Georgia treatment facility will use membranes to boost performance
Major construction recently began on the 32 mgd Big Creek Water Reclamation Facility in Roswell, Georgia. Set to replace Fulton County’s existing 24 mgd facility of the same name, the new Big Creek WRF will ensure adequate regional treatment capacity through at least 2050, improve the quality of wastewater discharged to the environment, and significantly reduce odors. Delivered by means of the progressive design-build approach, the $300 million project is being undertaken by a joint venture comprising the construction company Archer Western, a subsidiary of the Walsh Group, and the engineering consulting firm Brown and Caldwell.
Originally constructed in 1971, the existing 24 mgd tertiary treatment facility is nearing the limits of its capabilities, says David Clark, P.E., the director of public works for Fulton County. “On a typical day, we’re bringing in about 20 to 22 mgd,” Clark says. “We’re at the capacity of the plant.” Meanwhile, the region served by the Big Creek WRF is growing rapidly, including high-density redevelopment in many areas. “Looking at all that for the next 30 years, we thought we needed at least 32 mgd of treatment capacity to accommodate the growth of the cities that we serve,” Clark says. In fact, the new facility was designed to ultimately be expanded to 38 mgd.
More than a decade ago, Fulton County began using membrane technology to treat wastewater at its Johns Creek Environmental Campus, also in Roswell. Based on the success it has had with membranes there, Fulton County decided to use membrane technology for the new Big Creek WRF. The decision would have a major effect on the overall direction of the project.
As a result of its more advanced treatment capabilities, the new Big Creek WRF, despite its increased treatment capacity, will discharge cleaner effluent into the Chattahoochee River.
During the initial conceptual design process, the project team assessed whether it would be possible to retrofit the treatment trains of the existing facility to incorporate membranes, says Kelly Comstock, P.E., M.ASCE, the design project manager for Brown and Caldwell. One idea was to reconfigure either the existing secondary clarifiers or biological nutrient removal basins so that they could accommodate the membranes. “We found that from an engineering perspective, that was achievable,” Comstock says.
However, such an approach would require taking each of the eight existing treatment trains out of service, retrofitting it with membranes, and starting it up. There was a “significant cost duration associated with that,” Comstock says. Meanwhile, the “potential for a permit violation would be huge,” he notes, because of the need to run two different treatment processes in parallel while each of the remaining treatment trains was revamped in the same manner. “When you start to weigh those risks, it became very clear that it didn’t make sense just to retrofit,” Comstock says.
Instead, the project team decided that it made “more sense to do greenfield construction and construct a new facility and decommission the existing one,” Comstock says. At the same time, the team found ways to reconfigure other aspects of the existing facility, including converting the biological nutrient removal system into aerobic digesters and transforming the secondary clarifiers into an 8 million gal. equalization basin. As a result, “it became clear that it was the best cost approach, the best risk approach, and the best schedule approach to do this new greenfield process,” Comstock says. “That was something that was not clearly evident as we went into that conceptual design process.”
The new Big Creek WRF will feature coarse screening, grit removal, primary clarification, fine screening, an advanced BNR system, a membrane tank, disinfection by means of ultraviolet light, and a post-aeration system to increase dissolved oxygen levels in the treated effluent before it is discharged to the Chattahoochee River. Solids will be digested aerobically, dewatered by means of screw pumps, and trucked away for disposal.
Including membranes in the treatment process affords two key benefits, Comstock says. The first involves the reliability of the microfiltration membranes that will remove all particles greater than 0.2 microns. “Membrane technology allows you to meet a more consistent water quality,” he says. “Whereas with some of the other approaches, such as secondary clarifiers and filters, they’re more apt to spikes and overloading that could impact the discharge quality.” By contrast, “there’s a consistency with permeate levels because it’s a fixed membrane pore size, so particles of a certain size are not passing through it,” Comstock notes.
The other benefit of membrane technology is that it can “operate at a much higher mixed liquor concentration” compared with more traditional treatment approaches, Comstock says. In its current configuration, the BNR system at the Big Creek WRF operates with a mixed liquor concentration of approximately 3,000 mg/L. By comparison, the combined BNR and membrane bioreactor system will be able to accommodate mixed liquor levels in the range of 9,000 to 10,000 mg/L. “That results in a much smaller footprint in terms of the BNR basins,” Comstock notes.
Although its Johns Creek facility uses hollow fiber membranes, Fulton County has opted to install flat plate membranes in the Big Creek WRF. “We decided to go that route for (the Big Creek project) primarily because we were expecting lower maintenance needs of flat plate membranes,” Clark says. “It’s been our experience, based on Johns Creek, that once you get over a certain level of daily treatment capacity, maintenance responsibilities associated with the hollow fiber (membranes) start becoming too great.”
As for the new BNR system, it has been designed for optimal flexibility when it comes to removing phosphorus. Operators will be able to use either a chemical or a biological process, depending on which is preferred at a particular time. “They can make a decision in how they want to operate the facility and have some flexibility there,” Comstock says. For chemical treatment, operators can add ferric chloride, whereas they can add a carbon source to carry out biological removal of phosphorous.
As a result of its more advanced treatment capabilities, the new Big Creek WRF, despite its increased treatment capacity, will discharge cleaner effluent into the Chattahoochee River. “By implementing this expansion, we’re actually producing less waste load to the river than is currently permitted,” Comstock says. This situation will hold true even when the facility begins operating at its future maximum capacity of 38 mgd.
Compared with what it currently is permitted to discharge, the facility is expected to achieve the following reductions in its effluent: 50 percent less biochemical oxygen demand, 45 percent less ammonia, 15 percent less total suspended solids, 50 percent less fecal coliform, and 40 percent less total phosphorus. “You don’t often see that, where a project to expand a wastewater plant actually cleans the river,” Comstock says. “It’s pretty unique.”
The new facility was also designed to address odor and lighting complaints from nearby neighborhoods. “At the end of the day, we want to be good neighbors,” Clark says. “We want to be as unnoticeable as possible.”
To this end, the project team identified the major odor sources at the site and moved them as far from the fence line as possible, says David Walker, Archer Western’s program manager for the Southeast. At the same time, the design included rigorous measures to limit odors. For example, the headworks, primary clarifiers, and fine-screening units were either enclosed completely or outfitted with covers “because those are typically the sources of some of the worst odors that come from a plant,” Walker says. Foul air from those facilities will be sent to scrubber systems that include granular activated carbon filters as well as chemical scrubbers. Similarly, the BNR tank, membrane system, digesters, and dewatering facility will be covered and have granular activated carbon treatment systems to clean their foul air.
The contract with Fulton County includes “very stringent requirements and guidelines we have to meet as well as penalties if we don’t reduce the fence-line odors,” Walker says. “The design is robust, and it will be proven and tested at the end of construction.”
Construction is being conducted as part of a “fairly unique” two-phased approach, Walker says. Begun last fall, the initial phase involved such site work as rough grading, utility relocations, and the installation of deep foundation piles while design progressed on the facility. Thanks to these efforts, “we were able to hit the ground running very quickly” at the start of the second phase, which began in late August, Walker says.
Overall, the project will take 46 months to construct and commission, with the new facility scheduled to begin operations in 2024.
This article first appeared in the November 2020 issue of Civil Engineering.