Which of the following is NOT a type of sewer system?

The correct choice indicates a system that does not typically exist within the framework of wastewater management. A combined system, storm system, and domestic (sanitary) system are established categories within wastewater infrastructure.

The combined system is designed to handle both wastewater and stormwater runoff within the same set of pipes, which can be efficient but also poses challenges during heavy rain events. The storm system, specifically, is dedicated to handling rainwater and excess runoff to prevent flooding and water quality issues. The domestic or sanitary system is focused on the collection and treatment of sewage from households and businesses, ensuring proper disposal and treatment of human waste.

In contrast, the term "recreational system" does not align with recognized classifications of sewer systems. Recreation typically refers to leisure activities and does not pertain to the management or conveyance of wastewater. Therefore, this choice does not fit within the established categories of sewer systems that are recognized in wastewater treatment and management practices.