Process Page: Award-Winning Hillsborough County Northwest Regional Water Reclamation Facility: Advanced Technology and Operational Excellence—

Greetings from the FWEA Wastewater Process Committee! This month’s column will highlight the Hillsborough County Northwest Regional Water Reclamation Facility. This facility won the Earle B. Phelps Award in the category of advanced wastewater treatment in 2021.

Jason Hopp

The Hillsborough County (county) Northwest Regional Water Reclamation Facility (NWRWRF), shown in Figure 1, is the largest of five wastewater treatment facilities operated by Hillsborough County Utilities. The facility was recognized by the Florida Water Environment Association (FWEA) in 2021 as the Earle B. Phelps Award winner in the advanced wastewater treatment category for outstanding operations and environmental stewardship.

The Earle B. Phelps Award is presented annually to the wastewater treatment plants that demonstrate exceptional effluent quality 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 environmental stewardship.

The Board of County Commissioners (BOCC) approved a regionalization plan in 2014 for the northwest portion of the county that included the expansion of the NWRWRF. As with many utilities in Florida, the catalyst was aging infrastructure and unprecedented population growth. At $193.3 million, the plan developed by the county was the largestever capital improvement project. The project would retire two aging wastewater plants and consolidate treatment at the NWRWRF. The Dale Mabry Wastewater Treatment Facility (WWTF) and River Oaks WWTF were both more than 40 years old and beyond their useful life.

These facilities were decommissioned and converted to master pump stations as part of the project. In addition to decommissioning these two facilities, the county would expand the existing NWRWRF from 10 million gallons per day (mgd) to 30 mgd to meet the region’s wastewater needs through 2040. The goal of the BOCC was to improve wastewater service, improve reliability, and minimize future rate impacts.

The expansion of the NWRWRF was the cornerstone of the project and provided many operational benefits. One of the main benefits included improved overall operations and treatment efficiencies by consolidating all of the plant maintenance and operations into one facility. Another benefit was the addition of flow equalization to reduce peak demands and promote a more-uniform treatment process.

The county also took exceptional measures to minimize the impacts to the community. The county utilized advanced technologies to minimize odors and noise, preserved natural habitats while using them as natural buffers, and even incorporated a multiuse recreational trail through the restored wetlands for the local residents.

Along with the NWRWRF being the county’s largest wastewater treatment plant, it’s also the most advanced. As part of the expansion, the county incorporated a 6-megawatt natural gas microturbine (Figure 2) to supply all the power to the facility, which provided resiliency, reliability, and redundancy of its power sources. It’s the first such facility in Florida, and also the first in the United States, to be fully powered by this kind of microturbine.

By generating its own power, the facility is capable of uninterrupted service, even when electricity in the area is knocked out by a natural disaster, such as a hurricane. The facility will continue to rely on land power and backup generators for redundancy purposes.

The plant expansion took approximately three years to finish and was substantially

completed in April 2020. During the construction process, the county’s operations staff worked very closely with the contractor to coordinate activities to minimize the impact on the treatment process. As a result, the facility maintained complete compliance during the entire construction period.

The NWRWRF utilizes a five-stage Bardenpho® Process to treat domestic wastewater to advanced wastewater treatment standards of 5 mg/L carbonaceous biochemical oxygen demand (CBOD), 5 mg/L total suspended solids (TSS), 3 mg/L TN, and 1 mg/L of TP. The influent enters the process through a headworks with three dual mechanical screening systems, a backup manual bar screen, and two forced-vortex grit removal systems prior to entering the flow equalization basin. Influent is then introduced to the 10 parallel biological nutrient removal (BNR) treatment trains, each consisting of a fermentation, first anoxic, aeration, second anoxic, and reaeration zone. The foul air from the headworks, equalization (EQ) basin, and the covered sections of the BNR is scrubbed through three odor control systems. Following the BNR, the activated sludge is sent to eight final clarifiers for settling, return sludge, and waste sludge.

An optional alum injection system is available before and after the clarifiers for chemical phosphorous removal on an asneeded basis. Secondary effluent from the clarifiers is then sent to 20 dual-media deep-bed filters. The tertiary effluent is sent to two dualchambered, covered chlorine contact chambers for final disinfection and dechlorination. The highly treated effluent is then transferred by a series of effluent pumps to onsite storage tanks, surface water discharge locations, and groundwater injection wells. The facility has four reclaimed water storage tanks, with a total of 20 million gallons (MG) of storage, three reject storage tanks with a total volume of 13 MG, and one lined 5-MG reject storage pond.

The residuals handling facility includes: S One holding tank with 470,000 gallons total capacity S Four gravity belt thickeners S Four aerobic digesters with 6 MG of total capacity S Four centrifuge dewatering units

Biosolids from both the NWRWRF and the Van Dyke WWTF are processed at this location (Figure 3). Residuals are transported to a permitted biosolids facility for treatment and disposal to a Class I or II landfill.

Figure 2. The 6-megawatt natural gas microturbine.

Figure 3. Northwest Regional Water Reclamation Facility process flow diagram.

The operations and maintenance staff at the NWRWRF take a tremendous amount of pride in providing the best possible effluent, while maintaining its equipment and aesthetics. The facility employs numerous technologies, including water information management software, supervisory control and data acquisition (SCADA), continuous monitoring analytical equipment, and cover systems to improve the wastewater treatment process, while also reducing maintenance requirements. The biological treatment system utilizes oxidation reduction potential (ORP) within the aeration basins of the BNR to control the

mechanical aerators. The ORP allows the facility to operate at a target oxidation state, rather than a dissolved oxygen (DO) set point, which is ideal for creating an environment that is optimal for complete nitrification and denitrification. Since ORP is a relative measurement and can vary from plant to plant, the county worked diligently to determine the optimum set points, which are key to producing low nutrients. The operations staff members, along with their internal process engineer, performed an 18-month study to profile the process and develop the current control strategy. The operators utilize two ORP probes per basin. One probe is set up at the midway point of the aeration basin for monitoring only, and Continued on page 32 Florida Water Resources Journal • August 2021 31

Continued from page 31 the second probe that controls the aerators is located at the effluent end of the aeration basin prior to entering the second anoxic zone.

By utilizing ORP, the facility has been able to significantly reduce the power related to aeration and has improved nitrogen removal, as compared to pre-expansion performance. The plant improvements associated with the expansion and the process control measures have enhanced biological phosphorous removal and eliminated the use of 400 to 500 gallons of alum per day.

In 2020, this control strategy consistently produced an effluent that was very low in TN and TP. The average effluent TN for the year was 1.54 mg/L, which is almost half of the regulatory limit; the average effluent TP was 0.10 mg/L and was achieved biologically without the use of any chemical addition. The Bardenpho Process also produced exceptional removal efficiencies for CBOD, TSS, TN, and TP, as detailed in Table 1.

The county has added some additional upgrades to the NWRWRF that have reduced maintenance requirements and provided considerable savings in chlorine. The county utilizes cover systems on its clarifiers to prevent algae growth and on the chlorine contact chambers (Figure 4) to eliminate environmental effects (ultraviolet [UV], wind, etc.) that can decrease the chlorine residual. The launder covers (Figure 5) on the clarifiers drastically reduced the maintenance requirements and improved effluent quality by preventing algae growth on the weirs.

Since the implementation of the covers, plant staff has eliminated the weekly cleaning requirements, while also removing the use of chlorine; the staff now only performs periodic inspections. The chlorine contact chambers were covered in an effort to reduce the chlorine demand, prevent algae growth, and eliminate staining due to iron oxidation. The covers have eliminated the need to clean the chambers and the chlorine demand has also been significantly reduced.

These covers, along with the exceptional effluent quality produced by the BNR, have allowed the operations staff to reduce the chlorine demand to approximately 1 mg/L and meet the regulatory limits for dichlorobromomethane and dibromochloromethane without the use of UV disinfection. The plant currently averages about 96 gallons of 12 percent sodium hypochlorite per day and per million gallons treated.

Table 1. Removal Efficiencies

Influent 232 217 46 5.4 N/A

Effluent <2 <1 1.54 0.10 0.42 % Removal 99.6% 99.8% 96.7% 98.1% N/A Permit Limit 5 5 3 1 2

0 0 0 0 0

The county and its staff at the NWRWRF are dedicated to providing the best possible wastewater treatment and protecting the environment. The performance of the facility and its staff speaks for itself and exemplifies what the Earle B. Phelps Award is all about.

Congratulations to the county and the NWRWRF staff for being selected as the best advanced WWTF in the state of Florida in 2021.

Jason Hopp is a sales representative with Heyward Florida Inc. in southwest Florida and a member of the FWEA Wastewater Process Committee. S