Siphon,Vented loops and Anti-siphone valves In Discharge Lines Of OWS and Sewage

What is Siphon ?
     Siphon is an inverted 'U' shape Pipe, which can causes a liquid to flow upward above the surface of a reservoir, with out pump. Because of gravity, taller column of water pulling down causes less pressure at the top of a siphon.
The mass of water entering the tube and flowing upward is equal to the mass of water flowing downwards and leaving the tube.The pull on the surface of liquids from a higher to lower altitude is known as siphoning. If you want to stop siphoning made a hole at the top of siphon loop, water column breaks and it stops flowing.
       

Vented Loops or Anti-Siphon Valve

           Vented loop or Anti-Siphon Valves are inverted U-shaped pipes (siphon) with a vent at the top. This helps to let air into the pipes when the system is not working. When the air comes into the pipes the water level drops away from the loop on both side, thus preventing a siphon from occurring. Vented loops are installed above the water line at any heel shown in fig.

Why Vented loops or Anti-siphon in Oily Water Separator and Sewage lines ?

            Ows and sewage discharge is below water line. If there is no vented loop, when OWS pump is off, the water running back into the bilge initiates a self-sustaining siphon. Which may leads to flooding of OWS with sea water. Here the vented loop comes in action, when the pumping is stopped, air is let into the pipes and the water level drops away from the loop on both side, thus preventing a siphon from occurring. You can simply say that it prevent back flow of water into the system

Image Credits: https://en.wikipedia.org/wiki/Siphon