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Introduction
This site is about the filtration and purification of water through the use of sand, gravel and naturally occuring processes and describes how to put together a small slow sand water filter for under eighty five dollars.. The results of the tests on water run through the filter described on this website can be found here. If you want to know how well the filter works this would be the best place to start. This is a work in progress and information will be added as more investigation is done. Also, read this blog for discussion, questions, and collaboration on slow sand filters.
If designed correctly these biological sand and gravel filters do not need any electricticy or petroleum products to enable their operation, they work because of the pull of gravity on water and naturally occuring biological processes. If the filter and the storage containers are positioned such that the filter is higher than the storage container/s the water will flow into them and a pitcher pump can be used to extract the water from the storage container. Pathogens, and to some extent inorganic chemical pollutants, can be removed from water through the use of biologically active layers of sand in an aquatic oxygen-rich environment often called a slow sand filter, or biosand filter. Both of these types of filters are very good examples of sustainable technology as they can work wihout electricty or petrochemicals and they do not pollute or produce unusable by-products, last many years, and can be built from recycled materials. If you intend to build a filter, please read ALL of the information on this website BEFORE you start. Many sources of water, particularly in the U.S., contain chemical pollutants that require more than just a slow sand or biosand filter for their complete removal, however this is the exception rather than the rule - for now. . . .
What is a slow sand filter?
A slow sand filter (often called a BioSand filter, or a biological sand filter) is used to filter and clean water through the use of layers of sand and biological action. As oxygen and organisms in water pass throught the sand, a biofilm - an aquatic biological strata of living organisms in the sand - forms in the top layers of the sand as water passes through the filter. This biofilm or 'schmutzdecke' extends from the surface of the sand down 2 or 3 cm although biological activity occurs with less density at greater distances from the surface of the sand. Through biological action and some sand filtration, pathogens are rendered harmless through complex naturally occuring proceses."Viruses are effectively removed or inactivated by slow sand filtration and soil passage, but they are more resistant to UV and coagulation combined with sedimentation (9, 43, 50)." 1 The complete operation of the biological action in a slow sand filter is complex and is currently being studied by scientists; however effective filters can be built by the average person.4 A slow sand filter must maintain a constant water level above the surface of the sand; and oxygen is necessary to maintain proper operation. Slow sand filters are now gaining popularity because they do not require chemicals or huge amounts of energy to function.
What is the difference between a slow sand filter and a BioSand filter?
These two names both refer to a water filter that uses naturally occuring biological processes in layers of sand and gravel to purify water. A slow sand filter usually has a constant flow of water through the sand and gravel, is quite large and used to purify water for municipal water distribution systems. The concept of slow sand filtration has been around alomost as long as civilization, but there have been many improvements on the deisgn and the understanding of operation. The BioSand filter, developed by a scientist from Canada, is a modification of the slow sand filter design . It is usually much smaller and can be operated intermittantly; and it is particularly well suited for use on an individual household basis. The biofilm layer in both types of filters must be continuously covered by water but the depth of the water on top of the sand is only about 1 inch in a BioSand filter because the intermittant flow of water limits the oxygen supply. Oxygen supply is critical to keep the biofilm alive and functioning in both types of filters. A slow sand filter can be small (50 gallons) or large (public water supply). A "BioSand filter" is usually small (65 gallons or less). In both types of filters the biofilm can take up to 3 weeks to become fully effective in a newly constructed filter.
How safe is the water from a slow sand filter / biosand filter? This is not a simple question to answer. These filters have been in use in the U.S. since 1829; and are becoming more popular because of their ability to produce clean water acceptable to new standards set by the EPA.14 Biological sand filters are very efficent at removing disease causing bacteria from water. They remove 99.9 percent of beaver fever cysts and cryptosporidum from water. These bacteria are very resistant to chlorine in conventional water treatment and are more effectively removed by slow sand / biosand filters. Some viruses are trapped in the biofilm as their bacterial hosts are killed when they are trapped and consumed in the biolayer, however the removal of viruses by slow sand filters / biosand filters is still being studied. This is water purification by the removal of harmful pathogens.These filters do not add inorganic chemicals to kill organisms. Most conventional public water supply systems add chemicals such as chlorine, or ozone to kill micro-organisms and as a result chemical toxins are produced (this DOES NOT happen in a biosand, or slow sand filter) and furthermore, not all viruses are completely inactivated by chlorine. ( Not as much is known about the inactivation of viruses as is known about the removal and inactivation of bacteria in slow sand / biosand filters )8,9,10,11 In the filter described on this website, all Ecoli are removed along with all coliform bacteria. Turbidity (cloudyness) is significantly reduced. If there is at least 3 feet of sand in the filter covered by water continuously and the simple design parameters and operating instructions found on this website and in many other places on the web are followed - and the water used is free from chemical contamination, the water will be safe to consume. Chemical pollutants such as heavy metals, pesticides, herbicides and petroleum contaminants are not removed as completely as pathogens and organic particles, however a significant percentage of this type of contamination is removed. IF YOU ARE PUTTING A FILTER TOGETHER FOR THE FIR5T TIME, HAVE THE WATER TESTED BEFORE YOU CONSUME ANY, and educate yourself about the operation, theory and maintenence of these filters. Many of the filter designs that are in use in parts of the world other than the U.S.have been tested and are known to work. Understanding the theory does not require a college degree. If you can read and / or use the internet, you should have enough knowledge to easily build and use one of these filters successfully. Once the filter is operational, the water is safe, but maintenence is critical and absolutely necessary, so educate yourself!! Disease causing pathogens can enter the water from anywhere. If you intend to consume the water from one of these filters, be absolutely certain you have a way to test the water. If you live near a creek or small stream, the water from that source can be purified by a slow sand filter and will be safe to drink - but caveat emptor - have the water tested before you drink it - if it flows through a suburban or urban area in the U.S. with dwelling units near it, pathogens in the water may be the least of the problem - pesticdes, herbicides, petroleum distillates and many other chemicals and poisions may be in the water caused by runoff from roads, roofs, drainfields, and yards that have been treated with pesticides, herbicides and wood preservatives. Since many populations in the U.S. have access to thousands of "products" that contain poisons, the situation in affluent neighborhoods the U.S. is not the same as the situation in other places. In many other areas of the world the main problem with the water is biological contamination, which is easily handled by a slow sand filter; however in the U.S. and some other "advanced industrialized cultures" chemical poisions are an enormous problem and are only partially removed by slow sand filters. If you intend to drink the water collected from a roof, I STRONGLY URGE YOU TO USE A "FIRST FLUSH DIVERTER"7; it will minimize the extent of dissolved chemicals (from dust, pollution and roof material) in the water collected after a dry spell. Most asphalt 3 tab roofing material has a coating of inert mineral material attached to the asphalt base and may not leach significant amounts of petroleum based chemicals until it is near the end of its life and starts breaking down, or when it is very new.5
Although the water from a slow sand filter is of very high quality, I would strongly recommend the use of a UV filter followed by a ten stage carbon filter, or a "drip filter" to filter the water from the slow sand filter that you intend to drink on a regular basis.
IMPORTANT!
The roofing material on many houses in the U.S. contains poisons that kill moss. These poisons will be in the water collected from these roofs and cannot be completely removed by a biosand filter. In this part of our state there are roofing companies who advertise a moss-kill product they will apply to your roof to kill "that nasty moss that will destroy your roof". Our roof, here in the forest, was covered with moss, and lasted 30 years. The part that was in the worst shape was the part exposed to the sun - the moss was not the problem. Go figure. Also, the more research I do, the more convinced I am that any composition shingle roof (3 tab) is not a good surface from which to draw drinking water, because of the petroleum based chemicals (bitumens) used in the manufacture of this type of roofing material; however, most asphalt 3 tab roofing material has a coating of inert mineral material attached to the asphalt base and may not leach significant amounts of petroleum based chemicals until it is near the end of its life and starts breaking down, or when it is very new.5 As the roof material wears out, increasing amounts of these chemicals may be in the water collected from these types of roof surfaces.5 At the best, with a first flush diverter this water may be ok for temporary use, but consumed over a long period of time will be very harmful unless extensive carbon filtering is used. The biosand filter will remove the harmful pathogens very efficently and will remove most of the non-organic particles,
however for drinking water the best type of roof is a metal roof called "Galvalume". It is coated with non-toxic baked on enamel. This is the type of roofing material that "green" buildings have installed.
If you have a "carport" or garage with metal roofing that is not rusted and has a non-toxic paint coating (most do) that would be a good place to start and if you intend to drink the water, I would still highly recommend a "first flush diverter". The roof will add no toxins however air pollution particles deposited during a dry spell will be in the first 15 or 20 gallons that run off the roof.
The level of chemical pollutants from roof water will vary - if you don't have built in moss-killing poison or zinc strips on your asphalt (3 tab) roof, and it is in the "middle" of its "life" with moss growing on it, a "first flush diverter"7 will eliminate most of the harmful chemicals originating from the roof material5 and atmospheric pollution. The danger from these chemicals, since many bio-accumulate, is in prolonged exposure (as in years). If you use a 10 stage carbon filter following the slow sand filter, your water quality will probably be acceptable. The slow sand filter / biosand filter will remove nearly all the bacteria so moss on a roof is not a concern. Run the water through the biosand filter first, the bacteria will clog a carbon filter quickly. Carbon filters do not remove all the bacteria; in fact over time, bacteria will grow in a carbon filter and produce water outflow that has more contamination than the inflow. Non-toxic metal roofing can be installed over existing composition roofing material. It is slightly more expensive, but is lighter, will outlast a composition roof by 2 to three times the number of years, stand up much better in a storm, will not pollute the water that runs off of it, and metal roofing is fireproof. There are non-toxic coatings that can be applied to composition roofs that will prolong the life of the roof and allow water not contaminated with herbicide or petroleum derivatives to be collected from the roof. Also, if you have a roof without embedded moss-killing poison,(composition, fiberglass (3 tab)) a plastic tarp (check to make sure the tarp does not contain poisons) draped over the part of the roof that delivers water to your filter will help to keep most of the petroleum pollutants to an acceptable level. Even if you take water from a "non-toxic" roof, there will be poison chemicals in the water from local air pollution and small traces from atmospheric pollution. The amount will vary depending on your location. A stream that flows year round is constantly refreshing the water. A roof often sits dry for weeks or months, so pollutants build up on the roof surface and are far more concentrated in the first rain event than they would ever be in a small stream or creek several days after the first rain following a dry spell. This is why a first flush diverter is absolutely necessary.
New!
Links to pages showing how to put rain catchment systems together:
An interesting rain barrel project page This site has very detailed specific information on constructing and setting up rain barrels - the first step in getting water from a roof.
Why would someone want to put one of these together?
If you live in an area where water is at all an issue then there is every reason to do this. The water that runs off your roof can be collected and run through one of these filters and then be safe to use for just about anything. however before you consume any roof water from a slow sand filter or biosand filter HAVE IT TESTED - and if you plan to use the water as your main supply use a 10 stage filter followed by a uv filter for the water you intend to consume. If you are on a public water supply system DO NOT HOOK YOUR FILTER OUTPUT UP TO YOUR PLUMBING.
The moss-killing poison, and/or petroleum based pollutants present in roofing material will end up in the water that that runs off of many, if not most roof asphalt surfaces and as a result will be concentrated in the ground around the houses that have them, or worse; in many cities this water goes directly to the storm drain where the chemicals do not have a chance to biodegrade (if indeed they actually do break down in the soil). From the storm drain, the water from thousands of roofs often goes directly into rivers and streams. Now that there are droughts in many places in the U.S. we, as a nation, have to face our own arrogance in neglecting to think ahead. If our roofing materials were not designed such as to leach poisons, there would be a lot more water available at far less expense for us all. But then hindsight is always 20/20 . . .
In the metro areas of most cities, during a dry spell water is rationed. If you have a flower garden, or a vegetable garden, this water from a slow sand filter can be used for that purpose. After you have set up the collection system and the filter the water is free. Ten 55 gallon drums can hold a lot of water. Simply collecting the water that runs off your roof is ok, but if anyone ingests it they could get very sick. Once the water has been run through a functioning slow sand filter it is much safer than untreated water. ( remember: The water from a roof is not the same as the water from a creek or small stream. ) If I lived in Arizona, Nevada, or any of the dry hot areas of the southwest U.S. I would consider something like this an absolute necessity, and I would have a storage tank of very large capacity to hold the filtered water.
Several inches of rain on the average roof can yield thousands of gallons of water. (do the math here). If you are careful about the material used on your roof this water could be used for drinking water, but I would have it tested first; however, as evidenced by the information at the links above, BioSand filters are now being used in many areas around the world to make drinking water safe.
At the risk of sounding ultra-paranoid, the following must be considered:
It is important to know the condition of the environment from which the water is being taken. The situation in the location of this filter is unique in that the water collection surface is surrounded by trees and a large wildlife area located in the northwestern part of the U.S. . If water from a slow sand filter system similar to this one in surroundings and design is to be used for consumption on a regular basis, it would be adviseable to run the water through a uv (ultra-violet) filter following the output from the slow sand filter In other areas of the world the situation will very likely be quite different.
The operation of these biological sand filters has been documented with repeated investigations and tests showing the same result: Disease causing organisms are removed through naturally occuring processes; and turbidity is reduced to the extent that the water is safe to consume. This website is documentation of one more independent observation that shows the same result as the others. When properly constructed and maintained, biological sand filters work as claimed regarding the removal of bacterial pathogenic organisms. All of The test results can be seen here (a large image file).
For those who are interested in the Laboratory doing the testing; I will supply the other details on request.
1. Presence of Noroviruses and Other Enteric Viruses in Sewage and Surface Waters in The Netherlands W. J. Lodder and A. M. de Roda Husman* Microbiological Laboratory for Health Protection, National Institute of Public Health and the Environment, Bilthoven, The Netherlands.
Corresponding author. Mailing address:
Microbiological Laboratory for Health Protection, National
Institute of Public Health and the Environment, P.O. Box 1,
NL-3720 BA Bilthoven, The Netherlands. Phone: 31 30 274 4325.
Fax: 31 30 274 4434. E-mail:
2. Coliform Bacteria and Drinking Water (Washington State Department of Health)
3. The "average" roof can collect many undesirable substances. Caution is advised.
4.The following abstracts were accessed: November 23, 2007 and would be very good reading especially for understanding the biological action in a (slow) sand water filter:
Biological and Physical Mechanisms in Slow Sand Filtration
Haarhoff, J; Cleasby, JL
IN: Slow Sand Filtration. American Society of Civil Engineers, New York. 1991. p 19-68, 11 fig, 10 tab, 54 ref.
Slow Sand Filtration: Influences of Selected Process Variables
Author(s): Bellamy, William D.; Hendricks, David W.; Logsdon, Gary S.
Citation: Journal AWWA, Vol. 77 Iss. 12, December 1985, Page(s) 62-66
Bacterivory by a chrysophyte in slow sand filters
Monroe L. Weber-Shirk* and Richard I. Dick
School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853-3501, USA
Received 1 September 1997; accepted 1 June 1998. Available online 25 February 2000.
Information on roofing material:
5. A review of Methods for the Manufacture of Residential Roofing Materials. Hashem Akbari, Ronnen Levinson, and Paul Berdahl. Heat Island Group Lawrence Berkeley National Laboratory, Berkeley, Ca. 94720. A report prepared for: California Energy Commission PIER Program. June 2003.
6.Disclaimer: Use this information at your own risk. The Author assumes no responsibility whatsoever for any damages of any kind as a direct or indirect result of the use of any information on this website. The information provided here is free and published with the intent of sharing experience, and is not provided as an absolute solution to anything. This is a work in progress. Mistakes will likely be found. We reseve the right to remove this content or change it at any time we choose. You have been advised.
7. A first flush diverter WILL NOT safely eliminate bacteria or viruses from roof water, although some may be removed.
8. Read more about viruses here: freedrinkingwater.com/water-education2/87-water-disinfection2.htm
9.Mechanisms of inactivation of hepatitis A virus in water by chlorine dioxide: Jun Wen Li Corresponding Author Contact Information, E-mail The Corresponding Author, Zhong Tao Xin , Xin Wei Wang , Jin Lai Zheng and Fu Huan Chao Institute of Health and Environmental Medicine of Tianjin, 1 Da Li Road, Tianjin City 300050, People's Republic of China Received 15 April 2003; Revised 7 November 2003; accepted 13 December 2003. Available online 4 March 2004.
10. www.moh.govt.nz/moh.nsf/0/5A25BF765B400911CC25708F0002B5A8/$File/07-viruscompliance.doc
11. viruses that are not killed by chlorine in swimming pool water (caliciviruses): www.cumc.columbia.edu/dept/ps/2007/mid/2006/transcript_02_mid37.pdf
14. "BACTERIVORY BY A CHRYSOPHYTE IN SLOW SAND FILTERS" MONROE L. WEBER-SHIRK and RICHARD I. DICK
School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853-3501, U.S.A.
Wat. Res. Vol. 33, No. 3, pp. 631-638, 1999.
This site is about the filtration and purification of water through the use of sand, gravel and naturally occuring processes and describes how to put together a small slow sand water filter for under eighty five dollars.. The results of the tests on water run through the filter described on this website can be found here. If you want to know how well the filter works this would be the best place to start. This is a work in progress and information will be added as more investigation is done. Also, read this blog for discussion, questions, and collaboration on slow sand filters.
If designed correctly these biological sand and gravel filters do not need any electricticy or petroleum products to enable their operation, they work because of the pull of gravity on water and naturally occuring biological processes. If the filter and the storage containers are positioned such that the filter is higher than the storage container/s the water will flow into them and a pitcher pump can be used to extract the water from the storage container. Pathogens, and to some extent inorganic chemical pollutants, can be removed from water through the use of biologically active layers of sand in an aquatic oxygen-rich environment often called a slow sand filter, or biosand filter. Both of these types of filters are very good examples of sustainable technology as they can work wihout electricty or petrochemicals and they do not pollute or produce unusable by-products, last many years, and can be built from recycled materials. If you intend to build a filter, please read ALL of the information on this website BEFORE you start. Many sources of water, particularly in the U.S., contain chemical pollutants that require more than just a slow sand or biosand filter for their complete removal, however this is the exception rather than the rule - for now. . . .
What is a slow sand filter?
A slow sand filter (often called a BioSand filter, or a biological sand filter) is used to filter and clean water through the use of layers of sand and biological action. As oxygen and organisms in water pass throught the sand, a biofilm - an aquatic biological strata of living organisms in the sand - forms in the top layers of the sand as water passes through the filter. This biofilm or 'schmutzdecke' extends from the surface of the sand down 2 or 3 cm although biological activity occurs with less density at greater distances from the surface of the sand. Through biological action and some sand filtration, pathogens are rendered harmless through complex naturally occuring proceses."Viruses are effectively removed or inactivated by slow sand filtration and soil passage, but they are more resistant to UV and coagulation combined with sedimentation (9, 43, 50)." 1 The complete operation of the biological action in a slow sand filter is complex and is currently being studied by scientists; however effective filters can be built by the average person.4 A slow sand filter must maintain a constant water level above the surface of the sand; and oxygen is necessary to maintain proper operation. Slow sand filters are now gaining popularity because they do not require chemicals or huge amounts of energy to function.
What is the difference between a slow sand filter and a BioSand filter?
These two names both refer to a water filter that uses naturally occuring biological processes in layers of sand and gravel to purify water. A slow sand filter usually has a constant flow of water through the sand and gravel, is quite large and used to purify water for municipal water distribution systems. The concept of slow sand filtration has been around alomost as long as civilization, but there have been many improvements on the deisgn and the understanding of operation. The BioSand filter, developed by a scientist from Canada, is a modification of the slow sand filter design . It is usually much smaller and can be operated intermittantly; and it is particularly well suited for use on an individual household basis. The biofilm layer in both types of filters must be continuously covered by water but the depth of the water on top of the sand is only about 1 inch in a BioSand filter because the intermittant flow of water limits the oxygen supply. Oxygen supply is critical to keep the biofilm alive and functioning in both types of filters. A slow sand filter can be small (50 gallons) or large (public water supply). A "BioSand filter" is usually small (65 gallons or less). In both types of filters the biofilm can take up to 3 weeks to become fully effective in a newly constructed filter.
How safe is the water from a slow sand filter / biosand filter? This is not a simple question to answer. These filters have been in use in the U.S. since 1829; and are becoming more popular because of their ability to produce clean water acceptable to new standards set by the EPA.14 Biological sand filters are very efficent at removing disease causing bacteria from water. They remove 99.9 percent of beaver fever cysts and cryptosporidum from water. These bacteria are very resistant to chlorine in conventional water treatment and are more effectively removed by slow sand / biosand filters. Some viruses are trapped in the biofilm as their bacterial hosts are killed when they are trapped and consumed in the biolayer, however the removal of viruses by slow sand filters / biosand filters is still being studied. This is water purification by the removal of harmful pathogens.These filters do not add inorganic chemicals to kill organisms. Most conventional public water supply systems add chemicals such as chlorine, or ozone to kill micro-organisms and as a result chemical toxins are produced (this DOES NOT happen in a biosand, or slow sand filter) and furthermore, not all viruses are completely inactivated by chlorine. ( Not as much is known about the inactivation of viruses as is known about the removal and inactivation of bacteria in slow sand / biosand filters )8,9,10,11 In the filter described on this website, all Ecoli are removed along with all coliform bacteria. Turbidity (cloudyness) is significantly reduced. If there is at least 3 feet of sand in the filter covered by water continuously and the simple design parameters and operating instructions found on this website and in many other places on the web are followed - and the water used is free from chemical contamination, the water will be safe to consume. Chemical pollutants such as heavy metals, pesticides, herbicides and petroleum contaminants are not removed as completely as pathogens and organic particles, however a significant percentage of this type of contamination is removed. IF YOU ARE PUTTING A FILTER TOGETHER FOR THE FIR5T TIME, HAVE THE WATER TESTED BEFORE YOU CONSUME ANY, and educate yourself about the operation, theory and maintenence of these filters. Many of the filter designs that are in use in parts of the world other than the U.S.have been tested and are known to work. Understanding the theory does not require a college degree. If you can read and / or use the internet, you should have enough knowledge to easily build and use one of these filters successfully. Once the filter is operational, the water is safe, but maintenence is critical and absolutely necessary, so educate yourself!! Disease causing pathogens can enter the water from anywhere. If you intend to consume the water from one of these filters, be absolutely certain you have a way to test the water. If you live near a creek or small stream, the water from that source can be purified by a slow sand filter and will be safe to drink - but caveat emptor - have the water tested before you drink it - if it flows through a suburban or urban area in the U.S. with dwelling units near it, pathogens in the water may be the least of the problem - pesticdes, herbicides, petroleum distillates and many other chemicals and poisions may be in the water caused by runoff from roads, roofs, drainfields, and yards that have been treated with pesticides, herbicides and wood preservatives. Since many populations in the U.S. have access to thousands of "products" that contain poisons, the situation in affluent neighborhoods the U.S. is not the same as the situation in other places. In many other areas of the world the main problem with the water is biological contamination, which is easily handled by a slow sand filter; however in the U.S. and some other "advanced industrialized cultures" chemical poisions are an enormous problem and are only partially removed by slow sand filters. If you intend to drink the water collected from a roof, I STRONGLY URGE YOU TO USE A "FIRST FLUSH DIVERTER"7; it will minimize the extent of dissolved chemicals (from dust, pollution and roof material) in the water collected after a dry spell. Most asphalt 3 tab roofing material has a coating of inert mineral material attached to the asphalt base and may not leach significant amounts of petroleum based chemicals until it is near the end of its life and starts breaking down, or when it is very new.5
Although the water from a slow sand filter is of very high quality, I would strongly recommend the use of a UV filter followed by a ten stage carbon filter, or a "drip filter" to filter the water from the slow sand filter that you intend to drink on a regular basis.
IMPORTANT!
The roofing material on many houses in the U.S. contains poisons that kill moss. These poisons will be in the water collected from these roofs and cannot be completely removed by a biosand filter. In this part of our state there are roofing companies who advertise a moss-kill product they will apply to your roof to kill "that nasty moss that will destroy your roof". Our roof, here in the forest, was covered with moss, and lasted 30 years. The part that was in the worst shape was the part exposed to the sun - the moss was not the problem. Go figure. Also, the more research I do, the more convinced I am that any composition shingle roof (3 tab) is not a good surface from which to draw drinking water, because of the petroleum based chemicals (bitumens) used in the manufacture of this type of roofing material; however, most asphalt 3 tab roofing material has a coating of inert mineral material attached to the asphalt base and may not leach significant amounts of petroleum based chemicals until it is near the end of its life and starts breaking down, or when it is very new.5 As the roof material wears out, increasing amounts of these chemicals may be in the water collected from these types of roof surfaces.5 At the best, with a first flush diverter this water may be ok for temporary use, but consumed over a long period of time will be very harmful unless extensive carbon filtering is used. The biosand filter will remove the harmful pathogens very efficently and will remove most of the non-organic particles,
however for drinking water the best type of roof is a metal roof called "Galvalume". It is coated with non-toxic baked on enamel. This is the type of roofing material that "green" buildings have installed.
If you have a "carport" or garage with metal roofing that is not rusted and has a non-toxic paint coating (most do) that would be a good place to start and if you intend to drink the water, I would still highly recommend a "first flush diverter". The roof will add no toxins however air pollution particles deposited during a dry spell will be in the first 15 or 20 gallons that run off the roof.
The level of chemical pollutants from roof water will vary - if you don't have built in moss-killing poison or zinc strips on your asphalt (3 tab) roof, and it is in the "middle" of its "life" with moss growing on it, a "first flush diverter"7 will eliminate most of the harmful chemicals originating from the roof material5 and atmospheric pollution. The danger from these chemicals, since many bio-accumulate, is in prolonged exposure (as in years). If you use a 10 stage carbon filter following the slow sand filter, your water quality will probably be acceptable. The slow sand filter / biosand filter will remove nearly all the bacteria so moss on a roof is not a concern. Run the water through the biosand filter first, the bacteria will clog a carbon filter quickly. Carbon filters do not remove all the bacteria; in fact over time, bacteria will grow in a carbon filter and produce water outflow that has more contamination than the inflow. Non-toxic metal roofing can be installed over existing composition roofing material. It is slightly more expensive, but is lighter, will outlast a composition roof by 2 to three times the number of years, stand up much better in a storm, will not pollute the water that runs off of it, and metal roofing is fireproof. There are non-toxic coatings that can be applied to composition roofs that will prolong the life of the roof and allow water not contaminated with herbicide or petroleum derivatives to be collected from the roof. Also, if you have a roof without embedded moss-killing poison,(composition, fiberglass (3 tab)) a plastic tarp (check to make sure the tarp does not contain poisons) draped over the part of the roof that delivers water to your filter will help to keep most of the petroleum pollutants to an acceptable level. Even if you take water from a "non-toxic" roof, there will be poison chemicals in the water from local air pollution and small traces from atmospheric pollution. The amount will vary depending on your location. A stream that flows year round is constantly refreshing the water. A roof often sits dry for weeks or months, so pollutants build up on the roof surface and are far more concentrated in the first rain event than they would ever be in a small stream or creek several days after the first rain following a dry spell. This is why a first flush diverter is absolutely necessary.
New!
Links to pages showing how to put rain catchment systems together:
An interesting rain barrel project page This site has very detailed specific information on constructing and setting up rain barrels - the first step in getting water from a roof.
Why would someone want to put one of these together?
If you live in an area where water is at all an issue then there is every reason to do this. The water that runs off your roof can be collected and run through one of these filters and then be safe to use for just about anything. however before you consume any roof water from a slow sand filter or biosand filter HAVE IT TESTED - and if you plan to use the water as your main supply use a 10 stage filter followed by a uv filter for the water you intend to consume. If you are on a public water supply system DO NOT HOOK YOUR FILTER OUTPUT UP TO YOUR PLUMBING.
The moss-killing poison, and/or petroleum based pollutants present in roofing material will end up in the water that that runs off of many, if not most roof asphalt surfaces and as a result will be concentrated in the ground around the houses that have them, or worse; in many cities this water goes directly to the storm drain where the chemicals do not have a chance to biodegrade (if indeed they actually do break down in the soil). From the storm drain, the water from thousands of roofs often goes directly into rivers and streams. Now that there are droughts in many places in the U.S. we, as a nation, have to face our own arrogance in neglecting to think ahead. If our roofing materials were not designed such as to leach poisons, there would be a lot more water available at far less expense for us all. But then hindsight is always 20/20 . . .
In the metro areas of most cities, during a dry spell water is rationed. If you have a flower garden, or a vegetable garden, this water from a slow sand filter can be used for that purpose. After you have set up the collection system and the filter the water is free. Ten 55 gallon drums can hold a lot of water. Simply collecting the water that runs off your roof is ok, but if anyone ingests it they could get very sick. Once the water has been run through a functioning slow sand filter it is much safer than untreated water. ( remember: The water from a roof is not the same as the water from a creek or small stream. ) If I lived in Arizona, Nevada, or any of the dry hot areas of the southwest U.S. I would consider something like this an absolute necessity, and I would have a storage tank of very large capacity to hold the filtered water.
Several inches of rain on the average roof can yield thousands of gallons of water. (do the math here). If you are careful about the material used on your roof this water could be used for drinking water, but I would have it tested first; however, as evidenced by the information at the links above, BioSand filters are now being used in many areas around the world to make drinking water safe.
At the risk of sounding ultra-paranoid, the following must be considered:
It is important to know the condition of the environment from which the water is being taken. The situation in the location of this filter is unique in that the water collection surface is surrounded by trees and a large wildlife area located in the northwestern part of the U.S. . If water from a slow sand filter system similar to this one in surroundings and design is to be used for consumption on a regular basis, it would be adviseable to run the water through a uv (ultra-violet) filter following the output from the slow sand filter In other areas of the world the situation will very likely be quite different.
The operation of these biological sand filters has been documented with repeated investigations and tests showing the same result: Disease causing organisms are removed through naturally occuring processes; and turbidity is reduced to the extent that the water is safe to consume. This website is documentation of one more independent observation that shows the same result as the others. When properly constructed and maintained, biological sand filters work as claimed regarding the removal of bacterial pathogenic organisms. All of The test results can be seen here (a large image file).
For those who are interested in the Laboratory doing the testing; I will supply the other details on request.
1. Presence of Noroviruses and Other Enteric Viruses in Sewage and Surface Waters in The Netherlands W. J. Lodder and A. M. de Roda Husman* Microbiological Laboratory for Health Protection, National Institute of Public Health and the Environment, Bilthoven, The Netherlands.
Corresponding author. Mailing address:
Microbiological Laboratory for Health Protection, National
Institute of Public Health and the Environment, P.O. Box 1,
NL-3720 BA Bilthoven, The Netherlands. Phone: 31 30 274 4325.
Fax: 31 30 274 4434. E-mail:
2. Coliform Bacteria and Drinking Water (Washington State Department of Health)
3. The "average" roof can collect many undesirable substances. Caution is advised.
4.The following abstracts were accessed: November 23, 2007 and would be very good reading especially for understanding the biological action in a (slow) sand water filter:
Biological and Physical Mechanisms in Slow Sand Filtration
Haarhoff, J; Cleasby, JL
IN: Slow Sand Filtration. American Society of Civil Engineers, New York. 1991. p 19-68, 11 fig, 10 tab, 54 ref.
Slow Sand Filtration: Influences of Selected Process Variables
Author(s): Bellamy, William D.; Hendricks, David W.; Logsdon, Gary S.
Citation: Journal AWWA, Vol. 77 Iss. 12, December 1985, Page(s) 62-66
Bacterivory by a chrysophyte in slow sand filters
Monroe L. Weber-Shirk* and Richard I. Dick
School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853-3501, USA
Received 1 September 1997; accepted 1 June 1998. Available online 25 February 2000.
Information on roofing material:
5. A review of Methods for the Manufacture of Residential Roofing Materials. Hashem Akbari, Ronnen Levinson, and Paul Berdahl. Heat Island Group Lawrence Berkeley National Laboratory, Berkeley, Ca. 94720. A report prepared for: California Energy Commission PIER Program. June 2003.
6.Disclaimer: Use this information at your own risk. The Author assumes no responsibility whatsoever for any damages of any kind as a direct or indirect result of the use of any information on this website. The information provided here is free and published with the intent of sharing experience, and is not provided as an absolute solution to anything. This is a work in progress. Mistakes will likely be found. We reseve the right to remove this content or change it at any time we choose. You have been advised.
7. A first flush diverter WILL NOT safely eliminate bacteria or viruses from roof water, although some may be removed.
8. Read more about viruses here: freedrinkingwater.com/water-education2/87-water-disinfection2.htm
9.Mechanisms of inactivation of hepatitis A virus in water by chlorine dioxide: Jun Wen Li Corresponding Author Contact Information, E-mail The Corresponding Author, Zhong Tao Xin , Xin Wei Wang , Jin Lai Zheng and Fu Huan Chao Institute of Health and Environmental Medicine of Tianjin, 1 Da Li Road, Tianjin City 300050, People's Republic of China Received 15 April 2003; Revised 7 November 2003; accepted 13 December 2003. Available online 4 March 2004.
10. www.moh.govt.nz/moh.nsf/0/5A25BF765B400911CC25708F0002B5A8/$File/07-viruscompliance.doc
11. viruses that are not killed by chlorine in swimming pool water (caliciviruses): www.cumc.columbia.edu/dept/ps/2007/mid/2006/transcript_02_mid37.pdf
14. "BACTERIVORY BY A CHRYSOPHYTE IN SLOW SAND FILTERS" MONROE L. WEBER-SHIRK and RICHARD I. DICK
School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853-3501, U.S.A.
Wat. Res. Vol. 33, No. 3, pp. 631-638, 1999.
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