Is it possible to do this? Currently we have a line from the dishwasher to the air gap and from the air gap to the garbage disposal. I want to install a reverse osmosis system but we only have the one air gap hole and I want to use that for the reverse osmosis faucet (which also has an air gap). Would it be safe to combine the reverse osmosis AND the dishwasher waste line to the air gap faucet and have it run down to the garbage disposal? I notice that the waste line to and from the reverse osmosis is pretty small while the one to and from the dishwasher is pretty wide.
You have two options which are equally acceptable.
Option one is a dual inlet air gap. It provides two inlets - one for dishwasher, and another for a water purification system. Installation is straight forward - you remove the existing air gap and put the new one in its place. You can find such a device here.
Your other option is to purchase a faucet for your water filter that includes a built-in air gap, such as this. Note that if you go this route, you will have two drain lines that both need to connect to the garbage disposal. You can accomplish this by using a wye fitting to join the two drain lines into one line, which you would then connect to the disposal. A barbed wye fitting should be available at your local home improvement store for a minimal cost. Don't forget the hose clamps.
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This would be a bad idea because debris from the drain flowing up the line would quickly create blockage in an RO sized air gap. An RO faucet is designed to be sleek and minimal (for appearance purposes) it is an insufficient opening for the application of dishwasher-drain air gap.
The reason the RO reject-to-drain air gap can be so small is because the reject water is low pressure from an RO system, there's less back pressure to pull in large pieces of debris from the drain line (someone feel free to correct me on the fluid dynamics here).
In short if you want to remain within code (and possibly avoid flooding the cabinet with dishwasher drain water, depending on your dishwasher), then you'll need to cut another hole in the sink top.