MERRIMACK — More than $22 million in upgrades to the town’s wastewater treatment plant are being proposed over the next two years, which is expected to increase sewer rates substantially.

If approved by voters next month, a portion of the overall project — at a cost of $13.1 million — will include improvements to the town’s main wastewater treatment plant, as well as two pump stations.

“We are planning to have a comprehensive upgrade of all of the components,” said Sarita Croce, assistant director of public works.

The facility is about 40 years old and some of the equipment is beyond its useful life span, according to Croce.

“We have several goals with this project,” she told voters during the town’s deliberative session this week.

The improvements are necessary in order to correct safety concerns with existing equipment, fix code-related deficiencies and address issues related to flushable wipes that are entering the plant, said Croce.

Flushable wipes are currently the biggest problem facing the sewer system, as the degradable wipes are not breaking down as originally intended, she explained.

The first phase of the upgrades includes $13.1 million for designing the project and updating two external pump stations, the main pump station and constructing a new screening building.

Kyle Fox, director of public works, said the second phase will include $9.5 million for water clarifiers, a compost facility and more.

In April, voters will be asked to approve the first $13.1 million bond, followed by a second bond request of $9.5 million in April 2020, said Fox.

A two-thirds majority vote is needed to move forward with the bonds.

“It is not funded out of the property tax base. It is only funded by the users of the sewer system,” Fox said of the bonds.

He is proposing to implement sewer rate increases throughout the next four years, if the bonds are approved. Rates would increase by 12 percent each of the first three years, and increase by 5 percent the final year, explained Fox.

Currently, users are paying $242 a year in sewer costs. Once the four years of rate increases are complete, those users will be paying $347 a year, he said.

Despite the projected increase, Fox said Merrimack sewer rates are still low compared to elsewhere. Once increases have been implemented, Merrimack’s rates will be 12th lowest out of 85 New Hampshire communities, he said.

Voting day is April 9. The polls will be open 7 a.m. to 7 p.m.