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Old 25-06-2015, 05:31   #1
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Dual vent on holding tank

Boat is 2009 Beneteau 40. 25 gal tall holding tank is behind shower wall, below side deck. Deck pump out port in bottom, macerator, fill from electric head and vent in top. All ports in tank are 1" or 1.5". Vent fitting has 90 deg. 1/2" hose barb off side. About 3 ft. of hose to reverse facing small clam shell small opening vent. Wire mesh has been removed from vent and it flows as best a small vent can. Tank odor is a problem.
Plan is to remove vent fitting in top of tank, install a tee with the branch port of the tee horizontal. Reduce this to ¾” hose bard, install new vent to match ¾” line. Install reducer bushing in top run port of tee with street elbow just under deck. Will install new vent about 1 foot forward of existing vent to connect to top of tee. From reducer bushing or street elbow, install standpipe that would extend thru the tee and into the tank several inches. While underway, air would enter forward vent, flow thru the standpipe into the tank and exit thru annular area around standpipe and out branch port to rear vent. Probably install plastic clam shell over forward vent to help build some pressure.
After 7 months in the Bahamas, we are in Fort Pierce FL getting some bottom work done. I am having difficulty finding anything other than fuel type vents that are bigger than the 1/2" that came on the boat. Looking for thoughts and suggestions on the dual vents. Using a thru hull mushroom type fitting has been suggested. Changing out the head is not an option.
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Old 25-06-2015, 05:58   #2
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Re: Dual vent on holding tank

I have a 40 gal holding tank and the vent tube I put up my main mast its about 1/4 inch and works fine . Why so big ?
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Old 25-06-2015, 06:23   #3
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Re: Dual vent on holding tank

Why limit yourself to the traditional metal vents. I've installed several using a plastic tru hull fitting. bigger is better I prefer one inch and a half
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Old 25-06-2015, 06:43   #4
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Re: Dual vent on holding tank

Raritan Marine, the toilet and holding tank manufacturer, has a publication dealing with holding tank installation and venting as well as the use of various odor control chemicals. They recommend the use of two holding tank vents, locating one on each side of the hull as there is almost always a pressure differential between one side of the hull and the other, even when anchored or in the slip. This provides air flow thru the tank, even when the boat isn't moving.

I can vouch that the system is very effective when used with the correct additives.

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Old 25-06-2015, 07:09   #5
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Re: Dual vent on holding tank

From Peggy Hall (and as quoted by Raritan):

"... The tank vent line should be as short, as straight and as horizontal as possible, with no sags, no arches and no bends. The minimum I.D of the hose (which is the “standard” size in use today, but for no reason other than being “standard” in fresh water and fuel tanks) is 5/8”; we recommend that it be at least ¾”.
Ideally, it should be no more than 3’ long. If it has to be longer or if running the vent line uphill more than 45 degrees off horizontal can’t be avoided or if it’s impossible to run a vent line that does not go around a corner, increase the size of the vent line to 1” or even larger.
If, for instance on a sailboat, the line must go up to the deck, install a second vent line in order to create cross ventilation or install some mechanical means of forcing air through the tank ..."

ODOR CONTROL ➥ http://home.online.no/~lha-h/forum/phall.pdf

http://www.raritaneng.com/pdf_files/...nk/L165htw.pdf
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Old 25-06-2015, 07:55   #6
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Re: Dual vent on holding tank

Following the Gospel of Peggy Hall, I use TWO 1 1/2 inch diameter vent hoses that lead directly overboard on either side of the forward portion of the main hull. Since the tank uses aerobic digestion bacteria, and one side of the hull always provides positive pressure, the little beasties always get plenty of fresh oxygen. Hence, no smell. Read the sites that GordMay has kindly provided (and that Peggy Hall composed).
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Old 25-06-2015, 09:42   #7
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Re: Dual vent on holding tank

As long as y'all are taking my name in vain...

Plan is to remove vent fitting in top of tank, install a tee with the branch port of the tee horizontal. Reduce this to ¾” hose bard, install new vent to match ¾” line.

Won't work...air will just travel back and forth across the top of tee instead through the tank.

You may not need two vents...just shortening and straightening a single larger diameter vent line (easy to install using a li'l doodad called the Uniseal UNISEAL ) may be enough solve the odor problem. Or, you may need to do nothing more than straighten and shorten the vent you have and replace the thru-hull...and switch to a tank product that isn't lethal to every living organism on the planet or totally ineffective. Send me an email if you'd like to talk a bit.

Quote:
Originally Posted by motion30 View Post
Why limit yourself to the traditional metal vents. I've installed several using a plastic tru hull fitting. bigger is better I prefer one inch and a half
I've always recommended a plain ol' bulkhead or mushroom thru-hull for two reasons...it allows a lot more air exchange and also lets you stick a hose nozzle up against it and back flush the vent line to prevent blockage.

I have a 40 gal holding tank and the vent tube I put up my main mast its about 1/4 inch and works fine . Why so big ?

That arrangement creates an anaerobic "septic" tank that by its very anaerobic condition, has to stink to high heaven (literally to high heaven on your boat). By running the vent up the mast, you aren't even attempting to MANAGE the system, you've just sent the odor out into the atmosphere high enough above deck to keep it away from those on board your boat....but not always from all those downwind of you.

"Standard" size for all tank vents--water, fuel and waste--is 5/8".. a 1/4" is half that size--too small to adequately vent methane, which although odorless is high flammable. Lightning hitting your mast in the right conditions could be kinda interesting! It's also likely to be nylon water hose, which permeates in a heartbeat and also deteriorates, letting gasses escape into the boat.

Why so large? Because the key to odor elimination is oxygen. When organic matter breaks down in an anaerobic (airless...think septic tanks and stagnant swamps) environment, they generate hydrogen sulfide, sulfur dioxide--stinky gasses that are toxic, even lethal in high enough concentration--and methane. But when organic matter breaks down AEROBICALLY (oxygenated...think compost piles and running streams), it converts to CO2, which is odorless. A 5/8" vent line, especially one that takes a tour of the boat before exiting the hull, but even a relatively straight one that ends in a metal cap with a small slit in it, can allow enough fresh air exchange to keep the tank aerobic...but a larger, shorter, straighter vent that ends in an open through hull can.
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Old 25-06-2015, 11:38   #8
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Re: Dual vent on holding tank

I have a large stainless steel tank with a single vent, and no problems. I use a tank addidtive...its blue, can't recall the name. I think its the cheapest stuff at west marine. It has a west marine label/brand. I figure if there's one think west marine knows how to do, its how to make crap more desirable.

I mix about 100ml with a litre of water in an old nalgene container (with bpa). It stays next to the head, and after each use, about a half cup of the diluted stuff goes in. This leaves a nice fresh scent at the head (and blue water), and keeps the head tank smelling fresh...or rather, no smell at all.

I know this isn't the info you are looking for, but wanted to share my setup with others.
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Old 25-06-2015, 18:31   #9
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Re: Dual vent on holding tank

Thanks for so many great replies. A little more about the system. Tank is 2/3 of the way back on starboard side. Top of tall skinny tank would barely have room for an additional fitting in the top. Getting a second vent hose to the port side would be over 15 ft long and have to go thru the bilge, at least a 3 ft drop on height. This is not a good option

Peggy, I did not do a good enough job of describing what I was thinking about doing. The new vent would be connected by a smaller stand pipe around 3/4" inside the tee that would extend 4" plus into the tank. The existing vent would exit thru the horizontal oriented branch. While this would not guarantee total circulation of air, it would be much more than just passing over the top of the tank thru the tee.

As the current vent and new vent would be around 3 foot above water level, there would be minimum water splash at this level and we do not sail with the rail in the water. Going to a 1" vent will be the next project. If that does not improve the tank smell, a second vent using some form of standpipe to get air into the tank would be the next project. Will update as project progresses.
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Old 25-06-2015, 18:32   #10
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Re: Dual vent on holding tank

I had odour problems and rebuilt the whole system 2 years ago after reading all I could find. Twin vents were sometimes recomended but I couldn't fit two easily. Don't forget to use the best odour resistant pipe on the vent. Mine is a 1" single vent going as straight up as possible with no downward bends exiting on the cabin side. Use a standpipe of rigid PVC tube from the top to near the bottom of the tank to suck the contents out so the tank contents doesn't sit in a bottom exit drainage pipe. A tank bottom side drain will never remove all the contents as it is 1,1/2" diameter plus room for the fitting. My new tank has a V shaped bottom so almost all the contents could be removed via the stand pipe. Use the best quality expensive flexible odour resistant piping everywhere. It's also easier to work with. Rigid PVC and elbows can be used in places. There are domestic plumbing tails that can connect PVC to flexible. Don't have dips in pipes where waste can remain. And an excellent tip from microbiologist Tom who has a boat near my berth. Flush out the whole system with fresh water before finishing sailing and leaving the boat. Apparently sea water contains bacteria that produce sulphur dioxide, the rotten egg smell, when left standing in the system. This fresh water flushing really works well. Tom also recommends Bio Magic additive but I haven't needed to try that. My hand basin drains into the toilet so it's easy to put a few litres of fresh water through the system. When emptying at sea a thorough sea water flush through the pipes a few times is fine as it will not be sitting long unused.

Now after doing all that I have no odour problem whatsoever. But importantly with your vent use the best quality odour resistant piping taking it as straight up as possible. I think the piping I used was Triton from memory. The smallest size available was 1" for the vent. It's thick and rubbery but easy to fit. I use liquid soap to slide it on fittings and double clip where possible. I found a lot of this information on line written by John Wilson from General Marine Services Auckland NZ. I'm sure a Google search will turn up the article. I purchased many of my materials there also as GMS are nearby.
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Old 25-06-2015, 20:49   #11
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Re: Dual vent on holding tank

Quote:
Originally Posted by trishark View Post
Peggy, I did not do a good enough job of describing what I was thinking about doing. The new vent would be connected by a smaller stand pipe around 3/4" inside the tee that would extend 4" plus into the tank. The existing vent would exit thru the horizontal oriented branch. While this would not guarantee total circulation of air, it would be much more than just passing over the top of the tank thru the tee.
That wouldn't work either unless you install a fan to force air down the standpipe. We're dealing with ambient air that isn't under any pressure...it's gonna take the line of least resistance, and ambient air doesn't move vertically very well. Passive ventilation works best in short, straight lines that are as horizontal as possible.

[/QUOTE]As the current vent and new vent would be around 3 foot above water level, there would be minimum water splash at this level and we do not sail with the rail in the water. Going to a 1" vent will be the next project. [/QUOTE]

If it's short enough (5' max, 3' or less is better), straight enough (if a bend is unavoidable, make it a gentle one even if you have to make the line a bit longer than optimal)...doesn't rise more sharply than 45 degrees, AND you use good tank product (My favorite these days is Odorlos (Odorlos which is more widely available in RV supply stores and than in marine stores--and for a LOT less than marine store prices) a single vent should work. Since the thru-hull isn't ever likely to spend time under water, put a forward facing clam shell on it-a nice large high one--to function as an air scoop when you're underway or at anchor.

Btw... The holding tank is almost never the source of odor INSIDE the boat...because odor inside the tank has only one place to go: out the vent when the toilet is flushed or the boat rocks. So if you're trying to elimnate odor inside the boat, nothing you do to the tank or the tank vent will help. The most common sources of odor inside the boat are permeated sanitation hoses and/or wet dirty bilges...sumps...trapped water below the sole somewhere. The only cure for permeated hoses is new hose...dirty bilges and sumps only require some detergent, water and effort--but NO BLEACH!
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Old 26-06-2015, 04:44   #12
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Re: Dual vent on holding tank

Correction: Trident was the brand of sanitation hose I used. Not Triton. Also the correct term is dip tube not standpipe for the PVC pipe which is fitted in the top of a holding tank and going down near the bottom used to draw the contents out from above.
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Old 26-06-2015, 05:29   #13
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Re: Dual vent on holding tank

Quote:
Originally Posted by peghall View Post
Btw... The holding tank is almost never the source of odor INSIDE the boat...because odor inside the tank has only one place to go: out the vent when the toilet is flushed or the boat rocks. So if you're trying to elimnate odor inside the boat, nothing you do to the tank or the tank vent will help.
Not quite. Depending on the boat design and vent placement, vent odors get back into the boat and are the major source of odor.

For example, many catamaran vents are installed under the bridgedeck or on an inside hull. Odors from the vent blow under the bridgedeck and get sucked into the cockpit and cabin because of the venturi caused by the cabin bulkhead.

Rear cabin monohulls often have a similar situation with their aft heads.

While Peggy's explanation of the microbiology and chemistry of holding tanks is correct in theory, in practice it can be impossible to setup and maintain an aerobic system. It is just the nature of the physical limitations for some boats and designs. Talking to many we meet, this impossibility may be the majority.

If you fall into this impossible category, then an activated carbon vent filter works like magic. No need to pay $80-100 for one, because all they consist of is a piece of PVC pipe filled with cheap activated carbon. You can build your own for a few bucks.

After many years of attempting to make and keep aerobic septic systems, I just made some vent filters and the smell problem was completely solved immediately upon installing them.

For instructions on how I made them, look here: Holding tank vent filter | East of the Equator | M&M

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Old 26-06-2015, 05:57   #14
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Re: Dual vent on holding tank

Quote:
Originally Posted by trishark View Post
Boat is 2009 Beneteau 40. 25 gal tall holding tank is behind shower wall, below side deck. Deck pump out port in bottom, macerator, fill from electric head and vent in top. All ports in tank are 1" or 1.5". Vent fitting has 90 deg. 1/2" hose barb off side. About 3 ft. of hose to reverse facing small clam shell small opening vent. Wire mesh has been removed from vent and it flows as best a small vent can. Tank odor is a problem.
Plan is to remove vent fitting in top of tank, install a tee with the branch port of the tee horizontal. Reduce this to ¾” hose bard, install new vent to match ¾” line. Install reducer bushing in top run port of tee with street elbow just under deck. Will install new vent about 1 foot forward of existing vent to connect to top of tee. From reducer bushing or street elbow, install standpipe that would extend thru the tee and into the tank several inches. While underway, air would enter forward vent, flow thru the standpipe into the tank and exit thru annular area around standpipe and out branch port to rear vent. Probably install plastic clam shell over forward vent to help build some pressure.
After 7 months in the Bahamas, we are in Fort Pierce FL getting some bottom work done. I am having difficulty finding anything other than fuel type vents that are bigger than the 1/2" that came on the boat. Looking for thoughts and suggestions on the dual vents. Using a thru hull mushroom type fitting has been suggested. Changing out the head is not an option.
It's the anaerobic breakdown that gives the odor. So your thinking to improve venting is logical. Introducing crossflow is a good idea. It certainly works on high volume systems.

However most odor issues tend to emanate from leeching from old hoses, leaks and pooling of effluent in the hose runs.

Before you redesign your vent system I'd confirm that there aren't other issues.

I did chase an issue in one of my raritan heads where the delrin base was leeching odor through a crack that didnt leak effluent. I had to pressure test it to find it. Seawater tends to degrade those older plastics over time.

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Old 26-06-2015, 07:51   #15
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Re: Dual vent on holding tank

Thank you Peggy Hall. You can lead them to water........

I am in the process of overhauling my boat. I have removed my holding tank and all its related plumbing so that I could access, fully, that portion of my bow where the bow roller and some pedestal fasteners live unmolested. After thirty-some years, I figured it might be a good idea to inspect and repaint this rarely seen locale. There have been no surprises. A very small amount of mold on the 1 1/2 inch hose that connects to the foredeck pumpout fitting, and some degree of dust. After I repaint the locker I will be installing that outrageously expensive Shields Poly-X (Practical Sailor's review in 2012: "After only a year of testing, we recommend the Trident 101/102 as the budget pick. The easy to install, easy to clean—but pricey—Poly-X gets the nod for Best Choice"). I've installed it on other boats and I'm impressed. Besides, I won't be replacing it in my remaining lifetime.
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