Holding Tank Vent Filters--Why Carbon Lasts Much Longer Than Laboratory Testing Suggests

Warning: Reading this article may cause a mild ice cream headache in non-chemists.

The primary interaction of carbon with organic vapors is surface absorption, and the published Dometic tests explore this. The test method they used uses only hydrogen sulfide plus nitrogen at steady flow and is designed to measure only adsorption capacity. However, carbon presents 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:

1/2O2 + H2S ---> S +2H2O

While this reaction eventually fouls the carbon--I have observed sulfur crystals fouling the inlet side of my vent filter after several years--it does extend its life by many times. The carbon bed must be sufficient in size, as this is a slow reaction in most activated carbons, and oxygen is only available in small amounts, provided by thermal in-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. Again, the standard ASTM method used by Dometic is not appropriate for estimating carbon life when catalytic reactions are present, because the method substitutes nitrogen for air, eliminating oxygen from the process.

Additionally, the carbon does not need to remove sulfide—or any other odor, for that matter—on a continuous flow basis to attain odor control; it need only temporarily absorb and delay the peak load for a few minutes while the toilet is being flushed. If the filter absorbs the sulfide load only temporarily and bleeds it off over a period of hours, noticeable odors are eliminated. Continuous flow laboratory testing does not measure this “time-delay” influence on surges. In industrial practice, it is not unusual to see carbon beds that have become saturated on a continuous flow basis within weeks continue to serve very well as peak absorbers for many years.

There are limits. Eventually the carbon becomes fouled by non-volatile reaction product--as I said, I have seen sulfur deposits in my filter when it was spent--and damaged by acid build up. Additionally, the bed must be large enough for these slower processes to function.

I hope your headache isn't too bad. But I like to explain why the difference between one person's "theory" and  reality is so difference. Generally, it is not because science is wrong, but rather because their theory was incomplete. 

(In 35 years as a chemical engineer and wastewater guy, I got to work with carbon a good bit. More than marine holding tank guys, I'm betting. I learned where most of the bodies are buried.)

(What got me thinking about this? I've got a couple of articles in Good Old Boat this coming month, one of them on holding tanks. Another guy wrote a good article on safety bypasses, but he got the carbon adsorption bit wrong. Not a big deal, it just reminded me of this chemist detail.)