Rachael,Sludge can also be settlement of particles in the water supply. Turbidity (suspended matter) will settle on the bottom of a tank while the water is still. Take a glass, fill it from the tap, pick up a small amount of dirt and shake the glass. You will see a cloudy mixture, let ti settle for a few minutes and sludge will form on the bottom of the glass. The same thing happens in a water tank. Add to what Bud said and you have the whole picture. Why is it so? Mossy: What is sludge made of?