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Re: How big a holding tank vs. camping
"What I fail to understand is having fresh water flush. "
Seawater = soup. All sorts of live things are in it, along with plenty of nutrients for them. So the seawater in your head, in the hoses, etc. is alive, and if it sits and gets stagnant, it will get stagnant and develop odors, slime, etc.
The theory behind a fresh water flush is that it clears all that soup out of the bowl and hoses, so there's much less growing and much less that can make an odor there.
Add good ventilation to keep down odors in the holding tank itself...and in theory you can have an odorless system.