Lagoon treatment has been referred to as a “black box” because the specific mechanisms of treatment are hidden below the surface. Computational fluid dynamics (CFD) modeling provides a peek inside this black box, demonstrating how various elements of a facility (such as depth, baffles, influent and effluent structures, aeration and mixing) work together to either help or hinder wastewater treatment.
In this article, we’ll discuss CFD as a tool to optimize lagoon systems, why we commissioned CFD studies of our MARS Aerator, and what the results say about the MARS’ mixing capability.
What is Computational Fluid Dynamics (CFD)?
CFD is a science that combines computational software, physics, and math to visualize how a fluid or gas behaves in a certain circumstance. In wastewater treatment, CFD has long been used to solve hydraulics issues. Now that the technology has become more common and less expensive, CFD is being used to model more aspects of wastewater treatment, including nutrient removal and anaerobic digestion.
Recent studies about CFD in lagoon systems include a comparison of CFD’s accuracy to scale model tracer data; how CFD modeling can be used predict the rate and distribution of sludge accumulation; and how the modeling of individual aerators (surface aerators in this study) can be extrapolated to determine the overall hydraulic performance of a lagoon.
We commissioned a CFD study of the MARS Aerator with a few goals in mind:
- Verify our existing data about the mixing capability of the MARS unit
- Determine which minor tweaks to the design of the MARS can improve its performance, instead of using tedious trial-and-error approach
- Map the area of influence to develop a logic for aerator placement
- Compile data that will allow us to calculate precise performance in a particular facility
CFD Modeling Lagoon Aeration: The MARS Mission
Triplepoint’s MARS Aerator combines mixing and aeration in a portable unit. Coarse bubbles are released at the bottom of the central static tube, creating a draft that pulls water and organic matter up from the basin floor and through the unit, creating a highly agitated water column that mixes and circulates the entire lagoon.
Diffusers surrounding the central static tube release high surface area fine bubbles to maximize oxygen transfer efficiency while minimizing energy consumption.
This Double Bubble technology, combining the robust mixing of coarse bubbles and the superior oxygenation of fine bubbles in a single unit, creates the ideal environment for biological treatment.
Based on independent and internal testing along with hundreds of successful installations, we know the MARS Aerator works, but in order to improve it more expediently, we decided to use CFD modeling to rapidly test potential design modifications. Moreover, we aim to use CFD modeling to help us determine optimum unit placement and spacing to accomplish each lagoon’s treatment objectives.
The CFD study of our MARS Aeration diffuser was designed to answer the following questions:
- How well does the MARS pull water through the central static tube? How strong is the lift force?
- How does water behave around the entirety of the unit? How wide is the area of influence?
- How accurate were our original calculations in reflecting MARS’ mixing capability?
The Original MARS Test
We already had a pretty good idea of the MARS Aerator’s mixing capabilities from previous testing we’d performed. The CFD study would give us a chance to compare our original results against a sophisticated computer model.
In 2011, we performed a dynamic wet pumpage test of our aerator, which has ten fine bubble diffusers surrounding the coarse bubble static tube.
The MARS was placed at the bottom of a 12 foot deep clean water basin; air was supplied by a compressor. Using a portable flow meter, 21 velocity measurements were taken from points across the unit and between and below the diffusers. (This sounds very elegant, but it was actually one of our guys, in a pool, holding his breath while he took all these measurements.)
The final calculations of the dynamic wet pumpage test demonstrated that each MARS moved about 7,000 gallons per minute—robust mixing sufficient to keep lagoon solids in suspension and prevent settling.
MARS CFD Test
The CFD test was performed on a MARS 750T Aerator, which has a central static tube surrounded by ten EPDM fine bubble diffuser membranes. Airflow was provided by a compressor. The chart below shows the simulation parameters.
The simulations were designed to provide visualization of the MARS’ performance: the velocity of water vertically and horizontally; flow vectors around the unit and across the bottom of the basin; how the coarse and fine bubble regions interact; and the effect on the water’s surface. The diagram below, a cutplane (a 2 dimensional slice of a 3 dimensional model) of the center of the MARS unit, illustrates the vorticity, or mixing force, of the coarse bubble tube.
The final calculations of the dynamic wet pumpage test had demonstrated that each MARS moved about 7,000 gallons per minute. The results of the CFD modeling demonstrated that each MARS moves over 7,800 gallons per minute, significantly better than what our GIP (guy in a pool) study showed.
In an upcoming article, we’ll look at the results of the study in more detail and share the animations that show the effect MARS has on the water column.