Which systems routinely require a pump tank?

The correct answer highlights the systems that typically necessitate a pump tank in their design. In the context of on-site sewage facilities, certain systems need a pump tank due to their operational characteristics and site requirements.

Surface and drip irrigation systems often involve situations where effluent needs to be distributed over a specific area or elevated landscape, which means that gravity alone cannot handle the movement of wastewater from the treatment unit to the discharge field. A pump tank is utilized to ensure that effluent can be efficiently pumped to the irrigation system, allowing for proper application rates and coverage.

Similarly, sand filter systems also may require a pump tank to maintain appropriate hydraulic head for effluent distribution and to manage the effluent application rates. This is particularly crucial when the sand filter is located at or above the level of the treatment unit or in situations where the elevation prevents gravity flow to the disposal area.

In contrast, the other options include systems that either naturally utilize gravity flow, as seen in standard absorptive drainfields and gravelless pipe systems, or do not typically require the boosting capabilities of a pump tank. Therefore, the necessity of a pump tank arises specifically in the context of ensuring effective distribution and management of effluent for surface and drip irrigation and sand filters.