With a closed system the water enters the colon machine directly from the plumbing system to be purified and pressure regulated, and the evacuated waste matter exits directly from the the client's colon to the waste portion of the office plumbing.
At no time does the inflow or outflow come in contact with the air.
In contrast, an open system is more like an enema, where a container is filled, often via a tube connected from the sink and introduced into the client's colon by a pencil thin rectal tube. When the client eliminates, the waste matter is often directed to the toilet through an opening in the board or table where they received the colonic.
I think it's fair to say that a series of enemas can produce results equivalent to those achieved from an open system colonic.
I'll elaborate some more in a reply to the one preceding mine.