The one thing Islanders could rely on with certainty during the power outage was the presence of clean running water. And according to published reports, it is some of the cleanest water in the region.

But when Mercer Island’s drinking water travels from Seattle to a couple of large holding tanks on the Island, it tends to pick up some dirt along the way.

Earlier this month, the city hired three divers to plunge into the city’s two green, 4-million gallon water supply tanks to inspect the interior conditions and remove the quarter-inch of sediment that had collected at the bottom.

It was the first time the city employed this method to clean the tanks located near St. Monica’s Catholic church on 88th Avenue S.E. and S.E. 43rd Street. Traditionally, city maintenance employees drained the tanks to clean them. According to Terry Smith, the city’s utilities operation manager, maintenance crews used to flush the water from each tank, putting one out of service while it was cleaned. After finishing one tank, they would refill it, then start on the next one.

On Dec. 7, three men working for a company called Liquivision Technologies, based out of Klamath Falls, Ore., took turns swimming to the bottom of the tanks with a 2.5-foot wide vacuum to suck up the dirt. Total cost was $9,000.

“This is a really good efficient way to clean these tanks,” Terry Smith said. “Nothing gets drained. No water is wasted. And nothing is discharged at all.”

The dive crew uses a fully enclosed dry suit so there is no human contact with the water and they spray a small concentration of bleach water disinfectant onto the suit and gear before entering the tank. Before the water even gets to the Island it is screened for debris, such as twigs or leaves, disinfected with chlorine, fluoridated, and treated to reduce corrosion by adding a small amount of lime at a treatment facility along the Cedar River.

In 2004, Seattle Public Utilities added two more steps in water treatment at the this facility. These are ozonation and ultraviolet light disinfection. The ozone treatment process greatly improves the musty taste that sometimes occurs in SPU ’s water supply, according to the city’s 2005 annual water quality report.

According to the quality report, 12 Island homes were tested in 2005 for lead and copper analysis. The city did not receive any violations that year and the report also notes that the city’s compliance with all state and federal drinking water laws remains exemplary.

The water is stored in the tanks in case the supply line from Seattle breaks in a natural disaster. If that happened, Smith said the 8-million gallons of water can provide up to four days of supply in the summer and a maximum of six days in the winter.

In all, it took three days to remove the quarter inch of sediment along the bottom of both tanks. In addition to cleaning the tanks, the city also received an inspection of the tanks’ conditions from the inside.

“We begin with a top side inspection before the diver even enters the water,” Peterson said. “We inspect the ladder on the way up and the hatch. We’re looking at the rust grade inside the tank and the overall condition and the screens for the vents.”

If something is found wrong during the inspection, then the team fixes it. If there is nothing wrong, then they just do the cleaning process, Peterson said.

The sediment is the result of a very small amount of suspended particles making it through the filtration systems and collecting over time as the water is delivered and consumed on Mercer Island. The water comes from the Cedar River Watershed and the Tolt River Watershed and is delivered through Seattle Public Utilities. Roughly 26 cities and water districts rely on a supply of stored water from these two watersheds to meet most of the daily needs of about 1.3 million people in the region.

For any questions call the city’s Water Quality Information Line (206) 236-3566.