A regulator gate usually closes when the sewage reaches a predetermined:

The correct answer is based on the function of a regulator gate within sewage treatment systems. Regulator gates are devices used to control the flow of wastewater in various parts of a treatment facility, specifically to manage the levels of sewage in channels or basins.

When referencing a predetermined elevation, it highlights the physical level of sewage flow within the system. When the liquid reaches this specific height, it indicates the need to close the gate to prevent overflow, ensure proper treatment, and maintain system integrity. Regulating flow based on elevation helps manage hydraulic conditions and can protect downstream processes from overload or contamination.

The other options, while relevant to various aspects of sewage treatment, do not directly correlate with the operational principle of a regulator gate. Velocity relates to how fast sewage moves, pressure pertains to the force within the system, and temperature addresses the thermal conditions of the sewage, none of which directly dictate the opening or closing of a regulator gate as a function of flow regulation.