As various regions of the country struggle with stressed drinking water supplies, direct potable reuse of wastewater is gaining acceptance.
Various hurdles stand in the way, and one of them is cost. There’s no question that technology can treat wastewater to meet drinking water standards; the question is how. One obvious solution is reverse osmosis, but that can be expensive.
The Gwinnett County (Georgia) Department of Water Resources in 2017 completed a pilot study to test the feasibility of a different method: two-stage ozone-biological filtration, without RO. The project found that the method appears feasible, in terms of both treatment quality and cost. The results have implications not just for Gwinnett County (in the Atlanta metro area), but for utilities in any water-scarce area.
The project was conducted in collaboration with the Water Research Foundation, and co-principal investigators from CDM Smith (Jennifer Hooper, P.E., senior environmental engineer with CDM Smith) and Stantec (Katherine Bell, Ph.D., P.E., BCEE, now water strategy leader with Brown and Caldwell).
It has received two major honors: a 2018 Excellence in Environmental Engineering and Science Award from the American Academy of Environmental Engineers and Scientists and a 2018 WateReuse Excellence Award in the Transformational Innovation category from the WateReuse Association.
The principal investigator was Denise Funk, P.E., BCEE, division director of research and development with the Gwinnett County Department of Water Resources. She talked about the project (Water Research Foundation Project Reuse 15-11) in an interview with Treatment Plant Operator.
TPO: What does the Gwinnett County water treatment picture look like today?
Funk: We currently practice indirect potable reuse. We treat our wastewater to a very high level through advanced processes and return it to Lake Lanier, which is the source of water supply for our two drinking water treatment plants.
TPO: Why did the county undertake the investigation of direct potable reuse?
Funk: We have very robust treatment trains at our water reclamation facility and our water treatment facilities. So we decided to do research to look at skipping the lake for a portion of our advanced treated water.
TPO: In brief, what did the project consist of?
Funk: We purchased a pilot plant that is a small-scale version of our drinking water treatment train and ran blends of our advanced treated wastewater with lake water through our drinking water process. That’s direct potable reuse. We tested different blends, different filter media and different filtration rates.
TPO: Why does Gwinnett County face the potential for water supply shortages?
Funk: Metropolitan Atlanta lies at the headwaters of six major river basins. Because we are at the headwaters, the drainage basin for our reservoir is very small. In addition, our geology consists of clay over bedrock, which means access to groundwater is limited. So we rely on surface water. Our average rainfall is 50 inches a year, but because of our storage issues, if we get less than that, we’re in severe drought pretty quickly, within a year or so.
TPO: If indirect potable reuse is serving the county well, why is it beneficial to explore direct potable reuse?
Funk: Right now, we only have one source of water supply. So if we’re able to use our advanced treated reclaim water through direct potable reuse, that’s another source. It’s a potential way to diversify our water supply portfolio. We don’t have a plan to do this full-scale, but there was a great opportunity with the treatment processes we already have to do this investigation. We wanted the information for ourselves, but we also wanted to advance the science for the U.S. and the world on using ozone-biofiltration as an alternative to RO.
TPO: What processes are used at the county water treatment facilities?
Funk: Both facilities have the same treatment process. They are direct filtration plants because the water quality from Lake Lanier is very good. The first step is pre-ozonation, followed by coagulation and flocculation, and then direct filtration on deep-bed anthracite over sand. We also operate our filters biologically, so we remove dissolved organics as well as particulates.
TPO: What is the treatment process at the water reclamation facility?
Funk: The F. Wayne Hill Water Resources Center starts with a traditional activated sludge process, with biological nutrient removal for nitrogen and phosphorus. After the secondary clarifiers, the flow splits into two trains. The first has chemical clarification for additional phosphorus removal and then anthracite over sand media filters. The other train is treated the same except that is has ultrafiltration membranes instead of granular media filters. The two flows then come back together for pre-ozone followed by biological filtration with spent granular activated carbon, or GAC. The final step before discharge is disinfection with ozone.
TPO: How would you characterize the quality of the water after the advanced treatment?
Funk: The water is very high quality with turbidity that’s a lot lower than our 0.5 NTU limit, and it’s a lot lower than the lake water (about 1 NTU). We did a lot of testing for microbials in the treated water — Cryptosporidium, Giardia, Legionella and some others — and everything was nondetect.
TPO: How was the pilot study performed?
Funk: We purchased two pilot plants, which we still have and are using today on other projects. They’re located at the Shoal Creek Filter Plant. Our waterline from the F. Wayne Hill facility runs nearby, so we were able to tap in and run a 2-inch line over to the water plant to feed our pilot plant for the study. The operational phase of the study started in August 2016 and continued through June 2017.
TPO: How was the water treated in the pilot process?
Funk: The pilot plant had a blending tank where we added the F. Wayne Hill water to the lake water. We then injected ozone for a 30-minute contact time, about the same as in our full-scale plant. Then we had chemical addition, coagulation and flocculation, followed by four filter columns where we could test different filtration rates. Three of the filters had anthracite from our full-scale plant, and the other one had spent GAC. So we compared the performance of the anthracite against the GAC, and we used three columns to look at different filtration rates.
TPO: What was the scale of the pilot plant?
Funk: It was a very small flow rate. The Shoal Creek plant can process 98 mgd at peak flow, and the pilot plant can process 6 gpm, so it’s about 10,000 to one. The full-scale plant can serve about 800,000 people, whereas the pilot plant could serve 80 people, although we didn’t put any of the water from that plant into our system. It was just for testing.
TPO: What did you learn from the study as it relates to the county’s needs?
Funk: We looked at different blend ratios, the lowest being 15 percent advanced treated water to 85 percent lake water. At that blend, we found that the finished water from the pilot plant met all of the drinking water standards and health advisory limits that we evaluated in the study. At higher blends, we had some issues with nitrates and bromates, but we feel we could optimize our treatment processes in order to reduce those.
TPO: What did this study reveal about treatment costs?
Funk: We did a cost analysis comparing two-stage ozone-biofiltration versus an RO-based treatment train. For utilities that are inland, the RO-based approach is expensive because it creates a brine concentrate that has to be managed. Mechanical evaporation of brine is challenging and expensive. We analyzed capital and operating costs using our 15 percent blend over a 30-year time frame. The ozone-biofiltration-based train had a cost 2.5 times less than the RO-based approach.
TPO: Do you have any plans to deploy direct potable reuse based on this study?
Funk: We don’t have any plans to go full-scale right now. We do want to look at some of the issues and explore them further. We’d like to do some more testing.
TPO: In a larger sense, what are the implications of this study for other utilities with water-scarcity issues?
Funk: It means that a non-RO approach to potable reuse is definitely feasible, but that also depends on source water quality. If a utility has problems with nitrate or total dissolved solids, an RO approach may be necessary. There has to be some thought for each individual system to determine whether a non-RO approach is appropriate.
TPO: Would you expect any of the usual public acceptance issues if Gwinnett County were to adopt direct potable reuse in the future?
Funk: Yes. It’s really important to educate the public so that they understand the treatment processes and the science behind them. Public outreach and education is always a key component for acceptance of these projects.
