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, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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