It’s autumn again—time for football, apple cider, falling leaves, and wastewater lagoon turnover. Although less extreme than the spring transition, cooler fall temperatures cause destratification and may trigger lagoon turnover. In this short article, we’ll talk about causes of lagoon turnover and how to prevent it.
What Is Autumn Lagoon Turnover?
An improperly mixed wastewater lagoon will settle into layers during summer, with denser, cooler water at the bottom and warmer water at the top, which is known as thermal stratification.
This thermal stratification can lead to lagoon short-circuiting, where influent, taking the path of least resistance, rides along the warmer top layer of the water, and flows, virtually untreated, out with the effluent.
In spring and fall, the ambient air temperature warms or cools the top layer of water, creating “thermal instability.” As the layers approach a uniform temperature, they begin to mix and destratify. In the fall, the now-cooler surface water sinks down, displacing the warmer water at the bottom and dislodging the accumulated, septic solids. The lagoon is said to “turn over”—the top layer of water sinks to the bottom and the sludge can rise to the top, causing floating sludge mats and noxious odors.
Problems Caused by Autumn Lagoon Turnover
- Floating sludge: As the colder water sinks to the bottom of the lagoon, it dislodges the solids settled at the bottom, which have been quietly anaerobically digesting. The anaerobic digestion process releases gas as a byproduct, which becomes entrained in the sludge. Once dislodged, the gas trapped in the sludge causes it to rise from the bottom of the lagoon and float. For more about sludge, read Causes and Effects of Wastewater Lagoon Sludge Explained.
- Intense Odors: With the rising gas byproduct of anaerobic digestion (most notably H2S—a malodorous sulfur), unpleasant lagoon odors are released into the atmosphere all at once. These odors, coupled with those of the floating sludge mat, are strong during lagoon turnover. If picked up by the wind, these odors will garner fervent complaints (from both neighbors and staff).
- Lagoon Treatment Suffers: Lagoon turnover can be a sign that your lagoon is septic. In other words, it can be an indication that your lagoon contains a lower DO concentration, and is breaking down nutrients via anaerobic digestion—a very slow and smelly process. Expect a spike in effluent BOD, TSS, and other treatment parameters at some point in the near future if corrective action is not taken.
How to Prevent Seasonal Lagoon Turnover
To prevent lagoon turnover in spring and fall, you need to combat the causes of the problem itself:
- Increase circulation to prevent stratification: By horizontally circulating the water, the lagoon layers will never be able to fully stratify. With both a homogeneous lagoon water temperature and environment, no fall lagoon turnover will occur.
- Increase dissolved oxygen levels to combat low DO. One of the easiest ways to do this is to add aeration to your lagoon. By maintaining an aerobic environment you will limit the production of noxious H2S gases.
- Increase mixing to prevent sludge buildup on the bottom. Increasing circulation and adding DO is all well and good, but the best way to prevent floating sludge and fall lagoon turnover is to limit the accumulation of sludge in the first place. Proper lagoon mixing puts sludge up into the water column where it can break down aerobically and odor-free.