Health officials said water in the northern part of Montgomery County was safe to drink Wednesday night, after two tests concluded it contained no harmful bacteria.

A health advisory had been in effect for the area since Monday morning, following a major water main break in Derwood on Sunday. Residents were told to boil water before ingesting it, and restaurants were forced to shut down.

On Wednesday morning, officials eventually reopened restaurants under limited conditions when a first test for bacteria returned negative. The second test returned negative at 8:45 Wednesday night, prompting officials to lift the advisory.

Lynn Riggins, a spokesperson for the Washington Suburban Sanitary Commission, said there was a health concern because the rupture significantly lowered pressure in parts of the system, potentially allowing harmful groundwater to leak through small holes in the pipes. Many of the pipes are more than 50 years old.

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She said they were confident the water was safe once the pressure was restored Monday morning, but performed the two tests at the request of the Maryland Department of the Environment. She added that this was the first boiled-water advisory in the WSSC’s 90-year history as the main water utility in the area.

Restaurants reopened Wednesday under “strict public health precautions” ordered by the county health officer. These included boiling or bleaching water before use in kitchens, washing hands with bottled water and turning off all vending machines and drinking fountains.

Mary Anderson, a spokesperson for Montgomery County Health and Human Services, said they informed restaurants of the updates by dispatching inspectors and speaking to the media. They also created a call center, which received 678 calls as of Wednesday afternoon — half from the public, half from restaurant owners.