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.
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 polypropylene 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
holding tanks have a wall thickness of 9mm - 3/8")
Holding tank systems
Notes, hints and tips.
1. 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.)
2. When the outlet of the tank is in the
side, drop hose below the bottom of the tank to enable complete
emptying of contents.
3. Avoid pipe droops. Support with suitable
clips or bulkheads.
4. Chamfer inside of fittings to allow a
smooth run.
5. 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.
