When sewage enters your septic tank, it settles into 3 layers.
The Scum floats on top.
The Effluent or grey water goes through the middle.
The Sludge sinks to the bottom.
Eventually the scum and sludge will reach levels that cause your septic tank to operate incorrectly, this is why regular Maintenance is required , to check the levels and Pump out the Scum and Sludge when needed.