I am getting back flow from washing machine into slop sink. Can a check valve be installed horizontially between sink and washing machine.

Thanks for any advice in advance.

A check valve just hides the problem and also may stop the draining for the washer. The washer drainage is trying vent itself through the slop sink.
So if there are no clogs in the line and its not vented
The focus here is to vent the line
This is a better approach then a check valve and can be used if the code in your area allows it
Again Check to see if there is venting on that washer discharge line and if you can naturally vent it that would be the best approach. If to hard to vent then hers an option that's better than a check valve
Air Admittance Valves

Plumb Pak at Lowe's: Studor Mini Vent Air Admittance Valve

Signed 21 Boat

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