A limited amount of solids suspension data is available for square tanks because the corners are likely to accumulate some settled solids. However, based on available design information, suspension of 3 wt% solids 50 microns in diameter, with a specific gravity of 2.0, in a 15 meter by 15 meter square tank with a 5 meter liquid level is practical.
 
For off-bottom suspension, a 2.5-meter diameter, 4-blade, 45-degree pitched-blade turbine with 0.5 meter wide blades needs to operate at 25 rpm (0.42 rps). That impeller and operating speed will require at least a 15 KW motor.
 
If all you want is to move the solids around on the bottom (on-bottom motion), the speed could be reduced to 20 rpm (0.33 rps), with a 7 KW motor. However, with on-bottom motion you would expect some accumulation of solids, especially in the corners, or if they stick together.
 
Certainly, higher levels of mixing intensity would be required for larger or denser particles. Some accumulation of solids may occur in the bottom corners, even with the off-bottom suspension mixer recommendation. Because the application is described as an effluent vessel, minimal mixing recommendations are provided. More intense mixing would be required for any significant processing of the solids.