Real-world Advice from Holding Tank Makers


Before beginning a series of tests, we always involve the manufacturers. They have a wealth of information they are happy to share, in the interest of reducing problems with their products. In general, their advice forms a consensus.

Getting Started

Start with a clean tank. Many of these chemicals work in very different ways, and mixing them will be wasteful at best and harmful at worst. If you’ve chosen the disinfecting chemical, emptying the tank and giving a quick flush with the hose should be adequate. If you’re using a bio-augmentation approach, multiple flushes are required after using any disinfecting chemical.

Head Operation

Keep the doses steady. Don’t throw a bunch in at the start, and then nothing, and then try to catch up. Steady doses according to the manufacturer’s recommendation are required. Proper use of bio-augmentation chemistry on boats that are seldom used can be a challenge, as the chemicals can be exhausted while you are away.

Increased Tank Ventilation

More air in the holding tank is a help for any of the bio-augmentation or bacterial treatments. At a minimum, the vent should be ¾-inch diameter, no longer than 3 to 5 feet and rise no more than 18 inches above the holding tank. However, bigger is better, and 1½-inch vents are often recommended.

Use in Combination with Vent Filters

Vent filters can do a fine job of controlling odors, but at the cost of some increase in tank solids and significant increases in hydrogen sulfide generation (though you won’t be exposed to it). It has been suggested that Odorlos can be used occasionally in a tank fitted with a vent filter to control solids. Does it work? Based on our testing, it helps, though not as much as fresh air. However, we can ONLY recommend bio-augmentation products for use in combination with carbon filters, lacking sufficient information regarding the interaction of formaldehyde and other sterilizing chemicals with carbon. Use in combination with Type I MSDs. Stick to manufacturer recommendations. Chemical reaction is possible and could cause damage. Vent filters can’t be used with Raritan Type I MSDs because of potential for carbon over-heating.

Caution: The exploding holding tank

Sealed holding tanks can build significant levels of methane and hydrogen sulfide. More than 20 percent methane is considered explosive. We did not register such high levels in our test tanks. Use caution when working near a tank opening, or drilling holes. Hydrogen sulfide is very toxic; the safe eight-hour exposure limit is 10 ppm; it is considered immediately dangerous to life and health at levels over 100 ppm. A single breath can cause unconsciousness at 300 ppm, and we measured levels as high as 370 ppm in untreated tanks with poor ventilation.

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him by email at