Holding Tank Odors vs. Carbon Vent Filter

 rev.4-16-2017



PDQ did many things right, but they chose the holding tank vent location very poorly, at least on my boat. Anytime the boat is on the hook and aligned with the wind everything emitted from the vent is swept between the hulls, up into the salon, and out through the cockpit. We are aware of every flush within seconds.

Good holding tank ventilation--promoting good aerobic conditions--really helps. Unfortunately, our vent is practically at the dining room table. Even though our vent is plenty short (15 inches horizontal and 15 inches vertical in 3/4-inch tubing), it is still foul. I could simply move it, but then it would be too long.

Adding chemicals is easy and helps. In fact, it is a part of my solution. I've tested many brands (Oderlos, KO, WM, Camco, Thetford--See Practical Sailor reviews) and some really work. Camco Ultra Concentrate, Odorlos, Forespar Refresh, and No-Flex are favorites. You can add some with the first use after pump-out, or you can add some each day, as a bowl cleaner. This really works, treating the problem from two ends.

Some say the carbon filters are too expensive, $125/year and up. Well, not actually.

• They actually last 2-5 years based upon user experience. One-year replacement is only manufacturer guidance.
• Refills (if you make a re-fillable unit or refill a commercial units) are about $7 and take a few minutes; no longer than replacing the unit.
• A user-built unit can be built a bit over size, promising longer life.

The longer life is due to catalytic activity within the filter (oxygen breaks down the H2S into S in the presence of an active carbon surface) and regeneration in the off season (bleed-off during low load); we see both effects in industry. The ASTM laboratory method folks used to project life has 2 fatal flaws: it uses nitrogen instead of air (no oxygen), and it takes hours instead of months (to little time for catalysis and no reduced load periods).

Carbon vent filters can present an over- or under-pressure hazard, causing holding tank explosions and implosions if plugged; this has happened when the vent--filter or no--became plugged with mud dauber nests or poop. But this is a matter of proper installation. Certainly, just cutting the hose and inserting a filter in the line can be asking for trouble; refinery explosions have started that way. Did the accident investigation, have the tee shirt.

What are the installation requirements?

• The unit should be placed above the through hull so that it cannot get wet. Carbon does not function for gas absorption as well when wet, and a salt coating doesn't help either.
• There should be a by-pass so that if the holding tank over-flows or sloshes the filter will not be contaminated.
• The bypass must stop holding tank gases but pass overflows at low pressure drop.
• There should be a bypass vent (in and out) to protect the holding tank if the filter plugs.
• The bypass and back-up vents must be simple to maintain and not fail if dirty, corroded, or frozen. 

This system has been in service for seven years. The only change has been to replace the clear tubing with 3/4-inch white sanitation hose, to reduce permeation and eliminate kinking.

Simple enough. Our installation, in words:

• The vent comes from the holding tank to a tee. The vertical leg goes through a simple trap, like that under every home sink; this provides for by flow pass and while holding back sewer gas, just as it does under your home sink. The trap is a simple loop of hose. After the loop it exits the original through-hull.
• The horizontal leg of the tee leads to the inlet of the filter, through the filter, and exits the same through hull via a tee.
• The trap is filled with water (glycol in the winter). The trap is clear hose so it's obvious if it goes dry. Generally seawater splashes in and maintains the level, but if not, it is simple to refill. Additionally, if splashing does not fill the trap, glycol can be used , which will not evaporate. The clear hose was later replaced with Shields 148 white hose; although the clear was allowed us to see what was going with the trap, it soon began to kink and also permiated, allowing some odor in the compartment. The sanitation hose solved all of this. White sanitation hose is a bear to work with even in small sizes (the ends must be heated in hot water to fit) but it's about the only sanitation hose available in 3/4-inch.

The photo below shows the refillable end of the filter (2-inch Fernco no-hub stretched over the cap).

The filter is built from 12 inches of 2-inch PVC pipe; both ends are capped and 3/4-inch NPT fittings tapped in. The lower cap is cemented on, while the upper is sealed with a no-hub fitting. The varnished bracket was simple and a pleasure to build--two 2-inch PVC conduit clamps (Home Depot has these) would have been simpler, lighter, and less fun. The hose is clear and certainly permeable to stink; however, my holding tank is in a bulkheaded compartment so it does not matter. If the location were otherwise, I would still keep the loop clear, but I would make the other hoses sanitary grade to reduce odor and improve durability (much of the hose was late replaced with 148 series sanitation hose, as the vinyl kinked over time). The carbon is from an aquarium store; while not optimized for this application, it's available and suitable. Pelletized carbon will allow better airflow and better resist clogging than the fine granules more commonly used for water treatment. An air permeable open cell sponge supports the carbon at both ends, distributing air and keeping the carbon in place; the aquarium store should have that too. Pack the carbon firmly, tap it gently on a counter top for a minute to settle it in, use tight sponges, and slant the filter at least 15 degrees, all to forestall future settling and resultant by-passing. If you can hear anything when shaking the filter, you did something wrong.

 This test filter has been in service on a holding tank test stand since April 2011. I intentionally sourced everything from Home Depot or Petco so that the design would be simple to recreate. The hose is 5/8-inch.

Identical, except I made this one refillable (one end is attached with a boot instead of glue). In this case the hose is 3/4-inch (OEM spec on this boat). This one is in service on a PDQ 32, also since April 2011.

Note: We replaced much of the clear vinyl tubing with Sheilds 148 because of kinking problems. It's a bear to work with, even in small sizes, but it's about the only sanitation hose available in 3/4-inch.

The cost for the complete project was about $25, including the hose modifications, but the might-need drawer supplied a good number of fittings and tapping saved a few hoses and a few fittings. I think it might have been ~ $50 if everything came from the store. Carbon life has proven to be about four years, and service takes about 10 minutes and cost $7.

The down sides?

• If you have a monohull this may not work for you, a consequence of leaning over on the rail. Catamarans don't heel. You may have difficulty mounting the filter high enough. Power boats generally do not have this problem, but often they mount everything low under the floor and it fails because of that. Dumb.
• Back flushing the vent is more difficult. However, we've never needed to, since we don't overfill the tank and don't heel. This could be solved with a valve to isolate the filter (only the end near the outlet would require protection).
• Solids reduction is not as good as it is with chemicals and fresh air, but no worse than with no treatment. We've not had trouble pumping out. My suspicion is that serious pump-out problems are caused by non-flushable materials.

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DIY Note,  2-16-2012
Internet posts on refilling filters. There are a bunch of them out there, and there are some falicies. I doubt any of them are getting full life. Please see my description, above.

• Don't cut the filter in the middle or even a short distance from the end; it must be cut at the end cap. Unless it is cut at the end, you will never get the unit completely full, which means the carbon will settle to one side and the stink will by-pass. In my design the inner pipe extends to within 1/2-inch of the end cap, the pipe is filled a bit proud, and the foam insert compresses when the unit is assembled. 
• No effective by-pass for waste. Folks mount filters in a location that allows it to get wet; that is the primary reason they need service. This guy gave some thought to the problem, but perhaps doesn't have the space for a proper cure.
• Pack the carbon in. Gently tapping the unit on a hard surface for several minutes while holding it vertically will settle the carbon. Put the end cap on in such a way that the foam supports at each end are compressed. The pressure will keep the carbon in place.