Principles of 'Fill and Draw' Operation for Wastewater Treatment
Lee Wastewater Treatment Facility (WWTF), located in Massachusetts, has one of the most ecofriendly, efficient treatment facilities in New England.
Lee Wastewater Treatment Facility (WWTF), located in Massachusetts, has one of the most ecofriendly, efficient treatment facilities in New England. The plant received an award for its excellent treatment performance in 2012 from the Environmental Protection Agency (EPA), four years after start-up. Lee WWTF met a new EPA standard for removing phosphorus, well below the regulatory 0.2 mg/L limit.
The newly built, state-of-the-art sewage treatment plant went online in March 2008 with a four-basin AquaSBR® system from Aqua-Aerobic Systems Inc. coupled with two 8-disk AquaDisk® filters to obtain ultra-low nutrient effluent levels of 0.1 mg/L TP and 3.0 mg/L TN (without supplemental carbon addition). The new treatment system was also designed with the flexibility to support multiple-point chemical injection.
Effluent from the AquaDisk filters is sent to ultraviolet (UV) disinfection and is reused for makeup water for polymer system flushing and washing down the plant prior to discharging into the Housatonic River. Since the river supports fish and wildlife habitat, as well as contact recreation, it’s crucial that the plant meet its ultra-low nutrient effluent.
|Aerial view of the two-basin AquaSBR® system and buildings housing the two AquaDisk® filters and other treatment equipment.|
The small town of Lee (pop. 5,200 in 2014) accepts residential septage from Berkshire County for treatment as a form of revenue to offset its operating expenses but limits it to 1.4 percent of the incoming sewage flow to consistently achieve its low effluent nutrient objective.
The new facility replaced a 40-year-old, 1 MGD extended aeration activated sludge system, which was approaching its design life and was designed for a TP limit of 1.0 mg/L. The town chose Aqua-Aerobic Systems’ technologies due to their energy-efficiency and capability of meeting future stringent permit limits. Aqua-Aerobic provided operators with two weeks of on-site, hands-on training so that the staff could adapt to the new equipment and SCADA system.
The AquaSBR system operates on a simple concept of introducing a quantity of waste to a reactor, treating the waste in an adequate time period, and subsequently discharging a volume of effluent plus waste sludge that is equal to the original volume of waste introduced to the reactor. This “Fill and Draw” principle of operation comprises five basic elements: Fill, React, Settle, Decant, and Sludge Waste. The system may be designed to include seven individual phases of operation but the inclusion or duration of any individual phase is based upon specific waste characteristics and effluent objectives.
|Superintendent Alan Zerbato visually inspects one of the AquaDisk filter units. Photo copyright Ed Collier Photography 2014.|
Where nutrient removal is required, a simple adjustment to the SBR’s operating strategies permits nitrification, denitrification, and biological phosphorus removal. Optimum performance is attained when two or more reactors are utilized in a predetermined sequence of operation.
Clarified effluent from the AquaSBR system enters the filter and flows by gravity through the cloth media of the stationary hollow disks. The filtrate exits through the hollow shaft that supports the individual disks and flows to the effluent channel. As solids accumulate on the surface of the media, the water level surrounding the disks rises. Once a predetermined level is reached, the disks rotate and the media surface is automatically vacuum-backwashed clean. Heavier solids settle to the bottom of the tank and are then pumped to a digester or to the plant headworks.
Aqua-Aerobic Systems is exhibiting at WEFTEC.16, Booth 829. For more information, visit www.aqua-aerobic.com.