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View Full Version : Why would you connect a dishwasher to a disposal?


friedo
09-29-2006, 02:05 AM
I am considering the purchase of a garbage disposal for my currently un-mechanized sink. I've been building the necessary Google Muscles1 by reading various web pages on how to install a disposal. They all mention that the disposal comes with an optional inlet for the dishwasher drain hose, but they never tell you why.

So what in the hell is that thing for? Why would the dishwasher need to go through the disposal?


1. Google muscles (http://boards.straightdope.com/sdmb/showthread.php?t=386759)

WhyNot
09-29-2006, 02:11 AM
Because then you don't even have to scrape your dishes, much less rinse them, before you put them in your dishwasher. The bits and pieces and chunks of food get chopped up by the disposal before going into your pipes, same as if you'd put them in the sink. My mom's dishwasher runs through her disposal, and she just picks the plates up from the table and loads them as is. OK, she takes out bones and such first, but she doesn't scrape 'em. She loves it.

Billdo
09-29-2006, 04:04 AM
In many kitchens, the dishwasher drain will connect to the drain plumbing under the sink. When you remove the sink drain plumbing to replace it with the disposal, you would normally need someplace to connect the dishwasher drain, and most disposal units convinently have an inlet available to do just that.

I believe that in connecting the dishwasher drain without a disposal, you need to have an air gap to make sure the dishwasher drains properly. The disposal connections I've seen have the inlet in a collar around the top of the disposal, and the open sink drain above the disposal obviates the need for a separate air gap.

Renee
09-29-2006, 06:42 AM
I believe that in connecting the dishwasher drain without a disposal, you need to have an air gap to make sure the dishwasher drains properly. The disposal connections I've seen have the inlet in a collar around the top of the disposal, and the open sink drain above the disposal obviates the need for a separate air gap.

I've seen many dishwashers connected to disposals w/o air gaps, including a new house that passed a plumbing inspection, so I'm not sure this is true.

What you do have to be sure of is to knock out the hard plastic seal in the garbage disposal before connecting the dishwasher (we accidentally didn't do that once; oops).

LSLGuy
09-29-2006, 07:25 AM
I've seen many dishwashers connected to disposals w/o air gaps, including a new house that passed a plumbing inspection, so I'm not sure this is true. ...You're right there is no air gap between the dishwasher & the side of the disposal. The air gap is the inside of the disposal.

The drain line just dumps the dishwasher output into the chamber of the disposal which is open to the air through the sink drain. This prevents siphoning & other nasties.

R. P. McMurphy
09-29-2006, 08:01 AM
One problem that occurs with garbage disposals is that the operators don't run enough water through them to flush the particulates fully out of the disposal and past the drain elbow. By running the dishwasher waste through the disposal it gets fully flushed on a regular basis.

It is important though, to not leave a bunch of unprocessed garbage in the disposal while the dishwasher is running. The waste may start to back up into the sink.

When operating a garbage disposal don't overload it, let it run until everything is well processed and keep the water running after stopping the disposer to assure that it gets fully flushed.

Rhythmdvl
09-29-2006, 08:47 AM
You're right there is no air gap between the dishwasher & the side of the disposal. The air gap is the inside of the disposal.

The drain line just dumps the dishwasher output into the chamber of the disposal which is open to the air through the sink drain. This prevents siphoning & other nasties.


Please forgive any petulance you might perceive in my contradicting you, for though I dare not outrightly suggest you are in error, I nonetheless received a fair amount of advice directly contrary to your post. Of course, volume alone does not make for authority in such matters of fact, so I offer this for your perusal and personal consideration:

I asked: Is an air-gap required with a disposal? (http://boards.straightdope.com/sdmb/showthread.php?t=363683)

Dag Otto
09-29-2006, 12:12 PM
You're right there is no air gap between the dishwasher & the side of the disposal. The air gap is the inside of the disposal.

The drain line just dumps the dishwasher output into the chamber of the disposal which is open to the air through the sink drain. This prevents siphoning & other nasties.


Wrong. Like I posted in the other thread Rhythmdvl linked to, the disposal is directly connected to the sewer. It's very possible in the event of a drain clog for the disposal to be full of sewage. No air gap, and a dishwasher connected directly to the disposal can siphon the sewage. Heck, it doesn't even have to be flooded with sewage, the inside of a disposal isn't something you would consider clean when the drains are working properly.

Use an air gap, installed above the flood rim of the fixture that the drain is attached to. The 'air gap' that LSLGuy refers to is below the flood rim of the sink. That's the reason it is not an acceptable air gap for cross-connection control.

Billdo
09-29-2006, 12:51 PM
I've seen many dishwashers connected to disposals w/o air gaps, including a new house that passed a plumbing inspection, so I'm not sure this is true.

I guess I wasn't all that clear in what I posted. You either need to have the dishwasher drain connected with an air gap OR connected through a disposal.

When you connect it through the disposal, the diswasher waste water gets ejected into the open chamber of the disposal unit (below the sink drain opening) and then drains out the bottom of the disposal. Because the sink drain is open to the air, this creates the effect of an air gap without the need to install a separate dedicated air gap.

One thing I have learned from experience is that if your dishwasher is not draining, check that food or other waste has not become jammed in the connection between the dishwasher drain line and the disposal unit.

jjimm
09-29-2006, 12:55 PM
I'm confused. Does the disposal unit run constantly while the dishwasher is on, or does the food junk just sit there in the unit until the next time you run it?

Mama Zappa
09-29-2006, 01:27 PM
I'm confused. Does the disposal unit run constantly while the dishwasher is on, or does the food junk just sit there in the unit until the next time you run it?
No - the disposal doesn't run at all while the dishwasher is running unless of of course you happen to be using the disposal at the same time. Presumably it would get flushed by the water used to rinse the dishes so there wouldn't be a great deal of food junk.

And whatever food junk might be there, would be pretty thoroughly pulverized - most dishwashers sold in the US have a mechanism for that, though some imported brands don't (and that variety may be more common in the UK).

Dag Otto
09-29-2006, 01:29 PM
Because the sink drain is open to the air, this creates the effect of an air gap without the need to install a separate dedicated air gap.

It sure looks like an air gap, but it's not because it's below the flood rim of the sink. It offers zero protection from cross contamination from the sewer to the dishwasher. None.

Cheesesteak
09-29-2006, 02:12 PM
It sure looks like an air gap, but it's not because it's below the flood rim of the sink. It offers zero protection from cross contamination from the sewer to the dishwasher. None.I don't think the air gap has anything to do with cross contamination. Your dishwasher basin is chock full of dirty stinking soap filled water whenever it's running, I don't think that getting a bit of backwash from the sink is going to suddenly create new contamination issues.

However, if your sink is filled up with water on the disposal side due to a clog, the water will run back down the DW drain line to the basin, if you don't have a gap or a loop that goes high enough. If it drains into the DW, it will eventually overflow or cause worse problems.

I've been told the disposal connection is to ensure that the disposal gets a nice hot, soap filled rinse on a regular basis.

Rick
09-29-2006, 05:47 PM
FWIW the installation instructions for my dishwasher said not to use an air gap.
The DW before said too use one..
I am assuming that the guys that made the DW know what they are talking about.
$.02