Mixing Limited Tanks

In aerobic tanks, the aeration equipment is designed to both deliver oxygen and mix the contents of the basin. When more air is required to mix the basin contents than is needed to satisfy the oxygen demand, the conditions are referred to as mixing limited. Decoupling oxygen demand from mixing requirements provides the operational flexibility to reduce the airflow rate to meet the oxygen required for plant loading.

A BioMix Compressed Gas Mixing System can be interlaced with diffused aeration grids, allowing simultaneous operation and eliminating over-aeration — an outcome no mechanical mixing device can provide. This results in BNR process control benefits and significant energy savings.

application sheet

Causes & Solutions

What causes mixing limited conditions?
  • Underloading compared to design conditions
  • Daily or seasonal load fluctuations
  • Hydraulic limitations that prevent taking trains out of service
  • Oxygen demand that is diminished by upstream aerobic zones
How does excessive aeration impact a treatment plant?
  • Residual dissolved oxygen in RAS and mixed liquor return that inhibit BNR
  • Poor settling
  • Excess treatment chemicals in post-anoxic zones
  • Wasted energy
Why is BioMix the right solution?

The requirement for the aeration system to provide the air necessary to meet the process oxygen demand and the mixing air demand can be eliminated by decoupling oxygen demand from mixing requirements, thus providing the operational flexibility to reduce the airflow rate to meet the air required for just the plant loading. A BioMix Compressed Gas Mixing System can be interlaced with diffused aeration grids to provide mixing, allowing simultaneous operation with the aeration system at low aeration rates or independent operation. This results in:

  • Promotion of anoxic and anaerobic conditions to enable BNR
  • Elimination of high dissolved oxygen in IMLR
  • Conditions that promote good settling in secondary clarifiers
  • Stable dissolved oxygen profile throughout the aeration tank
  • Significant energy savings