An excess of mineral deposits ...

An excess of mineral deposits in the water that flows through your system can cause black water to emerge from cooling tower blowdown. Magnesium, manganese, and iron are common minerals that build up in your water system over time. When these minerals oxidize—i.e. mix with oxygen and turn black, which in turn causes the water that flows out of your CT to appear black as well. All you need is a water filtration system that removes excess minerals from your water before it recirculate back to system.