Sewer design terminology — what is an inverted siphon? Which of the following best describes an inverted siphon in wastewater conveyance systems?

Introduction / Context:
Sewers are ordinarily designed to flow by gravity following the hydraulic grade line (HGL). Where natural or man-made obstacles exist (rivers, utilities, road/rail embankments), the sewer may need to dip below the HGL and then rise again, forming what is commonly called an inverted siphon or depressed sewer.

Given Data / Assumptions:

• Gravity conveyance is preferred; pumps are avoided if possible.
• Continuous flow must be maintained without clogging.
• Air release/venting and flushing velocities are considered in design.

Concept / Approach:
An inverted siphon is not a true siphon; it operates under pressure flow because the sewer crown is below the HGL. It usually consists of one or more parallel barrels, designed to maintain scouring velocities over a wide flow range. Entry and exit structures allow flow splitting and recombination, and air valves or vents prevent air locking.

Step-by-Step Solution:
Identify the need: crossing under an obstacle while maintaining conveyance.Describe configuration: depressed sewer segment below HGL with suitable inlet/outlet structures.Confirm that normal sewer ventilation and maintenance concerns are addressed by design features.

Verification / Alternative check:
Standard wastewater engineering texts depict inverted siphons as depressed pressure-flow sections used to pass beneath obstructions without pumps.

Why Other Options Are Wrong:
Septic distribution devices and overflow preventers relate to on-site systems and water tanks, not sewers.
Crown corrosion mitigation involves ventilation and material choices, not a siphon.
Vacuum pumps are different equipment altogether.

Common Pitfalls:

• Assuming it functions by siphonic action (it does not; it is pressure flow).
• Undersizing barrels, leading to sediment deposition and maintenance issues.

Final Answer:
A section of sewer dropped below the hydraulic grade line to pass under an obstacle