Final answer:
Sludge buildup in cooling towers is mainly due to the accumulation of airborne dust and debris as well as bacterial growth, which contribute to system fouling and blockage.
Step-by-step explanation:
The cause of the problem of sludge buildup associated with cooling towers is primarily due to B) Airborne dust and debris and C) Bacterial growth. These factors contribute to the fouling and blockage within the systems. Airborne particles can become suspended in the water of the cooling tower and settle out to form sludge. Additionally, the warm, moist environment of a cooling tower is ideal for bacterial growth, which can increase the rate of sludge buildup. Hardness-forming salts like calcium and magnesium can also result in scale deposits when water temperatures increase, which can exacerbate the sludge problem.