The Mattabassett District Wastewater Treatment Plant, Cromwell, CT

Plant History and General Description:

The Mattabassett District Wastewater Treatment Plant was opened on December 16th, 1968 after two years and $130 million of construction. On average, it processes 12 to 21 million gallons of wastewater per day from New Britain, Berlin, Cromwell, Middletown, Newington, Rocky Hill and Farmington. This plant is the the fifth largest facility in Connecticut, remains in the top ten for lowest costs per gallon of wastewater treated, and is one of the most efficient. It is manned around the clock.

The Mattabassett District Wastewater Treatment Plant was built to support the growing population in the surrounding area. Prior to the facilities completion, the New Britain wastewater treatment plant was being over extended, leading to insufficiently treated waste that polluted both the Mattabassett and Connecticut Rivers. Once World War II hit, industrial plants ran more frequently, and thus more wastewater treatment was needed in the area. The Connecticut Water Resource Commission ordered the creation of the Mattabassett District, which would use a cooperative to eliminate pollution in both rivers listed above.

As time progressed, the facility required updates as a result of Connecticut DEEP issuing treatment requirements. In the Summer of 1989, a $78 million secondary treatment facility was completed. Specifically, DEEP aimed to reduce nitrogen discharge into the Connecticut River by 64%. In 2002, the plant added mercury control. And in 2003, the facility, not designed to remove nitrogen, was renovated to remove over 50% of nitrogen. Additionally, the facility utilized a new outfall starting In February, 2007 so that the effluent, which heavily is cleaned, is introduced to wider segment of the river, allowing for easier and quicker assimilation.

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Most recently, the plant has added tertiary treatment and a plant expansion that took five and a half year and $106 million, completed in November, 2017.

This hefty upgrade allows for a 15 million gallons/day increase in capacity, and thus it is the 3rd largest facility in Connecticut. During this update, many new systems have been installed, such as Nitrogen Treatment Systems, two new Final Clarifiers, a Fluidized Bed Incinerator, centrifuge Dewatering Equipment, compact carbon canisters for odor control, new electrical systems, and many other. Since these upgrades, the health of the Mattabassett and Connecticut Rivers have been drastically improved,and the district has exceeded the state’s expectations.

The plant is built to process up to 21 million gallons/day, 600 million gallons/month, and 7 billion gallons/year. It includes, four primary tanks, six aeration tanks, and six final clarifiers, each containing 1 million gallons, two half million gallon sludge storage tanks, 23 air blowers up to 12,000 cfm, 98 pumps up to 15 mgd, 150 motors up to 400 hp, three 2,400-volt 400-hp blowers for the aeration system, six 200-hp 15-million gallon/day raw sewage pumps, two 1.5-megawatt emergency generators, and one 1.25-megawatt emergency generator. The plan is complete with its Supervisory Control and Data Acquisition (SCADA) System with programmable logic controllers, that allows one person to manage the entire facility alone.

Configuration of Treatment Facility (flow diagram):

As stated earlier, the Mattabassett District Water Pollution Control Facility is designed to treat an average wastewater flow of 12 to 21 million gallons per day with a peak flow of 80 MGD. Influent comes from industrial, commercial, and residential sources, as well as septage and liquid biosolids from other plants. The plant also functions in a way very similar to the Durham Wastewater Treatment Plant, as outlined in the subsequent paragraphs.

The first step, as in most wastewater treatment plants, is bar screening. The wastewater from the sewers arrives at the pump station where two bar screens filter large, filter clogging materials out of the mixture. This debris is then shredded, incinerated, and then stored in an ash holding pond before disposed of by landfill. Then, the flow of the wastewater is slowed as it arrives at the two detritors. As a result of the slower flow, grit, and other heavy particles sink rather quickly, are removed by collection arms, and finally pushed by ejectors to a storage tank in the Dewatering building for combustion.

The comminutors grind the existing large floatable solids, and two parshall flumes measure the flow. Then the wastewater flows through an aerated distribution channel that prevents the deposition of solids, and the flow is split into two of four Primary Clarifiers. Here, the holding time is 2.5 hours, allowing sufficient time for the organic solids and excess secondary biological solids settle. The product is biosolid wastes, which are pumped into storage tanks where they meet the waste from the detritors.

Next, the secondary treatment and the nitrification/denitrification process begins. The water flows to the aeration tanks, where bacteria use their biological processes to remove dissolved organic and nitrogen matter. Once this process is completed, the liquid biomass flows to the secondary clarifier which recycles even more biomass. Here the clear liquid and the biomass are separated yet again. The biomass is pumped to the storage tanks and the clear liquid flows to the mixing chamber. Here, chlorine is added that kills the harmful bacteria. At this point in the process, the water is treated at the 95% to 99% levels of all monitored parameter, significantly higher than required by the government. Finally, the disinfected effluent is expelled from a 160-foot long diffuser pipe that is 20 feet below the river surface and 260 feet from shore. Between May 1 and October 31, the months that people typically are involved in water contact sports, the liquid treatment process disinfects the effluent.

While the processes listed above are happening, the biosolids- the solid waste from the clarifiers and the grit from the grit chamber- must be treated. The Mattabassett District Water Pollution Control Facility uses belt filter presses to remove a quarter of the water from the waste. The dewatered biosolids are then pumped to the fluidized bed incinerator where they are combusted in 19 tons of sand at 1400° Fahrenheit. To ensure that no odors and pathogens and to minimize the mercury emissions, the gases are heated to 1550° Fahrenheit. The ash is then disposed of in the ash lagoons or at local landfills as daily cover material. 25 to 30 tons/day

An odor control system was completed in 2005 after 14 years of construction that has resulted in zero odor complaints. “These systems consist of a Wet Chemical Scrubber to treat odors from the Biosolids Storage Tanks, six Biofilters (consisting of a three foot deep bed of wood chips) to clean the air from the Liquid Treatment Processes, and a state-of-the-art proprietary Biological filter. This system consists of a 12 foot diameter 20 foot high proprietary philcon unit followed by four, two each in parallel, 6x6x20 foot Biofilters. This unit treats the Odorous air from the Belt Filter Presses, and the Odorous Air from the Dewatering Building.” Additionally, odor control chemicals, such as sodium permanganate, is used to limit odors from the the aeration tanks. Overall, the district has spent over $6.3 million on odor control systems adding wet chemical scrubbers and biological odor control systems.

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The Mattabassett District Wastewater Treatment Plant, Cromwell, CT. (2022, May 10). Retrieved from

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