CALAIS – Residents can go ahead and drink the water now that the city’s engineering firm has determined it is safe.

Last week, residents learned that high levels of bacteria, yeast or mold were detected in the city’s water supply, and the city’s former engineer suggested that further tests be conducted.

The Maine Drinking Water Program and the Maine Rural Water Association were consulted, and neither E. coli nor fecal coliform was found in the water, so a boil order was not needed.

But the city did learn that a “common background bacteria” could have caused the “swamp water” smell that some residents have experienced.

In a letter to the city, William Olver of Olver Associates Inc. in Winterport said that based on the preliminary results by the state, the city’s water is fine.

“The [heterotrophic plate count] results represent background natural bacteria that do not appear to be a cause for concern,” Olver said. “You will note that all total coliform and E. coli test results came back at zero colonies.”

Olver said that the concern raised last week was premature “and may have caused undue anxiety among the public. The more extensive sampling conducted last week throughout the water distribution system does not suggest any bacteria or safety issues that would warrant the need for a boil order in Calais at this time.”

In the past, Calais bought its water from neighboring St. Stephen, New Brunswick. A change in U.S. water standards forced the city to look elsewhere for its supply.

Two years ago, the city drilled wells and last year it switched systems.

Because in some instances the water flowed in an opposite direction from its former flow for the first time in more than 100 years, sediment was dislodged and caused water-quality problems.