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A growing population and increased pollution led to the formation of The Mattabassett District.

The 1935 New Britain wastewater treatment plant had been designed to treat 9 million gallons of water per day, yet it was often deluged with up to 30 million. This resulted in partially treated wastes, which subsequently polluted the Mattabassett and Connecticut Rivers. During and following World War II, industrial plants ran at near capacity. The problem had grown severe and something had to be done.

The State Water Resource Commission ordered New Britain to take action to eliminate pollution in both rivers. This provided the impetus to establish an economic, cooperative approach to wastewater treatment: The Mattabassett District.

In 1961, Berlin and Cromwell joined New Britain to form The Mattabassett District. The $130 million* construction of the original plant began in 1966; it was completed and went into operation in December 1968.

In 1989, improvements were made to the treatment process with the addition of a secondary treatment facility at a cost of $78 million*.
(* Figures based on today's dollars.)

Since then, the Connecticut DEEP has issued new statewide and District specific treatment requirements. These have resulted in additional improvements to our facility's treatment capability. The statewide requirement is the DEEP's goal to reduce the amount of nitrogen discharged from Connecticut wastewater treatment facilities by 64%. And, The District specific requirement is to improve the mixing of The District's treated effluent with the Connecticut River.

In 2003, in response to the nitrogen removal requirement, The District successfully modified a facility, not designed to remove nitrogen, to remove nitrogen by over 50%. (See: What's New)

In February, 2007 The District's new outfall went online. As a result, The District's treated effluent discharge is no longer visible, and most importantly its treated effluent mixes over a wider segment of the river, assimilating more quickly. (See: What's New)

The upgrades made be The Mattabassett District over the past four decades have not only restored and improved the Mattabassett and Connecticut Rivers, but have provided a standard of excellence for the region. The Mattabassett District has exceeded its original planners' expectations of wastewater treatment and purification as it continues to refine and improve its operations.

In July 2015, the District completed a large expansion, the Nitrogen Upgrade Project, which increased the flow process capacity to 35 MGD. The main improvements included; a fluidizer bed incinerator with advanced emission controls, three highly efficient dewatering centrifuges, a side stream reactor for nitrogen removal, turbo air compressors for the aeration process, new odor control systems, upgrade/expansion to the plants SCADA system and more.

The design phase was completed and publicly bid by Wright-Pierce in 2011.  The construction project, also publicly bid, was awarded to C.H. Nickerson and a Notice to Proceed was issued on April 13, 2012.   The upgraded facility is rated for an average flow of 35 MGD with enhanced nitrogen removal, which represents an increase from the pre- construction 20 MGD capacity.  The peak hydraulic capacity was increased from 80 MGD to 110 MGD.  The upgrade allows for additional wastewater flow from the City of Middletown to be conveyed to Mattabassett District for treatment.   

In November 2019, the City of Middletown is officially tied in and the District now receives 100% of the cities flows.