COLUMBUS — Mayor Jim Bulkley got straight to the point Monday night while explaining a study that shows wastewater rates should be raised by 40 percent over the next four years.

“Nobody likes to raise rates, but I think all of you like your stool to flush and your water to come to your house,” he said.

The study completed by Kirkham Michael — the same firm that reviewed the city’s water and wastewater rates in 1997, 2003, 2008 and 2013 — recommends 10 percent increases to wastewater fees for three consecutive years beginning in fiscal year 2018-19. The wastewater rate charged to local businesses and residences is already scheduled to jump by 10 percent next fiscal year, which begins Oct. 1.

If the projections hold true — city fees are approved each year as part of the annual budget — wastewater rates will increase a cumulative 134 percent over an eight-year span. Hikes of 15, 15, 10, 10 and 10 percent were approved following the last rate study in 2013.

The reason for the significant jump was also easy for the mayor to explain.

Work continues on a multimillion-dollar expansion at the wastewater treatment plant that’s needed to ensure the facility can handle future residential and industrial growth, replace aging equipment and move the entire plant to the protected side of the Loup River levee.

The $11.25 million third phase of the project is expected to be completed next year with the fourth and final phase following at an estimated cost of $12 million to $13 million.

“I think everybody needs to understand the huge investment we’re making in our wastewater treatment program right now,” Bulkley said.

“This is something that’s pretty darn necessary for the city of Columbus to continue to grow and prosper,” the mayor added.

Because the water and wastewater departments are operated as an enterprise fund — meaning they’re supported by user fees without a property tax subsidy — rates must be increased to cover the additional debt issued for the treatment plant expansion.

Operating expenses are also projected to rise from $2.64 million in 2017-18 to $3.01 million in 2021-22, according to the study.

Bulkley said the city is trying to be proactive by expanding the treatment plant, which handles about 1.2 billion gallons of wastewater annually, while ensuring there’s enough revenue to maintain the facility in the future.

The average residential user (10,000 gallons) currently pays $45.27 per month in wastewater fees, according to the study, with the average commercial customer (65,000 gallons) paying $255.92.

Water and wastewater fees are included on the same bill from the city.

The study does not recommend increasing water rates, although they’re already scheduled to go up 1 percent next fiscal year as part of the last review.