I understand that many will have their holding tanks
in the bilge
(and therefore pumping is the only option), but for what it is worth, we have the following arrangement in each head:
Electric-flush toilet with macerator pump at base, pumps waste up into top of holding tank.
Deck pump-out goes straight into the top of each tank from the deck fitting.
At the bottom of each tank is another outlet going straight to a below-waterline seacock. Each tank empties in a few seconds when that seacock is opened, and being at the bottom, the emptying current
takes everything, leaving the tank fully empty (75mm/3" hose and valve - there is a big rush when it is opened).
What can go wrong?
1. The 'joker' (non-return) valve between the macerator and the top of the holding tank can from time to time get a little material stopping a complete seal.
No big problem as the volume of the hose above is nothing like the volume of the toilet pan, so it can't overflow. Just flush again, and it usually is fine.
When it eventually needs servicing, you just make sure the tank is empty before opening up the valve
2. The macerator pump can need a service
Again, no problem - just undo the four screws, remove, service
and back (if you empty the pan properly, there is very little water
to run out - and it is essentially clean anyway). I also keep a spare pump for a quick change-over should the problem be more than just a seal.
3. Lower hose falls off, or tank fitting cracks, dumping a full tank into the bilge.
With double hose clamps on all fittings, and regular inspections, this should not be a problem (hope I don't live to regret saying that !).
4. Tank itself cracks.
are very solid HDPE tanks, mounted on a shelf moulded into the hull
(so well-supported). Again, with regular inspection
, the chance is very slight, and we generally don't sail with full tanks - especially if we are bashing into it.
As a further note, while we are considering converting to F/W flushing
, at the end of each season, I fill each tank with de-scaler (via flushing
it into the pan so the hoses get the treatment also), then go for a lumpy sail, or at least a few hard tacks to slosh it around. It then sits overnight before emptying. That seems to remove any scale build-up in the hoses and the tanks.