Black/Grey Water

[StandUpGuy]

So help me understand please. I have been reading a little about black and grey water. If one has a toilet and a sink you have black and grey water right? It seems that all I read is two seperate tanks for each. Why is this? If both are being emptied at the dumping station, why two tanks instead of one? Sure I can see the advantage of two. One may be full and the other not. Other than that I do not see or understand why two.

[Roly Nelson]

I traveled full-time in my motorhome after I retired 17 years ago. Most of my over-nights were spent at relative's or close friend's houses. The grey water tank fills up much quicker than the black water, so here is what I did.

I found that if grey water is stored for any length of time, it stinks more than the black water. Therefore, I dumped it every day, often through a small hose into my host's flower bed or grass side yard. (No smell if dumped every day, no sweat). Often when I left, the grass was greener and taller at the end of the drain hose. Two tanks worked for me, and probably for many others who have factory installed dual tanks.

Another plus for two tanks, when dumping at a dump station, is that the grey water tank, when dumped after the black water, helps clean the "chunks" out of the sewer hose, especially when the grey water tank is intentionally filled to the top prior to dumping.

Now in the teardrop, all alone, a mason jar suits my fancy. Hey, we all gotta go sometime!
Roly

[StandUpGuy]

So it seems as what I suspected. Simply two different fill rates. You desire to dump the grey water often partially because you are not dumping it at a dump station. At least some of the time.

I am thinking my use of the trailer I am building will often be in two day stints on the weekend. I am probably going to do one run to the dump station I am thinking this will amount to about 10-12 gallons at the most. I am probably going to use detached wheeled tanks. So to have two tanks is twice the work really.

Am I missing something?

[mvperini]

the biggest reason is the sewer smell coming back up through the sink, if you only use one tank, with a black water tank when you flush the toilet that is the only time when the black water tank is open to the inside the trailer.

the only thing you will be missing is the smell unless you use only one tank

Mike

[StandUpGuy]

if you have an elbow then you have a gas barrier. Would not treating the sink like a real sink solve this?

[Roly Nelson]

I suppose codes require a P-trap on the drains on all commercial trailers and motorhomes. These traps only stop the sewer stink if they have water in them, and that halts the gas from entering the rig. A simple elbow, won't do the job. Just my 2 "scents".
Roly ~~

[StandUpGuy]

Exactly, so if the drains have traps then why not one tank? Seems to me it is cheaper and simpler and easier, for my application to have one tank.

[48Rob]

Exactly, so if the drains have traps then why not one tank? Seems to me it is cheaper and simpler and easier, for my application to have one tank.

Good logic...up to the point the single tank becomes full and backs up into the tub.
And really bad if raw sewage happens to be drawn into the handheld shower head that was left on the tub floor.

As others have pointed out, gray tanks can be filled very fast.
Most people don't want to hook up to the camper and drive it to a dump station to empty the tank, so they let a little run out.
You can't do that with the contents of a black water tank.

If you have the logistics worked out for your camping style, a single tank may be fine for you, but it just isn't for most people.

Rob

[StandUpGuy]

Really good points there. Since my plan is for the non built in tanks, I can use the one tank and see how it goes, then if I experience the issues with rapid fill up from the gray water, I can at that point, rework my drainage system to have two tanks.

So maybe I start with a single 16 gallon tank on wheels and then if needed make that my gray water tank and add a smaller tank for black water.

[fromeo]

Roly & Rob pretty much nailed the reasoning for the seperate tanks.

You may want to look into a "casette toilet" for your build, which is essentially a permanently mounted porta-potty - has it's own (removable from the outside for "dumping") tank. Then use the little guy on wheels for your grey needs.

When plumbing your sink(s) don't forget an air admittance valve on each sink "stack." The siphoning action of water draining can actually draw the p-trap dry. Seen here in the top right corner, kind of hiding behind the sink bowl:



- Frank

[StandUpGuy]

...
When plumbing your sink(s) don't forget an air admittance valve on each sink "stack." The siphoning action of water draining can actually draw the p-trap dry. Seen here in the top right corner, kind of hiding behind the sink bowl:


- Frank

I was wondering about that. Why this valve instead of simply running a vent through the outside wall? Because of fumes coming in from the outside? One could manage a vent in the roof I suppose.

[fromeo]

I was wondering about that. Why this valve instead of simply running a vent through the outside wall? Because of fumes coming in from the outside? One could manage a vent in the roof I suppose.

The tank needs an outside vent regardless - need some way for air to enter and exit the tank as the water level varies. I have permanent tanks, so venting through the roof was the obvious answer there.

Now, the air admittance valve is used because that pipe run from the sink to the tank is not vented to the outside. On my grey tank, I have 3 items draining into it - the galley sink, the bathroom sink, and the shower pan.

The shower pan is plumbed into the vent stack (it's only 6" from the shower drain to that pipe in my case) that runs up through the roof, so it does not get an air admittance valve.

The two sinks enter the tank seperately so they need the valve.

In theory you could run a vent through the roof (or even the wall) for each drain point. It's just not very practical. The valves allow you to "cheat" a little and run one vent, then the valves on any other pipe runs.

I'm not sure I'm explaining this very well with words. If a diagram would help, just let me know and I'll put something together explaining what I did. I also have a metric ton of pictures which may or may not help.

- Frank

[fromeo]

Another note - there are some different types of drains out there, often used in the pop-up trailers. Those are made to drain via hose and don't use a traditional trap. Something along those lines may also be useful for your external tank setup.

- Frank

[StandUpGuy]

...
The tank needs an outside vent regardless - need some way for air to enter and exit the tank as the water level varies. I have permanent tanks, so venting through the roof was the obvious answer there.

Now, the air admittance valve is used because that pipe run from the sink to the tank is not vented to the outside. On my grey tank, I have 3 items draining into it - the galley sink, the bathroom sink, and the shower pan.

The shower pan is plumbed into the vent stack (it's only 6" from the shower drain to that pipe in my case) that runs up through the roof, so it does not get an air admittance valve.

The two sinks enter the tank seperately so they need the valve.

In theory you could run a vent through the roof (or even the wall) for each drain point. It's just not very practical. The valves allow you to "cheat" a little and run one vent, then the valves on any other pipe runs.

I'm not sure I'm explaining this very well with words. If a diagram would help, just let me know and I'll put something together explaining what I did. I also have a metric ton of pictures which may or may not help.

- Frank

You can look at my build plans.

http://tnttt.com/viewtopic.php?t=43334

You will see my sink toilet and shower are inches from each other. I would just do as is in a house and have a main stack. In this case it does not have to be that large of a diameter. All drains will vent off of it and I will have no need for valves.

[fromeo]

You can look at my build plans.

http://tnttt.com/viewtopic.php?t=43334

You will see my sink toilet and shower are inches from each other. I would just do as is in a house and have a main stack. In this case it does not have to be that large of a diameter. All drains will vent off of it and I will have no need for valves.

Ah, ok! With your floorplan, that should doable.

Hopefully I didn't cause too much confusion..

- Frank