Holding Tank maintenance - winter 2011

forward cabin
Forward cabin layout
Waste products that should not be discharged in coastal waters (especially at anchor in cala's) so they are diverted to holding tanks until they can be discharged at sea. Typically there are two types, black water (toilet output) and grey water (sink discharge). Most Mediterranean countries prohibit the discharge of black water in coastal areas and a few (very few) also prohibit the discharge of grey water notably some parts of Greece who will be assured of clean seas devoid of all but local boats who don't appear to be subject to the excessive fines!

Our boat was commissioned in the UK where black water holding tanks are not a requirement but I had them factory fitted as an optional extra. The best quality flexible 38mm pipe used in boats has a guaranteed life of 5 years. That's very short compared to waste plumbing in a house. This year we had problems with both the forward and aft holding tanks. The tanks themselves are in good condition but the hoses, pumps and connections were causing odorous problems. The factory design for emptying the tanks at sea is by manual bilge pump operated from inside each head (toilet).

Forward head.

Due to the layout of the boat one of the walls of the forward toilet abuts the forward cabin, therefore the through-bulkhead manual pump is actually located under the bed in that cabin and operated from inside the toilet. This required two 38mm pipes connected to the pump which due to routing difficulties and time have developed pin-hole leaks.

forward tank valve
New valve fitted under the forward holding tank.
The forward holding tank is located almost directly over the through-hull outlet so I simplified the design to a direct gravity discharge controlled by an expensive manually operated plastic valve thus eliminating two pipes and a manual pump. In practice it took several attempts to get a completely watertight (excrement-tight) seal I tested with sea-water! The main problem was that the pipe from the through hull fitting didn't line up exactly with the fitting into the bottom of the tank causing stress on that joint. The final solution was to coat the whole joint with Sikaflex 291 sealant before screwing them together. After several trials I re-installed the covers. One down, one to go.


Aft head.

The aft tank was a different problem. The through-hull discharge is quite a distance from the tank so the pump is essential. I bought a maintenance kit for the Whale Gusher Titan Bilge Pump (the discharge pump) and re-furbished it before re-installing the tank.

After servicing the discharge pump I installed the holding tank, connected everything up and tested it (with fresh water!). All well.