The city of Yakima is making improvements at its wastewater plant that will save money and expand capacity to handle industrial waste, a step officials hope will spur economic development.

On average, residents and businesses in the Yakima area create a stream of wastewater that averages 10 million gallons a day, or about 15 Olympic-size swimming pools. Yet the plant, with an annual operating budget of about $22 million, supported by utility rates, sits mostly unnoticed behind the Kmart store off State Route 24 and strategically next to the Yakima River.

Unlike the police station or City Hall, most residents will never visit the plant, which is spread out over more than 100 acres.

“If the service stopped, they would notice it in a hurry,” said Dean Smith, the city’s wastewater utility project manager.

The plant, finished and operational in 1937, passed its 75th year of operation this year.

Plant officials say they are working on plans that will allow the facility to accommodate a larger volume of high-strength industrial waste from fruit processors or breweries, for example. That specialized waste digester — a $6.5 million project — is under construction now.

Without the new digester, the cost of other improvements to maintain the same capacity would be $10 million, the city estimated.

In addition, the move will reduce the amount of money spent on electricity to run the plant by a projected $70,000 a year.

Operating the wastewater plant requires a complex series of steps in order to stay in compliance with water quality regulations.

The water comes into the plant so dirty that it smells like stepping into a poorly kept outhouse. Schoolchildren on tours cringe when they step briefly inside the headworks building, where the incoming water passes through its first level of cleaning.

At that point, it contains almost every small item known to mankind, from socks to whatever people pour down the drain. A strainer grabs this stuff, which is then automatically placed in a waiting refuse container and carted off to the dump several times a month.

By the time the water is piped back in to the Yakima River, it’s clean enough for a flathead minnow to live in it for at least 96 hours. That’s one of the many quality tests the water must undergo for the plant to retain its operating permit through the state Department of Ecology.

Before the plant was built, wastewater went directly into the river. The city built the plant — which now serves an area with a population of 120,000 across Union Gap, Terrace Heights, Moxee and other parts of Yakima County outside the city limits — under threat of a lawsuit from Lower Valley cities.

Workers were hired through the federal Public Works Administration, the Depression-era program to provide jobs through public projects.

The design of the plant is different from many other wastewater operations in that an industrial line was installed some 60 years ago to serve downtown Yakima’s Fruit Row warehouse district. The Del Monte fruit processing plant is the only one that remains on the line, although the city is working on increasing capacity for the high-sugar waste produced by those type of operations.

Diverting that waste into its own part of the plant will mean the operation will support more industrial operations while continuing to treat residential wastewater, as well as saving on power costs. The new digester — where the waste is chewed on by microorganisms and turned into sludge that is used for fertilizer — should be running this summer.

The city currently pays a Lower Valley company to haul away the sludge, which is then sold to farmers as either wet or dry fertilizer. Plant officials say it would probably cost just as much for the city to market and distribute the product itself. But within the next couple of years, the city plans to start drying the sludge so that it can be sold directly from the plant.

Design of the dryer needed for that process has already started. The dryer will allow the plant to use more of the methane gas produced by the digesters. The methane is also used to heat the digesters to the 98-degree temperature required for the best processing of the waste. (That’s the same temperature as the human stomach.)

The bacteria that process the waste are human. “We just encourage the right environment for them to do it effectively,” Smith said.

One round of sludge will sit in the digester for about two weeks.

Staff members work with major producers of chemicals and grease, among other substances, to make sure concentration levels aren’t high enough to offset the bacterial balance.

“We can’t put in anything to kill the bacteria. It’s our friend,” Smith said.

• Mark Morey can be reached at 509-577-7671 or mmorey@yakimaherald.com.