Flush it and forget it. That’s the motto most of us live by.

But surging through the sewers of Johnson County each year is more than 6 billion gallons of raw sewage that is diverted to Kansas City wastewater treatment plants.

And that could end up costing Johnson County big.

Kansas City is under the gun from the federal government to fix overflows of billions of gallons of raw sewage into its streams each year.

The sewer rates for Kansas City residents are expected to quadruple in the next decade to pay for its $2.3 billion sewer project overhaul.

Johnson Countians are going to have to pay some of that tab, too. How much is still being negotiated and will be known later this summer.

“This does have the potential of having a significant impact on our ratepayers,” said John O’Neil, director of the county’s wastewater department. “We are keeping a close eye on this.”

Any rate increase would be spread among the county’s 133,000 customer accounts. The median Johnson County wastewater customer pays $39.42 every two months.

Up to now, it’s always been more cost effective for Johnson County Wastewater to divert nearly one third of its waste — 16.5 million gallons daily — to Kansas City for treatment.

But as Kansas City stares at the multi-billion dollar overhaul of its sewers — and the hefty price tag that comes with it — Johnson County may want to rethink its wastewater treatment strategy.

This summer, a group known as the Water Services Utility Funding Task Force will present recommendations for how to fund a plan to stop raw sewage overflows in Kansas City.It’s a major undertaking that is forcing Kansas City to face up to a long-ignored problem or pay millions in federal environmental clean-up fines.

Since the late 1980s, Johnson County has kept a close eye on costs to determine whether it makes more sense to continue the wholesale treatment contract with Kansas City; expand its Tomahawk Creek Treatment Plant on Mission Road south of Interstate 435; or build an entirely new treatment facility, a move that could cost taxpayers tens of millions of dollars, O’Neil said.

A study done three years ago showed that if rates would increase by more than 8 or 9 percent each year, then it would be less expensive in the long run for Johnson County to expand its treatment capacity rather than continue to pay Kansas City to treat its waste.

Kansas City already increased sewer rates by 9.5 percent beginning May 1, but the increase has not yet been passed on to Johnson County ratepayers because county officials are still working on next year’s budget, O’Neil said.

The rate increase is not part of Kansas City’s $2.3 billion sewer overhaul.

And if the task force comes back this summer and says the rate increase for Johnson County is in the 15- to 18-percent range or higher, then it will be more cost effective for the county to make capital improvements to its own sewer system, O’Neil said.

“We’re waiting for a final determination to give us a more firm projection of where they’re headed … and determine whether we should make improvements on our side to treat the flows here,” he said.

As engineers designing sewer systems know, waste flows downhill.

And if Johnson County receives a larger wastewater bill from Kansas City for treating some of the county’s waste, Olathe may receive a larger wastewater bill from Johnson County because some of the city’s waste is treated by the county.

Johnson County Chairwoman Annabeth Surbaugh and Commissioner Ed Peterson said the county will do what is in the best interest of its ratepayers.

“We are watching the rates so if it becomes more cost-effective for us to treat the water ourselves that’s the direction we’ll head,” Peterson said.