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.