How Does a Siphon Dose a Drainfield?

All your siphon questions answered. This is your guide to understanding how siphons work for drainfield dosing.

How Does a Siphon Dose a Drainfield?

Just as in applications requiring the use of pumps, it is important that there be a way to determine whether the flow delivered from the siphon is within the design specifications, so using digital cycle counters to establish the flow is a good practice.

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An increasing number of systems are installed with siphons, so it’s important to understand how they work and what problems you might encounter.

For those of you familiar with Consortium of Institutes for Decentralized Wastewater education materials, there are descriptions of how siphons operate in both the operation and maintenance service provider program manual and the installation of wastewater treatment system manual. These can be helpful references. 

A siphon delivers a set volume dose of effluent. The volume of effluent dosed depends on the size of the siphon bell and the height of the siphon trap. So, it is important that the siphon used be appropriate for the application intended. Any manufactured or custom-built siphon will not do; it needs to be matched with the necessary volume and delivery rates required for the application. 

In addition, the device will not function if the system component that effluent is being delivered to is not at a lower elevation of several feet. This should be common sense; just think of the last time you tried to siphon something out of a container — the liquid wouldn’t transfer until the elevation requirement was met.

How does it work?

During a siphon cycle, the siphon traps must be filled with water. When liquid rises above the open end of a pipe called a snifter or vent pipe, air is sealed in the bell and long leg of the siphon. As the fluid in the tank rises, the pressure on the confined air increases and forces water out of the long leg of the trap. When the air pressure is great enough to force all the water out of the long leg, trapped air escapes through the short leg to the air release vent pipe. At this point, the siphon has been “tripped” and fluid is discharged from the siphon until the liquid level in the tank drops to the bottom of the bell. Air is then drawn under the bell, which “breaks” the siphoning action, and the process begins again. 

Just as in applications requiring the use of pumps; it is important that there be a way to determine whether the flow delivered from the siphon is within the design specifications, so using digital cycle counters to establish the flow is a good practice. 

Likewise, having a high-water alarm to let the homeowner know that the siphon is not functioning is also good practice and is often required by state and local codes. The high-water alarm, however, will not identify one of the common problems with siphons when they continuously trickle rather than discharge the required dose. This is where looking at the cycle counter and inspecting the device is necessary. 



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