How long BI sewage improvements last depends on type of growth

Bainbridge Island is looking into $4 million expansion and improvements to its Wastewater Treatment Plant that will help deal with growth over the next several decades.

How long those improvements will be enough will depend on the type of growth.

BI Public Works director Chris Wierzbicki said at the City Council meeting Nov. 21 that commercial growth will take up more space in the plant than residential growth. And the type of commercial growth also is a factor.

It was said that a distillery, for example, takes up 100 times more of the plant than a domestic site, while a brewery takes up 50 times the space. However, there are high-technology pretreatment methods that can reduce those numbers. If those don’t happen, a plant upgrade could be needed sooner.

“This is not an exact science,” Wierzbicki said. “An optimized pretreatment program could reduce the load of the plant.”

Mayor Brenda Fantroy-Johnson said, “In layman’s term that means reducing the amount of poop,” to which everyone chuckled.

Wierzbicki said the city could require businesses to pretreat their waste to limit their contributions to the plant and make it more like a residential load. “There’s push and pull there,” he said, adding that could increase costs and scare some new businesses away. And, it could make it tough on some already here to stay in business.

He added the city could spend twice as much money and build for the ultimate growth, but what if that doesn’t happen?

“Nobody really knows what the future holds,” he said, adding something like a recession certainly could slow growth. On the other hand, if housing prices fall that could increase the growth rate. “We just can’t know.”

So, the city is taking a conservative approach, he said, planning for 1,100 residential units when the ultimate goal is 1,400.

Councilmember Jon Quitslund said unlike some on BI he doesn’t want all growth in Winslow, and that could affect the needs at the plant. “I believe that jamming the highest possible growth in Winslow is not the way to go.” He does want to see densification there, and the city needs to plan for the housing accommodations needed for that. But it also needs to plan for growth on other parts of BI.

Councilmember Leslie Schneider said the city is looking at getting a biodigester, which also would play a part in the needs at the wastewater treatment plant. Such a product could be a “big reduction in our carbon footprint,” she said. Instead of shipping solids elsewhere, they could be processed and used locally.

A PowerPoint presentation shows the plant has the hydraulic capacity to handle the liquid component of wastewater for up to 20 years. However, it is running short of capacity for solids of about 125 pounds per day. So, secondary treatment upgrades will add up to 25% additional solid waste capacity, or 560 more pounds per day.

Without the improvements, BI could reach its capacity in 2026 of pounds per day at 2,642. With the improvements, the maximum could be 3,200 pounds per day. With the improvements, that maximum amount may not be reached until 2044. In terms of residential units, the remaining capacity is about 200. With the upgrades, the system could handle 900 more residential units.

Along with increasing capacity, another goal is to reduce pollutants in effluent that is released into Puget Sound. Work will include replacing ultraviolet disinfection, adding diffusers, adding hydrocycloness and reconfiguring one of the tanks.

The city is expected to expand by 1,400 residential units over the next 20 years, meaning the upgrades would handle about 80% of that growth. The slide show concludes that more upgrades will be needed in the future to handle the total expected growth.

Wierzbicki said in 10-15 years the city will have to think about its next upgrade.