When can siphons be designed in relation to a sand filter and siphon chamber?

The correct answer is grounded in the principle of how siphons operate. Siphons rely on the difference in elevation to function effectively. When there is elevation between the siphon chamber and the sand filter, gravity helps create the necessary pressure differential that allows the siphon to draw fluid from the sand filter. This elevation change is critical for maintaining consistent flow and ensuring that wastewater can be moved through the system without requiring additional mechanical pumping.

In systems where siphons do not have this elevation difference, they may fail to operate correctly. Without sufficient gravity assist, the siphon can struggle to initiate or maintain flow, leading to potential backups or failures in the septic system. Hence, designing siphons with an elevation difference is essential for their successful implementation and operation, particularly in the context of septic systems that use sand filters and siphon chambers.

The other scenarios presented do not provide the conditions necessary for proper siphon function, emphasizing the importance of elevation in these systems.