Gravity discharge holding tank - will this be OK?

[Anthony]

Hi All,

As she is now based in the Med, I really want to add a holding tank, and have come up with the following layout that (hopefully) you can see HERE

The bottom of the tank will be about or slightly lower than sea level. I presume this is OK, although most documentation I have seen shows them as being clearly above sea level.

Basically I will just be adding the tank inline in the outlet pipe, just before the outlet seacock; and putting a filtered breather, inspection hatch, and level guage on it.

With regards to the 'U' bend plastic anti syphon thingy in the outlet pipe, it has some sort of air valve thing on the top (that seems to have been maybe DIY modified), whereas most others I have seen actually have a breather pipe coming from that valve thingy. Would it make sense to fit a new 'U' anti syphon thingy with a breather pipe, going into the filter along with the breather from the tank?

I was not planning on a deck mounted pump out point, as I have yet to see a pump out station in any marina that I have been to in the med. It will only be a small Vetus 42L tank as not much room, but should be enough for the two of us to use whilst in an anchorage for a night or two.

Any tips / advice / experience very welcome, espically as planning on getting the parts in advance as not much out there.

Many thanks!

Anthony

PS sorry about my lack of drawing ability /forums/images/graemlins/blush.gif

 

[Bejasus]

can't see a GRAVITY discharge being possible. However, the simplest installation would be as below, using a manual pump between the tank discharge and the seacock. Note that the discharge pipe has to rise above both the toilet and the outside sea height with the anti-syphon valve at the top of the discharge pipe. To directly discharge at sea, you will need a 'Y' valve between the toilet discharge pipe and the discharge line before it climbs to the anti-syphone valve. These images are from Jabsco but the same applies regardless. Hope this helps.
Parts available in the UK from here or even Vetus could supply.

'Y' valve

Vented Anti-syphon loop. Anti-siphon safety devices for all marine toilets installed below the vessel’s static or heeled waterline. Simple valve system ensures that water cannot siphon over pipe loops.

Manual Toilet Waste Pump

 

[HeadMistress]

Your drawing is fine...I can see what you have in mind, and there are a couple of problems with it:

Unless the tank is above the waterline, it can't drain via gravity...instead, sea water will rise in the tank to the waterline. That's a dandy way to rinse out the tank, but makes it hard to empty it without a pump.

Forget the filter in the vent line...filters actually help to create the very problem they're sold to solve. Oxygen is the key to odor elimination...when organic matter breaks down aerobically (oxygenated), it doesn't stink...it's only when it breaks down anaerobically (without oxygen) that it can create noxious hydrogen sulfide gasses. A filter traps those gasses, but it also prevents any exchange of air through the vent line, causing the tank to become anaerobic. So not only do they create the problem they're sold to solve, but they're also expensive...about £50 apiece. They only last a year at most...are toast immediately--and can block the vent--if they get wet...which, because heeling can cause tank contents to spill out the vent line, makes filters inadvisable on sailboats.

Raising the tank above the waterline will allow the vent line to be closer to horizontal, shorter and straighter (and btw, I recommend a 1" or even 1.5" vent line), which is what's needed to keep the tank aerobic and therefore odor-free. For a more complete explanation of this, go to the "sanitation guide" on the Tek-Tank site here: http://www.tek-tanks.com/sanitation/sanitation_index.html

As for your vented loop ("U bend plastic anti syphon thingy").... There should be an air valve in the top of it that only allows air to be pulled INTO the line, nothing out. The air valves do need periodic cleaning/replacement. It's only when air valves are either missing or have failed and begun to squirt that people who don't know any better put a "breather tube" onto it to catch the squirting. The problem with that solution: the tube and hole in the top of the loop quickly become clogged with what squirts out...turning the vented loop into an UNvented loop that no longer has any ability to break a siphon.

You asked, "Would it make sense to fit a new 'U' anti syphon thingy with a breather pipe, going into the filter along with the breather from the tank?"

No...for several reasons: waste would squirt into the filter charcoal (at least until the "breather" clogs)...the charcoal cannot get wet. But even though you're going to eliminate the filter anyway, it's still not a good idea--not only for the reasons I've already mentioned, but because venting into the vent line can clog up the tank vent line.

You'll want a shutoff valve in the overboard discharge line as close to the tank as you can get it--preferably the actual hose connection fitting that goes into the tank. This will allow you to use the tank without filling the discharge hose with sewage too, and also allow you add water to the tank to rinse it out.

Finally, I think you'll be MUCH happier with a rigid tank than with a bladder, for reasons that should become clear when you read the aforementioned sanitation guide on the Tek-Tank site.

Questions?

 

[Rick]

I've put essentially the same into my boat (and helped half a dozen others) to meet the new pooh regs here in Queensland Australia.

The differences are that my tank is above waterline, and the breather doesn't have the carbon filter. Tanks were made from glass and epoxy (about 2.5kg glass/m2) over a male mould, with all penetrations in the lid except the outlet.

 

[mhouse]

Re: Gravity discharge holding tank - Works ok

The tank I fitted 3 yrs ago is above the water level and drains by gravity fine. The toilet flush pump is ok to lift the 2 feet into the top of the tank and when the bottom valve is open it can free flow without additional pumping.
I also recommend TEK Tanks

 

[pvb]

One change I\'d make...

I have a gravity discharge holding tank, fitted as original equipment by Hallberg-Rassy. It works fine. It's about a foot or so above the waterline.

One change I'd make to your proposed layout would be to pipe the outlet from the toilet to the <u>bottom</u> of the holding tank, rather than to the top. This is the arrangement on my boat. The advantage is that everytime you flush the toilet, the contents of the holding tank get agitated, helping to break up solids and ensuring that the outlet pipe doesn't get clogged.

As Peggy said, you can leave out the charcoal filter.

 

[JackFrobisher]

Interesting. I was thinking about something similar and had supposed that it might be possible, by closing the vent pipe with a valve, to use the pressure from the pump to empty the tank even though the discharge point is below sea level (and before anyone jumps in, I do realise that too much pressure could make the tank go bang ... with unpleasant results). Releasing the vent valve would then allow sea water in to do a degree of rinsing.

I think it needs a lot of care.

 

[waterrat365]

We use holding tanks in the US by law, but there are a lot of variations on them. Basically, we are supposed to have deck-level fittings for the pumpout stations on them for use in marinas and closed harbors, but many people also have the ability to pump them out overboard below the water line while offshore. My boat has such a pump to empty the contents of the tank, which is at or just below waterline. BTW, I found a plastic trapezoidal tank that fit perfectly in the space under the V-berth.I glued it in place with expanding polyurthane foam-in-a-can (no pun intended).The disadvantage of this system, which is pretty universal here, is that if the pump fails, you could have a hell of a mess. If you keep the pump higher than the tank, you can repair the pump w/o much fuss, however. I've repaired my pump twice with little trouble.

A friend of mine has a boat with a 12 gal gravity-type holding tank system (I think its too small - mine is 18-gal) which he swears works liike a charm. The toilet pumps into the top of the tank, and empties out from the bottom by gravity. The bottom of the tank is about a foot above the waterline, with a valve to open and discharge the contents, even when under sail.

BTW, ordinary toilets bust up the sewage pretty effecively, and on my boat, even after 10 years of installation and use, there is no sediment at the bottom, with weekly pumpouts the norm. Although I have a filter on the vent (which does work) it is not necessary and actually cost more than the tank did.

Also: pay close attention - I do not recommend the type of holding tank with a clean out, especially not the plastic tanks (which are perfect otherwise). When the tank is full the sheer weight of the contents will deform the top of the tank just enough to cause theseal to open on the clean-out just enough for odor, and worst of all sewage, to escape. That can be really bad, as I have discovered the hard way. NO CLEANOUT. Just remember you heard it first from me.

Happy sailing, Cliff Moore S/V PELORUS, Keyport, NJ

 

[HeadMistress]

"I do not recommend the type of holding tank with a clean out, especially not the plastic tanks (which are perfect otherwise). When the tank is full the sheer weight of the contents will deform the top of the tank just enough to cause theseal to open on the clean-out just enough for odor, and worst of all sewage, to escape. That can be really bad, as I have discovered the hard way..."

I disagree, Cliff. There's no way that the weight of the contents can deform the TOP of a tank unless the vent is blocked, causing the tank to become pressurized. And it would take a considerable amount of pressure to force anything out around an inspection port cap that's sealed with a gasket or o-ring as it should be. So your problem had to have been caused by continuing to pump the toilet against a blocked tank vent. Had the pressure not been able to escape via the inspection port cap, you'd have had a geyser when you opened the pumpout cap...or worse yet, a burst tank.

 

[roger]

Bit of lateral thinking - you can get your holding tank to discharge up, down or sideways without a pump by just holding the contents until the contents ferment and build up pressure and opening the right pipe (at bottom of tank).
Best to fit a pressure gauge though - or it might just burst.

 

[HeadMistress]

Definitely bit of lateral thinking, Roger... /forums/images/graemlins/smile.gif But I don't think it would work. For one thing, the bacteria in the tank would prob'ly die before they could generate enough gasses to create enough pressure. For another, pressure has a way of escaping wherever it can...so before the tank had enoug pressure in it to evacuate on its own, there'd most likely be an eruption in the toilet.

However, you're not the the first person to come up with lateral ideas when it comes to holding tanks...I know of one person who CLAIMS to evacuate his tank by closing off the vent, then using tire pump to pump compressed air into the tank till it comes pressurized enough to shoot the contents out the deck pumpout fitting. I'd love to be on hand--but definitely at a distance!--to see that...

 

[Birdseye]

Used exactly that system and it worked very well. Best to make sure there is an inch or 2 clearance between the bottom of the outlet pipe and the bottom of the tank. Loop the outlet pipe above the water line then you can start the syphon going and forget it. Best to leave the seacock permanently open - I found that if I closed it the weak solution of poo etc caught inside did a power of no good to the Blakes seacock. All those curries etc.

Another tip that worked well was to use solvent welded ABS pipe from B&Q wherever poss - avoid the smell that you get from flexibles however expensive.

 

[lille_bee]

we too have a gravity-emptied holding tank and find it invaluable. As I have already mentioned in a previous post, I firmly believe in keeping things simple to ensure they get used: emptying a pump-out tank is a chore, but emptying a gravity one is just a matter of opening a seacock, counting to 20 and closing it again, without ever breaking into a sweat; so it really DOES get done every day, thus minimising clogging-up risks.
Although we've never come across a deck pump-out facility in Greece either, we are very glad that we fitted a deck pump-out hole as it comes in very handy for flushing out the tank with buckets of seawater every now and then...
We opted for a custom-made stainless steel tank, even though the jury is still out on whether corrosion is going to be a problem; the bottom of the tank is above the waterline, but the hull outlet & seacock are just below it. We've tried with and without a filter: doesn't seem to make much difference IMHO...

 

[HeadMistress]

Urine will get to your steel tank from the inside long before salt air from the outside or even sea water. Urine is so much more corrosive than salt that you'll actually extend its life by flushing it out with sea water EVERY time you dump it instead of leaving any waste to sit in it between the times you need to use it.

If you have a salt water washdown, it can be used to rinse out the tank.

 

[roger]

Self emptying holding tank

This happened in Sweden, witnessed by a friend of mine. Motor cruiser arrived at the tank emtying station at Bullando.
Owner? opens deck fitting and great fountain of poo erupts .. What Blaster Bates referred to as A shower of Sh*** over Shropshire. It covered him, the boat and the harbour. The harbour authorities were mainly concerned with the pollution of the water.
Probable cause guessed.
Emptying pipe goes of course to the bottom of the tank.
Overfull holding tank leads to blockage of breather filter.
Consequent pressurisation -- and release.
I suppose Headmistress you would consider it a tribute to the construction quality and leak tight nature of the installation.
I reall do not want that to happen to me!!

 

[HeadMistress]

Re: Self emptying holding tank

I can tell you from firsthand experience that you don't! 'Cuz it happened to me once...YEARS ago, before I got into the marine sanitation business. I dove off the boat into the water without a thought for clothing, shoes, watch or sunglasses while it was still erupting...then climbed back on the dock and hosed off the boat and the dock.

The tank vent is last thing that ever occurs to anyone when the toilet becomes harder and harder to pump...they think it's a clog and keep pumping to try to clear it, which only pressurizes the tank more. If they're lucky, a spew out the deck pumpout fitting is their only punishment...'cuz enough pressure can turn a joker valve inside out and erupt back through the toilet all over the head as well as the hapless user--who has no escape from it. Or blow out a fitting (more common with bladders than rigid tanks)...or even burst the tank. A blocked vent will also prevent the tank from being pumped out or dumped, 'cuz without a source of air to replace contents being pulled out, the pump pulls a vacuum that prevents anything from being pulled out. A particularly strong pumpout pulling against a blocked vent can implode a tank.

The tank vent is also the last thing anyone thinks of after a spew...they mistakenly look for a clog everywhere else instead.

So it's VERY important to make sure the tank vent remains open! The two most common places for a blockage to occur are the vent thru-hull--due to waste spilling out due to heeling, insects, dust, pollen etc clogging the small opening--and the connection on the tank--both the tank fitting and that end of the hose, from overfilling and/or heeling...the piping in between rarely becomes impacted. Check the thru-hull regularly...scrape out anything you find in it...and backflush the whole vent line every time you pumpout and/or wash the boat.

 

[waterrat365]

Re: Self emptying holding tank

Headmistress: trust me - my plastic holding tank full to the brim weighs in around 180# and when that bad boy is full to the brim, believe me, it deforms just enough to pop the seal on the clean-out just enough to leak - and smell. I learned that the hard way. Like many installations, it's supported on the sides and one end by the shape of the hull and the bottom hangs freely. If it were supported at the bottom and sides, it might resist deformation better, who knows? Too late now. BTW, the vent was perfectly clear and free of kinks- it had about 3 or 4 inches of sewage in it, which cleared when I pumped out the tank. Also, the plastic material the tank was made of (your guess is as good as mine) made it impossable to permenantly seal as nothing sticks to it. I had a liberal application of silicone, but it leaked anyway. This is a far more common problem than you would guess, as I have heard of it before from others who also learned the hard way. I imagine the best way to seal it is with heat, by melting it, which is beyond my capacity. Better is have no clean out to begin with. My solution - pump out when the level is 2 or3" from the top, or at about 16 gallons on an 18 gallon tank. I've resealed and so far so good - its been about 3 years since then, but you never forget the smell. Happy sailiong, Cliff Moore S/V PELORUS, Keyport, NJ

 

[HeadMistress]

Re: Self emptying holding tank

Your tank is polyethyelene...and you're right, no sealant will bond it to permanently. Unless the female inspection port ring is spinwelded (a type of heat welding) to seal it, the correct way to install an inspection port is by using screws, with a rubber gasket under the flange on the tank. There should be an o-ring under the flange on the cap is what's needed to seal an inspection port.

'Twas that 3-4" of waste in the vent line that blocked the vent caused your tank to become pressurized....it was just luck that it hadn't yet solidified in the connection on the tank...that the pumpout suction was able to clear it instead of imploding the tank.

It also sounds from your description that the tank is very flimsy--walls too thin to support the weight of the contents. That's not uncommon among OEM tanks 'cuz many boat builders use the cheapest supplier, resulting in tanks that are barely 1.4"...some even with walls as thin as 1/8". A good quality tank has minimum 3/8" walls, strong enough to support the weight of the contents (waste and waste weigh 8.333 lbs/gal) without the need to support the sides. When I was in that business (I sold my company in 1999), we replaced a lot of OEM and cheap aftermarket tanks that had cracked simply due to the weakening of the walls caused by flexing and bulging...as well as good number of 'em that had burst or imploded...it doesn't take much to crack a thin tank.