HONOLULU — Federal environmental regulators said Wednesday they've ordered Honolulu and the operator of the city's only landfill to take steps to prevent medical waste and other debris from flowing into the ocean again during heavy rains.

In January, medical waste flowed into the ocean off the Waianae Coast when the landfill's water diversion systems could not handle heavy rain runoff. Ko Olina and other nearby beaches had to be closed until the debris was cleared.

The U.S. Environmental Protection Agency ordered corrective action shortly after that incident but officials have determined further steps are needed.

The new order requires landfill operator Waste Management Inc. to separate storm water generated outside the landfill from storm water generated inside. This will allow the operator to better control water from heavy rains.

"We will be closely monitoring the work so runoff from future storms is properly controlled and residents' health and Oahu's coastal waters are protected," Jared Blumenfeld, EPA's Regional Administrator for the Pacific Southwest, said in a statement.

The EPA said its ongoing review found the landfill violated the Clean Water Act and a permit that regulates storm water discharge.

The landfill must also evaluate the adequacy of its storm water detention basin pond and its enhanced storm water monitoring plan. It must submit a report on its review to the EPA. The agency is also requiring the landfill to submit monthly monitoring reports.

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The city and Waste Management did not immediately respond to messages requesting comment.

The landfill is due to close in 2012. The city had wanted to keep it open for another 15 years, but the state Land Use Commission rejected that plan.

State Department of Health tests of the ocean near the runoff site last January indicated "very high" bacteria levels consistent with polluted runoff from a storm. The city posted warning signs at a Nanakuli surf spot known as Tracks and down the coast to the harbor and marina entrance past Ko Olina.