On a barrier island along the North Carolina coast, a seasonal tourist population creates a large wastewater load for a sensitive environment. Some scientists became curious about how well this wastewater is treated for nitrogen, so they did a study.
They looked at large onsite treatment systems using aeration with an anoxic tank for denitrification (called “extended aeration” by the scientists), sequencing batch reactors and advanced media filtration. The results of their work were published in the August 2019 issue of Environmental Management.
Pumper talked with Michael O’Driscoll, associate professor in the Department of Coastal Studies at East Carolina University in Greenville, North Carolina. O’Driscoll worked with co-authors Eban Bean at the University of Florida, Charlie Humphrey at East Carolina University, and graduate student Nick Mahoney, now with the Environmental Health Division of Union County, North Carolina.
Pumper: How did you pick this subject?
O’Driscoll: We had been doing a lot of work on conventional septic systems and on some of the challenges as we start to see more frequent storms in some of our coastal areas. Early on we noticed there were a lot of our coastal areas that used onsite wastewater management. The smaller residential septic systems wouldn’t be capable of dealing with large seasonal changes in population. So we were interested in the package plants, but there wasn’t much information about those.
In places where there are hotels or condominiums, when we started to look into it, we recognized a lot of those are on package plants. The barrier island we looked at, Bogue Banks, has about 40 package plant systems.
One of the issues we have in these coastal areas is that they’re very sensitive to nutrient inputs. We wanted to understand what sort of nutrient inputs were getting into the aquifer system.
Pumper: Obviously algae blooms have been prominent in the news, but are the sensitive locations in your area the brackish bays behind the islands?
O’Driscoll: Yes, they’re more sensitive if the water sits there for a while. During sunny conditions, if there’s excess nutrients, blooms will happen. There are also concerns with nitrogen in places where it affects the water supply. If you have a nitrate in there, it can cause problems, such as blue baby syndrome.
Pumper: How large were the package plants you studied?
O’Driscoll: The largest systems were permitted to treat up to about 100,000 gallons per day. They tend to serve condominiums, hotels — seasonal, tourist-related housing.
Pumper: When you did sampling, did you measure influent against effluent?
O’Driscoll: Yes. When we look at the water use out there, there’s about three times the water use in the summer because there are a lot of tourists who come in. One of the questions we had to begin with was if the nutrient load would be high in the summer because of that.
Pumper: So how efficient were those package plants at removing nitrogen?
O’Driscoll: We measured only seven out of the 40 systems because they were very resource-intensive to monitor. On average, they were about 74% effective. The range was 55% to 89% removal. In the effluent, we had about 17 mg/L total nitrogen during winter, and in summer it dropped to about 7 mg/L. For this paper, we also looked at a range of systems from around the world, and that average nitrogen removal was about 47%.
Pumper: Did the systems on the island do better because licensed operators were involved?
O’Driscoll: I think that’s one part of it — paying attention to the system. What was interesting for the extended aeration plants was that during the summer months when there was higher loading, they started to treat more efficiently. So I think another part is when organic loads and temperatures are higher, they can treat nitrogen better. During the winter months, when a lot of the population isn’t there and temperatures are cold, the plants didn’t work as well. There also isn’t a steady amount of waste going into the systems then, whereas in the summer months there are more stable flows.
Pumper: Did one type of system perform better than another?
O’Driscoll: We couldn’t really tease that out because we had just a couple sequencing batch reactors and advanced media filtration systems, and we had three of the extended aeration (systems). We just wanted to get an idea, on average, of the loadings from these systems.
The extended aeration had the best treatment for two of the plants, but one had lower treatment efficiency. To really compare across those systems, we would probably need a much larger sample size. It happened that the biggest systems were the extended aeration systems. The others didn’t have as much loading, but they were also generating much less waste.
Pumper: How much nitrogen treatment is there in the sand along the coast?
O’Driscoll: That’s the really good question. What you could see is what’s going into the aquifer. But exactly how much is making it out to the sound or the ocean — that’s still an open question. There’s some indication that after a few hundred meters, there’s a combination of dilution and some other denitrification mechanism when there’s organic matter in the soil and low oxygen. If it’s just a sandy aquifer, the nitrogen can be very mobile.
Pumper: What’s your next step?
O’Driscoll: One thing we’re trying to do now is review the literature and better understand the systems that will be more effective in coastal areas experiencing sea level rise and more frequent flooding. In other words, what kind of systems do you get better nitrogen treatment from in those situations?
