why separate tanks for grey and black water?

jplante4 wrote:

dave17352 wrote:
I actually explained what I do wrong. I leave the cap on the sewer outlet and open both the black and grey valve.

I did it this way...once. Then I realized that I had a mixture of black and grey water between the valves and the cap that had no place to go except into the waste bay. Holding the hose above tank level accomplishes the same thing with no mess.

My outlet is outside. I simply close the valves and open the little hose cap and drain what little is in the line into the hose then connect the hose and drain. No problem at all for me. I know some people simply hook a valve right to the end of there dump valve accomplishing the same thing.

dave17352 wrote:
I actually explained what I do wrong. I leave the cap on the sewer outlet and open both the black and grey valve.

I did it this way...once. Then I realized that I had a mixture of black and grey water between the valves and the cap that had no place to go except into the waste bay. Holding the hose above tank level accomplishes the same thing with no mess.

porkslap wrote:
Hi Folks,

Let's say I'm building my own RV. Is there a reason I can't use the same tank for grey and black water?

-Tom

Many times I can camp for a few weeks in one location, and dump my grey tank on the local plants (into a hole about 12" deep that is buried within about 1 hour to prevent flies from discovering the water). Sometimes in a National Park, or other developed campground (where you might have neighbors within 600 feet of you) you can not dump the grey water, and must do things like dump shower water into the black tank to extend your time before going to the dump station.

Also if you ever camped, and had the grey water back up into the shower, you will understand the NEED to not have black water in the grey tank.

Back in the 70's a lot of RV's only had one tank. And they might have backed up into the "Wet bath" when camping, and make a turn or drive down a winding road, and you could splash out a lot of water into the shower pan.

That is why RIVA requires two tanks in most RV's.

My buddy designed his RV and it's storage tanks. His grey water is mounted above the black water tank, and he has the ability to drain his 150 gallon grey tank independent of his 100 gallon black tank, or drain the grey into the black tank, to wash it out better.

So he can drain his black tank, then wash about 50 gallons of grey water into the black tank, and then drain that (removing all the solids) and then rinse again with the remaining grey water. Any time he is dry camping, and grey is say 90% full and black is only 10% full, he can drain some from black to grey and camp another week or so.

I think his fresh water tank is a stainless steel 150 gallon tank. He rarely needs to fill it completely, unless planning 3 weeks of dry camping!

For tanks, try this website. Tank-Depot.com

Good luck with the planning!

Fred.

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Dutch_12078 wrote:

jplante4 wrote:

dave17352 wrote:
I agree with all the posters here. Now on occasion I have equalized my tanks when I need more grey water room by opening the grey valve and leaving the black tank valve closed. This brings them to the same level.

I don't see how this equalizes the level. If you were to open both valves and hold the stinky slinky above the top of the tanks, then the fullest tank would sluice into the emptiest tank. Hopefully, the grey tank is fullest.:E

I do this to provide a little extra liquid to flush out the black tank. Be sure to hole the hose high.

I added a third "Main" valve to my dump setup. To equalize the tanks, I just leave it closed while opening the grey and black valves. When dumping, I open the black and main valves, and when the black tank is empty, give it a quick rinse by closing the main valve and opening the grey valve for a couple of minutes before opening the main valve again to drain the grey water.

I actually explained what I do wrong. I leave the cap on the sewer outlet and open both the black and grey valve. Both of my tanks are mounted at the same level in my TC holding area. With both valves open the tanks equalize. Like I said if doing this I know I will not have have sewer water in the shower because I know both holding tanks can hold the entire 46 gallon fresh tanks contents. I checked this to make sure before doing this. Works great if needed. But usually this is not necessary but when it is it works great.

Many large motorcoaches (commercial rather than RV use) have a single large waste tank. That makes it a "black tank."

There's some history. at one time, at late as the 60s, there was only a waste tank for "black" waste and "gray" drained to the ground or a bucket. Some types of RVs, including tent trailers, still do this. Gray tanks started showing up with two factors: RVs got showers, washing machines, other facilities that produced large amounts of gray waste, and not coincidentally, more and more places prohibited draining all this wastewater to the ground, so tanks had to be installed to contain it at those places.

The gray waste tank has probably been put into the RVIA standards, but as a home builder, you are not a RVIA member and won't be paying them to put their sticker in your rig. Those commercial coaches with one big tank? Manufacturers also not RVIA members, not building to RVIA standards (usually well beyond that) and not looking to license a RVIA sticker.

When you are at a site with full hookups, and you have two tanks, you can leave the grey tank valve open and take long showers and not have to worry about filling the tank up. Also, most of us put chemicals in the black tank to control the smell. If you only had one tank and you were having to empty it every couple of days, you'll be putting chemicals back in every couple of days. I can go a week easy, probably longer, without having to dump the black tank.

If you only have one tank, you can't leave the valve open when you have full hookups, because you need to keep some water in the tank so you don't create a dried out mountain of human waste in the tank.

You'd have to keep close watch on the tank level. If you don't, you'll have sewage in your shower and sinks.

For a home builder handling and installing a 50 gallon grey tank and a 25 gallon black tank should be easier than one 75 gallon tank.

In my experience, and talking to literally hundreds of folks, several dozen full-timers - the combined capacity of the waste tank(s) need to be at least five to ten gallons over the capacity of the fresh water tank.

Most people I've met will have to empty the grey tank twice, sometimes three times, before they need to empty the black tank. Even folks who use plenty of water to flush the toilet.

(Of course we aren't boondockers who are careful about every once of water they put in the tanks.)

Most of us do things like leave the water running to brush our teeth, rinse dishes, etc. Which puts more water into the grey tank, certainly more than we should.

Our first rig had one tank, everything drained into that tank

No reason to separate except for practical logistics of plumbing and tank size.

Although if the kids fill the grey you can still use the toilet if it has a separate tank.

If the common tank is filled.... the tub could get gnarly as you finish washing dishes.