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I'm currently attempting to design a water recycling solution for a carwash in a tropical country that currently uses up to 30m$^3$ (8,000 gallons US) of water in 12 hours.

Based on my research in the subject the best way of filtering this water is by using wetlands after having passed the water through an initial filtering stage to get rid of any plastics in it, however although I have been looking around the web I haven't found any equation I could use to calculate the flowrate of water through a wetland.

You can find an image of the wetland filter I'm thinking of using here: Schematic of the Horizontal Subsurface Flow Constructed Wetland (Wikipedia).

Just in case it's not clear, I am not attempting to make the water drinkable I just want to make the water clean enough so it can be used to wash cars again. The water will be stored at most 12 hours during the night so it can be used again in the morning.

Any advice in the matter would be greatly appreciated.

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What you're really looking at is a fairly basic groundwater flow problem. What you need are hydraulic conductivity values for the "gravel" sediment in question. Given the fixed 1% grade, what you want to find is the "tank" length that maximizes pollutant residence time, and is relatively easy to calculate.

I believe that this is the basic design around which your illustration comes from:

Horizontal Subsurface Flow CW

I don't know how you're going to solve the problem of finding and keeping enough water in your system during the dry season. You're system depends on growing plants!

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Nice 'green' idea, but I don't think it will work. Wetlands are good for filtering out solids. Dissolved molecules, like detergents will take much longer to degrade by microbial and UV degradation, by which time coloured organics in the wetland will develop, like fulvic, humic and tannic acids. These might be OK for washing dark coloured cars, but I doubt if you will get away with this on white or light-coloured cars. If you have the roof area then rainwater harvesting in a large tank would be a better bet - also, the rainwater would be soft, so less cleaning agent would be necessary.

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    $\begingroup$ We are already collecting rainwater to try and reduce our water consumption, however this only works during the rainy season when people tend to avoid washing cars so our water usage drops anyway. More people wash their cars during the dry season when collecting rainwater isn't possible. The country in which I live in has also had pretty bad draughts the last couple of years so it is something we need to do something about. What other alternatives would you suggest? $\endgroup$ – patrickdamery Dec 28 '16 at 18:11
  • $\begingroup$ What if I could recycle the water for, say, 3 days and then dump the recycled water and use fresh water again to avoid the humic and tannic acids problem? If that would be acceptable it'd still mean I'd be able to save 24k gallons of water $\endgroup$ – patrickdamery Dec 28 '16 at 18:13

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