Which component is essential in defining a septic tank system?

The essential component in defining a septic tank system is a septic tank and a subsurface disposal field. A septic tank serves as the primary treatment unit where solid waste settles at the bottom, allowing liquid effluent to flow into the subsurface disposal field, also known as a drain field. This combination is crucial because the septic tank processes the waste, while the subsurface disposal field facilitates the final treatment and dispersal of the effluent into the soil, allowing it to be naturally filtered and treated.

The other options involve components of wastewater management systems but do not collectively represent a standard septic tank system. For instance, a holding tank and treatment plant are not typical components of a conventional septic tank system; instead, they are generally utilized in more advanced or alternative wastewater treatment systems. A drainage field and inspection port, while relevant, do not include the septic tank itself, which is a fundamental aspect of the system. Likewise, a settlement pond and a pump pertain more to other types of treatment methods and may not necessarily align with standard septic systems used for residential properties.