorine injections, a task made more urgent after the potentially deadly bacteria sickened a child, officials said on Tuesday.

Tests first showed the bacteria in the water supply on Mercer Island on Sept. 26, forcing area schools to close for a day as health officials collected thousands of samples to try to locate the source of the contamination.

The alert was lifted last week in the city of 24,000 people only to be reimposed after another water sample test on Thursday again indicated the presence of E. coli. Officials said later a Mercer Island child had fallen ill from the bacteria.

"We know this incident has been challenging, and we thank islanders for their patience," Mayor Bruce Bassett said in a statement. "We are all eager to see life return to normal, but our first priority is protecting the health of our citizens."

The city, whose businesses and residents have been under orders to boil tap water, said it had flushed extra volumes of water and more than double the normal concentrations of E. coli-killing chlorine through its roughly 100 miles of water pipes.

Since then, 90 water-sample tests over five days have turned up clear of the fecal coliform bacteria that indicate the presence of sewage or animal waste in the water supply.

City spokesman Ross Freeman said the boil-water order could be lifted as early as Wednesday when Mercer Island officials meet with the state's health department.

Freeman said officials could lift the ban if they see a sixth day of clear water and the completion of system-wide inspections. They were also looking for longer-term safety proposals, such as a permanent chlorine-injection system at water reservoirs.

The cause of the bacteria, which can lead to severe illness, such as kidney failure, or even death, remains unknown, Freeman said.

More than half of the suburb's restaurants were operating with limited menus, and the city was helping small-business owners apply for a "Disaster Assistance Loan program," the city's statement said.


(Reporting by Eric M. Johnson; Editing by Cynthia Johnston and Peter Cooney)