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Siphon Prevention In Waste Water System

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Posted 08 October 2019 - 11:11 AM



I'm designing a waste water treatment system, as part of it we'll be gravity draining water from the bottom of a tank, to a pit that's several feet below it.  The line that goes from the bottom of the tank to the pit exits subsurface (below water) in the pit, and will have a few 3/8" anti-siphon holes drilled just above the water level in the pit.  The line also has a high point / trap that goes above the bottom of the tank to keep a level in the tank at all times.


I've attached a file that shows a rough drawing of the system.


We start out with ΔH1 -- the level of the water in the tank that's above the high point in the drain piping.  I would think worst case that level in the tank would drop down until ΔH1=0 and no further.  Worst case if there's some type of vacuum in the drian line, there are anti-siphon holes down towards the pit that would take in air and break the vacuum. (ΔH1=5 ft at start on page 1, ΔH1=0 ft final on page 2)


One of my colleagues is telling me that that this high point / trap design, and the anti-siphon holes won't prevent the tank from draining and that we need an air-intake to break vacuum in the line and prevent the tank from being sucked out to the pit. (this design shown on page 2)


I don't believe this is the case, but I don't know how to prove it.


I would prefer not to install this air-intake piping as worst case it would be a leak point if we get excessive pressures in the tank which is a closed vessel and worst case could run liquid full.


Can anyone help me wrap my head around this, or advise whether the existing design of trap piping plus anti-siphon holes will prevent vacuum and complete draining of the tank?

Attached Files

Edited by CH3CH2OH, 08 October 2019 - 11:39 AM.

#2 Pilesar


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Posted 08 October 2019 - 12:31 PM

Consider that the anti-siphon holes are below the source tank. If you were to cut off the drain line at the level of the anti-siphon holes, could a siphon still occur? If so, then the holes will not be adequate.



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Posted 08 October 2019 - 01:04 PM

That's a good point, typically anti-siphon holes are above the source liquid level.


In this case , if I cut off the drain line just above the anti-siphon holes, the line would no longer be sub-surface and I would eliminate the seal created to generate a vacuum in that vertical section of line, air would suck in through the bottom of the pipe and break the vacuum, which is what I'm attempting to do with the anti-siphon holes -- so if I'm thinking about it right, the anti-siphon holes will work.


My head still hurts though :)

#4 breizh


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Posted 08 October 2019 - 09:42 PM

hi ,

why don't you keep the discharge pipe above the surface of the pit ?  I will give a cut off the outlet @ 45 degrees for air ingress.

My 2 cents


Edited by breizh, 10 October 2019 - 03:10 AM.

#5 thorium90


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Posted 09 October 2019 - 08:22 AM

If its wastewater, are you sure those holes are not going to just plug up in a year? Would this thing stop working after some time?

I encourage you to think about any potential problems that may occur and how it will work in the long run as well.

#6 katmar


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Posted 10 October 2019 - 02:43 AM

The siphon breaking holes in the line entering the pit are not intended to stop water siphoning *into* the pit. They are there for the case where the supply tank is below the pit and you want to prevent the liquid siphoning *back* into the supply tank if/when the transfer pump fails. In your case, if the 25 ft downleg into the pit runs full above the anti-siphon holes there will still be sufficient static head to cause a siphon. There will be no "vacuum" at the anti-siphon holes and no air will be drawn into the pipe at this point.

Depending on the line sizes and lengths, the arrangement on Page 1 is very likely to totally empty the tank by siphoning it into the pit. This is true whether you have the siphon breaking holes or even if the downleg into the pit is cut off above the liquid surface. If the 25 ft downleg into the pit is of large enough diameter to ensure that it does not run full, and the pipe terminates above the surface, then there will be no static head in the downleg and it will not cause a siphon. However, this is an unusual way to prevent a siphon and requires very careful pipe sizing.

Your colleague who has recommended the vent pipe is (almost) correct. This is the usual arrangement for an application like you have. But it is not guaranteed to work in your case because the supply tank can become pressurized. You should take the vent pipe, which is more accurately called a pressure equalization pipe, back into the supply tank above the liquid surface.


A potential problem of using this pressure equalization pipe arrangement is that when the liquid level in the tank drops below the high point in the drain line (which will be where the equalization line joins it) then the gas from the tank will displace the liquid in the drain line. This could cause the gas from the tank to escape via the pit.  If the pressure in the tank is not too high it can be prevented by having the end of the downleg sufficiently below the surface of the pit to maintain the seal.

Edited by katmar, 10 October 2019 - 04:38 AM.

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