Process Page: Award-Winning Polk County Northeast Regional Wastewater Treatment

Greetings from the FWEA Wastewater Process Committee! This month’s column will highlight the Polk County Northeast Regional Wastewater Treatment Facility, which won the 2021 Earle B. Phelps Award in the advanced secondary category for facilities with a design capacity from 5 million gallons per day (mgd) to less than 10 mgd.

Award-Winning Polk County Northeast Regional Wastewater Treatment Facility: Advanced Treatment Through Operational Efficiency

Bartt C. Booz

The Northeast Regional Wastewater Treatment Facility (NERWWTF) is the newest, mostsophisticated, and largest of three regional plants currently in Polk County (county). The facility was recognized by the Florida Water Environment Association (FWEA) in 2021 as the Earle B. Phelps Award winner in the advanced secondary category for outstanding nutrient removal, secondary clarification, filtration, and high-level disinfection.

The Earle B. Phelps Award is presented annually to wastewater treatment plants that demonstrate exceptional advanced secondary treatment throughout the year and maintain the highest removal of major pollution-causing constituents, such as total nitrogen (TN) and total phosphorous (TP). This award demonstrates the county’s dedication and commitment to protecting the environment and serving the community.

Past Expansion

The original NERWWTF was built in 2001 with a capacity of 3 mgd. Due to the growth in the area and increasing waste loadings, the county identified in 2006 that the NERWWTF needed a significant expansion to 6 mgd due to rapid growth along the Highway 27 corridor. In 2012, construction was completed to expand the facility to 6 mgd, with a build-out capacity of 9 mgd.

The expansion included a state-of-the-art biological nutrient removal (BNR)/oxidation ditch treatment facility. Other facility improvements of the 2012 upgrade included automated sequencing for digestion, biosolids dewatering, a supervisory control and data acquisition (SCADA) system operable throughout the plant from tablets, and aquifer recharge.

Current Expansion

In 2019, a capacity evaluation of the NERWWTF identified the need to add secondary clarification capacity to the facility to accommodate current and future flows and loading, and address Class 1 reliability requirements. In 2020, the county began construction of two new 100-foot-diameter secondary clarifiers (Figure 1) to expand the facility’s secondary clarification process from a capacity of 6 to 9 mgd. The project also includes a new return activated sludge/waste activated sludge pumping station, new screenings wash presses (Figure 2) for the existing screening structure, hydraulic improvements, and site stormwater/ drainage improvements. Construction of the project is expected to be complete by June 2022.

Process Overview

Influent wastewater is screened with perforated plate mechanical screens at the screening structure before flowing to the BNR basin with flow equalization capability. The existing BNR contains selectable anoxic and aerobic zones, where aeration is accomplished with fine bubble diffusers prior to flowing to one of the two oxidation ditches, also with anoxic and aerobic zones. Following secondary biological treatment, the mixed liquor flows to three secondary clarifiers for settling, return sludge, and waste sludge.

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Upon completion of the secondary clarifier upgrade in 2022, the operations staff will have more flexibility to take clarifiers down for maintenance. Treated effluent from the final clarifiers is filtered through one of four parallel deep bed filters. The filters can be dosed with chlorine to deter algae growth. A polymer solution may also be added to the influent flow as a filtering aid, but it’s rarely used. Turbidity monitoring for compliance purposes takes place in the clear well of the deep bed filter before the disinfection process.

Following filtration and turbidity monitoring, sodium hypochlorite is injected in a rapid-mix chamber before flowing through slide gates into the chlorine contact chambers (CCCs). Effluent from the CCC effluent channel is monitored for total residual chlorine (TRC) before flowing by gravity into the effluent pump station wet well, where it’s pumped to ground storage reservoirs (GSRs). High-service pumps convey reuse water from the GSRs to the reclaimed water distribution system.

The facility is equipped with a state-of-theart biosolids facility. Two screw presses process aerated, digested sludge at approximately 95 gallons per minute (gpm) and dewater liquid sludge from approximately 1 percent to provide 15 to 17 percent cake. The biosolids are then hauled to the local landfill for disposal.

While the BNR can be run with many differing options for treatment, Figure 3 shows a block depiction of the NERWWTF in its current operational scheme. Note that the grayed-out structures are not in use, and only the mainstream treatment train is shown.

Table 1 summarizes the typical plant loadings and effluent quality.

Figure 3. Block depiction of the Northeast Regional Wastewater Treatment Facility in its current operational scheme.

Table 1. Plant Loadings and Effluent Quality

Advanced Secondary Treatment

The biological treatment system utilizes oxidation reduction potential (ORP), both within the BNR and the oxidation ditches, as the process control parameter for aeration and anoxic areas. The facility previously utilized dissolved oxygen (DO) to control the oxidation ditch’s aerator speeds. By utilizing ORP, the NERWWTF has been able to significantly reduce power consumption and chemicals while providing far greater nitrogen removal, as compared to pre-expansion performance. While there is not a “set” level for ORP, the county’s operators worked to determine the optimum set points for the facility. This is the key to the low TN levels and enhanced treatment.

The facility does not use carbon addition in the deep bed filters, but it has been able to average effluent TN of less than 1.8 mg/L for the past year. This represents a 97 percent removal of TN as compared to the 61 mg/L influent. Using ORP for process control, the power utilized per influent volume treated dropped by 12 percent from preexpansion levels and the total chlorine utilized per influent volume dropped by 49 percent. The net savings between the pre- and post-expansion operations from chemical and power represent nearly $120,000 annually, while the effluent quality from the plant has never been better or more consistent.

One of the major enhancements that the county implemented to optimize the chlorine disinfection efficiency and to further reduce chlorine consumption was to add shade balls to the CCCs (Figure 4). While approximately 65 percent of chlorine savings are attributed to the county’s operation of the mainstream process, approximately 35 percent of chlorine savings came from the addition of shade balls.

The NERWWTF operators experimented with the balls in the chlorine contact chambers to reduce chemical costs in August 2013. During the testing, it was discovered that the shade balls not only reduced chlorine consumption, they also stabilized the residual by occupying the area of chlorinated water exposed to the environment (ultraviolet [UV], wind, etc.). The balls virtually eliminated algae growth and iron oxidation that previously built up on the channel walls. The use of the balls has all but eliminated cleaning of the chambers, except where the channels are exposed to sunlight. To keep the balls in place, there is grating installed at the end of each contact chamber before the outfall.

Figure 4. View of chlorine contact chamber with shade balls (April 2015).

Figure 5. Pictured (left to right): Edward Clark, Todd Tysinger, James Welch, and Charles Nichols Jr. Not pictured: Nathan Silveira, Oswaldo Lopez, Bryan Osborne, and Richard Moore.

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Personnel

The Florida Department of Environmental Protection (FDEP) requires that the plant be staffed with state-certified operators for a minimum of 16 hours per day, seven days a week. The facility currently has three Level A operators, two Level B operators, and two Level C operators on staff. The operations staff is comprised of Edward Clark, Todd Tysinger, James Welch, Charles Nichols Jr., Nathan Silveira, Oswaldo Lopez, Bryan Osborne, and Richard Moore (Figure 5).

The dedication and hard work of the entire NERWWTF operations, maintenance, and electrical/instrumentation staff resulted in the facility consistently producing exceptional effluent quality, exceeding all regulatory requirements, and reducing chemical usage and energy consumption.

The 2021 Earle B. Phelps Award for advanced secondary facilities is well-earned by this dedicated team!

Bartt C. Booz, P.E., is senior project manager with Wright-Pierce in Maitland. S