# Flexible holding tank installation



## sarafinadh (Jun 16, 2009)

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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## badsanta (Oct 16, 2006)

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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## xsboats (Oct 2, 2007)

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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## xort (Aug 4, 2006)

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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## Jeff_H (Feb 26, 2000)

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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## sailingdog (Mar 19, 2006)

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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## chef2sail (Nov 27, 2007)

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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## xort (Aug 4, 2006)

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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## sarafinadh (Jun 16, 2009)

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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## xort (Aug 4, 2006)

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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## SVPrairieRose (Oct 10, 2009)

sarafinadh said:


> These are the tanks I am looking at;
> 
> 25 gal. Gray Water Pillow Tank (FDT-GW25)
> 
> ...


By what you are describing here it would seem contrary to what everyone is saying. The common idea is that a vent must vent and would therefore have to be at the top of the tank. If you have a common pump out hose with a vented cap this will not work because the pump out hose runs to the bottom of the tank or bag. It would seem to me that if enough pressure built you would actually have a poop volcano on deck.


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## sarafinadh (Jun 16, 2009)

You do not have to have the pumpout on the bottom of a flexible tank. In fact I think that might be problematic to do so. As the bag filled it would have to form itse;f around the contures of the exit hose. I would rather that bottom of the bag be laying nice and smooth along a flush surface. All of the drawings I have seen have the pumpout on top. The bag expands to fit the amount of liquid/waste in it. There isn't a huge amount of air space on top.

So rather than a 3/4" vent you would have a full 1 1/2" vent with my (imagined) system.

As gasses built up they would escape thru the hose. any air space would have to be created by tying up and securing the pumpout hose in some manner that would create space in the tank above the fill line.

Think about how a ziplock bag full of soup would act.


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## xort (Aug 4, 2006)

When I described my output fitting as being on the bottom of the bag, my bag is hanging on its side, vertical. The input and vent are at the top of the bag and the output is near the bottom of the bag; all on the same side of the bag as it hangs against a wall. It is a Vetus with very large grommets along one end that I used to hang it from.

My forward bag does lay on the floor and all fittings are on top. This bag has the problem of the suction going straight to the bottom and sealing off the output fitting. I have to grab the fitting and keep it separated from the bottom of the bag. With good suction, that is hard to do.
That will change when I replace it.


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## genieskip (Jan 1, 2008)

Get a composting toilet and get rid of all these problems.


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## sarafinadh (Jun 16, 2009)

xort said:


> When I described my output fitting as being on the bottom of the bag, my bag is hanging on its side, vertical. The input and vent are at the top of the bag and the output is near the bottom of the bag; all on the same side of the bag as it hangs against a wall. It is a Vetus with very large grommets along one end that I used to hang it from.


now that is a useful image!

I am thinking about our space and imagining hanging it along the side... That could work. It would be at sort of an angle along the hull in the port side of the v berth. I think I have like 30 inches of drop there, along the hull.

like this;









where the orange is the waste flow to overboard discharge and holding tank with Y valve, green is the pumpout discharge and blue is the vent.

I would LOVE to do a composting head, and that's in the plans for the next boat (that we plan to cruise on in 3 to 4 years). This boat is being equipt as a bay and coastal cruiser and we don't have the budget at this point.


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## mitiempo (Sep 19, 2008)

I'm trying to think about how a ziplock bag of soup would act but you're not holding soup in it. Soup doesn't gas and expand. I don't see how the combination fuel fill/vent fittings will work. You want the pumpout on deck and the vent on the hull side high up. The fittings you linked to aren't watertight as they state, couldn't be with the vent included. I can't help but think that you'd be much better off with a plastic tank as there are less problems to deal with. With a few exceptions everybody is using a solid tank for very good reasons.
Brian


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## Omatako (Sep 14, 2003)

sarafinadh said:


> _ 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. _


Seems lik all the advocates for hard tanks have missed the original point (see above).

Whilst it is probably correct to say that hard tanks in the long run are better, there is often space in a boat that cannot accommodate any form of hard tank and yet can take a sizeable bladder. This is what the OP is asking.

I have two bladders for the two heads in my boat. One is in such a space that a solid tank is simply not practical. The other is a space that will easily accommodate a solid tank of considerable proportions. Eventually I will replace that bladder with a box tank but right now both bladders are fine and present no problems at all. I have no idea how old they are - I've only had the boat for two years.

So in short, to the OP, if the space will only hold a bladder, fit one. They're not the monsters that some folks paint them. Just be sure that you fit it in a way that you can help with discharge if the bladder skin opposite the outlet is sucked against the pipe. This is often a problem.


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## xort (Aug 4, 2006)

Adding to Oma...make sure that not only do you have some access to the bladder but hat there is ample space for the bladder to expand without rubbing on anything. Mine came a bit closer to an overboard drain that I expected but still several inches away when full.

You're attaching to the inside cabinetry wall, not the hull itself, correct?


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## sailingdog (Mar 19, 2006)

I'd point out that you probably want to pad or cushion the area on which the bladder will sit, so as to protect it from chafe and puncture.


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## xort (Aug 4, 2006)

sailingdog said:


> I'd point out that you probably want to pad or cushion the area on which the bladder will sit, so as to protect it from chafe and puncture.


I'd totally forgot; I sprayed some of that spray foam insulation from a can into the bottom of the space. It was too deep a Vee so I filled it in and leveled it out a bit, providing a better base for the bag


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## sarafinadh (Jun 16, 2009)

Omatako said:


> So in short, to the OP, if the space will only hold a bladder, fit one. They're not the monsters that some folks paint them. Just be sure that you fit it in a way that you can help with discharge if the bladder skin opposite the outlet is sucked against the pipe. This is often a problem.


Bless you ; -)

I know this is not the optimal way to go, but it is the practical way for our little hole in the water. The feedback here has helped me get a solid lock on how to install it. The issue with the bag sucking up to the drain port and needing to be lower down if possible has knocked my imaginary little pumpout/vent thingy out of consideration... oh well.. SOME day I am gonna have a great idea that works and hasn't already been thought of!

Definitely plan to line and pad the space to protect the bladder. The spray foam is a clever idea.

I think the best way I can secure it is to the underside of the vberth inside the tabbing. There isn't a wall between the locker it is going in and the one next to it. The vberth has the water tank where most have that removable piece with a cushion to make a useless v shaped setee. Ours is built solid across and the side of the water tank, which also needs replacement but is a better candidate for a hard tank, is right there. Can't hang it on the tank ; -)

I was planning on letting the locker, with it's typical triangular v shape, determine the shape the bag takes as it fills. There isn't room really unless I had a bag made just for our space (which would blow the cost savings out of the water) to keep it from coming up against the hull towards the bow and against the water tank that defines the center limit of the space. I think I can keep it from sliding around and chafing, but I don't think I can hang it in such a way as to not touch anything except the hull it is against.

a problem?

One more thought. If I have to put the vent in my deck anyway, I can't think of any reason to not run 2 lines (as suggested) and bring them to a Y that would allow them to both be vented thru the same vent. might only make a small increase, but it sounds like any air is good air.


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## mitiempo (Sep 19, 2008)

Spray foam can be hard to control and you might end up having to carve to the shape you want. You could use Ethafoam, a closed cell foam that is available in sheets in different thicknesses and can be glued where you want it easily with spray glue or contact cement. It's also good for insulating hulls and decks.
Brian


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## Omatako (Sep 14, 2003)

sarafinadh said:


> I think I can keep it from sliding around and chafing, but I don't think I can hang it in such a way as to not touch anything except the hull it is against.
> 
> a problem?


My bags are both protected by a layer of ordinary domestic carpet, properly fitted to cover any potential hazards. One bag hangs from a bulkhead, the other simply lies in the cavity directly on the hull. So far, no problems and they have probably been there for several years (I've only owned the boat for 2).

Obviously if it is to lie flat in the space, have it oriented with the outlet at the lower end facing upwards

I would be hesitant to suggest teeing off breathers from a holding tank and a water/fuel tank for fear of contamination.


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## sarafinadh (Jun 16, 2009)

Sorry, not clear. I was not proposing venting different tanks to the same outlet. I thought I might put 2 vent lines on the ONEwaste tank and T them to one vent outlet. To increase air flow and aerobic activity.

Sound better?

; -)


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## mitiempo (Sep 19, 2008)

Yes. it's better than the thought of connecting potable water and waste, but if they go to one outlet you might as well only have one vent in my opinion. For 2 vents to work there has to be the ability of flow in one and out the other.
Brian


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## GaryHLucas (Mar 24, 2008)

I just removed the flexible Nauta tank from my 25 year old boat. Two reasons, first it did have a smell to the tank. Second it did not pump out well. The pump out fitting was on top, and sucks right down tight to the bottom and seeals off. Trying tro hold it was impossible, and doing that just caused the vent inlet to do the same thing. I believe if the fittings had ridges on the inside radiating outward to prevent sealing against the other side of the tank this wouldn't be problem. I actually though about placing a short piece of stainless chain in the fittings held in place by a pin across the end of the fitting where it would be inside the hose. However I work in waste water, we buy lots of tanks, I got a great price from Ronco!

Gary H. Lucas


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## roline (Apr 7, 2000)

Bladder tanks do work for holing tanks, and yes they can have the holding tank stench. My boat came with a new one, and I did replace it with a hard tank. When the bladder holding tank starts to stink, less folks will sail with you so it doesn't fill as fast. IF you put one in, add good ventilation for it.


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## xort (Aug 4, 2006)

Sarah

The spray foam insulation does have a mind of it's own and it is VERY sticky. Use it sparingly and be prepared to cut it to shape.
You don't HAVE to hang the bladder, it worked well for me in the one head because the space was more vertical than horizontal. The Vetus bag has grommets. My forward installation is more of a horizontal space so the bag lays down. That bag does give me more problems with the pumpout sucking down onto the opposite inside surface and sealing off. I plan on redoing that whole space and will try to tie up the pumpout fitting to keep it from sucking down.

I don't know why the old forward bag does not stink.


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## blt2ski (May 5, 2005)

Sarah,

For your use, a bladder sounds like the thing to do. When and IF the bladder starts to smell, just like my hoses do after 25 yrs of use, replace them! Not that it is the same, but I have the original bladder style water tank in my 85 Jeanneau, working fine! 

There is not a perfect single solution for everything, sometimes we need to be creative. You are doing just that! 

Marty


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## sarafinadh (Jun 16, 2009)

well my creativity will get a good work out. Today I got a 15 gallon Nauta tank at a swap for 90 bucks, never used. Has the 3/4" vent, so I will be adding the two 1 1/2" fill and empty nipples. That will let me position them optimally. The tank has grommets already so I will be able to hang it. God, I can hardly wait to not HAVE to haul it out to the marina building in the middle of the night!


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