Holding Tank Vent Filter

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by Ralph Caruso


I just finished building a vent filter for the holding tank. The breakthrough thought came from my wife, while we were discussing the possibilities, and I asked what the filter medium could be. She immediately suggested activated charcoal, which makes perfect sense, and I was soon off to Hechingers for parts. I built a U-tube structure with 2" PVC sanitary piping and 3/4" elbow end fittings to connect into the vent tubing where it rises up past the electrical panel in the aft-most cuddy on the port side. The U is about 12" long, and 8" wide, and is filled with a mixture of straight activated charcoal and something called "Ammocarb". These came from a pet store, where they are sold in bulk to people who own fish. They are used to filter out the same noxious gases in fish tanks. I have never seen the commercial product, and didn't know how big the absorber bed is, so I figured that a bed that was 2" in diameter by about 30" long should be fine. If not, I can extend it by adding some more loops. The U also has a side tap so that I can empty and refill it when the charcoal is exhausted. The whole thing cost about $20 in parts,and about $10 for the charcoal. And I can re-fill it very cheaply. I filled one leg with Ammocarb, and the other with straight charcoal. It works very well, and we now don't have to worry about stinking up a whole raft of boats any more!