Do I need a vent for greywater?

I'm diverting my greywater to trees in mulch basins. Do I need a p trap for my sinks and vents? Or could I just use a screen on the outlet?

Hi Yen,

Yes you will need p traps and vents, unless you want your plumbing venting into your house. Even if your pipes are draing somewhere uncontained, those grey water pipes will still smell, and the p trap stops the smell from just wafting up into your house every time you run the water. The vents give the air in the pipes a place to go, besides bubbling up through your p trap, when you introduce water into the system which causes pressure differenc. You won't have as much pressure from introducing water with your open system, but in your case with the outflow end sitting open to the air, just the wind blowing could cause a pressure difference in your system that could bubble smelly up through the p traps without the proper venting. So I would recomend using both p traps and vents if possible. Pluse p traps will help keep vermin out of your house. Since Im sure you don't want bugs or rats crawling up through there, with your outflow just sitting open outside.

Hope that helps!

R. Steele wrote:Hi Yen,

Yes you will need p traps and vents, unless you want your plumbing venting into your house. Even if your pipes are draing somewhere uncontained, those grey water pipes will still smell, and the p trap stops the smell from just wafting up into your house every time you run the water. The vents give the air in the pipes a place to go, besides bubbling up through your p trap, when you introduce water into the system which causes pressure differenc. You won't have as much pressure from introducing water with your open system, but in your case with the outflow end sitting open to the air, just the wind blowing could cause a pressure difference in your system that could bubble smelly up through the p traps without the proper venting. So I would recomend using both p traps and vents if possible. Pluse p traps will help keep vermin out of your house. Since Im sure you don't want bugs or rats crawling up through there, with your outflow just sitting open outside.

Hope that helps!

Thank you for your kind reply.

The vent pipe has to be higher than the house, is that correct?

Would a screen stop the vermin and bugs? I saw this in Art Ludwigs book.

DO you think there would be smell? As the greywater pipe is only 5 meters?

One option for venting if running a vent up a wall and thru a ceiling/roof is a studor vent. It's a one way air admittance valve installed right past a p-trap and as high up above the trap as allowable. I have two of them in my house where I was unable to run a vent through an exterior wall, and they work. Air admittance valves are very common in locations such as a sink in a kitchen island where there's no real practical way to run a traditional vent.

My shower, sink and washing machine are all connected to one 4 inch pipe. Is it OK to put a trap on the outside as all plumbing is already under the slab?

Hi Yen,

A screen will stop vermin and bugs. It's hard to predict the smells, because its hard to predict the constituents of your gray water or how long that moisture will lurk in the pipes growing funk. Traditional plumbing vents are ran up in the walls through the ceiling then the roof. Im not familiar with other styles of vents. If you can I would do p traps, vents and the screen. That just me though, as it can save alot of hassles down the road by eliminating funky phyiscs in plumbing system. If your system is small, you might be able to get away with one vent, but that depends on the phyiscs of your individual system.

Hi Yen,

If you already have your plumbing in a slab, it's most likely to late to install the p traps without busting concrete, except maybe on your sink. The p trap is essentially the first thing the water flows through once it leaves your sink or tub, and would be conected conveniently right below the drain. The sink is high enough you could still install a p trap, but the bathtub is to low. Also your washing machine drain is a straight drop, with the p trap typically in the floor where it connects to your plumbing pipes on grade.

To put a p trap in any other locations, like outside, would defeat the purpose, as the function of a p trap is to provide a water barrier with enough resistance, that with proper venting, you don't get a back flow of air coming up out of your drain when other drains introduce water into your system. If your worried about bugs, you'll be best served using a stainless steel screen on your outlet.

The good news is with your open system, and being gray water only. You won't have much if any methane build up in your pipes, meaning it most likely won't create a serious hazard from not having the p traps in place.

This is for educational purposes only, and shouldn't replace the advice of building professionals or required building codes.

Hope that helps.

Thank you for all your answers and input. I'm thinking of a design. I will run it by you guys when I have a better picture.