WOBURN - The cost of a new Rag Rock water storage tank was estimated at $5.9 million by a representative of the water consulting firm hired by the city.

Speaking to the City Council's Water Committee this week, Mayor Thomas McLaughlin said the estimate was just that, but city officials are clearly working quickly to replace the old water tank, which was shut down for two days earlier this month after routine tests detected the presence of E. Coli bacteria.

Carol Rego, an engineer with Camp Dresser McKee, the firm that was hired to advise the city on water issues, said the proposed replacement of the Rag Rock tank "is not just a coincidence" to the contamination incident, but was already on a preliminary list of water-related projects for the fall.

According to Department of Public Works Supt. Frederick Russell, the "sanitization" of the tank began on Tuesday, and he expects the tank will be back on line by the middle of next week.

Russell said safeguards have been installed to ensure another calamity like the one in early March doesn't happen again. The discovery of bacteria in the Rag Rock tank prompted the city to advise residents to boil their tap water for about 48 hours while the tank was drained.

Rego said the new Rag Rock tank will be made of concrete, which "has a longer life" than steel and should provide the residents with cleaner water because there is less of a chance for internal contamination.

The new tank will be located "a little bit further away" from the homes near the existing tank, and "a little bit higher elevation," which should provide more pressure to the system, though not enough for most homeowners to notice it.

The current tank is also flawed in that the cylindrical container was built first, then a roof was installed later, a common practice years ago.

"It doesn't have the same integrity," said Rego.

The old tank was also originally painted with lead-based paint, another common practice that was ceased years ago for obvious health reasons.

The new tank will also feature swails and detention basins that will contain any overflow. Presently, there are no topographical safeguards in place to protect the residents from any water coming from the tank, in case of a major leak.

The city plans to petition the state Department of Environmental Protection to incorporate funding for the new Rag Rock tank into a bond issue for on-going repairs to the city's water distribution system and water treatment plant.