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Rainwater Harvesting:
Frequently Asked Questions

© 2005-2016 Doug Pushard, All Rights Reserved

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1. How do you harvest rainwater?
2. Where do you get the water?
3. What is the best way of harvesting rain?
4. Why should I harvest rainwater?
5. Do I need pumps to harvest rainwater?
6. Can I use drip irrigation or soaker hoses with a rainwater?
7. How big a yard can I water?
8. How big are rain barrels?
9. I want more pressure, how should I raise it?
10. Can I water my grass with rainwater?
11. Is water quality a problem in rainwater systems, and how can I improve water quality?
12. What is Ozone?
13. How does Ozone work?
14. How often should I replace my UV bulb?
15. Why don't I have to insulate a correctly buried tank?
16. What is potable water?
17. What is LEED?
18. I am researching metal roofing systems for a client who is building a LEED residence and wants to harvest rainwater. Do you have recommendations for metal roofing systems for irrigation and/or potable water?
19. How do I incorporate UV in a building?
20. Are there difference is roof efficiencies in capturing rain?
21. Do other countries require rainwater catchment?
22. How do I prevent ice build up or ice dams from forming and damaging the gutter system?
23. What is the difference in filter screens between mesh and microns?
24. When do I use a regulator versus a limiting valve?
25. What are and how do I convert CCFs to gallons?
26. Does a UV system remove iron from water?
27. Where does the .623 number come from?
28. How much rainwater should I harvest?
29. How can I filter out algae?
30. Do I really need UV for domestic rainwater use?
31. Can I harvest rain in freezing weather?
32. How long can I store rainwater that has been purified?
33. How can I tell if my UV unit is working?

34. Is it possible to kill bacteria with a solar powered UV light?


1. How do you harvest rainwater?
As amazing as it sounds, this is a question that comes up often. It must be theword harvest that is causing the confusion.

Per Webster’s dictionary harvest is defined as 1) to gather in or 2) to accumulate a store of. Consequently to harvest rainwater means to gather it in.

In my Great Aunt’s house in Maine, this was done with an old wood barrel. It was put beside the house and rainwater drained into it. We would use this water to drink, clean the dishes and everything else you can think of. Today it can still be caught with barrels or any type of water tanks, large and small, but I would advise against drinking it straight out of the barrel. Visit our FREE Tank Viewer.

2. Where do you get the water?
Fortunately it comes FREE from the sky. They still have not figured out a way to privatize rain yet. Although I am sure someone is actually thinking about those air space rights.

3. What is the best way of harvesting rain?
Catch it in anything that holds waters. Many landscape or garden stores can tell you where to buy barrels. Here in Austin we have a wide variety, with some selling new barrels and other selling barrels that have been recycled. In the Austin yellow pages I found them by calling a few landscapers and even one of the gutter installers advertised rainwater collection. Some cities offer incentives, so also check with your local water company.

Once you have a container, simply put the barrel beneath where the rainwater runs off your roof and you have started harvesting.

Look for barrels that have a faucet attachment where you can attach a hose to use the captured water for your yard.

4. Why should I harvest rainwater?
Good question and probably the answer is going to be different for different folks. Some will like the idea of not paying the utility company for something that is FREE. Or maybe it is because rainwater is typically better for the plants. Or it could be that you don’t like big bond issues to pay for new water treatment plants. There are lots of possible reasons.

With me it was a little bit of all the above. But also it was a project I could do personally and knew it would have a net positive impact on the environment. My system makes me feel great when I am out watering or just looking at our

5. Do I need pumps to harvest rainwater?
Maybe. If you just have a small barrel and you are using an attached hose or
soaker line, no pump is likely required. If you have a barrel or tank that is
above ground level by a just a few feet, and you are on a flat piece of land, and have a small yard; probably no pump will be needed.

If your tank is at ground level and you need to move the water up any slope then you will likely need a pump.

However, sometimes you can get enough water pressure in a closed-looped water collection system to supply the pressure required, even to drive a sprinkler system. Consult a local specialist to determine what is going to work for you.

6. Can I use drip irrigation or soaker hoses with a rainwater?
Yes, but I recommend you install an inexpensive line filter.

Most irrigation stores sell inline sprinkler filters. This is a simple device that screws into the lines prior to your irrigation system and cleans out the large leaves and other stuff (i.e. sometimes referred to as particulates) out of the water so it does not clog your lines. Check out the vendors page for information on several inexpensive filters.

7. How big a yard can I water?
It depends, simple question with no simple answer. Can you have rain barrels at various places around the yard? How much rain can you capture? What type of plants do you have? How much do you want to spend on tanks?

The Austin Wildflower Center has 2 - 25,000 gallon (i.e. 7.5 - 94,625 liters)
tanks and 3 cisterns. They estimate they can capture up to 300,000 gallons (i.e. 1,135,500 liters) a year and they estimate their system meets 10-15% of their annual needs. The Austin Wildflower Center covers 279 acres and displays over 500 species of native plants and has become a popular destination for knowledge-seeking gardeners and nature-loving tourists.

As you can see, systems can be very large and water an extremely large area. It is just a matter of how much you really, really want to capture and how much you want to spend.

8. How big are rain barrels?
Thanks for the soft ball question, I needed it. Rain barrels vary in size from a few gallons/liters to about 100 gallons (i.e. 378 liters). Most barrels are around 50-60 gallons (i.e. 189 - 227 liters).

Rainwater tanks run from several hundred gallons/liters to many thousand gallons (i.e. 7,000 – 75,000 liters).

My tanks are about 2,000 gallons each (i.e. 7,570 liters each).

9. I want more pressure, how should I raise it?
Raising your barrel or tanks or by installing a pump. Every foot you raise your storage tank increases the pressure about 0.433 psi or less because of pressure loss due to friction (1 psi ~ 2.31 feet of fresh water head). It generally takes only a few feet to be able to use a hose or drip system, but it takes a lot to run a sprinkler. Raising your rain barrel can be a quick and cheap way to increase your pressure.

10. Can I water my grass with rainwater?
Yes, but grass usually takes a lot of water. A typical lawn requires about 3,000 gallons (i.e. 11,355 liters) a month. This means you would need some large tanks to hold the water, especially in drier climates. Additionally, you would need a large surface area to capture the rain.

However, rain barrels can and should be used to augment your watering. This will cut your watering bill and be better for your grass.

I recommend before going with big tanks to water your lawn you look at reducing your outdoor water consumption. Going to local vegetation, drought hearty plants and then installing either drip irrigation or soaker hose will reduce your water consumption. It will be less costly, since you then need smaller tanks.

But remember, rainwater is still free. The constraint is the cost of the tank.

11. Is water quality a problem in rainwater systems?
Yes, as it is increasingly with all of today's water systems. Rain water is generally free of harmful minerals and in most cases chemicals, but can be adversely effected by air pollutants and/or contaminated by animals in the catchment area. Due to increasing levels of pollutants, city and bottled water providers are increasingly turning to use of sophisticated treatment processes and chemincals to ensure a quality product. Consequently, rainwater for drinking should be carefully stored and treated prior to consumption. Several technlogies exist for home treatment including: ozone sterilization, UV and distillation.

I remember as a small kid drinking rainwater straight out of the barrel at my great aunt's house. But that was in very rural Maine many, many moons ago. Now we have both increased pollution and increased awareness. Pollution can add undesirable elements to the water. And now we know that open water sources can harbor bacteria and other health threatening organisms.

So if your rainwater is to be consumed by you or others (i.e. potable water) then it must be treated. There are many ways to treat rainwater (e.g. UV, Ozone, etc) and several articles on the Harvest2o.com website go into these options in great depth.

12. What is Ozone?
Ozone is activated oxygen. It is a natural purifier. Its clean, fresh scent is often noticed after a heavy rain. Ozone is a powerful oxidant that can safely be used to purify water. It is an alternative to using chlorine. It is a stable, yet powerful, bactericide, viricide and bleaching agent. It has been used in Europe to purify drinking water for over 100 years.

13. How does Ozone work?
Ozone was first discovered in the 1840's. By combining the ultraviolet rays from the sun, with air, ozone is generated. In 1906, the city of Nice, France; built the first municipal water purification plant utilizing ozone. Today, there are over 2000 plants worldwide using ozone to purify drinking water. It is a stable yet powerful bactericide and bleaching agent. It reacts chemically with other minerals and compounds to remove or filter them out of the water.

14. How often should I replace my UV bulb?
Most manufacturers recommend about 9,000 or 12 months of continuous use. The bulb may appear to be working (i.e. emitting visable light), but may not be strong enough to emit at the required 254 nm wavelength; so don't trust a simple visual test. Additionally, clean the glass whenever you are replacing the bulb as any blockage can greatly impair the effectiveness of the UV light.

15. Why don't I have to insulate a correctly buried tank?
The earth absorbs almost 50% of all solar energy and remains a nearly constant temperature of 50°F to 70°F depending on geographic location. If the tank is buried below the frost line than the earth's natural heat will keep the water in the tank from freezing. Care must be taken with the pipes to make sure they are also below freezing level in the ground. Your local rainwater harvesting supplier or contractor can tell you how deep the tank needs to be buried

16. What is potable water?
Potable water is another name for fresh drinking water. Water of sufficient quality to serve as drinking water is often called potable water.

17. What is LEED?
LEED stands for Leadership in Energy and Environmental Design. It is a ranking system; whereby, buildings can be evaluated on a specific set of criteria against other buildings. The result is a set of points that rank a building's "sustainability". Water conservation is one of the ranking categories in the LEED evaluation process. To read an article on water and LEED.

18. I am researching metal roofing systems for a client who is building a LEED residence and wants to harvest rainwater. Do you have recommendations for metal roofing systems for irrigation and/or potable water?

First, thank you Alicia for the question. Ideally there would be a roofing material certified by the National Sanitation Foundation for potable use, but there is none as of yet. NSF has certified some epoxy coating but none for exterior use. You can find out more about these materials at NSF Protocol 151.

As for as metal, galvanized metal, painted or unpainted with a nontoxic paint, are common. Other roofing type materials include terra cotta tile, slate, and fiberglass. Roofing materials should have as little toxic material on it as possible, to reduce leaching into the rain.

Make sure water does not sit on the roof for any length of time. This reduces leaching potential as well. Of course for irrigation purposes, almost any roof surface will work. Just keep the gutters and water clean.

And of course put in screens on gutters, a first flush device and filtration system to clean the water if it is to be used for potable water. Every municipality is different on what is allowed, so please check with local officials.

19. From a reader in Hong Kong, what sources are available on utilizing UV filtration or ozone treatment to make the rainwater potable?

Several sources exist. Below is a link to articles and the related links at the bottom of the article refers to a company that has good papers on this subject.


Please also check with local officials on what is allowee.

20. Are there differences in the efficiency of roof materials in capturing rainwater?

Not all of the rainwater that strikes a roof is collected: water is lost from evaporation, blowing wind, overflowing gutters, and leaky collection pipes, first-flush devices, and self-cleaning filters. The table below lists the differences of just the roof surfaces. A good article on this is The Capture System.

  • For a tile/metal roof assume a 95% runoff efficiency
  • For a concrete/asphalt roof assume a 95% runoff efficiency
  • For a gravel roof assume a 70% runoff efficiency
  • For a bare soil roof assume a 75% runoff efficiency
  • For a grass roof assume a 17% runoff efficiency

21. Do other countries require rainwater harvesting?

Yes. Australia is a great example of where harvesting rainwater is required in several states on new construction. A good article on this is Harvesting Rainwater Downunder.

22. How do I prevent ice build up or ice dams from forming and damaging the gutter system?

Depending on the angle of the roof, a roof snow scraper to manually clean the roof or an electric roof de-icing cable like the ones from EasyHeat in Connecticut or the SRF-RG Commercial Grade Roof & Gutter Heat Trace Cable from Chromalox in Pennsylvania.

23. What is the difference in filter screens between mesh filter and microns?

Different measures for the same thing. Below is a conversion from mesh to microns. These typically are used to tell you how small the opening is in the filter.


24. When do I use a regulator versus a limiting valve?

Limit valves are used in constant pressure situations and typically installed prior to irrigation valves. Limit valves are best installed with filters before them. Regulators are typically installed after valves.

25. My water company uses a unit of measurement CCF, what is this and how do I convert it to gallons?

Some water companies bill in gallons and others bill in CCF. CCF is 100s of cubic feet of water. One (1) CCF equals 748 gallons of water. To get gallons, multiply CCF by 748.

26. Does a UV system remove iron from water?

Unfortunately UV will not help with minerals. UV disrupts bacteria and makes them harmless to humans if implemented properly. For iron you will need a filtration system and these would include a redox (reduction/oxidation), reverse osmosis, or a distillation system. A great book on this subject is The Drinking Water Book.

27. Where does the .623 number come from when calculating how much rain I can capture?

The formula for figuring out how much rain water you can collect off your roof is roof square footage x .623 gallons per square inch of rainfall x annual rainfall. Square footage of the roof of the house x amount of rain and the last variable is .623. A cubic foot of water being equal to 7.48 gallons, which when divided by 12 (i.e. inches in a foot) equals .623 gallons per inch of rain.

28. How much rainwater should I harvest?

The short answer is how much do you need. Then there are various methods determine this amount. The easiest is past water bills. If inside and outside this, average the water bills to determine what your monthly average would be. If outside then it largely depends on square footage landscaped, where you live and what you are watering. Some very rough, but simple guidelines are:

Daily Consumption

  • Per Person 50 -100 gallons
  • Gardens/Lawns 600 gallons per 1,000 square feet
  • Young Trees 15 gallons
  • Small Animals .25 gallons per 25 pounds
  • Dairy Cattle 20 gallons
  • Range Cattle 15 gallons

There are more detailed methods of determining plant water usage based on transpiration or counting the number of sprinkler or drip heads used. These can yield more exact numbers. Additionally, it is possible to base it on local averages or what your neighbors use. In short there is no one 'right way' to determine water usage, but several. Determine which is going to be easiest for you or I will gladly do some math for you to help you size your requirements. >> More on Correctly Sizing Your Tank

29. How can I filter out algae?

Algae is good for the plants, but very bad for the emitters so that is the dilemma. So it is a matter of balancing the filtering. Algae can be as small as .5 micron. The smaller stuff in algae is bacteria and beneficial to plants. A filter is recommended with every drip irrigation system because it removes sediment and other particles that are large enough to clog the emitters. Most tape and drip emitters can not handle up to anything large than this. So at least a 200 mesh filter MUST be used on any T-Tape system. There are, of course, smaller micron filters (i.e. a 200 mesh equals a 73 micron filter) that will take out much more. However, these are generally used in drinking water systems and not irrigation systems. Most irrigation filters you get will be this matter out without a problem.

30. Do I really need UV for dometic rainwater use?

Unfortunately rainwater will have active bacteria in it in almost 100% of the cases. This bacteria needs to be taken care of either with filtration or purification.

The norm for rainwater systems is UV or Ozone. Chlorination, RO, and distillation are also options however, not commonly used in rainwater systems. A 1 micron filter will not remove all bacteria, but most. I have read publications by the CDC that it needs to be a .3 micron to remove almost all bacteria. Of course this would not remove viruses.

The linked article below gives a good overview of the various options. I have also linked a book below that is very useful, with great charts and tables.


As far as carbon you are correct. I only recommend them when more removal is required.

I typically recommend .25 micron (sediment removal), a 5 micron (most bacteria and dust), a granulated carbon filter (chemicals, chlorine, bad smells and taste) and finally a UV light (disrupt DNA of bacteria making it harmless). This combination removes bacteria, most viruses, and cysts.

31. Can I harvest rain in freezing weather?

Freezing weather poses several problems for harvesting rainwater and is usually discouraged unless underground cisterns are installed. Even then the pumps and fittings need to be freeze protected.

Of course, cold temperature versus hard freezes are very different and the strategies to use that can be imployed are also different.

32.How long can I store purified rainwater?

Rainwater that has been properly purified (i.e. UV, Ozone, chemical) can be stored indefinitely as long as it is stored properly in an airtight container or system (i.e. pressure tank and piping).

It is the opening of faucets or top of containers that could possibly introduce bacteria into the system.

33. How call I tell if my UV unit is working?

UV unlike many other treatment processes leaves no detectable trace (e.g. smell like chlorine or ozone).

New units now include a Ultra Violet Transparency (UVT) monitor. These units also include a solenoid valve that shuts off the water supply if the UVT falls below it's recommended operating level.

These units are more expensive, but I highly recommend them.

Otherwise, your only way of knowing if your unit is really working is to perform a bacteria test.

34. Is it possible to kill bacteria with a solar powered UV light?

Yes, there are several UV suppliers that sell 24v UV systems. I have several installed and work exclusively off solar power and batteries. With these systems it is critical that they do not stay ON 24 hours a day. Some type of timer is required to turn on and off the UV bulb; otherwise you will drain you batteries.?





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Taking on Water

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Drinking Water

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