When a toilet is flushed, liquids and solids leave the bowl and travel by gravity down the waste line which connects the toilet to the septic tank.
Separation into a liquid covered by a crust of solid matter occurs with a natural decomposition process continually taking place in a healthy septic tank. With enough use, liquids fill the tank to the level of the outlet.
Nutrient rich liquid spills through this outlet and travels again by gravity to distribution pits and slotted long effluent lines designed to naturally purify, distribute and disperse moisture through soil absorption, plant growth and evaporation.
Solids remain in the tank and build up a thick crust over time which ceases to function because of this blockage, it needs pumping out. Cleaning chemical and blue dye disinfectants placed in septic tanks via the toilet bowl interfere with the simple decomposition process by killing the useful bacteria.