Old thread, but something new to report.
I was told by more than a few that vent filters were too expensive, since the replacement cost was over $100 and they only lasted a year. But my math and chemistry suggested a different story, and in fact, my $15 filter lasted more than 3 years, and the refill was ~ $5 and less than 5 minutes ( I designed for easy changes). A good deal, I think. I sail most weekends and cruise
about 1 month each year, in 1-2 week increments.
3 years ago published this...
Why Carbon Can Last Much Longer Than Laboratory Testing Suggests
The primary interaction of carbon with organic vapors is surface absorption, as the Dometic tests explored. However, carbon is a very complex surface and presents very complex behaviors. In the presence of fresh air we have catalytic removal of sulfur by this simplified mechanism:
3O2 + 2H2S ----> (SO4)-2 + S + 2H2O
While this reaction eventually fouls the carbon, it does extend its life by many times. The carbon bed must be sufficient in size, as this is a slow reaction in most carbons, and oxygen is only available in small amounts, provided by thermal breathing of the tank, inflow during pump outs, and slow bidirectional flow in the vent hose. The oxygen requirement is easily met, however, as it is many times less than that required to support aerobic tank conditions. Standard ASTM methods (used by Dometic) are not appropriate for estimating carbon life when catalytic reactions are present, because the method substitutes nitrogen for air, eliminating oxygen from the process.
Among other things, the chemistry predicts that the fouled carbon, when spent, will have sulfur and sulfate on the surface. And that is exactly what I saw; black carbon though 80% of the bed depth
, and then whitened carbon at the tank inlet. I love chemistry.
So 3 years from now I will need to block out 5 minutes for a change-out.
Trouble with overflow into the filter? I have a cat and she doesn't heel much, but I do sail her hard. I installed a water
filled loop bypass (like a sink trap) and I don't overfill the tank. Trouble with pump-outs, clogged vent, or anything else? No. I do spray some treatment into the bowl frequently to keep it clean with out scrubbing, and that seem to keep odors in the tank manageable, even without good air exchange.