Anoxic Mixers

Anoxic Mixers are used in de-nitrification basins in waste water treatment plants. The process involves the de-nitrification of waste streams through the use of bacteria which breaks down the nitrate in the waste to use as an oxygen source (energy source).  This breakdown of nitrate from the waste stream releases oxygen and nitrogen gas.  The oxygen is consumed by the bacteria and the nitrogen gas releases to the atmosphere.  The waste stream then has acceptable nitrogen levels so the water can be discharged into the environment (streams, ponds, lakes, etc.).   

The basin is mixed but is not aerated (no oxygen is introduced into the basin) since the goal is for the bacteria to break down the nitrate which then provides oxygen to the bugs to live rather than use dissolved oxygen in the waste stream itself.  If dissolved oxygen is present in the waste stream, the bacteria will consume that oxygen first since it is readily available and provides more energy to the bacteria.  This results in less nitrate broken down and more nitrogen in the effluent from the plant which may be above set limits.

Anoxic Mixers are designed to:

  • Suspend solids which include waste sludge and provide complete basin control
  • Promote bacterial breakdown of nitrate so the nitrogen can escape
  • Eliminate air (i.e. oxygen) incorporation from the liquid surface (no swirl or vortex)

 

Typical Design Parameters for Anoxic Mixers

Classification

Blending and Motion

ChemScale

2-3

Impellers

 RL-3 Ragless Impeller

 XE-3 High Efficiency Impeller


The Anoxic basin may contain fibrous material which was not screened out of the waste stream or passed the grinders in the front end of the waste treatment plant.  This fibrous material has been found to agglomerate on hydrofoil (i.e. high efficiency) impellers which are typically used in these flow controlled applications.  The replacement of the hydrofoil impeller with the RL-3 Ragless Impeller will shed the fibrous material and prevent build-up, and therefore mechanical issues with the mixer. 

When the fibrous material builds up on hydrofoil impellers, the process flow pattern becomes more radial rather than axial in nature and a typical result is a swirl or vortex on the top surface.  This incorporates oxygen into the basin which the bacteria uses first and less of the nitrate is used as an oxygen source.  The use of the RL-3 will maintain the strong axial flow pattern and performance of the agitator which eliminates the swirl/vortex on the top surface, and hence the incorporation of oxygen into the basin.  This allows the bacteria to use nitrate as the primary source of energy and keeps the nitrogen levels in the effluent within permit levels.

 

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