Sunday, May 24, 2020

Preventing Holding Tank Collapse

Some time back I wrote a post about filtering the effluent gases coming out the holding tank vent at the stern.  As anyone knows, when the breeze is over the stern, the odor can be truly revolting, thus the need for the activated carbon filter.

It works beautifully, by the way.  Until, that is, the charcoal becomes saturated.

Then you simply discard the old charcoal and pour in a new batch.

But I digress.

While I was installing the filter, it dawned on me that, although I took many precautions to minimize the pressure drop it imposes on the vent line, those could be inadequate in one special circumstance:  When the tank is being pumped.

Holy cow - if the pressure drop was too great, the holding tank could collapse!  I don't want to contemplate what a mess that would be!  And Drew, from whom I got the idea for the filter, mentioned that he had seen circumstances where the charcoal actually reduced the H2S to elemental sulfur, plugging up the granules... even worse!

OK, something needed to be done.

I settled on a lightly spring loaded check valve installed in parallel with the filter.  Installed so that its flow direction was inbound, to allow incoming air to bypass the filter.  Here's what it looks like:

Filter with bypass check valve

I had some difficulty in locating a suitable check valve.  One without some kind of spring to hold it shut would leak holding tank off gases out the stern, rendering the filter essentially ineffective.  But if the spring were too strong, the holding tank would collapse before the valve opened.  I finally hit upon the perfect solution - a check valve designed to be installed in the "bubbler" lines in a hot tub - it opens at 1/4 psi - perfect!

Problem solved.



Drew Frye said...


I used a P-trap because I wanted it to be able to by-pass in the event of an overflow as well.

Drew Frye said...

How long did the carbon last?

Robert Salnick said...

The carbon lasted three years, Drew. There was no sign of elemental sulphur

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