So, every fixture has a trap and every trap has a vent… (two showers and the washing machine)

and then the drainpipes come together and then the single diverter valve sits downstream of the drainpipe confluence. Greywater flows into a single 55 gallon surge cistern in the basement and gets pumped out via a sump pump to the yard.

I installed an overflow protection drain high in the cistern so that, if the pump fails, greywater simply flows back into the sewer line.

My basic question is: should the surge tank overflow arm have a trap and vent?

Thoughts that make this decision harder than it seems:

  • The house is protected from sewer gasses by the trap installed at each fixture, so it seems unnecessary
  • Even if unnecessary, do I want the cistern exposed to sewer gasses and likely backwater bacteria? Small amounts of these bacteria get into the greywater anyway, so this concern seems a bit overblown.
  • Theoretically, without a cistern trap, would the pump pump sewer gasses into the yard?
  • If I do trap and vent the cistern, does the vent go before or after the trap?
    • With fixture trap and vent, vent goes after trap to prevent trap from being sucked dry, but in this case where the trap in question is reasonably far downstream, do I need to worry about flow from above pushing the trap dry? After the trap, in this case is (mostly) the larger 4″ sewer lateral. Hard to imagine this becoming full and creating suction.
    • Would this be considered double-trapping?

A follow up question has been created: can anyone discuss the usual source of the suction that creates a need for vents? Where specifically in the downstream plumbing does this suction tend to occur and how does this inform vent placement?

Thanks for any help the forum can provide!

Site won’t let me upload the 189kb jpg for some reason so here is a link to a drawing: http://www.anthonybaldor.com/wp-content/uploads/2016/07/GWQ.jpg


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I will answer your questions logistically, but also you should know that if you were to seek a permit for this type of system the local regulators may require other components, like a p-trap after the overflow.

Your tank should have a vent or some way for air to escape the tank. You can use a simple AAV, or just keep the hole for the pump wires loose to let air out. This will let air and any small amounts of gasses in the tank escape. You could connect this vent into the vent pipes  from other fixtures, but usually that is way too much work and it’s a lot easier to use an AAV (assuming the tank is not inside a living space!)

You do not need a p-trap on the overflow. Like you said, each fixture has its own vent, and the tank will be vented, so sewer gasses will not build up in your tank.

You do need a backwater valve on the overflow of the tank. This is a one-way check-valve that prevents sewage from backing up into your tank (and being pumped out into your yard) in case of a sewage clog downstream of your tank. Be sure to install the backwater valve in the horizontal position.

I have more description and details about this in my book, The Water Wise Home, if you want further references.

To answer your question about suction: think of a toilet, which has it’s trap built inside the toilet and located before the vent. When a toilet is flushed the rush of water pushes the existing water inside the bowl and the toilet’s internal trap down the drain and the resulting suction pulls the rest of the water out with it. Toilets automatically refill, so the trap is again full of water. If showers/sinks didn’t have a vent to equalize pressure in the pipes, the same thing would happen to their traps (emptying them), which would allow sewer gasses to enter the home. Codes specify where the vents should be located to prevent the traps from being sucked dry (not TOO close, and not too far either).  Hope this makes sense!

PS. Make sure to get a powerful enough pump. Typically sump pumps won’t last long in a greywater system b/c they’re not beefy enough. Something capable of pumping 3/4″ solids will be powerful enough, though some people use a smaller, 1/2″ solid capacity, rated pump.


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Thanks! Very helpful.

Can you please clarify/reconcile?:

  • “if you were to seek a permit for this type of system the local regulators may require other components, like a p-trap after the overflow.”
  • “You do not need a p-trap on the overflow.”

Just checked my sump pump: Solids rated to 1/8″ yikes! Thanks for the heads-up!

Begs the question: what is a sump pump for? I thought this was to keep your basement from flooding? Solids in the basement seem like they would be waaaay larger than solids that get past the shower drain!

Thanks again

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Not knowing what state/city/county you are in, I have no idea what your local regulator would want from such a system if you were to seek a permit. I thought you should be aware of this! Each person makes their own choices about getting a greywater permit or not, and some areas make it so difficult that it’s practically impossible to get one…. but not everywhere.

That said, I know that one place (San Jose, CA) has been requiring a p-trap on the overflow of a tank, even though no one in the greywater design/install community thinks it is necessary or even helpful.

Regarding the sump pump, they are for pumping water out of a basement. I don’t know why they don’t seem to hold up in a greywater system. Perhaps it’s the frequency they turn on (more often in a  greywater system than for water in basement?) or maybe it’s the hair or slime….

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