Wastewater Lagoon Turnover is the final installment of our 3-part blog series discussing the side effects of summer heat on wastewater lagoons. Our first installment focused on Lagoon Algae Control while our second installment focused on Lagoon Odor.
Lagoon turnover is when the sludge and water from the bottom of the lagoon rises to the top. Since the bottom of the lagoon is often where solids and less-oxygenated water tend to be, this can create a problematic situation for the rest of the lagoon. Here’s the how and why…
Why lagoon turnover happens during the summer:
- Low levels of DO (dissolved oxygen): As the water temperature increases, aerobic bacteria in your wastewater lagoon become increasingly active, and as a result, consume more oxygen. This leads to low lagoon DO. With less dissolved oxygen, anaerobic digestion increases.
- Thermal Stratification: Insufficiently mixed lagoons can experience “thermal stratification,” wherein three distinct layers form within the water column, separated by temperature and water density. The top layer contains the majority of the DO because of contact between the lagoon’s surface and the atmosphere, while the bottom layer is left without any oxygen to treat the suspended solids and nutrients. As the heat begins to warm the lagoon with seasonal changes, and as summer winds increase, the layers can mix and become more uniform. As a result, the oxygen in the upper layer becomes diluted into the total volume of the lagoon during lagoon turnover. Consequently, DO across the whole lagoon decreases, increasing anaerobic digestion and influencing sludge to float to the surface.
What happens to your wastewater lagoon because of lagoon turnover:
- Floating Biosolids: It’s not pretty. When you experience turnover, anaerobic digestion in the sludge biomass releases gas as a byproduct. This gas becomes entrained in the sludge, increasing its buoyancy and causing it to rise from the bottom of the lagoon.
- Intense Odors: They smell bad. With the rising gas byproduct of anaerobic digestion (most notably H2S —a malodorous sulfur), noxious 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.
How to prevent lagoon turnover in your wastewater lagoon:
Increase Lagoon Mixing & Circulation: The best way to stop lagoon turnover is to prevent thermal stratification from happening in the first place. The way to do this is to keep the wastewater lagoon thoroughly mixed from bottom to top throughout the year. By circulating the water in your lagoon, you keep the water column a relatively uniform temperature throughout the winter and summer months. Without differing temperatures to stratify the water column, the danger of lagoon turnover greatly decreases.
Depending on the depth of your lagoons and the type of aeration you have, this can be more or less difficult. If you have surface aerators and your wastewater lagoons are 10’ deep, there is nothing you can do to completely mix the water column. If, on the other hand, you have coarse bubble diffused aeration on the bottom of the lagoon, mixing the entire water column is much easier. Either way, the more thoroughly you mix and circulate your lagoon, the more likely you will be to prevent lagoon turnover from happening in your wastewater lagoon.