I think that there is no ...

I think that there is no "optimal" settings from the point of view of drying process. In any case you need to use a given amount of energy (heat) to evaporate 1 kg of water in the raw sludge. I can calculate for you the minimum quantity of energy to dry cold sludge with temp. T1 and initial x% of water to final hot sludge with temperature T2 and final water content y%. I say minimum heat quantity because in real device and in real process you loose some of it and the use of the heat (heat use efficiency) is less than 100%. The optimal settings you ask about can come only from the specific dryer construction (e.g. construction material etc.) and the highest temperature of the drying medium (air in fluidized bed dryer) should be stated to achieve the dry sludge of expected thermal degradation level. In the extreme case you could use the air heated to 500 C, dry and burn partially the sludge inside the dryer.