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DIY Holding Tank Vent Filter

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Vessel Info: The Midnight Sun
2007 41CMY
Volvo D6-370's
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DIY Holding Tank Vent Filter

Postby Midnightsun » September 11th, 2018, 1:52 pm

This topic came up on the forums so I figured might as well dig this up from 10 years ago which I design built and posted on another forum. Prices probably are too low since this was 10 years ago but it is still dirt cheap in comparison to what is sold on the market today both in finished product and refilling.

You will need to purchase the following.

From Home Depot or any hardware store

A 12" length of 2" abs plumbing pipe
(I had to by a 3ft length @ $5.67)

2 pieces, 2" abs slip x 2" female pipe thread fitting @ $1.46 ea

2 pieces, abs 2"MPT cap @ $0.82

2 pieces, 1/2" NPT x 3/4" PEX
I went all out on expenses for this one and paid $2.50 each for these solid brass babies. The PEX style fitting works better than a traditionally used barb as it is easier to remove the hose from the fitting when the unit needs recharging. However, if you are on a tight budget, they are available in plastic which would be 1/2" npt male x 1/2" male barb. Yes, even though vent lines are 5/8" the 1/2" barb works very well and is easier to reomve. These cost $0.54 each

2 pieces gear clamps @ $0.30 each

you will also need some foam cut to size to prevent the carbon from falling into the vent line.

In the picture below you can see all the parts I listed above


Next step would be to drill out the end cap with a 3/4" bit. Once drilled, you need to run a 1/2" NPT tap through it so you can screw in the hose adapter, Note the material of the cap is quit thick and makes for a very sturdy connection once tightened down.


The next step would be to cut the foam pieces which prevent the carbon from falling out the ends. This is easily done by standing the 12" pipe vertically on the foam and tracing with a felt marker then cutting it out with scissors. This works very well as the foam should be a little bigger in order to get a good snug fit.


Now take the 2 slip x pipe fittings and using abs cement glue them to the 12" pipe


Now take one of the foam pieces and push it into one end so that you will end up about flush with the cap when it is screwed on.


Screw cap on snugly by hand (no need to wrench on this) The flip around and fill with activated carbon to about 1" from where the bottom of the foam will sit, insert second foam piece and hand tighten the remaining cap in place. Here is the finished product


This is a very simple installation. All you need to do is locate the holding tank vent line, cut it in an easily accessible area and insert the tube attaching it with the gear clamps. You may want to attach the tube depending on where it is located. The carbon will last a year and is easily replaced by removing the unit, removing the foam, spilling out the old carbon and filling up with new.

Almost forgot, you can buy activated carbon at any pet store, eBay or Amazon as it is commonly used in aquarium filtration. Here is an example which should be good for about 5 years of refills.


Your dock neighbours will thank you, Happy flushing. :-D
Last edited by Midnightsun on September 30th, 2018, 5:29 am, edited 3 times in total.
Cheers, Hans
2007 Carver 41 CMY
Twin Volvo D6-370's
Montreal, Canada
Midnight Sun I Photos

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Re: DIY Holding Tank Vent Filter

Postby vineyardgray » September 12th, 2018, 11:06 pm

Thanks for the great idea! I'm going to take a look at the vent line on my 88 Santego this weekend. I don't notice any stink but this sure can't hurt for the price. :-)

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