Waste water tank sizing.

[Barn Owl]

My bus has a 120gal fresh, 60gal gray, and 60gal black. From my experience with this set up, it seems that there would be a better ratio than 50/50 for the waste water tanks.  To me it seems that it is very easy to produce a lot of gray water and not that easy to produce an equal amount of black water. After all, even after a lot of people doing #1 and #2 and with the TP thrown in I still feel the need to add water to make sure I get a good drain from my black water tank. I am thinking about adding a pump to run some gray water over to expand that tanks capacity and to keep from wasting fresh water. If one was in the process of a conversion is there a better ratio? I am thinking more of a 70%gray/30%black split. If I was to change mine, I would install tall vertical tanks so it would take less water to push everything out of the discharge (Does that make sense? More water vertical instead of horizontal). With my current setup it takes a lot of water to push solids. Any thoughts?


Photo of my setup:

[Barn Owl]

BTW, The top tank is the black one.

[Chaz]

When I get to "create" mine, it will be similar to yours except the black will be a bit smaller. BUT, I am also going to have the black on the bottom. And on the other end of the tanks, I am going to have a valved connection between the two so as to flush the black tank with the grey tank.
  I thought that one up myself, but I have also learned that people have been doing that already!! It makes sense to me!

   Just can't quite come up with a new idea,
        Chaz

[Buffalo SpaceShip]

Laryn, your experiences are why some converters install one combined black-gray tank, or go with a large gray and smaller black. Your setup looks nicely-done, so I don't know if I'd re-do it. I see two potential solutions

# 1 You could tie your two tanks together with some 3" pipe (which means the solids would find their way to the bottom tank). You could put a valve on it to only tie  gray to black when it becomes full.

#2 Put a small 12v utility pump to push gray water up to black tank as needed.

HTH,
Brian B.

[gumpy]

My thoughts... combine them. Connect the two tanks together. Change the inputs to the grey tank so the shower and sinks drain into the black. The vents are probably already combined, but make sure the grey vent is water tight as it will eventually hold water.
Use the combination as a single black tank.

Either that, or replace the two tanks with a single black tank of 120 gallons.

[TomC]

I'm still an advocate of the two tank system.  I have 130gal fresh, 85gal gray, 45gal black.  I used 45 gal since when I was trucking, I had a 20 gal black water tank in my sleeper that would fill up in a week with just me.  So 45 gal would get my wife and I a week also.  It's just about the right ratio.  Good Luck, TomC

[white-eagle]

i've got two tanks  although both are 100 (i haven't cheked, that's what i was told) the grey seems to fill up after a week to 10 days, black is ok.  i like the idea of emptying the black and flushing the hose with the grey.  blacks don't seem to empty well without help and a larger combined tank just means more crap (no pun intended) in a bigger screwed up tank.  even our spray fill doesn't seem to "flush" the tank well.

i think 1 big tank will just collect more solids and be harder to clean out.  my 2 cents.

[gumpy]

i think 1 big tank will just collect more solids and be harder to clean out.  my 2 cents.

Tom,

That's not the experience we've had so far. We have 150 gallon combined tank. There's so much extra liquid in there that the solids are literally obliterated by the sloshing, and it's just soup coming down the tube. I've not had one single problem dumping my tank. I usually let it get 3/4 or more full, but even when I dump when it's only 1/4 full, it just flows out. No chunks or anything hanging up.

My father-in-law has 85 gal combined, and when he first started using his, we were on the trip to AK, and didn't know how far it might be between dumps, so he was using water sparingly, so was not getting enough liquid into the mix. The first time he dumped, it was difficult, and we had to run water in the coach for some time to clear it out. After that, he used more water, and had no more problems.

We only have one valve to hook the hose to and mess with, also. Just pull it and let it all flow. 2 minutes to empty a full 150 gallons! Slicker than... well, you can finish that one yourself.

craig

[Buffalo SpaceShip]

blacks don't seem to empty well without help and a larger combined tank just means more crap (no pun intended) in a bigger screwed up tank.  even our spray fill doesn't seem to "flush" the tank well.

Sorry, but your logic seems counter-intuitive. Wouldn't it be the same amount of crap in a one-tank system... but more liquid on top of it? The reason your black tank doesn't empty well is because there's not enough liquids (like, gray liquids) to help push it all out. RV toilets are amazingly frugal with water, which doesn't help matters when it comes to solids vs. liquids.

I have a two-tank system that came with my conversion, but never understood the logic of their functionality. It seems it's only designed to create a cleaner hose, not a cleaner tank. Thankfully, the design of my tanks means I can backflush the black tank with the gray water... but when I'm doing it, it always begs the question: why they were separated in the first place? 

My logic, anyways,
Brian B.

[TomC]

The big difference I discovered to keep the black tank from clogging, after dumping I initially charge the tank with two full bowls of water from the toilet-which I figure is about 5 gal.  This is enough to fill the dump pipes coming out of the tank so no solids can compact in the pipe.  Since doing this, I've never had an emptying problem.  Good Luck, TomC

[Brian Diehl]

I'm with Craig on this one.  My single combined tank has been working great.  I've yet to have a single dump problem.  Of course, we don't see any need to "dump" gray water on the ground.  That is the only real serious "benefit" to a split system.  You read the rules and you decide.  For people with space issues a split system can help as they can move tanks around, but you still need the capacity.  I custom built my own tank based upon Craig's experiences when he built his.  I've been extremely happy with my very compact layout and the 115 gallons of capacity is more than enough.

[sivrtnge2]

I have a 2 tank system. Fresh 125 gallon and black/gray 125 gallon. I just reduced the sizes down from 160 fresh and 167 black/gray. My wife and I tested everything on a trial week vacation. Everything worked out great. No problem with dumping or leaving deposits in the tank. Also, as stated before, less than 2 minutes from full to empty on the black/gray tank. There was no problems with the larger tanks. I reduced them to retrofit my new basement a/c units. Just my penny and 1/2 !!!
--
BILL

[white-eagle]

I think i may have this figured out.  Most of you posting with no issues on your dump tanks don't have Eagles.  Eagles have such a smooth ride that the sloshing doesn't occur as much, therefore the sides of the tank aren't getting as much of a "rinse" to clean off the indicator wires so they indicate full more often and take more time to empty due to solids not being broken up.

Don't you other Eaglers agree? 

[Devin & Amy]

Mr. Eagle,

Mr. MCI says "OUCH THAT HURT."

just one question though, Do you have to put extra flooring in to hold up the tank because of the R**t?

Devin

[Dreamscape]

There's that awful word again, RUST. At least we don't have air bag leveling problems.  In fact the rust makes them go faster, lighter than when they were new. 

Paul