Treatment Process

Wastewater enters the Sewage Treatment Plant in two ways: 1) through the plant pump station and 2) through the west shore pump station. At these pump stations, wastewater flows through a bar screen, which removes large debris and rags, and from there gets pumped to the Headwork’s building. In the Headwork’s building, the wastewater from each pump station mixes and then flows through a grit chamber where smaller debris, known as grit, gets filtered out. The flow then splits and a portion flows to both the new plant and the existing plant.

  • Existing Plant
    The wastewater gets introduced to the activated sludge process which consists of several interrelated components: An aeration tank where the biological reactions occur, an aeration source that provides oxygen and mixing, and a clarifier where the solids settle and are separated from the treated wastewater. Aerobic bacteria thrive as they travel through the aeration tank and they multiply rapidly with sufficient food and oxygen. By the time the waste reaches the end of the tank (between four to eight hours), the bacteria has used most of the organic matter to produce new cells. Then in the clarifier tank, organisms settle to the bottom separating from the clearer water. This sludge, return activated sludge (RAS), is pumped back to the aeration tank where it is mixed with the incoming wastewater, or removed from the system as excess, waste activated sludge (WAS), in a process called wasting. The waste sludge is pressed out either through a Rotary Press or a Belt Filter Press, and then sent to a landfill. The relatively clear liquid above the sludge, the supernatant, is sent to contact tanks where it is chlorinated, to kill off any remaining bacteria or microbes, and then discharged to the river.


  • New Plant
    The wastewater gets introduced to the activated sludge process through the operation of a Sequential Batch Reactor (SBR) and is based on a fill-and-draw principle, which consists of five steps: fill, react, settle, decant, and idle.

    Fill: During this phase, the basin receives influent wastewater. The influent brings food to the microbes in the activated sludge, creating an environment for biochemical reactions to take place. Mixing and aeration can be varied during the fill phase to create the following three different scenarios: Static, Mixed, and Aerated Fill. Under a static-fill scenario, there is no mixing or aeration while the influent wastewater is entering the tank. Under a mixed-fill scenario, mechanical mixers are active, but the aerators remain off. The mixing action produces a uniform blend of influent wastewater and biomass. Because there is no aeration, an anoxic condition is present, which promotes denitrification. Under an aerated-fill scenario, both the aerators and the mechanical mixing unit are activated. The contents of the basin are aerated to convert the anoxic or anaerobic zone over to an aerobic zone. No adjustments to the aerated-fill cycle are needed to reduce organics and achieve nitrification. By switching the oxygen on and with the blowers, oxic and anoxic conditions are created, allowing for nitrification and denitrification. Dissolved oxygen (DO) should be monitored during this phase so it does not go over 0.2 mg/L. This ensures that an anoxic condition will occur during the idle phase.

    React: This phase allows for further reduction or "polishing" of wastewater parameters. During this phase, no wastewater enters the basin and the mechanical mixing and aeration units are on. Because there are no additional volume and organic loadings, the rate of organic removal increases dramatically. Most of the carbonaceous BOD removal occurs in the react phase. Further nitrification occurs by allowing the mixing and aeration to continue— the majority of denitrification takes place in the mixed-fill phase.

    Settle: During this phase, activated sludge is allowed to settle under quiescent conditions—no flow enters the basin and no aeration and mixing takes place. The activated sludge tends to settle as a flocculent mass, forming a distinctive interface with the clear supernatant. The sludge mass is called the sludge blanket. This phase is a critical part of the cycle, because if the solids do not settle rapidly, some sludge can be drawn off during the subsequent decant phase and thereby degrade effluent quality.

    Decant: During this phase, a decanter is used to remove the clear supernatant effluent. Once the settle phase is complete, a signal is sent to the decanter to initiate the opening of an effluent-discharge valve. There are floating and fixed-arm decanters. Floating decanters maintain the inlet orifice slightly below the water surface to minimize the removal of solids in the effluent removed during the decant phase. Floating decanters offer the operator flexibility to vary fill and draw volumes. Fixed-arm decanters are less expensive and can be designed to allow the operator to lower or raise the level of the decanter. It is optimal that the decanted volume is the same as the volume that enters the basin during the fill phase. It is also important that no surface foam or scum is decanted. The vertical distance from the decanter to the bottom of the tank should be maximized to avoid disturbing the settled biomass.

    Idle: This step occurs between the decant and the fill phases. The time varies, based on the influent flow rate and the operating strategy. During this phase, a small amount of activated sludge at the bottom of the SBR basin is pumped out—a process called wasting, which was explained above.

    Once the wastewater is decanted from the SBR Tanks, it then flows through channels that provide Ultraviolet disinfection (UV). This type of disinfection transfers electromagnetic energy from a mercury arc lamp to an organism's genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell's ability to reproduce and the organism eventually dies off. The effluent water is then discharged to the river.