Backflow Prevention Pictures The Air Gap is the most common, and most reliable, form of protection we have to prevent contamination of the drinking water (potable) system.  It works by preventing contact between the potable system and the drainage system.   The first two photographs show a water softener air gap functioning to prevent contact between over-flowing sewage and one end of the potable system (like the spout on a kitchen faucet).  This air gap saved the family a lot of trouble.

The water which rinses across the reverse osmosis membrane (to keep it
from clogging) is supposed to fall through an air-gap fitting above the
rim of the sink. In this installation, the tube intended to discharge
into the top of the air-gap simply loops, instead, from the filter
housing into the front of the sink drain.

Left as pictured, the sink could eventually drain, or back up, into the
filter system. The organisms in the drain would find a welcome home in
the de-chlorinated side of the system. They would then be portioned
into every glass, measuring cup, and coffee pot filled at that faucet.
Chronic and/or acute symptoms would follow and this condition could
continue to re-dose the consumer(s) while doctors and nurses look
elsewhere for the cause, since nearly everybody takes their drinking
water for granted.

Reverse osmosis systems should have three tubes running into/out-of the
bottom of the dispenser faucet: one supplies filtered water to the
consumer, one delivers membrane rinse-water to the top of the air-gap,
and the third receives that rinse water at the bottom of the air-gap
and runs it safely into the drain. Done this way, any water backing up
from a clog would fill the sink and escape across the rim of the sink
before any of it could contact the potable side of the system. Wet
counters and floors are supposed to be the worst of such a situation.
That's good, compared to a chronic stomach illness, or worse, which
waits patiently in dark places.