How will I know if my tank is full?

The question should be “How do I know how much sludge is in my tank?” The liquid level in your tank should always be at its normal level, which is at the outlet pipe (unless you are having a backup from the absorption area). So, lets say you have a 1000-gallon tank. The tank will be filled to 1000-gallons, at the level of the outlet pipe. When your tank is cleaned it will be empty. Once you use 1000-gallons (or however many gallons your tank holds to the outlet pipe), the tank will be filled to its normal level. The amount of sludge in the tank then determines when the tank is pumped, not when it is “full.”