Septic crust is caused by the solidification of fats, oils, and grease due to an inactive septic system. The crust clogs the septic pipes and causes backups into your house. Septic crust blocks the flow of air into an aerobic system, leading to oxygen-starved aerobic bacteria changing into anaerobic bacteria, which is less efficient at digesting organic waste. Anaerobic bacteria produces a dirty effluent and can, as a result, lead to ground water contamination if the drain field consists of sandy soils. Foul odors will become prevalent. Septic crust can be broken down by using bacterial additives to digest the crust. Properly diluted Hydrogen Peroxide can also be used to break down septic crust. If this fails, the only solution is to pump out the septic system.