CROMWELL — The Mattabassett District water pollution control facility will be undergoing a major renovation and expansion project and a Torrington-based company has been chosen to do the job, the company announced Thursday.

The Water Pollution Control Authority has selected C.H. Nickerson & Co.’s $93 million bid for the work.

The project is expected to take approximately three years, and in addition to the renovation and expansion, it will include an upgrade of the plant’s nitrogen removal equipment.

Nitrogen in discharge water coming from the plant, which eventually flows into Long Island Sound, causes algae blooms that consume dissolved oxygen in the water and suffocates aquatic life, according to a press release from Nickerson.

“C.H. Nickerson is pleased to lead this project, which will not only benefit the district and its ratepayers, but also have positive, long-term benefits for the environment,” said C.H. Nickerson President Jon Miller in a statement.

The renovation of the plant will upgrade much of the equipment there — some of the equipment is 40 years old. The new equipment will increase the facility’s flow capacity and operating efficiency, Miller said.

Brian Armet, executive director of the Mattabassett District, said Nickerson’s bid was the lowest the district received, and “met all of the qualifications” of the district’s criteria for selecting a contractor.

The Mattabassett facility currently handles sewage from New Britain, Berlin, Cromwell, Newington, Rocky Hill, Farmington and a portion of Middletown.

The facility is currently designed to treat an average of 20 million gallons of wastewater per day and serves 170,000 customers throughout its member towns.

The facility will remain fully operational throughout the renovation process. When the expansion is complete, Middletown can be fully incorporated into the Mattabassett District, according to Guy Russo, director of Middletown’s Water Pollution Control Authority.

Currently, about 40 percent of Middletown’s wastewater is treated at Mattabassett, with the remaining 60 percent treated at the city’s plant on River Road.

Between the late 1990s and early 2000s, the city conducted a study on the feasibility of modernizing and expanding the River Road facility.

Russo said the study concluded that it would be more cost effective for the city to join the Mattabassett District, and the state Department of Energy and Environmental Protection advocated that the city join the Mattabassett District as well.

Currently, Middletown is a contracted member of the district and is seeking to become a full member. The district would have to vote to accept Middletown as a full member. The district has yet to vote on Middletown’s request.

Until Middletown is voted in as a full member, only the portion of the city served by Mattabassett will be liable for repayment of the reconstruction costs, which will be built into ratepayer’s service fees, Russo said.

The state General Assembly passed a bill in 2011 which allows Middletown to be fully incorporated into this facility upon completion of the project and a stipulated $13 million buy-in, and Gov. Dannel Malloy signed off on Middletown’s petition to join.

The Middletown Common Council voted last June to approve joining the district, and the New Britain Common Council has approved the petition for Middletown to join. The Cromwell and Berlin boards of selectmen and the Mattabassett Board of Directors have yet to vote on it.

C.H. Nickerson & Company is a  general contracting and design-build solutions provider that specializes in water and wastewater treatment facilities and is a leader in environmental construction. The company is headquartered in Torrington, where it was founded in 1939.