Holding Tanks

As with all bandwagons, everyone who thinks he can make a buck off it, wants to jump on. Consequently, just about anything that will, ever has, or might hold liquid is being offered for sale as a holding tank. It’s an area of the boat where no one wants to spend money, in fact that’s true of the whole sanitation system.

Photo courtsey of Northshore Yachts

Photo courtesy of Northshore Yachts

Although you’ll see aluminium and stainless holding tanks, no metal of any kind should ever used to hold sewage. Urine is the most corrosive material that is possible to put next to any metal. If you are in doubt (ladies will have to take our word for it), notice the dividers between urinal stalls in men’s toilets. If that facility has been open for more than a week, no matter how clean and well-maintained it is, even though the dividers are stainless steel coated with enamel, you’ll see rust stains from the bolts that attach the dividers to the tile.

While the walls of a metal holding tank may last forever, the welds will typically begin to leak at a seam or a fitting in two to five years, and the tank will have to come out for repairs. Sailing boats especially are often fitted with flexible tanks also known as bladders. Bladders are invariably stuffed down any opening large enough to take it, and only rarely are the bladders secured in place. Since sailing boats are typically so much more ‘active” than houseboats or cruisers, heeling side to side, bladders move and chafe till they leak. Because the tank is in an inaccessible place, it is almost impossible to install the fittings correctly, and the tank is never checked or maintained. Some aren’t even vented, and it isn’t at all uncommon for a bladder to blow out its fittings. Furthermore it is all but impossible to control odour in a flexible tank. The very qualities that make bladders attractive to install, make them undesirable for use.

Rigid Polyethylene tanks as made by Tek-Tanks have sufficient wall thickness to prevent odour escaping through the wall. If the wall thickness doesn’t continue to increase with size, the tank walls will be too weak to support the eight pounds per gallon that sewage weighs (meaning a 40-gallon tank must support 320 pounds); it will bulge and, at the very least, create leaks at the fittings if it doesn’t actually crack. There are Polyethylene tanks being sold as holding tanks through most of the marine catalogues which have walls as thin as 1/8″. These are just not suitable. (All Tek-Tanks custom holding tanks have a wall thickness of 10mm)

Holding tank systems

Notes, hints and tips.

  • Holding tank vent pipe should be 1½” (38mm) ID. This will avoid blockage and possible implosion at pump out. It will also allow air to circulate within the tank thus helping with the natural biodegrading. (See section on Odour Control.)
  • When the outlet of the tank is in the side, drop hose below the bottom of the tank to enable complete emptying of contents.
  • Avoid pipe droops. Support with suitable clips or bulkheads.
  • Chamfer inside of fittings to allow a smooth run.
  • Avoid tight bends which can cause resistance and blockages in the pipe and minimise the number of bends.

Installation

When installing a system all connections should be double-clamped, only materials specified for marine sanitation should be used, and any below the waterline intake lines should include a seacock that is easily accessible by the boat owner. There are one or two heads on the market which require pressurised water and call for tapping into the on-board potable water supply. Allowing the sewage system to have any contact with potable water presents an unacceptable health hazard. The manufacturers assure us that check valves prevent any contact, but check valves can and do fail. Therefore we recommend that all systems either utilize raw water and a separate pump to pressurize it if necessary, or a separate on-board water tank to supply the head. Vented loops should be installed in all hoses to prevent backflow; if any part of the system is below the waterline vented loops must be installed.