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Old 12-21-2010, 04:10 PM   #1
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Just calculated how much to produce my own RO water

I've been doing some number crunching and came to some interesting conclusions that I thought could benefit some people who are getting into the hobby, like myself.

I've been reading and reading about how to setup my 65G tank and one thing that seemed to be general knowledge is that RO water is better to use than treated tap water. Will treated tap water work? For the most part. Will RO work? For sure. This of course requires either buying it from someone who makes it or buying an RO/DI setup. Option 2 seemed pricey to me so I found a LFS selling it. To my surprise they wanted .50 cents a gallon. To get my 65G going with sump water included would have been around $37.50 plus gas getting there and back. That's not even thinking about water changes that will be needed.

Next option was purchasing a RO/DI unit. I lucked out and found someone selling this SpectraPure unit on craigslist used for $50. I wanted something quick and they said filters were just changed a few months ago. I had a TDS test done on it to prove it was putting out 0ppm which it did. Anyways, on to the good stuff.

I checked my utilities bill to see what I'm paying for water. I'm paying $2.34 per 100cf for water and paying $3.80 per 100cf for sewage. Sewage is based off our water use. So I'm basically paying $6.14 per 100cf of water used. 100cf of water equals 748 gallons to put things in an easier measurement.

My water pressure at the RO unit is 76psi which helps a lot with efficiency. Water is cold this time of year so that doesn't help. After a few tests I figured that for every 1 gallon of good RO water I get I'm "wasting" 2.5 gallons. So basically it takes 3.5 gallons of water to make 1 good gallon.

748 / 3.5 = 213.714 gallons
213.714 gallons of good RO water costs me $6.14
$6.14 / 213.714 = 0.029 cents per gallon

So as you can see the per gallon cost is very minimal even including the "wasted" water. At this rate it will cost me $2.18 to initially fill my tank. To do a 10g water change will cost 0.29 cents. Doing that once a week for a year will cost $15.08. Even with the cost of replacing filters and membranes annual costs shouldn't be much over $50.

If I were to purchase RO from the LFS I'd be at around $260 doing 10g changes weekly for the year.

CONCLUSION: Even if you had to spend $200 on a new RO/DI unit it should pay for itself within the first year!

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Old 12-21-2010, 08:34 PM   #2
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Old 12-21-2010, 10:19 PM   #3
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Good explaination.....
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Old 12-21-2010, 10:25 PM   #4
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now calculate the price of salt into that! see how much per gallon it is, that'd be interesting to see how much i'm wasting on buying premade SW

(for a reef, if that matters)
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Old 12-21-2010, 10:39 PM   #5
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Well salt around here is 60ish a bucket which does about 160 gallons or so. Thats around .39 cents a gallon. That puts you at .42 cents a gallon or so.
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Old 12-21-2010, 10:47 PM   #6
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Also note that in *some* municipalities if you contact your water company they may opt to allow you a second water meter for the line that is not producing any sewage and not charge you for that on the second meter.
In the DC Metro Area? Check out GWAPA and WAMAS
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Old 12-21-2010, 11:18 PM   #7
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I think that is why some people meter their hose bib water separately. However, you want your RO to feed from the softened water line so that doesn't work.

That is a very good breakdown of the cost. I think I generated about 2000+ gallons my first year and spent $115 for the initial system and bought about $150 worth of DI resin, valves, tees, and tubing so I think I'm way ahead of cost of buying straight RO water at $0.37/gal at Wal-mart.

As for salt, I get a 200g box of Reef Crystals for about $52 through a group buy at LFS whenever they have it, I can usually get it for $56 from Foster & Smith if I buy enough, but I mix it to Salinity 35 which is 3 cups for 5 gallons instead of 2-1/2 cups, so that's 0.6 c/g so I get about 165 gallons of SW per box, or $0.33/gallon plus your $0.03/g water is about $0.36 per gallon mixed up 35 Salinity saltwater.

I'm sure the LFS SW is not 35 salinity and probably not Reef Crystals either though...
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
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Old 12-22-2010, 09:01 AM   #8
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Thanks for doing the math on that. It illustrates the point perfectly.

I do feel compelled to ask you to up your PWC to 15 gallons/week which is closer (sump included), but just over the recommended 10% weekly.
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Old 12-22-2010, 09:33 AM   #9
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Great work with the calculations. I'd bet it would be even more skewed if you added in the cost of gas driving to and from wal-mart or to and from your lfs every week for water.
"If your wife ever asks how much something was for your tank, just tell her you got it on sale and you had to have it! She'll understand"

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Old 12-22-2010, 09:47 AM   #10
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New calcs based on actual salt costs and %15 water changes.

$49 (Red Sea Pro treats 175g from my LFS) = .28 cents per gallon + .029 for water = .309 per gallon for mixed water.

%15PWC x 75G = 11.25g per week

11.25g x .309 = $3.48 per week for mixed water
11.25g x .029 = .33 cents per week for RO water

$3.48 x 52 = $180.96 for annual mixed water cost
.33 x 52 = $17.16 for annual RO water cost

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