Vent lines - how important is <45 degrees

[AJR4]

Hi Peggy,

I am in the final stages of installing a new holding tank/septic system on my Nauticat 32. Thank you for your advice in designing this hopefully smell-free system.

I have installed the holding tank below the V verth and plumbed the intake and output lines and was just getting ready to install the vent line (1" tubing) when I discovered that my original plan to route the vent line forward through the anchor locker at a shallow angle will not work unless I route the vent line vertically for about 12 inches and then it will be intruding inside the stateroom running along the forward wall until it can then move forward through the anchor locker, and eventually to a through hull near the bow. I really don't want a sewer vent hose running between the my feet an my wifes!

My alternative is to route it aft, following the other sewer lines into the back of the closet in the forward stateroom and then straight up for about 24 inches to a through hull. That way, no vent line is visible in the stateroom, as it would run essentially horizontally under the berth into the closet.

Either way, the total hose length will be about 10 feet. The anchor locker route was my only hope for keeping the angle of the vent line to less than 45 degrees. How important is that and WHY is it important? What do you recommend between my two alternatives?

Thanks!

 

[Peggie Hall HeadMistress]

Very important...and so is vent line length

Gasses--aerobic AND anaerobic--in the tank are heavier than air...so a 45 degree angle is the most they can travel and exchange passively...and 5' is about the max distance, even absolutely horizontal. So your plan won't work. However, I agree that running a waste tank vent line through your berth and between your feet is unacceptable...so I guess we need talk again about how to solve your problem. Aeration MAY be the only solution. Give me a shout..we'll figure out something.

 

[AJR4]

Re: Very important...and so is vent line length

Hmm...5 feet? Well, my motor home has a 40 gallon holding tank and the vent tube is about 8 feet long, straight up and it never has any odor issues. However, some fresh air gets in the tank with every flush, unlike (I assume) a marine head. But with these restrictions, I'd venture to say that almost ANY holding tank mounted under the V-Berth of a sailboat is guaranteed to have ventilation problems unless it has two vents. I'll give you a call, but for the benefit of the forum, is there any guideline about how many cubic feet of new fresh air needs to enter a holding tank per day to keep the aerobic critters alive?

 

[Peggie Hall HeadMistress]

I'm sure there are arcane formulae for that...

But I've never found it necessary to come up with one.

Well, my motor home has a 40 gallon holding tank and the vent tube is about 8 feet long, straight up and it never has any odor issues.

Low flying birds and airplanes have confirmed that? :dance: You don't have any odor issues because it's released so high into the air that you don't notice it. Some houseboat mfrs do the same thing--they run a tank vent through the roof and up another 4-6'. And, some sailboat builders run vent lines up the mast. You can certainly do any of those things on your boat if you want to. However, they all do make maintaining the vent line a bit challenging.

I'd venture to say that almost ANY holding tank mounted under the V-Berth of a sailboat is guaranteed to have ventilation problems unless it has two vents.

Haven't found that to be true, and it's been more than 20 years since I first talked a Pearson owner into letting us put new holes in both his tank and his boat for a new 1" vent line. His tank was under his v-berth...vent line came off the aft end with a 90 to the hull (2-3')...another 90 and a run of about 15' to ANOTHER 90 that went up to a vent thru-hull just below the toe rail that needed one more 90. That boat had odor INSIDE the boat from permeated hoses AND odor out the vent!

We replaced all his hoses and installed a new 1" vent fitting--just ONE--on the top of the tank at the bow end...ran the vent line forward about 4' to new bulkhead thru-hull fitting about 1' below the to rail. And then I kept my fingers crossed and held my breath for two weeks, waiting to hear whether it worked... 'Cuz not only had we done all this at my expense, I'd promised the owner that we'd make any and all changes he wanted us to make to correct it if it didn't work. But it did...and the reason it did--and always will if the vent line is at least 3/4" (1" or even larger is better), no longer than 5', straight as an arrow, doesn't rise more than 45 degrees AND goes forward (VERY important!)-- is because air is always being forced into the tank when the boat is underway or on an anchor or a mooring. And it doesn't require two vents...the air can travel both directions via a single vent.

Btw...glad you took me up on my offer to talk again...enjoyed the conversation!

 

[thinwater]

I'm sure there are arcane formulae for that...But I've never found it necessary to come up with one.

If Odorlos (sodium nitrate) release all of their oxygen by reduction of nitrate to nitrogen gas (which I doubt is true--some certainly goes only to nitrite) a 4 oz. package would release about 2.2 oz. of oxygen in 3 days, about 0.75 oz/day, about 0.5ft3 of oxygen, or about 2.5ft3 air/day.

The smallest aquarium pumps put out about 5ft3/hour, so clearly a "Sweet Tank" system has this requirements buried.

A typical houshold contributes about 200 gallons x 200 ppm BOD5 = 0.33 pounds of oxygen demand each day, but I imagine on a boat that would be less (no dishwasher, clothes washer etc.), so about 2-3 ounces would be about normal... which matches up well with the oxygen contribution of the Odorlos combined with any tank ventilation, no matter how twisted the routing. Bear in mind that the BOD5 value assumes complete break down and that the Odorlos figure assumed complete reduction to N2 gas, which is not true, so the errors are very likely equal and offsetting. There is also some air introduced every time the head is flushed.

Mixing is always a wildcard. A boat at dock is one thing, a boat be sailed is another. I think it is clear that a boat that is used will generally get better results.

So we have empirical evidence that suggests somewhere around 2-4 ft3 per day for up to 4 people on board should make everything happy from an odor standpoint.

__________________________

How much hose and what angles provide this? Wow, that's a question with a lot of variables, as Peggy has stated many times. I'm currently running some tests for Practical Sailor using both boat holding tanks and multiple identical shore-based holding tanks. We're investigating vent filters, vent line options, and chemicals. Though these are long term tests and the results are far from final, the feeling I'm getting is that Peggy's rules of thumb for hose diameter and routing are very reasonable; when the hose is smaller and vertical, odor and solids go up. If the routing meets some minimum requirements, the odor and the solids in the tank go down and pump-outs are less troublesome. I'm going to suggest that hoses smaller than 1-inch are asking for trouble, based upon both our dry-land and on-boat experience, though with perfect installation 3/4-inch can work.

What we are learning is that larger vent lines, vent filters, and chemicals all have their pluses and minuses, that all can be done right, and that all can be done wrong. Not too surprising.

 

[robertsapp]

Has there ever been any discussion of using a small fan of some kind, maybe solar powered, to keep a small volume of air continually flowing into the tank to keep it aerated? Then it wouldn't be as important to depend on passive air exchange to maintain proper oxygen levels in the tank.

 

[thinwater]

Has there ever been any discussion of using a small fan of some kind, maybe solar powered, to keep a small volume of air continually flowing into the tank to keep it aerated? Then it wouldn't be as important to depend on passive air exchange to maintain proper oxygen levels in the tank.

Try searching to find threads. Also google "sweet tank".
http://forums.sbo.sailboatowners.com/showthread.php?t=127872&highlight=holding+tank+fan

 

[Peggie Hall HeadMistress]

Aeration? Or ventilation?

Has there ever been any discussion of using a small fan of some kind, maybe solar powered, to keep a small volume of air continually flowing into the tank to keep it aerated? Then it wouldn't be as important to depend on passive air exchange to maintain proper oxygen levels in the tank.

An aeration oxygenates the tank contents...but I think you're asking about using a fan to increase ventilation in the tank ABOVE the surface.

Yes, it's done, prob'ly a lot more often than is necessary. It requires two vents, and the fan should be an exhaust fan, 'cuz it's a lot more efficient to pull air through the tank than to push air through it. The only problem is in finding a fan that small that a) runs on DC power...and b) can survive even a whole season in the highly corrosive environment in a tank vent line.

 

[thinwater]

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