There’s nothing complicated about the massive chambers of concrete being buried downtown, capable of storing millions of gallons of water.
“If the Romans cared about it, they would have done it,” said Marcia Davis, a principal engineer with the city who’s helped design the tanks over the past decade.
The city first turned to the idea of combined sewer overflow storage tanks in 2003, burying a chamber capable of storing 360,000 gallons of runoff near the Riverside Park Water Reclamation Facility. The idea was to buy time during heavy rains and major snowmelts, when the tanks are capable of storing untreated water that would otherwise stress the sewer pipes in areas of the city where the two systems haven’t been split.
In the years since, Spokane has buried another 14 tanks, with eight more scheduled or being built now. City officials say it’s the most cost-effective way to meet pollution standards for the river as determined by state and federal agencies.
How they work
If too much stormwater gets into the large sewer pipes, called interceptors, the whole system can back up, and pressure can cause the interceptors to burst, which is an even bigger problem. The major sewer pipes are fitted with weirs – holes built into the pipe wall that send the gushing runoff into another pipe that flows into the river if the water level gets too high.
The tanks are a stopgap measure of keeping that gushing water from tripping the weirs and sending untreated runoff flowing into the river. A former city engineer referred to the structures as “a bubble” in the system.
In 2017, the city is beginning construction on some of its largest tanks to date, including two downtown that will be able to hold more than 4 million gallons of water combined when they’re complete.
In the 1980s, the city split the sanitary sewer and stormwater systems in northern sections of Spokane at a cost of about $50 million, but the rocky soil on the South Hill and heavily paved downtown kept those areas from receiving the same treatment. The city expects to spend $144 million to put the entire storage system online, most of that funding coming from the purchase of so-called “green bonds” in 2014.
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