Poop system alternatives
There has been some good debate here.
As for my two pence, I am not a fan of composting heads for two primary reasons:
1. You have to carry around both your poop and the sawdust/peatmoss/whatever around with you all the time. Neither one is ejectable at sea (without some awkwardness). On a small boat I'd rather have a fireplace if I have to carry wood pellets and stuff. It'd be more beneficial to the boat & crew.
2. Emptying the toilet is an indelicate affair and ultimately, despite the pro-compost lobby's eco-responsibility arguments, no good for anyone. It's just garbage-- and for heaven's sake don't tell the staff what's in the bag you're carrying up the dock or you may find yourself evicted from the marina! At least pumping out at a proper dockside facility enables the sewage to be processed into usable byproducts-- and relegates the responsibility for said process to a party with more money and better lawyers.
I have a friend who carries a quart mixing cup into the toilet room for flushing. He's the one who taught me about saltwater gremlins in the lines (most of my experience has been on boats kept in rivers). So I am plumbing my toilet with a line from the freshwater tank, through a one-way valve, up to the loop and down to a valve that lets it into the seawater-flush line. By opening this valve and shutting off the seacock I can use my (copious) freshwater supply for the head. Best of all I can still flush the toilet when the boat is out of the water-- which is a great benefit to liveaboards!
I don't get Peggy Hall's idea of stopping up the sink when the seawater line is open. Do I understand that right? I do like the idea of flushing using the shower-- it's simple and easy. But if you are altering the plumbing on the boat for a more elegant solution, I think mine is a good idea. The loop is important because the backwater will fall down towards the tank and check valve. A level line would give the check valve some opportunity to fail someday, which would muck up the freshwater supply.
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