Cooling towers have a plastic fill that allows the water to have more surface contact with the air to cool the water by evaporation. This fill is usually in layers and has to be taken out to clean. The tower itself usually has about 1 to 2 foot of water in a basin and this is where you will also find mud sand and other debris the fans have picked up depending on the location of the tower. Most companies should change this plastic fill about every 5 years, and if you get to aggresive with a pressure washer you can tear it up. Hope this helps some, I work with cooling tower at my real job and maintain them on a day to day basis.
