Which of the following is NOT permitted to drain into a septic system?

A septic system is designed to treat and dispose of household wastewater through a natural process involving soil, bacteria, and other organisms. However, not all substances are suitable for drainage into a septic system, as some can disrupt its function or lead to contamination.

Automotive grease is a type of waste that is not permitted to drain into a septic system. It is harmful because it can clog the system, reduce the effectiveness of the biological treatment processes, and contribute to the accumulation of solids in the tank. This can lead to system failures, such as backups or the need for more frequent pumping.

In contrast, backwash from a water softener, cooling water, and condensate waste from a refrigeration system may have more limited or conditional acceptability, depending on local regulations and the specific characteristics of the substances. For instance, while backwash can contribute to increased sodium levels in the system, it is generally less harmful than automotive grease. Thus, it's clear that automotive grease significantly affects the operation of a septic system, making it the correct answer to identify as not permitted.