Award Winning Project

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Enhanced Nitrogen Removal
Mattabassett District Wastewater Treatment Facility Upgrade 

Award Winning Project

The Mattabassett District manages the Water Pollution Control Facility along the Connecticut River which serves multiple municipalities in the region. In 2004 the District completed a wastewater facilities study which evaluated treatment alternatives to achieve compliance with stringent nitrogen discharge requirements, as well as other upgrades required for this aging facility to handle peak flows of up to 110 MGD.

Wright-Pierce was hired to perform a value engineering assessment of the recommended nitrogen removal process and the resulting impacts to the facility’s hydraulic profile, and alternative sludge dewatering processes. Based on the evaluation, the recommended nitrogen removal process was changed to a 4-stage Bardenpho process with a side-stream reactor to treat the recycled filtrate from the sludge dewatering system.

The District subsequently hired Wright-Pierce to update their facilities plan design concepts, and to provide full engineering services for the comprehensive upgrade to the regional Water Pollution Control Facility.

Wright-Pierce worked closely with the Mattabassett District throughout the design process to position the District to receive CT DEEP Clean Water Funding. The design also included energy efficiency improvements, earning it more than $1 million in incentive rebates from Connecticut Light & Power (CL&P), now Eversource.

The facility upgrades include:

  • stringent nitrogen regulatory requirements compliance with 4-stage Bardenpho process and innovative side-stream reactor
  • accommodation of increased flows from the constituent communities, including a new member - Middletown
  • increased treatment capacity from 20 MGD to 35 MGD, with peak flow capacity to 110 MGD
  • new 35 dry ton/day fluidized bed sewage sludge incinerator (SSI)
  • first in the country, state-of-the-art emissions control to achieve compliance with new federal SSI regulatory requirements
  • aging equipment and systems upgrades
  • innovative piping configurations to address hydraulic bottlenecks to accommodate higher influent flows
  • chemically enhanced primary treatment for wet weather flows of up to 55 MGD
  • two new secondary clarifiers
  • new effluent pump station
  • new sludge dewatering system with three new centrifuges
  • new dewatered sludge conveying and pumping system
  • multiple new odor control systems
  • influent screening and grit removal upgrades
  • electrical system and instrumentation system upgrades
  • energy efficiency features
  • administrative and maintenance buildings expansions


State Department of Energy and Environmental Protection Commissioner Rob Klee spoke at the June 18th ribbon cutting ceremony. "Today you're putting online an expanded new treatment plant that will handle more water and clean it to higher standards and hold down costs for our communities and ratepayers."

Project Highlights
  • Large regional plant serving multiple Connecticut communities
  • Challenging site constraints
  • Rehabilitation and reuse of existing structures
  • Aggressive design schedule
  • Low level nitrogen removal to less than 3.5 mg/l
  • Innovative side-stream reactor to handle high internal recycle nitrogen loads and minimize aeration volume requirements
  • New on-site sludge incinerator
  • Energy efficiency improvements that qualified for more than $1 million in incentive rebates from Connecticut Light & Power (CL&P), now Eversource

For more information contact:

John W. Braccio, PE

Christopher N. Pierce, PE
Senior Project Manager