The city of Yakima is about eight months into a $7.7 million expansion of the wastewater treatment system, a project designed to handle difficult food processing waste, create new treatment capacity and bring efficiencies to the messy business of handling waste.

All sorts of small objects and strange substances — from socks to super sugary water — end up at the Yakima Regional Wastewater Treatment plant. The plant’s distinctive Geodesic domes are visible just off Interstate 82 near the exit for State Route 24. The domes were added in 1992 as part of an odor control system.

During canning season, when food processors put out large flows of wastewater, the treatment plant operates at full capacity. The system handles about 10 million gallons from residents and businesses on an average day.

“If a (large, industrial) customer wanted to hook up, we’d have to say ‘no’,” said Dean Smith, the city’s wastewater utility project manager.

The city is extending a pipeline to handle only food processors’ high-sugar wastewater and adding a specialized digester to treat it. The sewer extension from Yakima’s Fruit Row to near River Road and 23rd Avenue is expected to cost $1.2 million. The new digester’s price tag is budgeted at $6.5 million.

The changes will cut the plant’s energy costs by about $70,000.

“Because it’s just that one specific type of waste, it’s much more efficient” and cost-effective, Smith said.

The alternative — keeping the two streams together and upgrading the entire system — would cost $10 million to $15 million, he said.

The new digester is on schedule to start operating in July, about when pear canning starts. The city contracted with Columbia Energy and Environmental Services of Richland to install the system.

The process will be similar to how regular wastewater is treated. That starts by removing solid objects, sediments and some other substances. Bacteria do the heavy lifting during treatment, chewing on the wastewater, breaking down chemical bonds and effectively cleaning the fluids.

The water ultimately pumped out to the Yakima River is clean enough for a flathead minnow to live in. The state Department of Ecology actually tests that standard when the plant renews its operating permit.

The other byproduct is a sludge that the city currently pays a Lower Yakima Valley company to cart off. That sludge is processed and sold as fertilizer to farmers.

In March, the wastewater division plans to ask the Yakima City Council to approve a contract to purchase equipment that would let the plant process the sludge on site. That way the city wouldn’t have to pay to dispose of it, and could sell it as fertilizer.

That could be a money maker for the city in the end, but the primary incentive is being able to reliably dispose of the material. It is becoming harder and harder to find places willing to take the sludge, even though studies show that it is not harmful, Smith said.

“It’s the more sustainable option,” he said.

• Dan Catchpole can be reached at 509-759-7850 or dcatchpole@yakimaherald.com. Follow him on Twitter at twitter.com/dcatchpole.