You need to have an understanding of the biosolids disposal method that you will be using. Assuming that you have a land disposal of type B biosolids, you may use a dewatering press or centrifuge. Many operators are going with centrifuges because the presses are mechanical nightmares.
Auto Thermal Anaerobic Digestion" is used to produce a type A sludge. A type A sludge allows more or less unlimited biosolids disposal, while a type B biosolids disposal is limited to farm land disposal.
"Auto Thermal Anaerobic Digestion" is an expensive process that most do not use.
It is necessary to have a full understanding of your treatment process (BNR, tertiary, etc.), the nature of your wastewater, and the biosolids disposal options to develop a recommendation.
Equipment capacities are obtained from the manufacturers. You have to estimate the hours of operation. A small plant like this would probably operate the sludge system approximately 16 hours per week. There is a tradeoff between operating hours and press capacity.
Some small plants do not have sludge dewatering equipment, and rely on mobile dewatering operated by conract or sludge hauling.