On a Side Note: Holding Tanks

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Although none of the test tanks or the test boat’s holding tank accumulated solids sufficient to restrict pump-out, we observed significant differences. While vent filters did not increase tank solids beyond what is normal in a poorly ventilated and un-treated tank, some of the subject tanks did better than others.

The holding tank with a 1½-inch vent was effectively free of solids, most of the waste breaking down into liquid. Tanks with effective treatment chemicals  were also solids free, as was a tank fitted with a bubbler that aerated the waste liquid. However, tanks fitted with either vent filters or extended 5/8-inch vent lines had similar amounts of solids; less than was introduced, but still a noticeable amount. Treating tanks with vent filters using effective treatment chemicals also reduced tank solids, though most boaters won’t want the expense of both a vent filter and a treatment program. Further tests are in progress.

Because vent filters restrict airflow, they tend to generate more stink and to bottle it up. Any tendencies for hose permeation will be increased.

Some boats are prone to vent plugging, a consequence of insect nesting, and waste splashing or overfills. While bugs can nest anywhere, waste plugging is generally in the tank vent fitting, often because the vent fitting was not located on the top center of the tank, where splashing is least likely. Back flushing with a garden hose after each pump-out is often recommended, and vent filters complicate this practice. If the filter is installed in-line (not recommended), an annual inlet disconnection and flush should be a part of your commissioning exercise.

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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