As part of recent reader-submitted questions on dosing siphons, there was an inquiry about the squirt height in small-diameter pipe. There are a number of factors that determine what that height will be, so there’s not a simple — or single — answer. The best way to approach this is to look at potential siphon applications to see what is necessary.

The size of a siphon is determined by the diameter of its trap. The size will vary according to the manufacturer and the desired application. Typical sizes are 2 to 3 inches for onsite applications (although they can be larger). 

Along with size, siphons are set up to drawdown a set number of inches out of the tank relative to the desired dose volume. This is why when a siphon is replaced it must be replaced with one that meets the desired design criteria for the intended application. Replacing it with the first — or cheapest — siphon a service provider can find may not provide the right dose. 

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For pressure applications it is important to know the discharge rate through the piping. Again this is supplied by the manufacturer and is often provided as a maximum, minimum or average flow rate. The flow rate needs to match the piping and orifice requirements of the supply piping and the pressure distribution laterals. In order to replace a siphon for a pressure application, you must obtain this information from the system design. 

A typical flow rate for a 3-inch siphon would be 30 gpm. So, if the flow rate to fill the piping and match the discharge at the orifice needs to be greater, a larger siphon would have to be used.

Siphons can discharge to both gravity and pressure drainfields. Discharge to gravity can be done in two ways. Siphons can discharge to a distribution box or the first drop box in a sequence. From there the effluent is distributed to the trenches just as in a conventional system. The only criteria here is that delivery be less than 45 gpm to avoid any problems with filling the boxes too quickly. 

The second way siphons can discharge is through use of a hydrosplitter, which, depending on orifice size in the splitter, can deliver equal or unequal amounts of effluent to the individual trenches. The key here is that effluent back up to a height above the hydrosplitter that will provide the pressure necessary to operate the hydrosplitter. This means location of the hydrosplitter relative to the siphon is very important. 

Sizing siphons for pressurized below- or above-ground systems is similar to the use of a hydrosplitter. The pressure at the orifices (the squirt height) is provided by the elevation of the backfilled supply pipe while providing the necessary flow rate for the orifice discharge. This requires accounting for friction loss in the pipe as well as the elevation difference —similar to a pump — so for a typical residential mound the siphon would have to provide at least 1 foot of head at the pressure manifold (more would be desirable). 

Again, the siphon, piping and distribution system need to be part of an integrated design so any replacement would have to provide the same discharge rates and create a similar backup in the supply pipe for the system to function properly. 

While siphons have no moving parts that wear out and can be less expensive to install than a pump, you still need to consider these factors when properly sizing a siphon just as you would with a pump. 

For a complete list of alarms, controls and monitoring devices, visit www.onsiteinstaller.com/editorial/2014/07/alarms_controls_and_monitor_systems4.

About the Author

Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

This article is part of a series on dosing siphons:

• How the Heck Does a Siphon Dose a Drainfield?
• Tell Me More About These Dosing Siphons
• On Tap: Dosing Siphon Troubleshooting