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Question DetailsAsked on 12/18/2017

Can I vent a sani-flow pump to an outside wall when discharging graywater from a daylight basement kitchen.

There is a drain at 5' 6" off the floor to connect to. There is a utility basin for air gap for a washing machine if needed.

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You need expertise from a plumber for your specific situation - and I would run the layout schematic past the city/county inspector to be sure they will accept it before it is done, but here is my call on what I would be looking for if I were inspecting it:

1) you need a backflow prevention vent at the discharge point into the connection to the drain which you are going to connect into. If this is a graywater-only drain pipe (which hopefully daylights well away from the foundation, because buried drainfield-type graywater disposal systems from washers plug up REALLY quick with lint and soap scum), then the vent for that should be separate from the household DWV sewer vent pipe. Could not find anything in the plumbing code saying the vent does NOT have to meet the above-roof venting requirement just like normal DWV vent, though there are some "green" sites out there saying to use an AAC - air admittance vent - which is NOT, I repeat NOT the way to go because that just lets air in, not out also.

A graywater drain pipe will get quite stinky from the soap scum buildup as it goes black and "septic" - i.e. anaerobic, and can smell basically as bad as a sewer and generate gases like a sewer, so I would say both from safety and odor control purposes even if not required by code, go to above the roof with the venting. I remember about a 20' graywater line from our kitchen sink to a surface drain ditch to trees it watered when I was a kid - generated enough gas to totally blow out the trap liquid in the sink and REALLY stink up the house when the line got debris clogged so it could build up some pressure.

Also, you want to assume that connection WILL back up at some point in its history - so a tall vent (through the roof) is the safe way to handle that risk and prevent a major backflow or blockage potential back to the drain pump.

2) if the drain you are dumping into is the household DWV line, then this basement piping is part of a blackwater system by definition because they are connected, so you need to vent through the roof as for sewer - which could be connected to the household DWV stack vent. And pay attention to the requirement for standpipe or drop height into the drain line - typically 18-30" (depending on local code variations) pump discharge line standpipe/stack or stickup is required above the entry point to the DWV drain (with the vent at the top of that 18-30" standpipe above the DWV line you are going into from the top). Top entry, not side entry, into the DWV line as normal, to prevent blockage from solids in the DWV line.

3) kitchen sink and wash tub need to be connected to vent lines as usual shortly downstream from their traps - so connecting (presumably) to the pump discharge vent line. These vent connections are especially needed with a lift pump situation so it does not pull the liquids out of the traps - but have to be carefully located so they do not result in the pump pulling air instead of the liquid in the pipe to the pump. Wrongly located (like right at the pump inlet if the incoming pipes are not all gravity flow) can result in the pump immediately pulling air and not the liquids it is supposed to be pulling.

4) in many states kitchen sink and dishwasher discharge is considered blackwater, not graywater, because of the biologic loading. The EPA has recently issued guidance that kitchen sink/dishwasher discharge is blackwater for the purposes of boats and ships and for any source feeding into waterways, which combined with the recent Corps of Engineeers and EPA interpretation of basically any waterway, including surface runoff rills and ditches and simple overland runoff flow being part of navigable "waterways" because they drain into them, basically showed the handwriting on the wall (confirmed by an EPA "position paper", that the next step is universally defining kitchen waste drains as "blackwater" - so check carefully with local building official before going with a separate greywater discharge system. Some states have already taken this step.

5) even though the lift pump will have a backflow preventer as part of it, you need to consider what happens if the backflow valve fails. Having the airgap at the point where the discharge pipe meets the DWV line or grayewater discharge line out side the house is a start and limits the amount of backflow to what is in the discharge pipe. Sometimes I have oversized the drain lines from basement fixtures so they will hold that total amount of liquid in case it backflows, rather than having it overflow the fixtures. Of course, this is a more critical situation with true blackwater, but something to consider considering the material in the pipes and traps is pretty nasty even just from sinks and washers.

6) note a separate graywater discharge system is generally a big negative to buyers who are not familiar with them - the lift pump from the basement is bad enough, I would avoid aggravating it by not going to the DWV system with the graywater unless that will cause septic overload issues.

7) pump sizing for discharge line size, flow capacity enough to handle the input flow without backing up in sink or significantly in a washtub - and should be designed to handle maximum washer discharge AND dishwasher at same time in case washer end up being plumbed with a conventional standpipe leading to the pump rather than having the surge capacity of a washtub.

Answered 2 years ago by LCD

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