What does it mean to alter a septic system?

To alter a septic system means to make a change to the on-site sewage facility. This encompasses a broad range of modifications, which can include various actions such as altering the layout, changing the components, or modifying the treatment process of the system. The intent behind such alterations could be to improve efficiency, adapt to changes in regulations, or respond to different usage patterns.

In the context of the other choices, while repairing the system, upgrading its components, or expanding it do involve modifications, they each fall under specific categories of alteration. The broad definition of "alter" encompasses all these actions, making it the most suitable choice for understanding the full scope of changes that can be made to a septic system.