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Old 13-06-2018, 10:11   #1
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Here's one for Peggy - bladder holding tank

Looked at a boat yesterday and found that it had a bladder holding tank. All the right fittings so obviously made for that purpose.


Any comments, pro or con? Looked like some really thick rubber. There was some content, but no smell.


Bob
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Old 13-06-2018, 10:39   #2
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Re: Here's one for Peggy - bladder holding tank

Bladders are ok for water and even fuel, but not the best choice for waste holding because they're designed to hug the contents, which makes it impossible to maintain the tank aerobically--the key to preventing odor--not INSIDE the boat unless the tank is leaking, but odor out the tank vent from inside the tank.

Top quality bladders are more expensive than top quality rotomolded tanks, but only have about a 15 year lifespan vs indefinite for rigid tanks.

All that said...it's what's on the boat, apparently good quality and in excellent condition, so I wouldn't let it be a deal breaker. Use it till all the other upgrades, replacements and repairs you didn't know it needed are paid for and then address replacing it. We'll find a way to deal odor out the vent.

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Old 13-06-2018, 11:50   #3
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Re: Here's one for Peggy - bladder holding tank

But check to make sure it holds “water” before getting carried away.

I was once with a club that had a policy of not using the heads on the boats (don’t ask) but changed after some years. I assume that may have been a factor - in any case the bladder seams had dried and cracked. I know because I was the first to use a boat after the policy change.

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Old 13-06-2018, 12:26   #4
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Re: Here's one for Peggy - bladder holding tank

This one had something in it, you could squeeze it, but don't know what, may just have been water.


Peggie, I assume that since the bladder conforms to the contents, there is no vent?


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Old 13-06-2018, 13:55   #5
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Re: Here's one for Peggy - bladder holding tank

Quote:
Originally Posted by BobHorn View Post
Peggie, I assume that since the bladder conforms to the contents, there is no vent?
I'd think given the contents, not venting that tank could have some interesting consequences.
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Old 13-06-2018, 14:35   #6
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Re: Here's one for Peggy - bladder holding tank

There was one on our last boat. Always made me cringe that the seems would give way.

If we hadn't sold the boat, it would have been replaced with a composter.

If we were buying another boat and it had one, the price would be adjusted based on the assumption that it would have to be replaced.
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Old 13-06-2018, 16:05   #7
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Re: Here's one for Peggy - bladder holding tank

Neither bladder tanks or rigid tanks can maintain aerobiosis in the presence of organic waste. A little math will demonstrate that the oxygen in the headspace of a tank is insignificant compared to the oxygen demand of solid waste. Diffusion down the vent hose is also insignificant. (And yes, of course bladder tanks have vents.)
Forced aeration would be required to have aerobic metabolism in the tank and this would work equally well (or poorly) in either type.
This is where "Odorlos" and similar products come in: they add nitrate as a substitute for oxygen, and work equally well in bladder or rigid tanks.

OK, after that, I suppose I have to do the math... Assume a 25 gallon (94 L) tank, half full of sea water. (Of course, it's not sea water any more...)
That gives you about 0.4 grams of dissolved O2 in water (9 mg/L) and about 12 grams in the headspace (PV=nRT and 20% O2). The oxygen demand of "animal slurry" is going to vary but is somewhere around 10-80 grams per liter, depending on how much you've diluted it. All the oxygen in that tank was gone with the first poo or two.

A little bit of a "head" start over a bladder tank, but not so much that you'd notice. Maybe if you have a small crew and empty the tank every day, it'll stay mostly aerobic. Say, never go over a couple of gallons in a 25 gallon tank? Gee, I've almost talked myself into it. Sadly, I don't have room for that large a tank anyway.
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Old 13-06-2018, 18:12   #8
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Re: Here's one for Peggy - bladder holding tank

Peggie, I assume that since the bladder conforms to the contents, there is no vent?

Unvented bladders are legal for water and fuel, but USCG regs require that all black and gray water tanks be vented to the outside of the boat because organic matter generates gasses as it breaks down, one of which can be methane in an anaerobic environment, and methane is flammable.

So that bladder is almost certain to be vented...if not, it's illegal and dangerous.

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Old 13-06-2018, 18:21   #9
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Re: Here's one for Peggy - bladder holding tank

Neither bladder tanks or rigid tanks can maintain aerobiosis in the presence of organic waste. A little math will demonstrate that the oxygen in the headspace of a tank is insignificant compared to the oxygen demand of solid waste.

Actually, up to a depth of about 20" it is possible for passive ventilation above the surface to maintain tank contents aerobically. That does require a larger diameter vent line than the "standard" 5/8" on all water, fuel and waste tanks...and the vent line does have be relatively short, straight and rise no steeper than about 45 degrees, but when all those conditions are met, the tank does not generate odor. There are several tank products that help--Raritan K.O. which is live aerobic bacteria, Odorlos, in which the active ingredient is nitrates, and a relative newcomer, No-Flex Digester, which is getting rave reviews from everyone who tries it.

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Old 14-06-2018, 09:09   #10
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Re: Here's one for Peggy - bladder holding tank

"All that said...it's what's on the boat, apparently good quality and in excellent condition, so I wouldn't let it be a deal breaker. Use it till all the other upgrades, replacements and repairs you didn't know it needed are paid for and then address replacing it. We'll find a way to deal odor out the vent."

Ain't that the truth. . .priceless!
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Old 14-06-2018, 16:14   #11
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Re: Here's one for Peggy - bladder holding tank

I have seen many ideas on holding and water tankage on the cheap and easy--the best systems are the ones designed specifically for the vessel and I am in favour of the HD Polythene types, they seem to work fine.

Alternatives, depending on where one is cruising, may require non-discharge into the waterway of black water and grey water, and some areas are not happy with the chlorinated waste systems of sterilization of sewage before discharge.

I have used the Thetford and similar types of 20 litre holding portable sysetms--but they fill fairly quickly, and nothing beats a macerated system for making the waste easier to pump and store.

Simplest I have seen are a rack of five twenty litre blue plastic jerry cans with two outlets. The big outlet is for filling with pumped sewage and flush water. The smaller one goes to a carbon filter vented above decks. Each of the 20 litre bottles is pre-charged with Racassan or a similar sterilant, many are available at camping stores. It is easy to see how full the container is, and when close to filled, one moves the fill hose to the next container and replaces the caps on the filled one--which can be rowed ashore and emptied, rinsed and recharged.

Shower water can be kept for use in the heads if one requires it, but it will smell if not treated with Racassan.

So, freshwater tankage can be rigged the same way, five containers equates to about one hundred litres of fresh water, and the jerry cans are easily handled when full.

I am a fan for the container system. Each plastic jerry can slid into a pocket lined with carpet, and was secured by a drop-in rail across the front of the rack. Movement was minimal. I saw it used on a Wharram, and it solved a raft of problems where piped water was not available and water had to be brought aboard by dinghy.
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Old 14-06-2018, 21:30   #12
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Re: Here's one for Peggy - bladder holding tank

When I had my boat built I had them install a bladder tank for black water. I don’t remember exactly why but I replaced it with a rigid tank from Ronco after about a year.
I like bladders for carrying extra fuel if you need to extend range but somehow not for black water. A company called Aero Tech Labs makes excellent bladder tanks.
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