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the pointy end is the bow
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The wife requires one of those holding tank vent filters on our vent line. They run around $70 to replace. When I was shopping online for a better price, I came across an article by a fellow who made his own out of PVC pipe and activated charcoal from an aquarium supply store. I checked out our local pet store and found granulated activated charcoal. Before I spent too much time trying to save $40 bucks, I thought I would check to see if anyone else has done it and whether they would continue doing it.

How to recharge holding tank vent filter -- w/pics
 

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It sounds like reasonable experiment to me with very little downside. The pvc parts are cheap. I'm guessing you'll need a fine screen at both ends to keep the carbon granules in place. I could see cutting an oversize circle of screen and slot it from the edge to the center. Then pull the edges in to form a cone and let it expand into the pipe. This will leave a large surface area for air to flow through.
 

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The truth is that a filter in the vent line is just as likely to make things worse, rather than better. The great majority of holding tank odors are a direct result of inadequate venting. Reducing the airflow, by putting a filter in the line, only exacerbates the problem. What you need is a larger vent line that flows more air.

Get and read this book, or you will be chasing odors in vain for a long time to come!

Amazon.com: Get Rid of Boat Odors: A Boat Owner's Guide to Marine Sanitation Systems and Other Sources of Aggravation and Odor (9781892399151): Peggie Hall: Books
 

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The truth is that a filter in the vent line is just as likely to make things worse, rather than better. The great majority of holding tank odors are a direct result of inadequate venting. Reducing the airflow, by putting a filter in the line, only exacerbates the problem. What you need is a larger vent line that flows more air.
Even better, and along with a larger diameter and less restrictive vent, is to continuously pump a small amount of air into the bottom of the holding tank so that the 'fermentation' becomes aerobic. You need proper 'mass transfer' of air to affect and promote aerobic fermentation ... think of a quite small 12 vdc air pump discharging into a porous stone (aquarium supply stuff) at the very bottom of the tank. This is called 'sparging' which mixes air with the contents, as rising air bubbles keeps the tank mixed for better aeration.

There are many many 'types' of activated carbons, some for liquid applications and some for gaseous (odor) applications ... NorthAmerican Norit Corp. is the probable largest supplier of such activated carbon; suggest you go to their technical website and choose the correct carbon type for the correct application.
Further, such carbon chambers have to be correctly designed as the adsorption of odors is not an average of the amount of material in the 'chamber' but the odor is removed as a 'front' (isotere) similar to what happens as a cold front of air changes humidity conditions during a storm as the 'front' moves across the land. Such carbon swells in contact with humidity and the 'inlet' has to be correctly designed so that the entry humidity doesnt cause swelling of the carbon and thus block the 'entry' thus reducing the amount of 'front' of moisture/odor that enters the chamber. The commercially available 'odor vent filters' already have this all worked out so to get efficiency of removal.
 

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Or you can go to eBay and buy AirForce charcoal vent filters for about $46.00 per filter.
 
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