The tank is designed to be a sludge trap. It simply keeps the solid stuff that you put into it from your toilets, sinks, showers, and tubs from getting into the leach field. It does this by holding it back the solids for a period while letting the liquid pass through. There are baffles in the tank that slow down the movement of solids. Usually, the solids drop to the bottom of the tank, except for the materials that are lighter than water, which float to the top. In tanks manufactured since 1961, there are two compartments. These compartments ensure that what doesn’t get trapped in the first compartment will be held back in the second one. Over time, bacteria work on the solids to break them down into tiny pieces. The liquid containing the tiny pieces of solids is called effluent. Effluent leaves the tank and must be disposed of in a leaching device. Effluent is more than 99% water.