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Old 27-10-2014, 15:19   #1
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Holding Tank Vent Sizing

Hey alll, I'm re-doing the hose for the head/holding tank system. While doing research for this project I've seen some info that suggests the standard 1 vent of 1/2" or 3/4" diameter isn't really enough to really ventilate the tank. The research seem to indicate that at the least a bigger vent diameter or maybe two vents would be the way to go to really be able to keep the tank ventilated.
My thoughts on doing more to ventilate the tank mainly lie in keeping plenty of air flowing though to promote the good aerobic bacteria doing the job over the "bad/stinky" anerobic bacteria.

The end result of all this thinking and research is this question: would it be really beneficial to up the vent size to 1" and add a second 1" vent to this. Also, since the boat swings to the wind on a mooring I'm thinking of adding a small removable shroud to the windward vent thruhull to get a little more air through the tank while at the mooring.

Any thoughts? complete waste of time... or maybe a worthwhile thing?
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Old 27-10-2014, 15:22   #2
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Re: Holding Tank Vent Sizing

You will have to figure out how to connect these larger vent hoses to your holding tank but recalling Peggie Hall's book on marine sanitation, if you're willing to go to that trouble it should be worthwhile. I wouldn't bother with the shroud though, just normal vents.
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Old 27-10-2014, 15:24   #3
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Re: Holding Tank Vent Sizing

From my understanding your correct, the more air the better, with the ideal being airflow if that could be established.

Peggy would be the best to comment though
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Old 27-10-2014, 15:45   #4
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Re: Holding Tank Vent Sizing

A carbon/charcoal filter on the vents will make evening G&Ts in the cockpit much more enjoyable !
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Old 27-10-2014, 15:51   #5
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Re: Holding Tank Vent Sizing

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Originally Posted by steamgoat View Post
A carbon/charcoal filter on the vents will make evening G&Ts in the cockpit much more enjoyable !
A charcoal filter slows down air flow and would negate the advantage of the larger and or two vents. You either ventilate the tank and reduce odors or you put a filter on the vent to try to contain them.
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Old 27-10-2014, 15:55   #6
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Re: Holding Tank Vent Sizing

Ventilate it as much as you like, S*it still stinks !!
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Old 27-10-2014, 16:23   #7
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Re: Holding Tank Vent Sizing

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Ventilate it as much as you like, S*it still stinks !!
No, actually, you don't understand how a holding tank works. Yes, of course it stinks at first. With good aerobic bacterial action, though, that stink goes away very quickly. Put a filter on it and you kill the air flow that the aerobic bacteria need. So instead of a brief period of stink that clears quickly, you get filtered stink that lasts, and lasts, and lasts.

I'll take the former over the latter any day of the week, and twice on Sunday.
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Old 27-10-2014, 18:06   #8
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Re: Holding Tank Vent Sizing

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No, actually, you don't understand how a holding tank works. Yes, of course it stinks at first. With good aerobic bacterial action, though, that stink goes away very quickly. Put a filter on it and you kill the air flow that the aerobic bacteria need. So instead of a brief period of stink that clears quickly, you get filtered stink that lasts, and lasts, and lasts.

I'll take the former over the latter any day of the week, and twice on Sunday.
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Old 27-10-2014, 18:17   #9
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Re: Holding Tank Vent Sizing

And don't use a vent with a screen on it, use a simple mushroom thru hull.
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Old 27-10-2014, 18:54   #10
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Re: Holding Tank Vent Sizing

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Originally Posted by rwidman View Post
You will have to figure out how to connect these larger vent hoses to your holding tank but recalling Peggie Hall's book on marine sanitation, if you're willing to go to that trouble it should be worthwhile. I wouldn't bother with the shroud though, just normal vents.
Installing/relocating a fitting in a plastic tank is actually very easy these days, thanks to a li'l doodad called the Uniseal. UNISEAL Check out the instructions here. Uniseal Instructions Tank location and even just the location of the vent fitting can make a different....if you'd like to discuss your options, send me a PM.
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Old 27-10-2014, 19:05   #11
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Re: Holding Tank Vent Sizing

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Ventilate it as much as you like, S*it still stinks !!
Only if it's stored in anaerobic conditions, which regrettably is true of far toomany holding tanks.

It's only when organic matter breaks down ANAEROBICALLY--without oxygen--that it can generate stinky--and also highly toxic--gasses....sulphur dioxide, hydrogren sulfide, and methane (which contriary to popular belief is odorless). When organic matter breaks down AEROBICALLY--with oxygen--it converts to CO2, which is odorless. Filters only block odors that already exist...oxygen prevents odors from occurring in the first place.

Unfortunately, some systems are so badly designed (if you can call them "designs") by builders, yards and DIY owners that they can't be sufficiently ventilated to prevent odor without major expensive refitting...leaving a filter as the only solution.
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Old 27-10-2014, 19:21   #12
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Re: Holding Tank Vent Sizing

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Originally Posted by peghall View Post
Installing/relocating a fitting in a plastic tank is actually very easy these days, thanks to a li'l doodad called the Uniseal. UNISEAL Check out the instructions here. Uniseal Instructions Tank location and even just the location of the vent fitting can make a different....if you'd like to discuss your options, send me a PM.

PM sent, Thanks in advance for any help you may be able to provide! I bought your book a week or so ago and that's more or less where I got the idea that more ventilation would be beneficial.
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Old 27-10-2014, 20:18   #13
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Re: Holding Tank Vent Sizing

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Originally Posted by Yeti View Post
Hey alll, I'm re-doing the hose for the head/holding tank system. While doing research for this project I've seen some info that suggests the standard 1 vent of 1/2" or 3/4" diameter isn't really enough to really ventilate the tank. The research seem to indicate that at the least a bigger vent diameter or maybe two vents would be the way to go to really be able to keep the tank ventilated.
You might try different sizes and numbers of vents; an experiment to see what works best for you. Let's say you normally run a ratio of a half dozen no.1s to each no. 2, and that you pump out once a week. Deposit six 1s and one 2 in a toilet bowl size mixing bowl from you kitchen, add a bucket of seawater, tightly tape a cardboard cover over the bowl, punch a single 5/8" hole in it, and tape 5 or 6 feet of 5/8" garden hose to the hole. Over the course of a week sneek-a-sniff from time to time and record your results. You could do this placing your nose near the end of the hose and pressing down on the cardboard lid to simulate a flush into a holding tank. If 5/8" is not big enough, repeat the experiment with a fresh charge and simulate a larger vent using your wife's 1-1/4"vacuum cleaner hose. If that does not give you the desired result, you might try two 1-1/4" hoses by borrowing a second vacuum hose from a neighbor. Alternatively, you could try your 2" shop vac hose or maybe a piece of swimming pool hose if you find that a still bigger vent is required.

By doing the experiment you will know what works in your case (It may be ration dependent.) and what meets your expectations (which may be different from those of others). You won't have to depend on internet opinions. You will know what you need.
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Old 27-10-2014, 21:28   #14
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Re: Holding Tank Vent Sizing

Peggie Hall is too polite and demure. And many of the folks here are pretty dense. I built my holding tank system using her guidelines, many years ago. She advocates for an aerobic digestive system. For those who haven't quite figured it out yet, that means a holding tank that supports the consumption of anaerobic bacteria (found in fecal waste) by aerobic bacteria. You have to add the aerobic bacteria to the storage tank, AND you have to provide the little beasties with plenty of fresh air. That means a 1 1/2 inch hose bringing fresh air INTO the tank and ANOTHER 1 1/2 inch hose venting it OUT. It works a heck of a lot better than simply dumping a mixture of formaldehyde and perfume and blue dye into a tank to kill the anerobic bacteria, and allow the resulting waste gases formed to escape and make the surrounding area pretty smelly.

You don't even need to buy her book (BUT, YOU SHOULD, BECAUSE SHE DID HER HOMEWORK WHEN YOU DIDN'T) to have a great holding tank system.

Thank you Peggie, by the way, for the Uniseal info. I just removed my holding tank system to repaint the head compartment, and this item will make my replumbing much easier. Much appreciated.
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Old 28-10-2014, 08:08   #15
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Re: Holding Tank Vent Sizing

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Originally Posted by Roy M View Post
Peggie Hall is too polite and demure. And many of the folks here are pretty dense. I built my holding tank system using her guidelines, many years ago. She advocates for an aerobic digestive system. For those who haven't quite figured it out yet, that means a holding tank that supports the consumption of anaerobic bacteria (found in fecal waste) by aerobic bacteria. You have to add the aerobic bacteria to the storage tank, AND you have to provide the little beasties with plenty of fresh air. That means a 1 1/2 inch hose bringing fresh air INTO the tank and ANOTHER 1 1/2 inch hose venting it OUT. It works a heck of a lot better than simply dumping a mixture of formaldehyde and perfume and blue dye into a tank to kill the anerobic bacteria, and allow the resulting waste gases formed to escape and make the surrounding area pretty smelly.

You don't even need to buy her book (BUT, YOU SHOULD, BECAUSE SHE DID HER HOMEWORK WHEN YOU DIDN'T) to have a great holding tank system.

Thank you Peggie, by the way, for the Uniseal info. I just removed my holding tank system to repaint the head compartment, and this item will make my replumbing much easier. Much appreciated.
Roy M, How do those big vents exit the hull. Did you just use a mushroom thru hull? Do you put a cover on it to keep water out when you are healed? I would like to add another vent to mine but I was wondering how people did this. The only thing you see online is the little 5/8" vent outlets with the screen which seems to counter everything that is being recommended here. Any advice would be appreciated.

Jim
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