The answer to this mixer selection question probably had at least a half dozen valid answers, depending on the available gear reducers and operating speeds.  While the basin size and depth are reasonable, the shallow level makes a single mixer very impractical.
 
A practical design requires four mixers placed in the center of each quadrant of the basin.  Each mixer would have 0.8m diameter four-blade, straight-blade impellers (radial flow).  The impellers would have 0.16m wide blades and be located about 0.3m off the bottom.  The mixers would operate at 30 rpm and require 0.375kW motors.
 
Any design for a single mixer has an extremely small impeller relative to the basin size and may not keep liquid in the corners of the basin moving.  A possible single mixer design would have a 1.2m diameter, straight-blade impeller operating at 25 rpm and requiring a 1.5kW motor.
 
No simple formula for mixer design is practical because impeller diameter and speed must be matched for the basin geometry.  However, the total mixer torque should be about 0.001 N m / liter to keep the water moving.

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