Mixing anything in a horizontal, cylindrical vessel is difficult, especially when the length is more than two times the diameter.  The ends of the cylinder will see much less motion than the center, unless two, evenly spaced mixers are used.  Assuming that only one mixer can be installed in the center of the vessel, the objective would be to get all of the sludge to move, so a moderate degree of uniformity can be achieved.
 
Further a sludge with a 2.2 kg/liter density will almost certainly have considerable apparent viscosity.  If the fluid has additional non-Newtonian characteristics, such as a yield point, effective mixing may be impossible.  Assuming that the sludge can be mixed, using reasonable assumptions about physical properties and available mixing equipment, the following should be a reasonable recommendation.
 
The mixer should be equipped with a single four-blade, pitched-blade impeller, 250 mm in diameter.  That is an impeller with four flat-plate blades, 50 mm wide and mounted at a 45 degree angle.  The mixer should rotate at 300 rpm and would require a 1.1 KW motor.  If two mixers were installed in the vessel, then each mixer should have a 200 mm diameter impeller with a 0.5 KW motor.

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