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  #1  
Old 10-14-2009
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Flexible holding tank installation

OK, So we are to the plumbing system and beginning to purchase the parts. The question on the table is installation of flexible tanks for waste storage.

*caveat* we have done the reading and debating on the advisability of using a flexible tank and don't want to rehash that here. We feel that if the US military can fill them with diesel fuel and drop them from the air we can trust them with a bit of biowaste. WORST case is we have a nasty bilge cleanout. Best case is we have a reasonable sized tank in an inaccessible and currently unused area that solves our lack of any holding tank problem in a cost effective manner. We do plan on lining the space with a waterproof liner and padding to protect the tank, and securing it with tiedowns to prevent chafing.

I understand the physics behind a standard hard tank install; Line from head to tank, line from tank to pump out fitting, line from tank to vent. assorted loops etc to manage flow. The vent serves a number of functions. It allows the pumpout to proceed without creating a suction problem which would interfere with the pumpout. It allows a flow of air so the tank doesn't fill with gasses and burst. There's something about aerobic and anaerobic in there as well.

I have looked at flexible tank install instructions and there are 2 ways shown. One follows the standard set for a hard tank. The other is for a simpler setup with only a pumpout, no vent. The argument being that with a flexible tank you don't need to solve the suction issue. The tank collapses and inflates as needed. This seems to leave the build up of gas issue still to address. As I look at the 2 installations it occurs to me that for a hard tank you NEED 2 outlets to deal with the suction buildup. With a flexible tank why would you need 2? Why can't you install it with one outlet, sized for pumpout, that ALSO functions as a vent to prevent the gas buildup?

I looked around and found there are fuel deck plates that incorporate the fill and vent in the same unit. They allow for the correct 1 1/2" hose. Other than dealing with the label which would need to be changed so no numbnuts tried to pump diesel or gas into the holding tank...

Am I missing something here? Any brilliant insights?
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Old 10-14-2009
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Very well explained and thought out. The problem I see with the vent incorporated with the pump out tube, is you will not get suction. Because you are trying to create a suction to the deck pump out that has a vent line connected to it. You would have to turn off the vent hose some how and then you could have the possibility of collapsing the holding bladder and putting all that suction on one spot. I dont know if the bladder could invert and be sucked into the pump out hose. What kind of bladder are you going to use?
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Old 10-14-2009
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The other problem that I see is that those type deckfill/vents usually only work in vent mode when the deckfill cap is off. Best to check yours to see if this holds true with it's design.
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I added a bladder holding tank last year. So far it has worked great. I put in a 1.5" input connector at the top of the tank. I put in a 1.5" output connector near the bottom ofthe tank. I put in a .75" vent connector near the top of the tank. The pumpout at our local marina really sucks. I think there could be damage if there was no vent. Even then, there can be issues of having the pumpout connector hit the opposite wall of the tank and seal it off before everything is removed. To prevent that, I used heavy line to string up the hoses so they cannot collapse, especially the pump out hose. Our other holding tank is a 25 year old bladder and still does not smell at all. But it does have the suction issue and I have to hold the pumpout fitting to keep it from locking onto the opposite wall, sealing off the suction.

While you are at it, I recommend Trident 101 hose for sanitary lines. I used both the trident 101 and sealand odorlos hose, the trident was MUCH easier to fit on the fittings.
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Old 10-14-2009
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I have had bladder type holding tanks on two separate boats now, with the bladders made by two different manufacturers and removed both after a couple years. The problem was not with the tank itself leaking fluids (although there was a problem with the locking nut loosening on one over and over again and leaking a little) but with the fact that in both cases the odors get embedded in the fabric and permiated through to the surface and pretty soon the whole boat started to smell like sewage.

In both cases, I went to rigid plastic tanks and the smell went away. Ronco Plastics makes tanks in almost any shape that you can conceive of, at much cheaper prices than the higher quality bladders and will custom place the intake, vent, and pump outs to suit your installation.

As to the venting issue, the vent plays three roles, It 1. allows the tank to exhale gases, 2. Provides air so the microbes don't go anerobic, 3 acts as an overflow relief and over-full warning system if the tank accidently is filled beyond capacity. (BTW I have heard of bladders being over pressurized and busting. Just because millitary spec blatters survive being tossed from planes, it does not mean that boat holding tanks are built to Millitary spec.)

Even with a Bladder you want a vented tank since the bladder will fill with gasses over time and reduce the capacity of the bladder as the gasses displace the fluids.

On any kind of tank, the vent and fill tubes should be located on the top of the tank and the pump out connection should reach the bottom of the tank. With a bladder this means that the pumpout connection is on the bottom of the tank. With a rigid tank you can get fittings that enter the top of the tank but have a tube that reach the bottom of the tank. In an ideal world all of the connections should enter the tank from the top so that the tank does not need to be pumped dry if the discharge tube gets blocked.

Jeff
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Old 10-14-2009
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IMHO, you're a lot better off with a rigid plastic tank, primarily for the permeation problems JeffH has pointed out. Trying to compare flexible tank use for diesel fuel with that for sewage is comparing apples to oranges... the two uses have very DIFFERENT REQUIREMENTS.

While you don't necessarily need a vent line on a flexible tank, since it can collapse when being emptied and expand when being filled, it is very difficult to properly aerate a flexible tank, since they tend to conform to the shape and size of their contents... making good air circulation almost impossible. Without good air circulation, the contents will consist of primarily the noxious anaerobic sulfide-producing bacteria...and that will cause your boat to become rather malodorous....
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I agree with Jeff H as usual here. We just replaced a bladder tank with a rigid one. The problem with the bladder as Jeff stated is the permeability of the bladder to the sewage smell. The major importance of the vent whether bladder or solid tank is to allow enough oxygen into the tank to prevent anerobic and get the "sweet smelling aerobic bacteria in the tank. Also you do need a displacement of iar when the waste goes in which the vent serves to allow the air/ gas in the tank to escape while adding waste.

I also used the black trident 101 when we redid our waste system this year and it is great stuff. No smellls and somewhat better workable than the other brands.
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Sorry but the waste bladder in the forward head of my boat does not stink. It is a fuel tank bladder that looks VERY old. Probably 20+ years. I've had my nose in there very close to it and it does not stink. And I know that smell. I still have one hose that runs to that tank and I can smell it in the head cabinet when it gets warm out. About 10' of hose length away from the bladder. The hose stinks but the bladder does not. That hose will get replaced with trident 101 this winter. I will probalby replace the bladder too because it is showing wear after many many years.
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These are the tanks I am looking at;

25 gal. Gray Water Pillow Tank (FDT-GW25)

Vetus Flexible Waste Tank - 26.5 Gallon

Nauta Flexible Waste Holding Tanks at AhoyCaptain.com

These are the deck fills I am looking at;

Vented Gas Deck Fill & Tank Vent from Wholesale Marine

Redden Marine Supply Angled, Vented Deck Fill

I think the problem I have here is the application I am envisioning has not has a deck fill manufactured for it. I think what would work with the flexi tanks is a pumpout thru deck that had the vent built into the middle of the pumpout cap. So that the pumpout hose would function as the vent at all time except for during the actual pumpout. At that time the removal of the cap would in essence remove the vent as well and leave behind the opening to mate up with the pumpout station. I am not concerned about suction during pumpout because of the dynamic properties of the flexible tanks. Thinks about a juice pouch. as long as you keep sucking on the pouch and as long as there is juice in it you get something to drink. The pouch collapses around the fluid inside it and allows the fluid to flow to the point of pressure, the straw in the case of the juice. The pumpout in the case of the waste tank. An additional vent hose just complicates the issue.

This is what I am imagining;



Now I just need to figure out how to make one.

Might be easier to just put in the extra vent hole and run the darn hose...

rats.

I am interested any feed back on these or other flexible tanks. successes or failures!
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Last edited by sarafinadh; 10-14-2009 at 11:53 PM.
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the more air the better. The more air, the less smell you will have.
Those 2 combo fill/vents don't look to have much for air passage.
A better install would be TWO 3/4" vent lines. One forward and the other aft to allow more air flow thru the tank.
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