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Author Topic: Need some advice on tank configuration  (Read 1267 times)
Jeremy
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« on: July 18, 2013, 02:45:58 AM »

I've been doing some work on the black & grey tank set-up for my bus, and, having now actually bought a couple of tanks, my plans have completely changed from how I had originally decided to arrange things.

My bus is mid-engined so doesn't have any pass-through bays - instead it's got small bays on the sides, and a big rear bay. There's also a lot of empty, dead space in the middle of the bus between the gearbox and the rear axle, which is where 90% of the conversion stuff (batteries, tanks, calorifier, pumps, Webasto, electric water heater etc) will be, in order to keep the bays empty. Only the inverter and the converter will be in a bay.

Although an attractive empty space in theory, this central-area of the bus isn't an easy space to work with as access is a bit tricky (at least for big stuff like tanks), and there are various bits of mechanical bus gubbins in there as well; in order to get the required tank capacity my original idea was to have a medium-sized black tank, and string-together several small tanks to give the required grey tank capacity. But now that's all changed having bought a couple of tanks (actually three, but one probably won't now be used), which has made me think about the practicalities involved of mounting multiple tanks.

So the new plan is to move the spare wheel mount about three feet down the chassis and re-locate the big oil-bath air filter as well, which will give me room to mount one big tank and one medium-sized tank, as shown in the illustration below:
 


The black tank is about four-times the size of the grey tank unfortunately - I'd rather it be the other way around, but that's just how it has to be to fit the space. So my idea is to link the two tanks together right at the top, so that when the grey tank is full it overflows into the black tank. Is this a good idea? I realise that the black tank could theoretically overflow into the grey tank, but I'm sure that would never happen in practice.

The drain from the grey tank goes through a 2-way valve so it can either be emptied directly, or the water in the grey tank could be used to flush out the black tank when the black tank has been emptied.

Lastly, I can't decide whether it's better to for the sinks (basins) and shower to drain into the top of the grey tank or the bottom - if they drain into the bottom there will always be water sitting in the pipes (which seems wrong), but if it drains into the top the drain pipe from the shower will be at a very shallow angle, which seems wrong as well. Any thoughts on this, or anything else in the illustration?

Thanks in advance for any advice

Jeremy
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bevans6
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« Reply #1 on: July 18, 2013, 03:37:13 AM »

Inflow to the tank is at the top, outflow at the bottom, so drains enter at the top.  You can run a single daisy-chain drain and have fixtures feed it (except for the head) and also use it as a vent line - you should have a drain vent line on the drain run, and you can rely on that for the tank vents.  You need a separate vent for the black tank as well, keeping in mind that vents need to always rise, never fall.  I have a combined tank of about 100 gallons and I can't really grab the idea of why separate tanks are better, but if you have two the ability to use the gray to rinse the black is great.  I think the overflow is fine as well, I would do it at 75% of max height to make sure that the shower can drain in to it.  If you connect at the very top also that will vent the black tank into the grey and out the grey tank vent.  Shallow angle is fine as long as you always have it - parking nose down shouldn't make the grey tank empty into the shower, and make sure you have air after water with a good vent for the shower.   I might think on ways to combine the tanks all the time such that the black tank always has grey water in it but the grey tank never gets black waste backflowing to it, but at the end of the day I would probably do that by connecting  at 50% and allowing more grey water into the black tank and trusting to gravity to keep the poo where it belongs.  Not sure that is a good idea anyway.

Brian
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« Reply #2 on: July 18, 2013, 05:49:24 AM »

Here we go again. Sorry for bringing this up. Everyone knows I love all here....but I'm really feeling just one tank here...ie black/grey combo. Of they are going to potentially mix anyway because of the overflow, then legally you can never really drain your grey water on the ground when and where it is allowed. I know everyone says it isn't allowed anymore, but we are given permission to do this i many more situations than one would believe. I would just make one big black/grey combo and be done. If you are ever in a situation where you want to drain your grey onto the ground, put a "y" and a valve in each drain (shower, sink) so you can simply twist and drop it on the ground. I've done this and couldn't be happier.


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« Reply #3 on: July 18, 2013, 05:52:29 AM »

Hi Jeremy,

One thought to keep in mind... If the gray tank is forward of the black tank, we usually break harder

then we accelerate. This may cause black water/black fumes to push into the gray tank. I have delt

with this issue on an entertainer Prevost and the sink traps would actually bubble and fumes entered

the coach. Maybe a one way valve between the two tanks would solve this issue!

Nick-
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Jeremy
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« Reply #4 on: July 18, 2013, 06:32:37 AM »

As it happens the grey tank is slightly behind the black tank, but it's a good point to make. In fact I'd already been wondering about the issue of black tank fumes being able to get into the bus - the shower and sinks will all have proper (domestic) water traps on their drains, which I would think would prevent any fumes getting past, but I'd hate to find that they didn't. The toilet is a conventional domestic toilet with a water trap by the way - I know many would disagree with that choice, but I've read the arguments against using them in a bus and haven't been convinced.

The black tank will have a vent straight up to the roof, exiting via a mushroom vent (should have shown it on the illustration).

The logic of two separate tanks seems compelling to me - the grey tank will get far more use, and will need to be emptied far more often - and there are vastly more more places where tanks which only contain grey water can be emptied.

I think Brian's suggestion of putting the cross-over drain at 75% height rather than 100% height is probably a good idea. I'll have to think about how to do that though, because I was planning to use the moulded-in ports that the tanks already have right at the top.

Thanks for the thoughts so far

Jeremy
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« Reply #5 on: July 18, 2013, 07:51:37 AM »

Common wisdom says that the toilet must be mounted over the tank, but that is not necessarily true.  I have the toilet as much as 6 feet from the tank with no problems except perhaps using a bit more water to flush. If you are using a domestic toilet, that should not be an issue.

If the toilet location is driving your decisions, you might want to consider that.

My experience is that 30/70 or ever 25/75 black/grey works well.
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« Reply #6 on: July 18, 2013, 10:18:55 AM »

Over here this side of the pond there are virtually no places (I have never encountered one, for sure) that let you drain only grey and not black.  It's one hole in the ground and it takes what you have.  If you have places that are more common that let you drain only grey, and few places to drain grey and black, then that is indeed a compelling reason for two tanks, and for having the black tank larger.  If you set it up so that your cross-over is at 75%, that gives you a semi-automatic over-load relief for when the grey is too full - it will drain into the black automatically and as long as the black is below the height of the cross-over it won't flow back the other way.  The effect would be that you have a grey tank and a combined black/grey tank - best of both worlds!

Brian
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« Reply #7 on: July 18, 2013, 10:52:29 AM »

We have a dripper on out grey tank . Not that I have used it   wink wink  nudge nudge                                           dave
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« Reply #8 on: July 18, 2013, 12:40:04 PM »

We have one combined black/grey tank.  There is a grey water bypass though.  When dry camping in a forest, for example, we can run a small hose into nearby bushes and let the grey water out so we do not fill the black tank too quickly.  This makes a huge difference.
Since it appears that your drawing is linking the two tanks together with a valve, I am not sure the overflow is necessary.  However, if it is, I would consider on dump valve there also, so you can decide if you want the tanks to interchange fluids.  Driving up and down hills could definitely effect the way things go.  If left unprotected, the theoretic grey tank will really just be an auxiliary black tank.

I would think it is best the drains enter from as near the top of the tanks as possible.  Even if you did have them come in the bottom, the pipes will back up to the shower when the tank is full.

Of course, make sure that there is no way that black waste can get by that 2-way valve into the grey water discharge pipe.
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« Reply #9 on: July 18, 2013, 01:18:42 PM »

I like tall skinny tanks. I saw a Foretravel one time and the bottom of the tank wasn't much wider than the drain. The tank was actually more trapezoidal in shape. A high water column with a skinny floor would flush out solids and keep a cleaner tank. I have just the opposite setup and solids just gently lay themselves on the bottom in defiance to my wishes. I am also a big, big, fan of large grey water and small black water tanks or just combine them. My dump rate is probably around 10:1 grey:black. Being able to transfer water from grey to black is something I need fabricate.
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« Reply #10 on: July 19, 2013, 07:33:53 AM »

I would NOT interconnect the tanks. At least what I do-when I get that rotten egg smell coming from the gray tank, I use bleach to counter act the smell. Bleach is something you never want to introduce into the black tank because of the ammonia in urine interacting with bleach (ammonia gas). If you don't use bleach, I would still not interconnect them-instead use a small pump to pump the gray water to black tank when you want. Good Luck, TomC
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« Reply #11 on: July 19, 2013, 02:03:47 PM »

Anyone have any recommendations on what type or brand of pump to use to get the grey water into the black tank?
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« Reply #12 on: July 19, 2013, 02:16:55 PM »

 Your plan looks good to me. I prefer one tank but your setup amounts to the same thing. The more liquid in the black tank the better.

There is no reason not to input both tanks at the bottom. A bottom feed may even be better because you aren't exposed to the whole black tank surface every time the toilet flapper opens.

My 4104 commode is also on the opposite side of the bus from the tank (one tank only). The sewer pipe passes forward into the next bay, then across that bay back into the rearward bay and then into the bottom of the waste tank!! It has very little slope anywhere!! And there are two 90* Ls in the line??

This toilet sewer pipe is about 12' long and is also connected to the kitchen sink and shower near the commode!!

 According to everything I've read on the forums this won't work, but it does, even on an opposite slanted parking spot?? My setup violates all the rules laid out by the naysayers on the forum.
 
I probably wouldn't have done it that way but it was on the bus when I bought it and works fine.

The only thing I can figure that makes it work is that we always use plenty of water to flush. I don't mean great quantities, just a little extra. The more water in a black tank the better.

I also have a tank vent which makes a loop above the tank then back down through the floor. I'm not recommending you do this because the code police will get on my case, but it works for me and a few others.
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« Reply #13 on: July 24, 2013, 03:49:00 PM »

Hello Jeremy- I just installed my tanks similar to what you are planning, but instead of an open "overflow" from gray to black, I put another 2" pull valve. In your sketch, this would prevent the backflow you fear. At a dump station, I dump my black, then flush the gray through the black. Two tanks is better for us because we generate more gray than black and some campgrounds and racetracks have french drains that permit gray overflow. By installing an additional gray bypass drain, we can carry our black until we get to a dump station. Obviously, if the gray fills and I can't dump it, I can make room by dumping some into the black tank. I believe this setup has the unique advantage of a 2 tank system, or a 1 tank system, whichever you prefer!
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« Reply #14 on: July 24, 2013, 04:12:18 PM »

in a nutshell. .
I have equal black and grey, but I have a diverter valve that allows my kitchen sink to be selected to either the black or the grey. This allows me to add extra water to the black if it needs more to prevent sludge build up or if the grey is getting full. Also helps if the camp ground has on site showers.
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