Project: Harvest Rainwater with Sand Filters
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Here’s a great tip from Stephen and Rebekah Hren from their book The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit. Harvested rainwater needs filtration before it is potable. Pollution, particles from the air, debris from the collection system (your gutters) are not things you want to find in your tall glass of ice water. Instead of investing in a garage-sized Brita pitcher, Stephen and Rebekah have another idea: sand.
From The Carbon-Free Home:
Sand filters (also called biofilters) are a biological way of purifying drinking water. Low turbidity (suspended sediment in the water) is a requirement for sand filters to function effectively. Fortunately, a well-functioning rainwater-catchment system should meet this requirement. Sand filters can purify only small amounts of water at a time, as they are unpressurized and work using gravity, so purifying is limited to drinking water. Essentially a sand filter is a large drum filled with sand. Water enters the top and slowly percolates through. A thin, biologically active layer (called the hypogeal layer) quickly forms on top, feeding on the bits of organic residue and other impurities in the water. By the time the water has made it through the several feet of sand, it is potable and remarkably clean. Eventually, the hypogeal layer becomes too thick and needs to be either scraped off or destroyed by drying and backflushing (the water from the flush being disposed of into a nearby thirsty plant). A new one quickly forms and water filtration can continue.
Drawing courtesy of Dennis “Mad Man” Pacheco
March 9th, 2009 at 5:32 pm
What do you use to separate the sand from the bottom portion?
March 9th, 2009 at 8:45 pm
This basic design could be significantly improved with gutter screen to keep out leaves, a debris screen to filter out smaller particles and a diverter to remove the dirty water that comes immediately after a rain. I would raise the tank so it’s easier to fill water containers. Also, I would run the water through a better filter — a Big Berkey, etc. — to be safe.
March 10th, 2009 at 7:48 pm
[…] Chelsea Green » Blog Archive » Project: Harvest Rainwater with Sand Filters […]
March 16th, 2009 at 9:20 am
We’ve been building and installing sand filters in southern Nicaragua for several years (see http://www.flickr.com/photos/gringopinolero/sets/72157614341672003/ for pix of this year’s project). We use expensive volcanic sand, because with beach sand each grain has been smoothed by the continuous action of water, and what you want is “sharp, jagged” grains of sand to trap impurities. A student of mine and I built one in a lab at Simmons College in Boston, and she’s been sand-filtering the incredibly contaminated water of the Muddy River that runs through the Fenway, removing 98% of the pathogens. For the rainwater filter, do what we did: get “rock dust”–the residue or “sawdust” that comes from cutting granite and other types of stone. Your local building supplies place will carry it, such as Pirolli’s in Watertown, MA. For more on BioSand Filters, check out http://www.cawst.org, our colleagues up in Calgary.
March 16th, 2009 at 9:22 am
Ah yes, to answer JSG: put a layer of coarse gravel around the outflow pipe, then fine gravel above that, then the sand. This will keep the outflow from clogging up. DG
April 12th, 2009 at 4:08 pm
[…] Project: Harvest Rainwater with Sand Filters […]
April 14th, 2009 at 4:28 pm
[…] Project: Harvest Rainwater with Sand Filters […]
April 15th, 2009 at 3:31 am
[…] at least some of your water. Visit green book publisher Chelsea Green for more information on harvesting rainwater with sand filters and the book The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel […]
April 29th, 2009 at 11:14 am
For a typical rainwater harvesting system for an urban home, this sand filter method is very impractical. The flowrate of water through the sand will be very low versus the potential high rate of rainfall. Rainwater will end up overflowing this system due to the impedance by the sand. In addition, this unit would have to be a separate filtration unit since it wouldn’t hold much water at all.
This system may be great for areas of the world where other methods of filtration are too costly or too difficult to obtain materials for, but this is just not feasible for the majority of rainwater collection systems where people want water for only irrigation. The issue is storage volume not potable water quality water. We install rainwater collection system for potable water use but we use a filter and UV system to filter and disinfect the rainwater. We install large storage volumes in order to capture the rainwater in order to provide the household with water year round. Again, most people are just looking to collect some rainwater to offset the municipal water use they use for irrigation, not necessarily to drink it.
August 20th, 2009 at 8:16 pm
So i would like to do this filter, but i was wondering if there was a division after the layer of several feet of sand or does the sand go to the bottom?
And what exactly do i have to do for the hypogeal layer to form?
November 1st, 2009 at 10:08 pm
We use a 3000 gallon collection tank, then pump it to the house using a small jet pump on low pressure (20psi), it passes thru our old inground pool sand filter and into the basement to a custom manifold. The manifold allows us to use rain, well or city water all at a turn of a valve. The water then passes thru our basic whole house filter as does all our water. For our drinking and cooking water we use our Berkey and UV light also. Pressure is a little slow but we have adapted and are happy and healthy. We harvest creek water but it is specifically for the garden if rainwater isn’t available.
November 23rd, 2009 at 11:45 pm
Harvesting rainwater and filtering it with a slow sand filter (biosand filter, biological sand filter) is quite practical and will work. It does not matter if the flow rate is slow. Using a first flush diverter and having storage is the key. The website mentioned here has all the details - tests done by epa certified lab showing total removal of all Coliform bactera - filters have been in operation for over 2 years. There are far too many details to mention here. These filters work and they are the only - yes the only - way to completely remove beaver fever cysts without the use of dangerous amounts of chlorine or ozone. The slow sand filter is very “green” and does not need petro-chemicals or any chemicals to function. People just don’t understand them or just don’t know how they work. Water is purified in nature in the same way as in a slow sand filter.
February 14th, 2010 at 2:25 pm
how often does the sand and filtration need to be replaced?