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.