The addition of excess grease or oil from food preparation processes is not helpful to your septic system. When you wash or rinse cooking oils or grease in hot water at the sink, it starts to congeal on its way down the sewer line to the septic tank, and can form a hard, caked clog at the inlet tee or baffle in the tank. This can restrict the flow into the tank and result in a blockage and backup into the house. The grease can be put into suspension again when a hot load of laundry water and detergent enter the tank, and the grease will travel through the tank and congeal again.
Grease also causes a problem if there is a depression in the sewer line between the house and tank that allows water to collect. Grease and other solids will sit in the depressed portion of the pipe, and will cool and harden in place, eventually causing a backup into the house.
The problem is compounded if you have any filters in your system – either at the outlet of the 1st tank or a screened vault or Biotube filter at the lift station pump. These filters screen solids 1/8” and larger, and grease or oil in suspension adheres to the screen surface, reducing the screen size and resulting in premature plugging.
The secret with grease and oils is to keep a 1 lb. coffee can in your refrigerator, and pour all cooking grease and oil into the can after cooking and before washing. The contents of the can will not produce an odor while refrigerated. Once the can is full, put the plastic lid on and throw the can away in the trash.