Under what condition should check valves be used in dosing tanks?

Check valves are crucial in dosing tanks, especially in scenarios involving pressurized systems. They are designed to prevent backflow, ensuring that effluent does not return to the dosing tank after it has been pumped out to the drain field.

The specific condition where check valves should be used is when field lines are above the operating water level. In this instance, if the dosing tank were to lose pressure or if there was a disruption in operation, gravity could cause the liquid in the field lines to flow back into the tank. This backflow can dilute the effluent or possibly lead to system failures. Therefore, installing a check valve in this scenario is essential to maintain proper functionality and prevent contamination.

The other choices do not appropriately justify the necessity of a check valve. The size of the tank alone doesn't dictate the need for a check valve, nor does the method of feed unless it involves pressurization. Additionally, performing pumping of liquid waste does not inherently necessitate a check valve unless the backflow risk is present, which is primarily the case with above-level lines.