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Discussion Starter #1
I'm installing a holding tank setup on my Cal 2-27, which never had one.
Arrived at the last bit: installing the vent line. I plan to run the hose up to the cabin side since I figure that's the best location for maximum air and minimum water entry. This means I need a 90 degree elbow, but it seems the only "proper" vent fittings are these bulky chrome things for gas tanks. Using one would place the hose almost 3" away from the cabin side and probably scrape my scalp every time I stand up after finishing the business. They also have a very small opening with a heavy screen, meaning minimum air transfer.

Chrome gas tenk vent

In order to keep things as flush against the cabin side as possible, I figure I can use one of these things instead (5/8" version), sawing off excess on the outboard side. I could put a small clamshell vent cover over it to discourage creatures and green water from getting into it, but still have decent air transfer.

Plastic thru hull

Is this a bad idea? Any better options you know of?
 

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You may want to consider that it should be in the hull just below the deck as it is in 90 percent of boats. I would not put it up in the airstream where you may be able to smell the aroma streaming back to the cockpit while sailing. also if you do ever overflow the tank the deck may get a bit slippery and another reason to not have to big a hole.
 

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If you run the vent line up to the deck and make a loop back down to a fitting through the side of the boat (topsides), you'll keep both raw water ingress and odors from becoming a problem, along with reducing the likelihood that your tank discharges through it. However, if it ever did, it would be in the best place.

The best vent line is 1" in diameter. Smaller is not very effective. If you want ideal ventilation, which is best to keep the tank properly decomposing and odor-free, then install two vents on either side.
 

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Learning the HARD way...
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Vent lines are typically 5/8", and that is the size that most vent fittings are as well.

Your best bet is to install TWO vents, one on each side of the vessel, as this will encourage air flow through the hose. Connect a "T" fitting at, or near, the tank. I used a stainless steel flush fitting vent. It looks something like this;
 

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Here are the actual vents that I used;


I liked that I could unscrew the cover with an allen wrench to clean the screen.
 

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Discussion Starter #7
Thanks everyone. I hadn't considered just looking at other boats. Venting to the topside does make the most sense. I was worried about stinky seawater getting in when heeling hard, but if the vent faces aft that won't be a problem.

Unfortunately cross ventilation on this boat isn't a very practical option. Routing a 2nd hose from port, where the tank is, to starboard will introduce a lot of bends and low spots. We'll see how one 5/8 vent works for now. I'm counting on chemicals and bathroom-shy guests to minimize odors.

I also like the idea of larger vents and understand the value of more air, but have never seen it in practice. Wonder what vents people use, where they're routed, etc.
 

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one vent is fine on a small tank. the air is always moving in and out because the air temp changes from day to night. 5/8" is the standard size and plenty big for your size tank
 

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Holding tank chemicals are evil and can actually cause odors in the long run. I'm no tree hugger, but I also believe they are the one good reason we can't discharge overboard in reasonable locations.
 

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Correction. Sterilizing holding tank chemicals (those containing caustic, formadahyde, or related chemicals, typically blue and strong smelling) are evil. Why they are still on the market is beyond me, other than some folks just don't learn.

Chemicals designed to augment the bacterial action in the tank are almost universally good and more pleasant to use. I tested many for Practical Sailor, and they all helped. Typically these contain either nitrate, live cultures, spores, or enzymes. The nitrate-based formulas are, perhaps, most dependable; the culture/spore/enzyme treatments were sometimes amazing and sometimes mediocre.

The reason we can't discharge close to shore is simply that the waste is not free of human pathenogens; while fish certainly poop in the ocean, they don't carry cholera. No simple treatment chemical will solve this problem, for a number of reasons.
 

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......Chemicals designed to augment the bacterial action in the tank are almost universally good and more pleasant to use. I tested many for Practical Sailor, and they all helped. Typically these contain either nitrate, live cultures, spores, or enzymes.
Good correction, because I did not mean to imply that these were "chemicals". I suppose they are actually organic chemicals. Would love to hear what you found works best. I use Raritan KO and it seems to work as far as I can tell. Can't wait to hear your take.

The reason we can't discharge close to shore is simply that the waste is not free of human pathenogens; while fish certainly poop in the ocean, they don't carry cholera. No simple treatment chemical will solve this problem, for a number of reasons.
Agreed, however, the definition of close to shore is completely arbitrary. It's set at three miles only because that's how far the State's are authorized to regulate. I'm convinced that something less is just as safe. However, if someone would show me that it took 1000 miles for recreational boat waste to be safe, I would be fine. Show me the science.
 

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Our vent came plumbed with a 1/2" through-hull just below the toe rail.
I up-sized it to 3/4" in the exact same location, and added a Big Orange (or something like that Canadian) carbon canister filter and I don't smell a thing from the holding tank now...
 

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Eherlihy please could you share where you bought that piece. Can't seem to find in defender or west catalogues. Thanks
 

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There two types of bacteria: anaerobic (need absence of oxygen) and aerobic (need oxygen). Anaerobic bacteria smell, aerobic do not and they do not live together well. That is why you are venting and I suspect a filter just limits the air exchange. With good venting, anaerobic bacteria die (and stop smelling) and the aerobic flourish. There are several other basics to maintaining a holding tank.

This is the bible on the topic and a must read:

Get Rid of Boat Odors: A Boat Owners Guide to Marine Sanitation Systems and Other Sources of Aggravation and Odor: Peggie Hall: 9781892399151: Amazon.com: [email protected]@[email protected]@http://ecx.images-amazon.com/images/I/[email protected]@[email protected]@51c0oAGGw1L
 

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Outbound. Big Orange filters can be bought dirrect or found on Amazon.
Big Orange

Miniwaska. The nitrates used are generally inorganic (sodium or calcium nitrate), basically chemical fertilizers. Nitrate is useful because facilutative bacteria (those that can go either aerobic or anerobic, a 3rd type you did not mention, but perhaps the most important in this context) can used it as a virtual oxygen source.

KO is a live culture and is known to be quite effective (it was in the group I tested), though it does not have the shelf life of inorganic chemicals like Odorlos or Camco Ultra. Some of the bottles had been on the shelf too long and did not work as well.

There are obligate aneroic bacteria; yes they tend to smell, but in the presense of signifigant oxygen they do not thrive. There are obligate aerobic bacteria that must have oxygen; you won't find many of these in a holding tank--the aerobic bacteria are actually facilitative. So long as there is enough oxygen in the top layer of the tank, the stink in the lower parts of the tank have a good chance of being scrubbed by the bugs in the top. However, stir the tank up with a good rough sail, and there will be some stink until the top lay is reestablished. Of course, since the wind is up, this is not too noticable.

Does adding a filter reduce air flow and thus encourage skink within the tank? Yes. But the people with filters are generally quite happy, since they don't smell it. I just resently replaced the carbon in my filter; it had been over 3 years. The process took 5 minutes and cost ~ $2. The builder place the vent below window, and a vent filter elimianted odor, where chemicals and ventilation could only reduce it. It is important that the vent location be well considered, such that overflows and seawater cannot enter. Search my blog or read PS.

As for the notion that oxygen is required to break down waste, that is silly and not supported by observation; the waste digestors (used to reduce waste volume) at every sewage treatment plant are anearobic, tightly sealed from air; anerobic digestion produces less waste volume than aerobic digestion.
 

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Thanks found the filters but was asking about that particular SS vent. It matches what I have and was thinking about it being good to have. Seems vents clog all the time. Usually can clear by having a few short bursts from pump out lying flush on vent. Also can fill tank with water. Go for brisk sail beating to weather. When boat working and offshore open thru hull.
Having this type of vent allows that clearing by pump out whereas anything with a clamshell does not without some effort. Surprisingly clogged vents can lead to smell as get more anaerobes and less aerobic bacteria as pdq says. Found using fresh water to flush also makes a big difference.
 

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......As for the notion that oxygen is required to break down waste, that is silly....
I don't recall saying this. O2 is simply required for the non-smelly bacteria that break down waste.
 

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There is a much, much more in that book above. Such as flushing sufficient water to keep waste from "holding" in the waste lines, particularly if you have a riser up to a vented loop. Also, what you flush and don't flush is equally important. Even flushing vegetable oil to lube your head pump can skim across the top of the tank and prevent aerobic bacteria colonization. Really, all should read it.
 

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.....The nitrates used are generally inorganic (sodium or calcium nitrate), basically chemical fertilizers......
Sounds like something that would be very bad for the environment, but I don't know that.

Non organic chemicals in our holding tanks, in fact anything that doesn't decompose, is going into the environment somewhere, even if you pump out. In fact, in some places, it goes to muni treatment plants and ends up in the same water we can't discharge in. Not arguing for free discharging, just pointing out the disfunction.
 

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I don't recall saying this. O2 is simply required for the non-smelly bacteria that break down waste.
I did not mean to imply you were silly--your responses suport that you are not--but the view that air is needed for waste break down is widely held.

As for the second statement, 2 subtle corrections:
1. O2 is not needed for metabolism without smell, only an electron source, and the nitrate (NO3) in the chemicals can supply that. That is how the chemicals function.
2. Anaerobic bacteria break down waste very well. However, they produce reduced gases (H2S and CH4) rather than oxidized gases (CO2), resulting in the stink. I did considerable testing with holding tanks, and venting variations caused no important difference in the consistency of the waste (lubing with veggy oil did).

Not only can veggy oil make the smell worse, it also leads to clumping of the waste and build up on the walls.

Peggy is correct, also when she encourages folks to flush enough. Conserving water or economizing on strokes for any reason is probably the leading cause of pump-out troubles.
 
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