I installed a holding tank in Raasay soon after bringing the boat home. I was on a swinging mooring in a tidal estuary with a public beach ...
Also several hundred seals, mind you. Not sure about that one.
Since the installation, I've never quite managed to stop the heads from backfilling. I have high loops, a tank vent, and a siphon break, but still get occasional problems. Also smells. I know the non-return valves in the pump should stop this, but they really don't look like they're meant to be a tight seal.
I can always give the bowl a quick pump with the lid open, but generally the need for this only reveals itself just after a tack on a hard beat. Not when I have a moment that needs filled up with a little job. So to speak.
So, one of today's objectives was to fit a separate non return valve close to the pan. I wasn't looking forward to it.
At the planning stage (ever optimistic) I imagined doing this by reaching behind the bowl, but on site it was obvious I was going to have to take everything out to get at it well enough to do the job properly. Advanced stage 1 looked like this:
Unfortunately (?) I haven't been able to photographically represent the smell.
A little later things looked like this:
And the whole boat, plus a considerable part of Inverness Marina, now smells of Milton.
Preliminary tests suggest that this will solve the problem. I discourage the disposal of toilet paper via the heads (a convenient polythene bag is provided), so everything which isn't the byproduct of a medical emergency should pass through.
I'm sure there will be a few brisk Spring days which will test the theory ...