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post #1 of 16 Old 01-04-2010 Thread Starter
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Holding tank vent question

I'm thinking of building my own 15 gallon holding tank inspired by an example from the Gougeon Brothers: DIY-BOAT.COM - Durable DIY Holding Tank
and I have a question about venting if anyone would be so kind as to share their expertise. All the layout drawings I have come across show a simple air vent pipe out of the top of the tank and it exits via a thru hull just below deck level. My question is what happens when the tank gets full - will the waste water try to rise up the air vent and drip into the sea? Must a valve be installed to stop this? Or is there a valve in the thruhull fitting? Or is the Jabsco manual head pump not high pressure enough to force waste up to this height? I've read through quite a lot of postings and not found the answer but if I've overlooked something by all means let me know. Thank you.
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post #2 of 16 Old 01-04-2010
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Quote:
Originally Posted by johnseattle View Post
I'm thinking of building my own 15 gallon holding tank inspired by an example from the Gougeon Brothers: DIY-BOAT.COM - Durable DIY Holding Tank
and I have a question about venting if anyone would be so kind as to share their expertise. All the layout drawings I have come across show a simple air vent pipe out of the top of the tank and it exits via a thru hull just below deck level. My question is what happens when the tank gets full - will the waste water try to rise up the air vent and drip into the sea? Must a valve be installed to stop this? Or is there a valve in the thruhull fitting? Or is the Jabsco manual head pump not high pressure enough to force waste up to this height? I've read through quite a lot of postings and not found the answer but if I've overlooked something by all means let me know. Thank you.
Your holding tank will probably start leaking before anything could come out the vent, because the holding tank will be well below the vent...so no valve of any kind is needed or wanted. Run at least one 1" vent line, and two is preferable for good aeration of the tank. Air in the tank will not allow the stinky bacteria to grow nearly as well as no air in the tank, and the good bacteria likes air.
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post #3 of 16 Old 01-05-2010
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If you do a good job of the tank you could pump it full and right out the vent. There has to be a way to tell how full the tank is. That is one reason most tanks are translucent. Regulations call for a method of knowing when the tank is 3/4 full.
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post #4 of 16 Old 01-05-2010 Thread Starter
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I'm very glad you pointed out the regulation requiring a level monitor on the holding tank, thank you, I was unaware. A quick online search suggests these devices may be expensive and perhaps instead of building a tank myself it becomes economic to buy a factory-made translucent tank and obviate the problem. The simplest home-made level monitor I can conceive would be a piece of translucent tube positioned vertically on one side of the tank with right angle pipe joints at top and bottom. I don't know if this would comply but for an amateur like myself trying to keep things simple and legal perhaps it's best to go for a factory made tank or a porta potti. Thanks again.
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post #5 of 16 Old 01-05-2010
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There is another consideration concerning the vent. Each time the head is used, air will be expelled through the vent. If you spend much time using your boat at anchor into the breeze; then, this foul smell will always waft across your cockpit. For this reason you may wish to lead the vent hose aft. Lowering the outflow vent is another potential solution, but you'll need to take care to pervent any possible siphoning into the tank. 'take care and joy, Aythya crew
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post #6 of 16 Old 01-05-2010
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john
Just install a clear plastic port in the top. It allows for easier cleaning as well. I'm going to make a tank as well to take better advantage of the space and that is what I'll do. Ideally there are no holes anywhere except on the tank top.
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Last edited by mitiempo; 01-05-2010 at 10:52 AM. Reason: add
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post #7 of 16 Old 01-05-2010 Thread Starter
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There's a Sealand Tankwatch level monitor around $70 that I might try because my holding tank locker is not very easy to access. I guess as an alternative I could cut an inspection window into the side panel next to the tank but that would be a last resort - I really don't like cutting holes in bulkheads and other fixed woodwork. A previous owner cut holes all over the place for speakers, a CD player, the VHF etc and I've taken some care plugging up the holes by scarfing in matching wood patches.
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post #8 of 16 Old 01-05-2010
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I love that people enjoy playing with their poo so much. Seriously what was so bad about a bucket? Swathing your poo in chemicals, or spending all of the time and environmental impact of installing and maintaining a flushing toilet on a boat has to be a lot worse for the earth than a little poo out in the big blue sea.
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post #9 of 16 Old 01-05-2010
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While a bucket works offshore it is illegal to dump a bucket of crap within 3 miles of shore.

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post #10 of 16 Old 01-05-2010
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If you only use Raritan's 'KO' product as your tank treatment then there will be no nasty chemicals as 'KO' is a purely bacterial product.
Two vent lines are better then one.

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