Lagoon Diffuser Mixing Performance: CFD Study of the MARS

lagoon diffuserWe know good data leads to good decisions, so we’re always looking for ways to improve our data. Recently, we commissioned a CFD (Computational Fluid Dynamics) study of our MARS lagoon diffuser to refine our data about its performance.

A previous article, CFD Modeling Lagoon Aeration: A Peek Inside the Black Box, talked about how these sophisticated flow simulations can be an important tool in cracking the black box that is wastewater lagoon treatment. In this followup, we’ll take a close look at the results of the CFD study of our MARS lagoon diffuser—complete with cool animations—and highlight what it reveals about its performance.

The MARS Lagoon Diffuser

lagoon diffuserOur MARS Lagoon Diffuser is used in municipal and industrial applications to maximize biological treatment. The central coarse bubble static tube forcefully releases large bubbles that create a draft, pulling water and organic matter up and through the unit. Meanwhile, fine bubble diffusers release high-surface-area small bubbles to efficiently transfer oxygen.

The MARS is available in several models, varying in number and length of fine bubble diffuser tubes and the ratio of coarse to fine bubbles.

MARS CFD Test

The CFD test was performed on our most popular model, the MARS 750T Aerator, which has a central static tube surrounded by ten EPDM fine bubble diffuser membranes. Airflow was provided by a compressor; the air ratio was 10% coarse bubble to 90% fine bubble. The chart below shows the simulation parameters.

lagoon diffuser

This initial simulation (we plan to do more) was designed to answer the following questions:

  1. How well does the MARS pull water through the central static tube? How strong is the lift force?
  2. How does water behave around the entirety of the unit? How wide is the area of influence?
  3. How accurate were our original calculations in reflecting MARS’ mixing capability?
  4. Are there any areas of the MARS unit that can be tweaked to improve performance?

Following are some of the results of the MARS CFD study.

Horizontal Velocity of Water

lagoon diffuser

This first illustration demonstrates the horizontal pull on the water, with colors trending toward red in the areas of greatest velocity. The CFD study determined that the MARS draws in water from as far as nine meters away. The distribution of intensity shows the MARS is performing as designed: pulling water and organic material from a long distance but gently enough to not disrupt the bottom. The greater force beneath the unit keeps the diffuser membranes clear so sludge cannot accumulate and foul them.

Velocity Vectors: Center

In this central cutplane side view, arrows indicate the direction of the eddies and currents that MARS creates and its area of influence. This simulation is modeling a single MARS, so the swirled areas of greater intensity near the sides of the diagram are being created by just one unit. What’s notable in this illustration is the force with which the water is punching through the surface (shown in red). In real life, that surface churning is apparent in the following case study video.

MARS Lagoon Diffuser’s Effect on the Surface

This animation of the interrelation between the coarse bubble static tube and fine bubble diffusers illustrates the force the MARS exerts on the surface of the lagoon. Before the first coarse bubbles burst from the static tube, the surface already reflects the disturbance. Watch the surface as the coarse bubbles rise: It’s already roiling with turbulence. The fine bubbles rise much more slowly to maximize oxygen transfer.

Water Surface Animation

The water surface animation shows the effect of the MARS on the surface of the water, with colors trending toward yellow showing the strongest impact.

MARS Lagoon Diffuser Flow Rate Calculation

lagoon diffuser

Our determination that the MARS lagoon aerator moves 7,000 gallons a minute was derived from a dynamic wet pumpage test. The CFD modeling, which placed the MARS in the middle of a 1.5 m diameter sphere, concluded that each MARS actually moves 7,842.74 gallons a minute—significantly better. This level of mixing performance keeps solids in suspension to keep them from settling as sludge and ensures that the BOD-consuming microorganisms stay in contact with dissolved and suspended organic manager and the oxygen they need.

What’s Next

Now that we’ve validated the MARS Aerator’s mixing performance and are able to see exactly how water behaves around it, we’ll be further refining our data with additional CFD studies.

For a crash course on why mixing is critical to wastewater lagoon performance, watch our YouTube video, Lagoon Aeration 101. For more on the MARS lagoon diffuser, check out our MARS technical page, or download the brochure.

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