Water-tank ('box') design for gravity-feed water system - HELP

joel_bc(z6 BC)September 22, 2007

Hi. After many years of utilizing an old, multi-household gravity-feed system, I've convinced enough of my neighbors to do something about replacing it. The old concrete tank (we call them "boxes" in this part of N. America) is too small and the concrete has deteriorated on the inside so that too much silt adheres and we're getting bacterial and worm problems in the water.

The system is basically this: A concrete basin in a small creek withdraws water. A buried pipe conducts the water from the basin downhill for about 150 metres, where it feeds a 600-gallon concrete tank. The tank has a settling compartment to allow silt to settle out, and then also four distribution compartments, each of which has a gate valve and a pipe that heads downhill to the various homesteads.

The 600 gallon tank is too small, because at low-flow time (July to realy September) it refills too slowly in relation to the need for water by the homesteads. Yet even in the dry period water is overflowing this tank during the night, when there is no use of the water.

We're thinking of increasing the "buffer" (distribution-tank capacity) to at least 2400 gallons. Larger would be better in a way, except the available spots in the steep forestland area would not very well accommodate the location and construction of a huge tank.

So our need is for a good design that is large enough, easy to clean, and can equitably distribute water to four individual outflow pipes.

Anyone know of a good Web site, book, or set of mail-order plans? The more plans we find, the better we can make design decisions through comparison. Thanks.


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the "best" bet is to drain, dry and clean the old concrete tank. (muratic acid). Coat the inside with an couple of epoxy coats and make sure that the top of it is well covered to omit daylight. Part way down the feed lines for each of the homesteads, put in in-ground polyethelene cisterns. Sizes vary from 250 gallons to 2500 gallons. An 1100 gallon cistern runs about $1200 CDN.

Each homestead can buy their own, ( as a group buy) and size them according to their needs. As all the cisterns will be fed by the concrete tank, all households will recieve their "fair" daily allotment of water.

Average NA useage of water is 80 gallons per person, average Canadian useage is 120 gallons per person. Average African useage is 8 quarts per person.

It would be a good idea to test the existing system for leaks and overconsumption. Every gallon you guys take out of the stream is one less gallon for wildlife, fish and plants.

    Bookmark   September 24, 2007 at 2:06AM
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joel_bc(z6 BC)

Interesting idea, and of course we will consider it. Thank you.

A couple points: How stable is an epoxy coating? I'm thinking about the epoxy coming in contact with drinking/cooking water.

Another point, though, is that the 80 gallons or 120 that you say Americans or Canadians use per day, does that include irrigation water? We irrigate with it (gardens, fruit trees, chickens, goats. I doubt the figure for Africa includes the irrigation-water daily average. As to fish in the creek that might be deprived, both the biologists and we water users have confirmed there are no fish in this creek. In our case, too, the excess (grey) water goes back into the watershed, just at a lower point in elevation. Nevertheless, you bring up good points.

Thanks, again


    Bookmark   September 24, 2007 at 9:30AM
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epoxy is inert and more stable timewise than concrete or even cross linked polyethelene. Keep in mind that you want to use a Marine grade epoxy like West Systems.

If you do not want to use epoxy, if you clean and prep the tank really well, wash it down with muratic acid, you can then coat the tank with a bonding agent, and trowel on a parge coat of acrylic and fiber reinforced topping cement. Mix it up thick and stiff. You need to trowel it, not pour it. To get a smooth surface, keep trowelling and polishing the coat until it is rock hard.

That 120 gallons that Canadians use is just suburban use, and does not include farm irrigation water. The trick is to use the 80 or 120 gallon figure as an estimate to determine cistern size. Four homes sharing a 600 gallon tank, even though it is refilled during the day, is small if there are more than two people in each home.

You may want to consider irrigating at night, or adopting less consumptive irrigation methods like drip.

    Bookmark   September 24, 2007 at 12:04PM
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joel_bc(z6 BC)

jay, thanks again. You wrote (snipped): "Four homes sharing a 600 gallon tank, even though it is refilled during the day, is small if there are more than two people in each home." I didn't mention details before, but two of the lines are split (due to subdivisions of land parcels), and there are six families - of two to four people each - now using the system. So the need is even clearer, I believe.


    Bookmark   September 24, 2007 at 1:03PM
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The gold standard on tanks is stainless. I welded up 10 4x8x18guage 304 stainless sheets into a 2500 gallon cylindrical tank for $480 about twenty years ago. NO issues at all in that time. Stainless has doubled or more since. I'm not a particularly good welder, just dumb and ornery enough to ignore conventional wisdom. No regrets.

    Bookmark   October 11, 2007 at 3:21PM
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joel_bc(z6 BC)

Fruithack, a few questions:

Did you consider a formed-in-place concrete tank? And if so, why did you choose to go stainless steel instead?

Did you gas or arc weld your tank?

Did the tank require partitioning for multiple-household or multiple-building water use? If so, can you describe how you designed that?

Did you set the tank into a wood-frame reinforcing & support structure? Or....? (Partly, I'm wondering if the welded sheet metal was strong enough (or not) in itself to hold together over the years, with the constant pressure of the water volume inside it.)

This would be great to hear about. Thanks for posting.


    Bookmark   October 16, 2007 at 6:56PM
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I did'nt consider concrete. I've seen too many of these over the years. Too expensive, leak, etc. Mig welded with 2# spool gun, .030 308L Si wire, and trimix. No partitions, You're scaring me with frame, strength, etc, question. Cylinders are self supporting. Tensile strength of stainless is brutal. Pressure at bottom of gravity 8' tank is about 4psi. 18 guage metal was chosen because that was the thinnest my welding skills would accomodate. This was not a slam dunk, but doable with some hack tenacity and judgement.

    Bookmark   October 18, 2007 at 4:19PM
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Re: water usage and systems. We designed and installed a rainwater catchment system a year ago. The system includes four 550 gallon poly tanks that we covered with heavy black plastic and placed under a deck, a pump, 5 micron filter, UV filter, and water meter. Our average daily use is 58 gallons per day for 2 people who have lots and lots of of company.

We live in southeast Virginia on the Chesapeake Bay. Normal annual rainfall is about 48 inches per year. However, we are in a severe drought now so have had about half of normal annual rainfall since we installed the system. We have not run out of water yet, although we came close a couple of times. We do plan to add another 550 gallon tank.

I use a shallow dug well (36 inches wide, about 11 feet deep) for outside watering.

Rainwater harvesting or catchment systems are great. Once the system is in place, it's low maintenance and the water quality is excellent.

    Bookmark   October 27, 2007 at 11:20AM
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