If you need to keep the ...

If you need to keep the sludge from going septic then using coarse bubble diffusers is a good way to go. The only disadvantage would be if the sludge is going from retention to anaerobic digestion, where the residual DO may be a slight problem.  Another option would be to use a pump and recirculate the sludge - I think Red Valve markets systems like that. Pumping is less energy intensive than aeration.