Rainwater harvesting systems

[james.parera]

Hi All

I am very interested in the rain water collection system which is an environment friendly design application.
I am in my thesis year right now, and would like to know more about these systems.


we will highly appreciate your feed back.
Thanks in advance.
James

 

[Jimbo]

Well, it doesn't rain here in southern Calif. so we have no use for such a system. Maybe others have familiarity. I would think if you are smart enough to be at the "thesis" stage, you would be smart enough to hoof it over to the university library, or at least let google do the walking!

 

[Sponsor]

 

[Southern Man]

I just went to a Green Building seminar two weeks ago sponsered by an affilliate of Hanco pipe (www.hancor.com). They had a brief presentation on these systems. Basically you use a large diameter HDPE couugated pipe, 24 to 60", and bury it in the ground. They sell end cap, tees, elbows, plastic manholes, covers and such. I think it is a cost effective way to provide irrigation water, even on a residential scale, and for commercial work will score several LEED points.

 

[Valveman]

Usually takes a very large storage tank. It depends on the amount of rain fall you get, and the amount you need for irrigation.

 

[Gary Swart]

I'd file this with tankless water heaters. It's a great theory but there are many problems with the application. It might work in certain areas that received a fair amount of rain, was not too hot, had a small area to irrigate, and had a back up plan for drought.

 

[Southern Man]

I've got a spot at the bottom of my driveway that would fill up a 4000 gallon tank easly with most small storms around here. This is based on a catchment area of 8000 square feet (roof, driveway, most of my front lawn), 80% average runoff, and 1" of rain. That would allow me to water the lawn for 14 hours. Based on my water rates that is worth about $13. The payoff period is probably 20 years.

 

[Bob NH]

I am very interested in the rain water collection system which is an environment friendly design application.
I am in my thesis year right now, and would like to know more about these systems. James

"Environment friendly" is based on comparison of the value of the product to the cost of all resources (materials, labor, energy, and negative impacts caused by the activity) necessary to deliver an economically useful product. In an economic system such as we have in the United States, all of those things can be normalized to cost. Cost is a measure of the total resources that must be diverted from something else to accomplish an objective.

Everything should be reduced to cost per unit of water collected and applied to use,and that cost per unit of water should be compared to the cost for water from other sources and the value that is returned for the eventual use of that water.

If you are writing a thesis you can easily create a model of the water collection system, including the economics of the system, using Excel or other computer program.

Get the actual rainfall data for a few areas, develop a conceptual design for collection area and storage system, account for evaporation losses which you can find on the internet, and calculate the capital and operating cost for collection, storage, treatment, and delivery of the water.

Then calculate the cost per unit of water delivered, and the economic value of that water for some purpose, and compare it to the cost and economic value for alternative sources of water. The economic value of drinking water is greater than the economic value of water to irrigate a field of alfalfa.

When you get through with the project you should be able to publish conclusions, supported by analysis, of the climates and conditions for which rain water collection is the best (meaning most economical) means of supplying needed water that has some economic value.

Economic value exists only if someone is willing to pay more for the water than it costs to produce it.

 

[Southern Man]

Yup. "Do the math."

 

[TheRobster]

There's a whole PhD thesis on rainwater harvesting systems

 

[Southern Man]

Basically they are using the volume of the pipe for storage, plus the volume within the voids of the crushed stone fill and plastic liner. (The void space in clean crushed stone is about 40%.) The stone will support the pipe under a parking lot. For a residential area, say under a lawn, it may make more sense just to omit the liner and stone, and only use the pipe itself for storage volume. A "smart shopper" could probably find a great deal on a used fuel tank or corrugated steel pipe and create the volume required for less money.

A simple sump pump on the bottom of the tank could be used to draw water when needed.

 

[Alectrician]

1) Get a large flat surface to collect and divert rainwater into a large storage tank.

2) Wait for rain.

 

[osx-addict]

I ran across a site a while back that would calculate run-off from my roof based on size,etc.. I checked the amount of rain in my area (Los Angeles) on the average and calculated that with my roof size, I could collect ~20k gallons of water per year with my roof.. Obviously a series of tanks to hold that sort of quantity would be rather large for the average city dweller and underground would be best.. Anyway, just an FYI.

 

[Southern Man]

I ran across a site a while back that would calculate run-off from my roof based on size,etc.. I checked the amount of rain in my area (Los Angeles) on the average and calculated that with my roof size, I could collect ~20k gallons of water per year with my roof.. Obviously a series of tanks to hold that sort of quantity would be rather large for the average city dweller and underground would be best.. Anyway, just an FYI.

Inches of rain x square feet of collection area x "C" factor = volume collected. Just make sure your units are correct.

"C" is typically 98% for roofs and pavement, 12" to a foot, 7.48 gallons to a cubic foot.

Assuming a 1" rain and a 2000 square foot roof:

1/12 x 2000 x 7.48 x 0.98 = 1200 gallons

A typical garden sprinkler puts out 4 gallons per minute so this would run it for 5 hours.

Obviously this isn't for everyone. But if you've got a nice spot in your yard that collects from a large area that has room for a tank, and if you can get one for cheap and water is expensive, then its something that you might want to consider.

 

[osx-addict]

Southern--

I re-ran my numbers with your calculation method above and depending on what rainfall numbers you believe for my area (either 12.55 or 14.79), I get the following for my home :

12.55/12 x 3220 x 7.48 x 0.98 = 24,685 gallons
or
14.79/12 x 3220 x 7.48 x 0.98 = 29,091 gallons

So, I guess my 20k gallons wasn't too far off -- I was guestimating roof size but my above figured are more accurate (43.5'x74').

What I'd like to know is how much $$ would something like this cost.. I'd bet it aint cheap once you factor in the excavation costs, required gravel for backfill around the pipes,etc..

 

[Southern Man]

What you need to do is figure out how much water you would actually use and when, and compare that to when it usually rains. That way you can reduce the amount of storage required. Also you may have a paved area, such as a driveway, that can be drained into the tank, giving you more water available, either for more irrigation, or to reduce the tank size.

Also, it may be cheaper to eliminate the liner and crushed stone. The void volume within the stone is only 40%. That design is for a commercial application under pavement. In a residential project that may not make sense.

 

[Speedbump]

I think we lost the College Student long ago.

This is kind of like the guy who puts in a shallow well here in Florida so he will have water during the dry season. Well, in the wet season there is all kinds of surface water to be had with a little shallow well. Come the dry season when you really need the water, the surface vein is dry also. I mean; HELLO it's the rain that puts it there.

bob...

 

[Redwood]

Ummm I think the OP was a spammer dropping a link in the forum...