We fitted a black-water 55 litre holding tank to Selkie Dancer’s forward heads in 2008 while wintering in Almerimar. The rigid tank was custom made by Boat Wide (using Tek Tank technology) www.bwcm.com and is in the aft section of the locker above the sink.
The system is simple. The Lavac pumps everything into the top of holding tank. The tank drains directly to the sea via the existing sea cock. The tank is either in “holding” - outlet sea cock closed; or “open” – outlet seacock open. When in port or at anchor the forward heads are designated the “po heads” and the aft heads as the “pee heads” (sorry to be so basic).
The tank has two unfiltered air vents.
On the deck there is a cap atop the “Pump Out” shaft. This shaft is directly above the tank drain so you could “rod” the drain through the deck fitting if necessary.
Does it work? In my biased opinion – yes. There are no extra valves to open or close and the no additional pumping is required – it is a gravity system. That said there are occasional odours in the cockpit when fluid is being pumped up into the tank and tainted air comes out of the vents - filters would alleviate this but filters restrict airflow and compromise the efficient breakdown of material in the tank. Second, the tank has twice failed to drain when the sea cock was opened. The solution is simple - cork up the two air vents and then cautiously pump. The over pressure overcomes the restriction and all is well. But do pump with caution - you do not want an explosion!
Before ordering the tank I made a scale model out of thin plywood to ensure a good fit and to determine clearances. I am glad I did – the original fitted exactly but I could not get it through the door of the heads.
The amended version did although I had to take a door off to get it in.
Two unfiltered air vents. Mug indicates planned position of pump out connection
Top of tank as fitted. Shows two grey air vent lines, emergency corks, to the right the black access cover and grey pump out cap.