The Plant


The Stamford Water Pollution Control Facility is designed to treat an average wastewater flow of 24 million gallons per day (MGD) from the City of Stamford and the Town of Darien.

WPCA PictureThere are four steps to the treatment of wastewater at the Stamford WPCA. They are Preliminary, Primary, Advanced Nitrogen Removal, and Ultraviolet Disinfection.

  • The Preliminary Process removes large objects from the water after it first enters the facility. These are materials that cannot be treated and are called screenings.
  • The Primary process removes easily settled materials from the wastewater.
  • Advanced Nitrogen Removal is a biological process which removes organic pollutants and nitrogen.
  • Ultraviolet Disinfection is the final step in the process where the water is disinfected before exiting the facility into the East Branch of Stamford Harbor.


  • Preliminary Treatment

Wastewater entering the Stamford WPCA facility goes through the Raw Sewage Pumping Station. As it enters, it passes through the bar screens designed to remove objects from the water larger than 0.75 inch, such as rags, rocks, plastics, and pieces of wood. Five pumps transport the water to the primary clarifiers for Primary Treatment.

  • Primary Treatment

There are two primary clarifiers. Each tank is 130 ft in diameter with a 14 ft sidewall depth. Their function is to remove easily settled material (primary sludge) from the wastewater. Primary sludge consists of grit, inorganic and organic materials. The wastewater flows up the center column in the tank where it is evenly distributed, eventually overflowing the weirs. As this happens, velocity decreases and the settleable solids drop to the bottom where they are collected by a rake mechanism. The clarified wastewater overflows the weir into an effluent channel that conveys it to the next phase, Biological Treatment.

  • Biological Treatment

The biological reactors provide the various reaction zones where the proper mixing of microorganisms, nutrients, carbonaceous and nitrogenous organic matter and oxygen result in the maximum reduction of the organic matter (pollutants) and nitrogen.

There are two reactor trains. Each train is approximately 350 ft by 150 ft and has a volume of 4.9 million gallons. They each have a primary anoxic zone followed by an aerobic zone. At the discharge end of the each aerobic zone there are four recycle pumps which return nitrate to the primary anoxic zone for denitrification (removal of nitrogen). The total volume of the biological reactors, both old and new tanks, is close to 15 million gallons.

Primary effluent, return activated sludge and nitrate recycle enter the first (primary) anoxic zone. In this anoxic zone, nitrate is converted to nitrogen gas, but there is no removal of carbon-containing pollutants in that tank. The mixture, called mixed liquor, then flows to the aerobic zone followed by the secondary anoxic zone and finally a re-aeration zone. Activated sludge is wasted as mixed liquor from the end of the first aerobic zone. Mixed liquor exits the biological reactors over an adjustable weir and is divided equally among four secondary clarifiers. Methanol is added to enhance the biological nitrogen removal process.

There are four secondary clarifiers each 130 ft (39.6 m) in diameter with a sidewall depth of 13 ft (4.0 m). The sludge collected at the bottom of these clarifiers is withdrawn through a series of tubes mounted on the rake and deposited in a sludge well adjacent to the operations building. A large portion of the activated sludge is returned to the aeration system (RAS). Each of the secondary clarifier units is equipped  with the Stamford Baffle.

Secondary effluent overflows the perimeter weir, falls into the effluent channel and then through a pipe which conveys it to a collection well. The effluent then flows to the ultraviolet disinfection system.

  • Ultraviolet Disinfection

The disinfection basin is comprised of two channels containing the UV system. Each channel has two banks of ultraviolet lamps and each bank has 6 modules with 18 lamps per module. Secondary effluent flows through the two channels to be disinfected before discharging into the East Branch of Stamford Harbor.

  • Sludge Processing

Primary sludge which accumulates at the bottom of the primary clarifiers is continuously raked into the sludge hopper adjacent to the center column of the clarifier and pumped into the hydrocyclonic degritters to separate the grit from the sludge.  The grit removed from the sludge is disposed of in an approved landfill, and the degritted primary sludge is pumped to the thickener tanks.

There are three thickener tanks located at the Stamford WPCA facility.  Sludge enters the thickeners through the center column and the liquid fraction overflows the peripheral weir to an effluent channel which eventually discharges to the primary clarifiers.  This overflow has a high concentration of total suspended solids and biochemical oxygen demand (BOD) and thus, requires further treatment.

The thickened sludge (4% solids) is raked to the sludge hopper near the center column and transferred to the sludge blend tanks.  Waste activated sludge in the form of mixed liquor (1% solids), taken from the biological reactors, is thickened on gravity belt thickeners to about 4% solids and then conveyed to the blend tank where it is mixed with primary sludge.  The mixture is pumped to belt filter presses for dewatering. Prior to introduction onto the belt filter presses, the sludge is conditioned with a cationic polymer to produce large, strong flocs which in turn enable the release of the bound water from the sludge.  The sludge discharged from the belt presses has a concentration of 25% solids.

The liquid released from the pressing operation is conveyed to the head of the Plant for treatment.  The dewatered sludge (25% solid) is conveyed to the solids drying facility where it is converted to a Class A EQ biosolid, as defined by the CT DEEP and Federal EPA.  The dried, pelletized biosolid is currently being used in the fertilizer industry.