Which factor is NOT controlled in a dewatering centrifuge?

The correct choice highlights the element that is not controlled in a dewatering centrifuge, which is the type of sludge being processed. In the context of a dewatering centrifuge, operators often have limited control over the characteristics of the sludge being treated, as it can vary based on the source and composition of wastewater.

On the other hand, factors such as bowl speed, pool depth, and polymer addition are all adjustable parameters within the centrifuge system that can be manipulated to optimize the dewatering process. Bowl speed influences the centrifugal force applied to the sludge, which affects how effectively the liquid is separated from the solids. Pool depth is the depth of the liquid in the bowl, which can affect how well solids are captured and removed. Polymer addition is a critical factor in enhancing the flocculation of solids, thereby improving the overall efficiency of the dewatering process. Thus, the type of sludge remains a fixed characteristic that is inherent to the wastewater being treated, making it the aspect that is not actively controlled during the operation of a dewatering centrifuge.