Which material is usually not recommended for disposal in a septic system?

Fats and oils are typically not recommended for disposal in a septic system because they can lead to significant issues within the system. When fats and oils enter the septic tank, they do not break down easily. Instead, they tend to float to the top and form a scum layer that can hinder the normal function of the tank, reducing its capacity and affecting the overall treatment process. This buildup can also lead to clogs in the outlet filter or the drain field, causing backups and potentially expensive repairs or system failures.

In contrast, other materials listed, such as dish soap, human waste, and fruit and vegetable peels, are generally more compatible with the biological processes in a septic system. Dish soap, when used in moderation, can be broken down by bacteria in the tank. Human waste is the primary purpose of septic systems, and fruit and vegetable peels, while they should be minimized, can typically decompose without drastically affecting the system's function.