High Loop is not preventing sink backflow into dishwasher. Separate DW trap below sink trap.

[pk.diy]

When I moved into this house, the kitchen had a high(-ish) loop and no air gap.

After the kitchen drain is a long horizontal run to a shower, and then eventually to a septic tank. This run sometimes gets clogged.

During the first instance of a clog, the sink filled and we eventually had a backflow into the dishwasher. I was assuming that the mid-sink water level of the sink went above the not-that-high loop and thus backflowed.

At this point I got a longer dishwasher outlet hose and went as high as I could with the loop. It's now touching the underside of the countertop.

Yesterday the drain clogged again. The sink filled slightly but well below the level of the high loop. It stayed in the sink for maybe 20 minutes. Then the dishwasher started dripping; the sink water level dropped and it all came out the dishwasher.

I'm trying to understand how/why this happened. It seemed impossible due to the high loop being higher than the sink water level. I then saw some references online to the dishwasher's outlet/trap being below the sink's trap, as you can see in the picture below. Could that cause the issue?

Can someone explain the physics/hydrology of how this happens?

I know I can install an air gap. I may do that. Is there another fix for the high loop? Should I drain the dishwasher into the same trap as the sink?

Dishwasher outlet enters from DW at bottom right. High loops up to right side of sink. Extra slack. Goes into its own p-trap BELOW the level of the sink p-trap. Item at far left is a water heater, not a garbage disposal.

 

[pk.diy]

Any advice? Thanks

 

[Sponsor]

 

[GReynolds929]

Air gap is the best option. There's a lot of drain snaked around under there. If the drain hose does not go all the way up to the bottom of the counter and the backup in the sink is higher than the drain hose it can drain back into the dishwasher.

 

[pk.diy]

It does go up all the way to the bottom of the counter.

The backup in the sink was not higher than the high loop.

 

[clk1986]

I made an account just to ask about the dishwasher drain, since I've never seen a drain like this, seemingly specific for a dishwasher. I've seen the drain come up to an air gap then down to either the inlet on a garbage disposal or to a drain pipe directly below the sink hole with a dishwasher branch, to attach the dishwasher drain hose. So on initial thought, and the fact that this is septic, am I going out on a limb to say this is an older house and that "dishwasher drain" was a weekend warrior add on (maybe a previous owner or landlord) and is not up to code?

First thing I thought when I saw it though, is this would be one of those cases where having a plumber come in and redo all that would be worth it. But, in anycase, it seems like figuring out what's causing the drains to back up should be priority #1. If you can't afford that repair, then you probably won't like how much it costs to repair water damage.

Also, just noticed both of you are in WA. What a coincidence. heh