Pouring cooking oil down the drain causes "fatbergs" to form. As the grease cools, it solidifies, narrowing the pipes and trapping other debris. Chemical Overload:
A septic service will use a high-pressure water jetter (up to 4000 PSI). They insert a hose through the cleanout or the tank outlet and blast the clog. This is highly effective for grease and root intrusion. Never use a standard electric drain snake (auger) on a septic outlet—it can break the old clay or concrete baffles. clogged septic tank