Waste water from the garbage disposal when filled with water, will flow back from the disposal through the dishwasher waste hose and back into the dishwasher. Is there some sort of one-way valve that will prevent this?

You have a partial blockage in the horizontal drain line in the wall, the solution would be to dip the water out of the sink and remove the "J" bend from the trap and send a snake up into the wall, (see image) You will hit a bend about 8 inches in but once around that you only have to put out 5 feet more. After, flush with a tub of hot water.

Hi Ozad...

Could be that you have a blockage as Afaroo has suggested... could be.

I'm also thinking that it could be simpler than that... maybe? I'm wondering if the dishwasher hose connects directly to the disposal without having a high loop in place. Here, if the dishwasher hose doesn't loop up high under the sink, higher than the disposal, then water can simply drain back toward the dishwasher via gravity every time you use your sink or disposal... can be kind of yucky if you have clean dishes in there... ;)

Anyway, check the hose under the counter... see if it loops up high. Let us know... O.K?

MARK

Hi Ozad...

Could be that you have a blockage as Afaroo has suggested...could be.

I'm also thinking that it could be simpler than that...maybe? I'm wondering if the dishwasher hose connects directly to the disposal without having a high loop in place. Here, if the dishwasher hose doesn't loop up high under the sink, higher than the disposal, then water can simply drain back toward the dishwasher via gravity every time you use your sink or disposal....can be kinda yucky if you have clean dishes in there...;)

Anyway, check the hose under the counter...see if it loops up high. Let us know...O.K?

MARK

That was exactly the fix that was needed. Thank you

This is exactly what Air Gaps are for...

This is exactly what Air Gaps are for.....

Yes, Milo, it sure is. :)

Yeah! A high loop is the way to go.
Good luck, Tom