“Short-circuiting is the greatest deterrent to consistent pond performance. The importance of the hydraulic design of a pond system cannot be overemphasized.” That’s a quote from the EPA’s lagoon manual, Principles of Design and Operations of Wastewater Treatment Pond Systems for Plant Operators, Engineers, and Managers.
Short-circuiting occurs when wastewater flows through the lagoon unevenly, allowing some of the influent to take a short cut through the cell and bypass the treatment process. Since adequate retention time is critical to lagoon treatment, anything that shortens retention time can result in high BOD and TSS in effluent. In this article, we’ll describe the ways lagoon hydraulics can contribute to short-circuiting, how to diagnose it, and how it can be prevented.
What causes a lagoon to short circuit?
Short-circuiting has many causes:
1. Flaws in lagoon design: Lagoon hydraulics, such as the placement of influent and effluent pipes, can contribute to the likelihood of short-circuiting. A single influent pipe can forcefully push wastewater toward the effluent side of the pond. A lagoon oriented in such a way that it’s affected by wind is also subject to dead spots and short-circuiting.
2. Infiltration & inflow: A huge influx of stormwater can overwhelm the lagoon and disrupt the treatment process. For more about lagoon I&I, read our blog, Lagoon Infiltration and Inflow: Problems and Solutions.
3. Sludge accumulation: Sludge buildup, in addition to reducing lagoon capacity and thereby retention time, can also create channels that wastewater flows around.
4. Stratification: Stratification is when water settles into distinct layers due to differences in temperature (thermal stratification) or dissolved oxygen (DO) content (biological stratification). The colder, denser water sinks to the bottom of the lagoon, creating an anoxic zone with low DO and high BOD. The shorter winter days with less solar radiation and cold winds create some lagoon mixing, as colder surface water continually displaces the warmer water at the bottom. However, without aeration and mixing throughout the water column, the warmer influent doesn’t get incorporated into the overall environment of the lagoon. Taking the path of least resistance, the warm influent rides the thermocline, or thin dividing layer between strata, above the heavier, colder water, and doesn’t get treated.
Diagnosing short-circuiting…with oranges
We asked lagoon expert Steve Harris to tell us of something interesting he’s seen in his many years as a lagoon consultant. He related an experience with a new lagoon facility that appeared to have a short-circuiting problem:
A visual inspection of the flowpath led Steve to believe the lagoon was short-circuiting. To test it, he returned with a bucket of oranges, which he tossed into the influent side of the lagoon. Why oranges? “Oranges act like a drogue,” Steve says. “It’s not a perfect flowpath indicator, but it’s actually really good, because most of the orange floats beneath the surface of the water—about 90 percent or so—so the wind is not going to directly influence that orange.” The oranges confirmed his suspicion: The next day, all of them were floating near the effluent structure, demonstrating that the lagoon’s effective detention time was hours instead of days.
Some other methods of diagnosing short-circuiting:
- Temperature and DO reading: Measurements of temperature and DO levels taken vertically every 6” from the top of the lagoon to the bottom will indicate whether thermal or biological stratification exists. If there is stratification, there is likely short circuiting. Effluent warmer than the overall lagoon temperature is also an indicator of a short circuit.
- Effluent testing: A TSS:BOD5 ratio of 1—the same ratio as untreated wastewater—in the effluent indicates short circuiting. Normal treatment performance for lagoons is a TSS:BOD5 ratio of 1.5.
- Microscopic examination: Raw sewage and sludge particles in the effluent indicate a short circuit.
- Dye testing: If a short circuit is suspected, dye testing can confirm it and demonstrate where it is occurring. A nontoxic fluorometric tracer dye is introduced into the lagoon; twice-daily testing with a fluorometer tracks its progress, allowing the operator to calculate actual retention time and flow patterns.
How to prevent short-circuiting
At the facility described in the video, Steve determined that a misalignment of the aerators was causing short-circuiting and an accumulation of sludge, and recommended repositioning them to correct the problem. Here are some other ways to prevent short-circuiting:
- Improve lagoon design: In his Wastewater Lagoon Troubleshooting: An Operators Guide, Steve recommends running influent through a manifold to distribute the load more evenly. Influent and effluent pipes may need to be repositioned as well. The addition of baffles can prevent a short circuit by manipulating flow patterns to improve retention time and promote the settling of solids. Studies have shown that long, evenly spaced baffles 70% of the width of the lagoon provide the greatest efficiency; however, mathematical modeling may be required to determine the optimal size, number, and placement of baffles for a particular installation.
- Correct infiltration and inflow: Lagoon I&I needs to be corrected at the source, by sleuthing out and correcting breaches in the system. For more details, see Lagoon Infiltration and Inflow: Problems and Solutions.
- Deal with sludge: Excessive sludge buildup can be corrected with dredging and removal, bioaugmentation, or improved mixing and aeration. See Wastewater Sludge Treatment or Removal? for a closer look at options.
- Prevent stratification: Mixing and aeration throughout the water column prevent sludge buildup and stratification that can lead to short-circuiting.
Lagoon hydraulics: Small changes, big results
As lagoon expert Steve Harris notes, even small changes to correct short-circuiting can result in massive improvements in effluent quality. Learn how to diagnose and optimize your lagoon at a Lagoon Training Day with Steve Harris. He’ll teach you tips and tricks (like oranges!) that will help you to improve treatment in your current system, even without an infrastructure upgrade. He’ll be in Tulalip, Washington, on March 28; and Storm Lake and Cedar Falls, Iowa, on May 2–3, respectively. Register now!