In the holding tank, the key to odour control is the
vent line; it must allow a free exchange of fresh air for
the carbon dioxide generated by the sewage. Therefore,
those bladder tanks which have no vent are all but guaranteed
to stink; there's no source of air into them at all. Boat
builders, boat owners and boat yard personnel who install
holding tanks have always viewed the vent line only as a source
of enough air to allow the tank to be pumped out without collapsing
and an exhaust for methane (Many even believe methane to be
the source of odour.) Some take the attitude that tanks are
going to stink so the thing to do is run that vent line as
far from people areas: cockpits, sun decks, etc. as possible,
or make the line as small as possible. All of the above actually
create the very problem you want to solve.
Vent the tank with as short, straight, and
level a line as is possible, with no sags, no arches, and
no bends. The minimum I.D of the hose should be 1". Ideally,
it should be no more than 5' long. If it has to be substantially
longer, or if running the vent line uphill can't be avoided,
or if it's impossible to run a vent line that does not go
around a corner, increase the size of the vent line to 1½".
(In fact, when possible we put 1½" vent lines on all
installations. The British Standard MAS 101 actually specifies
an 1½" breather or a multiple of breathers meeting the
same cross sectional area as an 1½".) If, for instance
on a sailing boat, the line must go up to the deck, install
a second vent line in order to create cross ventilation, or
install some means of forcing air down into the tank. We prefer
to put holding tanks in the bow of sailboats, under the v-berth,
because the hull just behind the point of the bow is the only
place on the hull except the transom that will never be under
water when the boat is on heel; it's the perfect place to
install a vent line skin-fitting.
The vent fitting should not be the same type
as a fuel vent fitting, a cap with a slit in it, but should
be a straight open type.
A
vent filter may also be installed between the deck skin-fitting
and the tank. Use an inline filter if problems persist or
you have inherited an installation that may have not been
correctly installed in the first place. The MICROVENT activated carbon filter from Lee Sanitation is designed for
use with 1½" pipes and thus ensures a good flow of fresh
air.
