Facultative lagoon systems are common for wastewater treatment in small communities. In areas with inexpensive available land, this type of facility offers low operational and maintenance requirements and reliable treatment. However, new ammonia limits have put many communities in danger of noncompliance.
A small town upgraded their facultative lagoon system to meet Missouri’s new ammonia effluent limits. Watch the case study and read below for details.
This video case study was shot with a 360° camera—watch it with VR goggles for an immersive experience, or simply use your finger or mouse to rotate the view.
What Is a Facultative Lagoon?
The term facultative refers to organisms that can adapt to live in aerobic (with free oxygen) or anaerobic (without free oxygen) conditions. A facultative lagoon has aerobic, anaerobic, and anoxic (without free oxygen, but containing oxygen within compounds like NO3) layers, and bacteria that can operate in all conditions.
In the diagram above, taken from Julie Hartwig’s excellent webinar on Lagoon Microbiology (available here), the obligate aerobes (bacteria that require oxygen) are clustered near the top of the test tube, where the oxygen is; and the obligate anaerobes (or bacteria that can’t live in water with oxygen) are down in the bottom, where there is no oxygen. The facultative bacteria are found throughout the tube, with a slightly higher density at the top because they are more efficient in aerobic conditions.
The facultative lagoon acts as a clarifier, where heavier solids sink to the bottom to be digested anaerobically and the lighter solids remain in the water column to be treated aerobically. By definition, facultative lagoons are not mixed and aerated because to do so would disrupt the stratification that characterizes the facultative lagoon.
The aerobic zone—or top one to two feet—of a facultative lagoon contains dissolved oxygen provided by algae and wind action and performs most of the BOD and nutrient removal.
The anaerobic layer at the bottom of a facultative lagoon stores and slowly digests sludge and traps metals and nutrients.
Between these two zones is the anoxic, or facultative, zone which has some oxygen tied up in compounds like nitrate (NO3), nitrite (NO2), and sulfur (SO3). The anoxic zone helps to remove the odor-producing sulfur compounds generated by anaerobic digestion and also contributes some BOD and nutrient removal.
Facultative lagoons tend to be shallow—4 to 8 feet deep—and large, to provide surface area for oxygen transfer and to cultivate the algae that provide much of the oxygen. Hydraulic detention time is long—often upward of 120 days—to ensure sufficient treatment.
According to the EPA’s Wastewater Technology Fact Sheet on Facultative Lagoons, these systems are “partially reliable in performance.” Although BOD removal is usually satisfactory, facultative lagoons are unable to meet low effluent ammonia limits, especially during the winter, and will require an upgrade to remove ammonia.
Case Study: Benton Facultative Lagoon Ammonia Removal
Benton, Missouri, is a small town of 900 in the southeast corner of the state, just a few miles south of Cape Girardeau. Their wastewater is treated by a three-cell facultative lagoon system: two main lagoons separated by a berm with one lagoon divided into two cells. The lagoon system has an average flow of 83,000 gallons per day.
Like many Missouri municipalities, Benton was faced with new ammonia limits—1.2 mg/L in summer and 2.7 mg/L in winter—which went into effect in October 2019. Although otherwise providing adequate treatment, the lagoon system would be unable to meet these new limits, especially during the winter, when cold temperatures inhibit nitrifying bacteria.
Because Benton’s facultative lagoon system was providing adequate BOD treatment, it wasn’t necessary to add mechanical aeration to reduce BOD enough for nitrification.
The Solution: Triplepoint’s NitrOx®
Benton considered other technologies for lagoon ammonia removal, including in-lagoon media and covers for heat retention, but found them to be cost prohibitive. They opted instead for Triplepoint’s NitrOx System because:
- It is based on proven technologies—with decades of research studies and thousands of successful installations around the world.
- It is 2/3 the cost of available alternatives. Capital costs are lower because it integrates into the existing process rather than replacing it and only takes up a small footprint, which negates the need for additional land.
- It would preserve the ease of operation and low maintenance that Benton’s existing lagoon system provides. It is simple to operate, automated, self-cleaning, and flexible.
- Treatment would not be disrupted, because the lagoons wouldn’t need to be taken off-line for installation.
- It offers controlled, reliable nitrification: Because influent temperature is thermally regulated only when needed, energy costs are minimized.
NitrOx Small Footprint
The NitrOx Reactor was installed between Cells 1 and 2. After BOD treatment, effluent from Cell 1 is gravity-fed to the NitrOx; flow is then returned to Cell 2 for polishing. Unlike other lagoon ammonia removal technologies, NitrOx has a small footprint and tucks into the existing site.
To provide year round nitrification, a lagoon ammonia removal process must account for cold temperatures, because nitrifying bacteria are sensitive to cold. Other lagoon ammonia removal options compensate by increasing biomass—adding an additional lagoon cell filled with carrier media to encourage the growth of nitrifying bacteria.
Triplepoint’s patented NitrOx Lagoon Ammonia Reactor solves the temperature issue by heating the influent, only when needed and just enough to ensure biological nitrification. Because the temperature increase is minor, to a target between 36.5 and 41° F, and is only required a few months per year, energy costs are kept to a minimum. Temperature is regulated automatically, too, preserving the ease of operation of the lagoon system.
By choosing NitrOx, Benton did not have to purchase additional land or media, and would be assured of year round ammonia removal to meet their permit.