Which of the following is NOT a probable cause for excessive sloughing in a rotating biological contactor?

Excessive sloughing in a rotating biological contactor is typically caused by factors that disrupt the stability and health of the biofilm. High dissolved oxygen levels are generally beneficial for the microorganisms involved in the treatment process, as they support aerobic digestion and enhance the efficiency of the system. In most cases, these elevated oxygen levels promote growth rather than lead to sloughing.

On the other hand, factors like excessive pH changes can create an unsuitable environment for microbial life, causing stress and sloughing. Likewise, significant fluctuations in flow and organic loading can lead to instability in the biofilm, resulting in detachment. The presence of toxic materials can harm the microorganisms, further contributing to sloughing. Hence, high dissolved oxygen levels are not typically associated with causing excessive sloughing, making it the correct answer.