Only fairly clear water should exist between the scum and sludge layers. This water fills the second chamber of the tank where it becomes even more clear. It is this solids overflow which clogs soil pores and causes septic systems to fail. Bacteria must be present in the septic tank to digest the organic solids. Normal household waste provides enough bacteria to digest the solids, unless any harm is done to the bacteria.
Bacteria are very sensitive to environmental changes.
Many home-care products used in most homes today will destroy bacteria. Home Reference Book by Carson Dunlop 1999-2000. Optimized for Netscape 4, Explorer 4 or better at 800x600 resolution.