Holding tank vent filter

Back on inland waters in the States we are in holding tank territory.  Regulations are well and good but they don’t always come with a support system, so we all the more value the free pump-out boats here in Martin County Florida that come whenever you call.

Our heads are manual sea water pumps and the extra organic matter that goes into tank storage creates a powerful biology experiment.  Holding tanks have air vent lines and benefit from good airflow to avoid stinky anaerobic bacteria, yet that airflow sometimes wafts up into our cockpit and cabin.

After replacing any older hoses with smells still coming out of the vent line, we realized we needed an in-line carbon filter.  It really works!!!   

Here’s how ~ DIY courtesy of Mark:

Holding Tank Vent Filter

One problem with catamarans is the Venturi effect caused by air flowing over the cabin and creating a small low pressure area at the stern.  Since Manta put the holding tank vent thruhulls underneath the bridgedeck, this causes tank odors to be sucked back into the cockpit and cabin.

We spent quite a bit of time figuring this out.  We would have bad smells in the cabin, but couldn’t smell anything when we put our noses into the tank compartments and near the hoses.

So a vent filter was necessary.  You can buy vent filters, but they cost $90/each.  All they consist of is a PVC pipe filled with activated charcoal.

We are cheap, so we made two of them ourselves for a total of ~$30 of parts.  If you have some scrap PVC around, or can wait for an internet order, this cost could be much lower.

Here is what you need:

1. One foot long piece of 2” PVC pipe.  This can really be any length/diameter you want - I just decided this was a good size.

2. PVC end fittings to cap the pipe and fit a 5/8” hose barb converter.

3. A 5/8” hose barb/MPT converter for each end

4. A scotchbrite scrub pad to use as an end stop

5. Activated carbon pellets used in fish aquariums and air filters.  I bought ours at Petco ($15), but they are probably cheaper at aquarium stores.

6. PVC pipe cement

I chose to make these refillable by using PVC endcaps that can unscrew.  This added some parts cost.  In reality, I suspect this will last several years, after which I will probably just throw it away and make a new one anyhow.

For ours, I used a female PVC pipe-thread fitting on the tube. The mating half is a male pipe-thread fitting glued to a PVC pipe-3/4” converter.

Alternately, it would be simpler and cheaper to just glue the pipe-3/4” converter directly (and permanently) onto the filter pipe.  Or drill and tap a PVC cap to fit a hose barb.

Here is how to build it:

1. Glue the female pipe-thread fittings on both sides of the PVC pipe.

2. Screw in the hose barb converters to the PVC pipe-3/4” caps and glue the caps to the male pipe-thread fittings.

3. Cut out circles of scotchbrite pad and fit one inside each cap.  Screw one of the caps onto one end of the pipe.

4. Fill the other cap with carbon pellets and put another circle of scotchbrite pad on top to contain the pellets in the cap.

5. Fill the pipe with carbon pellets being sure to tap them down well by bumping the pipe up and down and rapping it on the side.

6. When the pellets are within an inch of the top and fully settled, screw on the other end cap.  The reason you filled this cap with pellets is that it isn’t possible to fill the tube and the cap with the cap already on.

You now should have a completed vent filter that doesn’t rattle freely when you shake it.  If it does rattle a lot, you have not filled it well.  Take it apart and put more carbon in it.  It is important that air doesn’t have a free channel through the pipe.

The port tank is easy because it takes a linear piece of pipe for the filter.  The starboard tank presents a problem because there is no room to fit a linear pipe.

So I made the starboard vent filter a vertical one by adding a 90* barb elbow on one end, and using 1/2” PVC pipe to bring the outlet back down to another 90* barb elbow.  This allows the vent to be fit to the tank vertically by cutting the vent line and installing the vent between the cut.

Parts needed - hose barbs, pipe, cap fittings, carbon pellets, scotchbrite circles

Endcap - endstop scotchbrite filter installed.  The other cap would also be filled with carbon pellets and capped with another circle of scotchbrite pad.

Finished port tank filter

Port tank filter installed

Starboard tank filter

Starboard tank filter installed


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