Empty of water, full of sound
April 15, 2014
By Greg Farrar
Water district empties tank for maintainence
The signature water tank for the Sammamish Plateau Water and Sewer District, instead of holding the usual reservoir of drinking water, became a giant echo chamber last week during a scheduled maintenance procedure.
The 2 million gallon tank at district headquarters on 228th Avenue Southeast in Sammamish, seen as a landmark to many residents for its forest mural paint job and location at the second-highest point in the city (the highest, in the Inglewood Hill neighborhood, at 22025 Northeast 12th Place also has a tank on it), was completely drained, pressure washed and inspected for the first time since 1993.
The district’s 2010 Water Comprehensive Plan, which calls for the procedure to take place every five years at each of SPWSD’s eight tanks, means there will not be as long an interval again.
Other inspections did take place during those 21 years, through the tank’s roof hatch using rafts to do ‘float’ inspections and through occasionally using scuba divers who would inspect the interior coatings and use underwater vacuums to remove accumulations of sediment.
To empty the tank, much of the water was drained off through normal demand, with the remainder dechlorinated and moved through the district’s drainage ponds into the stormwater system to be safe for the ecosystem.
While the 89-foot tall, 62-feet-4 inch diameter tank was empty, every sound inside was multiplied many times over before dying slowly away.
Two hatches at the base could be used for entering with lights and cleaning equipment. Those hatches will also provide access for the final procedure, sanitizing the interior after the inspections and maintenance are complete.
Workers found the inside to be free of rust and damage, but knew previously the inside walls were permanently stained with manganese deposits.
The naturally-occurring mineral is found in the aquifer the district’s wells tap into. Filtration systems remove the manganese before the water is delivered to customer homes.
The steel tank was constructed in 1977 and is well on its way to achieving the design life of 100 years, according to the water district.
The district’s other seven tanks, with a capacity of 21.1 million gallons, handled the 12-million gallon average daily usage during maintenance.