Drain System Vent

[Iceni John]

Here's what I'm doing.   I have two 2" vents in and out of each waste tank, specifically to ensure a good flow of air through both of them.   The gray and poo tanks are next to each other but separated by a frame rail, and their front vents are interconnected by a 2" pipe that loops under the frame rail.   The poo tank's rear vent pipe goes up and exits out of a forward-facing louver just below the roof gutter, and the gray tank's rear vent goes down to road level and vents out through a rear-facing venturi just ahead of the differential.   The theory is this  -  when driving, air is pushed into the poo tank's upper vent because of the forward-facing louver, and this air passes through both tanks and is sucked out at road level under the engine;  when parked, I'm hoping the airflow is reversed, with cool air drawn in to the gray tank's lower vent and exiting the poo tank's upper vent by means of natural convection.   At least, that's the idea!   My thought was to have a constant flow of fresh air through both tanks to help the aerobic bacteria there do their thing and keep smells to a minimum.

I never understand how single vent pipes can allow a good flow of fresh air through tanks.   I have no idea how well my crazy setup will work in practice, but it's worth a try!   And if I don't get enough natural airflow when parked, I can run a blower fan from my solar panels.

John

[RJ]

Our Angola Coach conversion MC-5C had a single, 100 gallon grey/black tank.

It had a single vent made of 1.5" PVC on the curbside coming out of the top of the tank, a 90^o elbow thru the rear baggage bay bulkhead/wall, another 90^o elbow pointing down, then a straight pipe terminating just above the bottom of the bulkhead, and finally a 45^o elbow pointing right between the rear duals.

In four years and 45,000 miles, we never had an issue.  The kitchen, shower and bathroom sink "p-traps" never lost water, and very rarely did we ever get "stink" outside the bus near the rear axle curbside.  We'd get "stink" by the rear axle streetside, but only when dumping the tank - everybody gets that! 

When we first got the bus, and before we totally figured out this holding tank thing, we'd get "stink" inside the coach every time somebody flushed, especially as the tank got over half-full.  After some study, and trying different remedies, we settled on septic tank enzyme packets available at any hardware store - that pretty well solved that problem, and at a fraction of the cost of "RV" holding tank products.

Angola converted the MCI on a brand new shell in 1978, this vent system has worked for over 40 years.

KISS principle, people! 

FWIW & HTH. . .

[buswarrior]

Iceni John is on the track.

The entire RV industry has put up with stinking toilet tanks for ever due to self inflicted wounds.

Without fresh air, the wrong stinky bacteria thrive, and the right smell free bacteria die.

It isn't about taking the stink away, or treating it, it is about creating the bacterial conditions to prevent stink in the first place.

The marine hobby has an expert. Peggy Hall, The Headmistress, has a book full of best practices and materials to keep the marine folks happy.

Larger vent pipe sizing, and two of them to promote some air exchange, early adopters cleansed their tanks of chemicals, and the smells are gone. Boats typically will put one vent pipe to the port, and the other to starboard to promote lazy air exchange.

The marine folks are also more inclined to use flexible hose in their sewer systems, most of the common ones quickly become sources of stink on their own, as they permeate. We are much more likely to be using household piping, which does not permeate, and typically don't have loops with sewage sitting in it.

Solutions are wherever you can find 'em?

happy coaching!
buswarrior

[chessie4905]

Use a little antifreeze. It won't evaporate.

[Jim Blackwood]

Use a little antifreeze. It won't evaporate.

Now there's a thought.
David, the trap we're talking about isn't on the main discharge, it's on the rim overflow. Probably something most busnuts don't even know existed.

I'm also pursuing the aerobic solution (conditions 1a, 2a and 3a). 1a is the most difficult however and depends on passive air movement. It also is hampered by heavier than air gases which will tend to accumulate in the tank. For extended boondocking in no-wind conditions this could become a problem. How do you solve it? Powering the bilge fan with a solar collector seems to be the most reliable solution but if it takes 1.5 amps to drive it that's not insignificant. Perhaps it doesn't need constant fresh air. One purge a day could be enough, and that almost certainly could be done using thermosiphon principles (hot air rises). Might be as simple as painting the omnidirectional vent vaccum cap black.

Jim

[richard5933]

Not every RV toilet has this overflow hole. There was not one on either of the Thetford toilets that we've had - I know since I'm the one that did the cleaning on ours.

Regarding adding an air intake vent...Be sure that there is a riser on that vent, otherwise you might find the contents of the tank sloshing out when the tank is in near full and/or the vehicle is in motion.

Jim - We rarely have any odor problems when camping unless the tank is getting full or one of our traps has sucked dry when we dump our tank.. The only time we routinely experience a problem is when we're in motion on the road. Since we have a single gray/black tank the contents are always quite fluid, and I'm guessing that helps somewhat over a regular black tank.

[6805eagleguy]

That's not bad, I was planning to weld up some stainless but it'll likely cost that much and more. Nearing completion of the fresh water tank, just have to get material for a cover and time to work on it.

Jim

Jim

I was planning the same but:
I talked to ardemco and he said that most of the tanks they make replace stainless, apparently gunk will build up on stainless and corrode right through it.
I am still researching that, though.

[Jim Blackwood]

Interesting. Let me know if you find out anything. I can see a plastic tank being the best for black water. Maybe even gray. For fresh though I expect the stainless will be hard to beat. (Especially since I already had most of it on hand.)

Bob, about those $200 tanks: Will he make them to my sketches? I'll be needing custom dimensions, but if he's willing to do that I'd have no problem buying them, I'd just need more information on things like type of material and wall thickness, as well as whether or not they need a box for support of the walls.

Jim

[6805eagleguy]

Yes he was talking about mostly the black tank. The grey is right in between  and for plain water the stainless will last forever.
If you have the stainless on hand, then that would be a good test.
The salesman thought they would last 10 or 15 years.

But then he was a salesman, after all..