The way the fresh water flush on my boat works is via the head sink.
The drain hose on the sink has a diverter valve, and in one position it
goes out via an above water thruhull, in the other, it drains into the
line that feeds the head pump. The line leading down to the raw water
line for the head is also vented, so that it can't siphon back out to
the sink.

Works really well, but I do have to remember to fill the fresh water
tank, which is fairly small on my boat, before a longer trip.

SD

On 2006-07-20 16:58:21 -0400, "Roger Long" said:

This business about not connecting FW systems to sanitary plumbing
reminds me of a USPH required and approved arrangment for FW flushing
systems.

It's called an "Air Gap" and is basically just a funnel with a pipe
outlet a few inches above it. The funnel goes to the tank for fresh
flush water. The valve can either be manual or perhaps controlled by a
float in the flush water tank. If something goes wrong with the
sanitary system, the water just backs up and overflows the funnel with
no way to get into the potable water.