A:  Regardless of what you do, the vent from a sewage holding tank will always emit sewage smells, especially when the head is used, which forces gases up and out of the tank. These gases usually dissipate rapidly. A good installation places the vent outlet as far as possible from the cockpit area, where it will be noticed least. Your problem may be as much the location of the vent as the smell itself.

An abnormally strong stench coming from the tank vent may have a couple of sources: Either the tank is so contaminated with sludge that it produces a cloud of sewage gas the moment liquid is introduced, or one of the hoses connected to the tank has a low-lying sag that traps sewage. This trapped sewage then festers, producing swamp gas that is pushed back into the tank and out the vent when the head is flushed.

With the first scenario, some chemical treatment may be available, but it sounds like you’ve already tried this without good result. Some chemicals damage seals in the head, so always be sure the product you’re pouring down the toilet is compatible with both the pump seals and the environment.

With the second scenario, sewage or vent lines may need replacing or at least re-mounting to reduce the source of the ill odour.
Given the age of your boat and the length of time it was sitting idle (possibly with sewage in it?), some of the sewage hoses may well have deteriorated or become so foul they need replacement.

Tank vent charcoal filters are available, but there’s debate over whether they’re a good way to control vent odour. If the filter restricts airflow to or from the tank and causes the tank to go anaerobic (bacterial action that does not require oxygen), it might produce more of the very odour the filter was installed to control.

An even more important concern is the possibility of liquid sewage being pumped up the vent by accident when the tank is full. Complete clogging of the filter or vent line means no more vent. Without a vent, and bacterial action going on in the tank, the tank will soon become pressurized, especially in warm weather. As you can imagine, this could have disastrous consequences, usually with the rubber outlet valve in the head failing and tank contents blown violently back through the toilet! If you ever overfill your holding tank so that sewage is forced out the vent, make sure the vent is clear afterwards.

All lines should be as short as possible and straight: no dips or loops that will trap sewage. Lines carrying sewage to the tank should slope down from the source to the tank. Lines carrying sewage from the tank and vent lines should slope up continuously from the tank to thru-hull fittings in order to prevent trapping sewage. Vent lines should be large (1/2" or more) and unrestricted.