Holding tank breather

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Mikedefieslife

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There's a respectable body of opinion stating that holding tanks should have a minimum of 25mm breather, preferably 38mm. I'm curious though, where does one get the deck/hull fittings for these?

The best I've seen so far is 19mm.
 
Mikedefieslife said:
There's a respectable body of opinion stating that holding tanks should have a minimum of 25mm breather, preferably 38mm. I'm curious though, where does one get the deck/hull fittings for these?

The best I've seen so far is 19mm.
Click to expand...

Yes the theory is that by keeping the tank well ventillated you avoid anaerobic conditions which lead to the "nasty smell microbes" taking over

Lee Sanitation are the people

https://www.leesan.com/shop/tank-accessories/breather-hull-fittings
 
Mikedefieslife said:
There's a respectable body of opinion stating that holding tanks should have a minimum of 25mm breather, preferably 38mm. I'm curious though, where does one get the deck/hull fittings for these?

The best I've seen so far is 19mm.
Click to expand...

Surely 1-1/2" (aka 38mm) skin fittings are pretty standard, for bilge pumps, galley sinks, etc?

I would be wary of specialist fittings with mesh, baffles, etc in them. If the contents of the tank does overflow up the breather for whatever reason, you want it to come out straight out and over the side rather than any solids getting stuck in the fitting.

We chartered a yacht in Greece a few years ago in which the holding tank breather was clearly blocked - the tank pressurised as you pumped the head and then the water was slowly forced back into the bowl against the valves. When the tank seacock was opened (offshore) the pressure was relieved and the head flushed normally, including the immediate cessation of the backflow trickle. Halfway through the week I happened to look up while swimming and noticed the edges of a wad of dried-out toilet paper protruding around the neat baffle in what must have been the vent fitting in the topside. Some earlier charterer must have over-filled it, and the bogroll got forced into the constricted end of the breather and plugged it.

Pete
 
vyv_cox said:
Peggy Hall also recommended that the skin fitting should face forward, helping to push air into the tank at anchor and when moving forward. Not sure that the physics agrees with this idea.
Click to expand...

I can't think of anywhere on a typical boat that you could achieve that without the risk of overflow onto the deck :)

Pete
 
I used a 38mm Skin fitting (brass) local chandeliers,
keep the pipe as close and as straight to the tank as possible
The reason being, as said before to allow oxygen into the tank which which aerobically decompose the contents , which should minimise smell. Small tanks will get away with smaller breathers (mostly)
but if your going for a larger tank then a larger breather is wanted .
Issues with water flowing back into the tank if you heel over can be limited with a clam shell cover.
 
I think that warmer temperatures of the US, often over 25C, make holding tank stink much more of a problem. Peggy Hall has written a good book, and I do not want to steal her thunder. But I will offer this:

1. The vent fitting should be a plain through hull, with no screen. This makes it easy to shoot a water hose through them to clean them out.
2. The larger fitting allows limited 2-way flow in the hose, so long as the run is short and has no low spots. Aim for a 45 degree angle.
3. More air in the tank really reduces stink. I've tested this for a series of articles for Practical Sailor. It does not eliminate stink, but often moving from 5/8" (common in US) to 1" reduces it enough.
4. Two vents are even better (fore and aft) but generally not needed. A single larger vent is enough in most cases.
5. Using a nitrate based chemical really helps. There are many, but Odorlos is a good starting point and should be available in the UK. The nitrate serves as an alternative oxygen source for the bacteria, reducing sulfide generation.

The warmer the climate, the bigger the issue.
http://sail-delmarva.blogspot.com/2017/06/100-best-buys-chapter-5.html
 
The Tek-tanks one looks like what I'm after. Thanks all.

I don't like the idea of the venting being a plain through-hull. Having a 38mm hole in the side of the hull leading directly to the tank is just asking for a tank full of sea water in anyone's forward head, even if you do raise the hose above the inlet. That or making a great colony for various insects.

I did get a bottle of odourlos last time I was in the UK (it seems pretty good). It doesn't seem to exist in the Med, and the UK companies won't post it. I've not come across anything similar on my travels.
 
My Moody 44 has a 38mm vent fitted as original standard. Outlet is high on the topsides just forward of the tank and has a clam shell cover. No idea if there is a screen on the outlet but I have no problems with smells or blockages
 
If building your own black water tank installation, try not only to make the vent as large as possible with charcoal filter for the reasons already mentioned. But also try to make the discharge outlet as high as possible just under the waterline so that it is hidden when at anchor in calm conditions, but is repeatedly drained and flushed with seawater while sailing.

If the outlet vent is of a decent diameter so that water can flow in and out easily you get your holding tank nicely cleaned out and stink free every time you go for a sail.
 
VicS said:
Yes the theory is that by keeping the tank well ventillated you avoid anaerobic conditions which lead to the "nasty smell microbes" taking over
Click to expand...
That doesn't make a lot of sense. A holding tank is a closed system apart from the vent - there's nothing to cause air to circulate from the atmosphere into the tank. Some oxygen will get in by diffusion, but there won't be any inward airflow. The vent provides a means for air to escape when the waste is pumped into the tank and to enter when the tank is pumped out. The Tek Tanks recommendation is to fit a charcoal filter in the vent to prevent the smell getting out.
 
Mikedefieslife said:
holding tanks should have a minimum of 25mm breather
Click to expand...

To me this sounds like a sensible figure to aim for, with regard to the capacity to let air in when discharging at a pump out station.
Ours is only 19mm, and while this is mostly fine, we have experienced a few pump out facilities where the suction force is particularly strong. On these occasions the flat sections of the stainless steel tank can be heard 'unbulging' from the decompression. No leaks yet, though...
 
andygc said:
That doesn't make a lot of sense. A holding tank is a closed system apart from the vent - there's nothing to cause air to circulate from the atmosphere into the tank. Some oxygen will get in by diffusion, but there won't be any inward airflow. The vent provides a means for air to escape when the waste is pumped into the tank and to enter when the tank is pumped out. The Tek Tanks recommendation is to fit a charcoal filter in the vent to prevent the smell getting out.[/QUOTE


Diffusion for this system can not be valued unless some serious formula is worked out , but after the waste has entered the system and the gases has expelled , from the vent then diffusion can commence in the other direction as diffusion can also be affected by the gradient which is the change in the value of a quantity e.g. concentration, pressure, or temperature with the change in another variable, usually distance.(the length of the pipe) pressure,(there will be a pressure variation from tank to outside) and temperature,(as waste breaks down there will be heat) all which will exist from the tank.
In such a small space as a waste tank their is not a lot need for large quantity of Oxygen , this is why when fitting a tank the air vent should have no bends, and be as short as you can get it to the side of the boat,and 38 mm to allow more diffusion to happen, also most waste tanks will get emptied more often that a typical home system which would require an air pump to feed oxygen down to the waste.
The carbon filter is to stop the smell of the waste discharge from flowing out the vent and killing the helmsman when some one has the runs., after the heads have been used the waste will settle down and oxygen can freely move in.
Click to expand...
 
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andygc said:
That doesn't make a lot of sense. A holding tank is a closed system apart from the vent - there's nothing to cause air to circulate from the atmosphere into the tank. Some oxygen will get in by diffusion, but there won't be any inward airflow. The vent provides a means for air to escape when the waste is pumped into the tank and to enter when the tank is pumped out. The Tek Tanks recommendation is to fit a charcoal filter in the vent to prevent the smell getting out.
Click to expand...

I have a 25mm inlet in the chain locker and an exhaust just below the toe rail. I hope that arrangement will generate a through flow of air.
 
Graham_Wright said:
I have a 25mm inlet in the chain locker and an exhaust just below the toe rail. I hope that arrangement will generate a through flow of air.
Click to expand...

quote 16
diffusion , so there is no need for a flow of air from one outlet to the other. that is why all waste tanks have one outlet , if on doubt google it on images Tank-1.JPG
the second outlet is optional pump out . less holes in a boat the better, this is my set up , without the pumpout
 
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I hate leading people to another forum, but there is simply a lot more information here:
https://forums.sailboatowners.com/index.php?threads/holding-tank-vent-charcoal-filter.193860/

I'm guessing it's because the US is warmer and they have more stink to deal with (when water temperatures drop to UK levels, the stink goes away). I've done years of side-by-side testing for magazine articles, so I'm not guessing.
1. A bigger vent does help. Convention becomes important, which has been overlooked by posters here.
2. Chemicals help, a lot. But they need to be the right ones. Look for nitrate.
3. Location of vents and vent filters is critical. If you can't find a place for a big vent, there is no way you will find a high location for carbon that will not get wet or clogged with poo. It's shame that builders consider tanks an afterthought.

Carbon really is the last choice. Chemicals and a larger vent are better. But here is a link describing it. It is NOT as simple as cutting the hose and sticking one in, not if you want it to work properly.
http://sail-delmarva.blogspot.com/2011/04/holding-tank-odors-vs-carbon-vent-filter.html
 
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