Water in glass
images by: David Tarsi






Frequently asked questions regarding slow sand filters (note 1)   (note 2-F)

1.   What is a slow sand filter?

2.   How does a slow sand filter work?

3.   Are slow sand filters safe?

4.   What are the disadvantages of a small slow sand filter?

5.   What are the advantages of a small slow sand filter?

6.   How do I keep air pockets from forming when buillding a small slow sand filter?

7.   How do I tell if the water is too muddy or silty to be filtered by a slow sand filter?

8.   What is an NTU?

9.   How much do small slow sand filters cost?

10. What kind of sand and gravel do slow sand filters use?

11. What size of sand and gravel do slow sand filters use?

12. How deep does the sand have to be in a slow sand filter?

13. What is "Uniformity coefficient"?

14. What does "log credit" mean in reference to pathogen removal in a slow sand filter?

15. Where can I find more information about slow sand filters?

16. Can roof water be made potable (suitable for people to drink) by a slow sand filter?

17. What is a first flush diverter?

18. Can I purify water from a creek, stream or pond using a slow sand filter?

19. What is "wet harrowing"?

20. Will a slow sand filter remove industrial pollution?

21. What is the difference between a biosand filter and a slow sand filter?

22. Can I purify roof water with a slow sand filter?

23. Where can I get plans for a slow sand filter?

24. Can I safely water my vegetable garden with roof water that has been purified by a slow sand filter?

25. Can I use a slow sand filter to purify tap water?

26. I have seen videos of slow sand filters in operation that show very dirty water being poured in and clean looking water flowing out. Is this realistic?

27. How is a slow sand filter "cleaned"?

28. What is the difference between rapid sand filtration and slow sand filtration?

29. What is COD (Chemical Oxygen Demand)?

30. Will a slow sand filter remove poisons in water from toxic algae blooms?

31. How fast does water move through a slow sand filter?

32. Does a slow sand filter work at near freezing temperatures?

33. How do I test a slow sand filter?

34. What does "effective size for filtration" and/or "effective size" mean in reference to sand for a slow sand water filter?

35. Can I use beach sand from the ocean shore in a slow sand filter?

36. Can I use beach sand found at a lake or river in a slow sand filter?


Important note: Although the filters described on this website that have been assembled by the author
have been shown to produce water free of coliform bacteria, and free of harmful chemicals;
THE FILTERS DESCRIBED ON THIS WEBSITE ARE NOT GUARANTEED TO PRODUCE POTABLE WATER.
The proper functioning of any of the filters described here is totally dependent on the owner.
This information is provided free without any assurance of accuracy. Use this information at your own risk.
The author reserves the right to change any of this information at any time without notice
You have been advised.
1.   What is a slow sand filter?
A slow sand filter is sometimes referred to as a "Biosand" filter, or a biological sand filter. All three of these names refer to a water filter that works using biological action in sand without adding any chemicals to the water. These filters are good examples of sustainable technology. They can operate without the use of electridty or petroleum based fuel and can be made from mostly recycled materials. A slow sand filter consists of a container with a system of pipes with holes drilled in them covered by about 6 inches of gravel, in turn covered by 3 feet of sand. Water is allowed to flow over the top of the sand and flow slowly down (because of the pull of gravity) through the sand and gravel to the pipes on the bottom. The water then flows back up (due to hydraulic pressure) through one output pipe to the level of the input water. After about 3 or 4 weeks a biological layer forms on the sand that traps and destroys harmful bacteria and viruses.  back to top

2.   How does a slow sand filter work?
When a slow sand filter is first put into operation or after it is "cleaned", a living "community" of aquatic aerobic, predatory microscopic organisms grow in the top 5 to 10 cm of wet sand and form what is called a "Schmutzdecke" or "biolayer" made of exocelluar polymers (complex proteins and carbohydrates) and living organisms consisting of diatoms, algae, bacteria, and zooplankton. This sand and biolayer must always be submerged under oxygen rich water (the filter must ALWAYS have water in it up to and covering all of the sand), and it is very effective at mechanically filtering very small particles out of the water flowing through it. Also, the living organisms in the biolayer literally "eat" pathogens in the water that get caught in the biolayer from a process know as "biological flocculation" (they stick to the biofilm). Some filtering also occurs because of the physical action of the sand below the biolayer. And additionally, the organisms in the biolayer produce substances that are toxic to pathogenic viruses and bacteria found in the water flowing through the filter. 58 (270-273; 424-432), 59, 60 Water must not flow through the filter faster than the biological action occurs in the Schmutzdecke. The organisms in the biolayer are also present at lower levels in the filter sand but they are not as numerous and don't form a biofilm. In small versions of slow sand filters the drainpipes at the bottom connect to a (usually pvc) pipe that runs out and up to an outlet several inches above the top of the sand. This way water drains slowly and never leaves the surface of the sand exposed to open air (this will kill the biolayer very quickly). The action of water seeking its own level is a key part of the filtering operation as it helps to regulate the speed and pressure at which the water passes through the sand. Although slow sand filters have been in operation since the early 1800's and have been studied extensively by modern scientists, the complete extent of the biological activity enabling their operation is not fully understood yet; however extensive tests have shown that slow sand filters remove viruses, bacteria, and chemicals as well as, and in some cases better than, modern complex filtering systems (slow sand filters are the most effective way to remove "Beaver fever organisms" from water).  back to top

3.   Are slow sand filters safe?61 (339-340; 345-346)
They can be quite safe but should be used with caution and sufficient knowledge of their operation. The effectiveness of a slow sand filter depends on the conditions in which they operate; and on the degree of pollution in the water to be filtered. Water must be allowed to run through them for at least 3 weeks before any is consumed. All parts of the filter must be kept clean - contamination can come from anywhere outside the filter. Water must be kept over the sand always, and the filter must be allowed to run again for 3 weeks after cleaning before consuming any water from them unless "wet harrowing" is used. Slow sand filters are able to provide very good safe water free from biological pathogens. This is a well established, although not well known, fact. Cryptosporidium cysts (similar to, but not the same thing as Beaver Fever (Giardia Lamblia), viruses, and coliform bacteria can all be removed from water by slow sand filters at rates often exceeding 99 percent13 30. It is common knowledge that because of world wide industrialization and lack of attention to the problems of pollution, both "tap" water and rainwater can contain varying amounts of harmful substances, depending on the water treatment process and geographical location. Biological sand filters can remove some harmful substances from water, and most certainly improve the quality of water that is run through them in all respects, but the exact nature of their ability to remove all industrial pollution from water has not been well established. However, ability of carbon filters to remove chemicals from water has been well established; and the addition of a high quality carbon filter to the output of a biological sand filter would be a good idea. Do not consume water from a DIY (do it yourself) slow sand filter unless the water has been tested and shown to be of acceptable biological and chemical quality; and it is monitored and tested regularly by someone who understands its operation. Also a uv filter on the output after the carbon filter should be used.   back to top

4.   What are the disadvantages of a small slow sand filter?
They do not remove 100 percent of the color or odor from water.
They require realtively non-turbid water to function properly.
They cannot function if they are frozen.
Water must be added regularly and the sand must stay covered by water continuously.
During construction care must be taken to prevent air pockets from forming in the sand - this will foul the filter and require removing the sand and removing all anaerobic bacteria and odors.
Industrial pollution is only partially removed from water by these filters.
These filters must be allowed to "ripen" (have water flow through them for at least 3 weeks) before they will purify water.
They are VERY heavy (1000 pounds plus) and much thought must go into where they will be located - it is almost impossible to move them once they are set up.  back to top

5.   What are the advantages of a small slow sand filter?
They are the best way to remove beaver fever bacteria from water.
They can remove over 99 percent of harmful bacteria from water.
They can remove from 91 to 99.999 percent of viruses from water.30
They improve water clarity.
They do not require the addition of chemicals to function.
They can be designed to function using only the power of gravity.
They can last for many years with only occasionally adding more sand to the top layer.
They are inexpensive.
They do not pollute.
They are sustainable technology.
They do not produce harmful byproducts as do chlorine and ozone purification processes.
Maintenance is simple and easily understood.
Their construction is easily understood.
They can be built from locally available materials.
They can operate in remote areas where no electrical power or petroleum energy is available.
They empower the individual.
The theory of operation is easily understood.  back to top

6.   How do I keep air pockets from forming when building a small slow sand filter?
Fill the container with water FIRST - then add the gravel and sand slowly waiting for all the bubbles to stop before putting in more sand. Have all the drain pipe and the ouput (outlet) pipe installed and tested to be sure it works and does not leak. Your last step should be adding the gravel and sand.  back to top

7.   How do I tell if the water is too muddy or silty to be filtered by a slow sand filter?
This is not an easy question to answer because there are some variables: How long has the filter has been running? How long will cloudy water be used in the filter? What is the turbidity level in NTU's (Nephelometric Turbidity Units) of the water, and does it change? Generally, if you can't see through a drinking glass of the water well enough to read your name printed 1 cm high on a white note card it is too silty to use in a slow sand filter. Water that is almost clear will probably work. Muddy water - that is water that is opaque - will not work in a slow sand filter. The accepted maximum turbidity level in NTU's for continuous operation is 20 - this would be water in a glass that looks slightly cloudy - almost clear. Less than 5 NTU's of turbidity cannot be noticed, and 200 NTU's of turbidity is the maximum that can be tolerated by a slow sand filter - this would be water that looks like a glass of non-filtered fresh squeezed lemon juice (without the seeds). The ideal turbidity would be less than 10.12
A good example of what turbidity levels look like can be found here.  back to top

8.   What is an NTU?
This is a measurement of the ability of light to pass through water. A detailed explanation is beyond the scope of this FAQ, but a good explanation of NTU's and how to calculate them can be found here.   back to top

9.   How much do small slow sand filters cost?
Very little or nothing. A small slow sand filter can be built for under 100 dollars.   back to top

10.  What kind of sand and gravel do slow sand filters use?
It is best to use sand and gravel that is NSF/ANSI 61 or AWWA 100 approved. If this is not available, know EXACTLY where the sand and gravel come from and EXACTLY what is in it - and what is NOT in it. Be certain there are no industrial chemicals in the sand and /or gravel. Things like lead, mercury, asbestos, zinc, petroleum derivatives, insecticides, pesticides, or storm runoff residue from road surfaces may be in sand, or on gravel. Be certain the sand is washed clean of any organic material, clay, mud, or slit. The sand should be washed until the water draining from it is clear. Play sand that has been sterilized can be purchased at most home improvement stores. It will work if the sand is fairly uniform in size. Builder's sand may work, but if the grains are too coarse and the size is not uniform (see faq 11) it may not work satisfactorily; also be certain chemicals have not been added if builder's sand is used. The following website has listings of NSF certified sand suppliers and processing facilities:
http://www.nsf.org/Certified/Common/Company.asp?Standard=061 Two of the companies listed at this nsf.org site have supplied the sand used in the filters described in the study described on this website They are: MANUFACTURERS MINERAL COMPANY in Washington state    and Cemex in California. They are wholesale. You will need to find a hardware store, or building supplier near you to order products from them.   back to top

11.  What size of sand and gravel do slow sand filters use?
The most important information to know is that coarse sand does not filter as well as fine sand; and, fine sand offers more resistance to the flow of water than coarse sand. Between .35 mm and .15 mm effective size with a uniformity coefficient of less than 2 is the desired (and most commonly used) range of sand sizes. 13 however all the sand in a layer should be the same effective size. The smaller effective size should be in the top 30 or 40 cm layer of sand. The gravel on the bottom should be large enough to not pass through the holes in the drain pipes and small enough to prevent sand from seeping into them. The drain pipes should be covered by at least 6 inches of gravel. Generally, the smaller the effective size of the sand used the better. If the effective size is too small however, water will not flow through the filter fast enough. If the effective size is too large the biolayer will not form effectively and the filter will not purify water.  back to top

12. How deep does the sand have to be in a slow sand filter?
The minimum depth of sand is 30 inches and some municipal filters are 48 inches or deeper. If the sand is coarse (.35 mm effective size or larger) increase the depth accordingly. There is a balance to be maintained. Most slow sand filters have layers of sand with the smaller effective size on the top 30 or 40 cm layer.  back to top

13. What is "Uniformity coefficient"?
The uniformity coefficient of sand is defined as a ratio: the size at which 60 percent (by weight) of a sand sample passes through a sieve (in other words 60 percent of the sand is finer than a given size) divided by the size at which 10 percent of the same sample (by weight) passes through a sieve (10 percent is finer than a given size). A uniformity coefficient of less than 2 indicates the grains of sand are suitable for use in a slow sand filter. As the number goes up the size differentiation becomes greater and the quality of the sand becomes less desirable for use in a slow sand filter.  back to top

14.  What does "log credit" mean in reference to pathogen removal in a slow sand filter?
This term is somewhat ambiguous, because what we really want is removal not necessarily just a "credit" for removal - so "log reduction" would be a better way to speak of removing pathogens from water. In some cases, "Log credit" may be used to refer to the inactivation of pathogens. It is important to note that slow sand filters remove pathogens; however chlorine or ozone treatment only kills them and they are still in the water - they are just not alive. "Log credit" or "log reduction" means: The log10 (the log relative to base 10) of the number of organisms per unit volume before filtering minus the log10 of the number of organisms per unit volume after filtering. For example 1000 beaver fever cysts before filtering per 100 ml of water as compared to 1 beaver fever cyst per 100 ml of water after filtering is the log10 of 1000 minus the log10 of 1 or 3 - 0 or 3. A very good filter. The corresponding percentage would be 999/1000 or 99.9 percent removal.   back to top

15. Where can I find more information about slow sand filters?
There is a website here that has much more info about these filters. Also, the "Literature Cited" link on the upper left side of this page has links to other sites and some scientific studies on biological sand filters.  back to top

16. Can roof water be made potable (suitable for people to drink) by a slow sand filter? 4,14,15,16,17,18,19,20,21
Under some conditions - possibly - but EXERCISE EXTREME CAUTION. DO NOT DRINK OR OTHERWISE CONSUME WATER FROM ANY OF THE FILTER DESIGNS OR SOURCES MENTIONED ON THIS WEBSITE. ANYTHING CAN BE IN WATER AND EVERY SITUATION IS DIFFERENT. YOUR LOCAL HEALTH DEPARTMENT IS THE FINAL AUTHORITY ON POTABLE WATER. If there are raccoons present, keep in mind that they carry roundworms in their digestive systems (Baylisascaris procyonis) that produce millions of tiny eggs which are deadly if ingested by humans.43, 44, 45 Although infection is very rare, it has happened and there is no cure. The eggs are 50 microns in size, so most commercial water filters will remove them easily. A properly operating slow sand filter should be able to remove them. Simple precautions will prevent infection. If there are racoons in your area, you must have your roof checked by a knowlegeable professional for the presence of a racoon latrine. If one is found, the best action is to consider water from that roof off limits until the entire area is decontaminated and the racoons are prevented from having access to the roof. Don't use water from a rain barrel until your roof has been checked for a racoon latrine.
Before doing anything more with water from a roof surface, have the surface material tested for asbestos. Roofing applied before 1980 may have asbestos embeded in it. Do not drink or even use water taken from this type of roof - asbestos is a known cancer causing material. If the roofing has asbestos in it DO NOT USE THE RUNOFF FROM IT FOR ANYTHING, and note there is likely asbestos in the soil around the house if there are any areas of damage on the roof. Composition roofing installed by reputable contractors after 1980 will probably not contain asbestos, as it was banned from use after 1980 in the U.S. .

Do not drink water taken from cedar shake roofs under any circumstances - there are naturally occurring tannins in cedar which can be harmful18. In addition to tannins, manufactured cedar shakes contain added wood preservatives, (some of which contain arsenic, copper, and other powerful poisons) that are extremely harmful19,20,21,31. Also any roof surface that has lead, copper, tin, or zinc is not suitable for potable water, although the slow sand filter will remove some of these chemical contaminants and nearly all of the biological contaminants from the water.

Use a first flush diverter (see FAQ 17 below).

Use a high quality 10 stage carbon filter at the point of use.

Have an EPA certified laboratory test the water for zinc, lead, copper, chromium, arsenic, asbestos, and tin.

Metal roofing with the brand name "Galvalume" or "Zincalume" is used in many areas to provide chemical free water for harvesting and consumption. These metals are alloys covered by non-toxic baked on enamel finishes. If they are not coated or if they are allowed to rust, they can contribute toxic metal compounds to runoff water.31. Also there is a roof coating available called CIMtm Industrial Membrane that is NSF 61 certified available from Sunshine supply Company, inc

New information as of 2010/01/05: Some composition and tar roofing (also referred to as asphalt roofing) may be ok for water harvesting. There are, however, a number of caveats that must be considered:

First, total Petroleum hydrocarbons (TPH) were found to be present at 2.9 mg/L of water, in the winter at temperatures of 32 degrees F in tests on water from the roof used in the study on this website. The slow sand filter (filter 1) in the study on this website removes those petroleum hydrocarbons from roof water down to less than one part per ten million - an amount probably harmless to healthy individuals. The TPH working group has done extensive research on Total Petroleum Hydrocarbons. The reference listed here has extensive information on this subject. Mentioned in this study is as much as 7 mg/L as toxic and as little as .01 mg/L as toxic in specific types of hydrocarbons.53 (pp 30,34,45,66,69) Since the slow sand filter takes the total to below the .01 mg/L, we can probably safely use the water in this case. It must be noted, however that this test was done in the winter months when there is likely to be much less hydrocarbon pollution present than there would be in the summer.

Secondly, asphalt is soluble in water above 20 degrees centigrade (68 degrees F), so if your roof surface is above 68 degrees F when water is collected from it, err on the side of caution and assume the water collected at this temperature contains hydrocarbons from the roofing material. Below 20 degrees centigrade there will not likely be enough dissolved hydrocarbon compounds to be of concern (asphalt has been used to seal reservoirs used for drinking water 17 (p.518)).

Additionally, some composition roofing has chemicals embedded in it, and roofing manufactured before 1980 may contain asbestos. Chemicals, such as zinc or copper are there to prevent the growth of moss. This type of roofing will not be suitable for any kind of water collection and should be removed before it poisons the entire yard. If there is moss growing on your roof, that is a good sign. If your roofing was manufactured after 1980 it probably does not contain asbestos.

Furthermore, in many areas the dust from the air pollution may pose more of a threat than the chemicals from the roofing material - as the sun heats up the roof, hydrocarbons become soluble in water and mix with the dust to form (possibly) even more toxic pollutants.

Another consideration is the coloring in the aggregate covering composition roofing. It can contain heavy metals (which produce the color) such as chromium, copper, or lead. Avoid green, or white colors. Brown colored roofing usually has iron which may not be as harmful as other metals.

Also, the majority of water soluble pollutants in the runoff from most composition roofing come from the pieces of roofing that break loose due to normal aging. These pieces are usually washed off the roof during a moderate rainfall event; however they may remain in the gutter, so they must be removed before using any of the water that runs through the gutter. 15,16,17

Another issue that must be considered is the diversity of complex hydrocarbons, sulfur compounds, and nitrogen compounds present in asphalt. The exact nature of the combination of these compounds depends on the source of the asphalt; so different roofing manufacturers' products will not contain the same Asphalt contaminants.17 (p. 518) The study quoted on this website on roofing materials pertains only to most roofing currently sold on the west coast of the U.S. .

A first flush diverter with a very large disgard amount such as 25 or 30 gallons per downspout (assuming at least 4 downspouts) per rain event on a 2000 square foot roof would be advisable. After the high volume discard first flush diverter is installed, at the very least, have the water tested for Polycyclic Aromatic Hydrocarbons, (PAH - Polycyclic Aromatic Hydrocarbons) and / or Hydrocarbons (TPH - Total Petroleum Hydrocarbons) (along with the standard EPA water quality tests which should include tests for copper, lead, zinc, iron, phosphates, nitrates, nitrogen compounds, and chemical oxygen demand) and do not use water from a composition, tar or asphalt roof if the temperature of the roof is above 68 degrees F. 4,14,15,16,17,31 For example; during the summer months the water from the roof in this study is of very poor quality.    However, water collected during the rainy season in the fall, winter and spring is of very high quality .   back to top


These are the tests that are required by the epa. They have already been done on the output water from the system we describe on this site as Filter 1. Filter 1 removes all of these 18 contaminants to levels exceeding the requirments set by the EPA (and the county in which we are located) for drinking water quality.:
Coliform bacteria
Arsenic
Zinc
Nitrites
Nitrates
Sodium
Silver
Barium
Cadmium
Selenium
Chromium
Mercury
Lead
Fluoride
Total Petroleum Hydrocarbons sometimes called TPH
Total Organic Carbon sometimes called TOC
E Coli bacteria
Fecal Coliform bacteria

Other contaminants that may be present (and should be tested for) but have not been tested for in this research include: asbestos, copper, tannins, Baylisascaris Procyonis (Raccoon roundworms), viruses of any type, and industrial chemicals. And furthermore, there may be anything else in roofwater runoff - anything - depending on where the roof is located.
At the very least, do a test for all of these contaminants mentioned above. You will need to continue testing regularly, because the quality of roof water will vary considerably from time to time, and even if the slow sand filter is working perfectly, it may not be able to remove all of the contaminants if the concentration is too high. Every roof, every situation, and every filter output will not necessisarily be the same.

17.  What is a first flush diverter?
This is an absolutely necessary part of a roofwater harvesting system and will keep concentrated toxic chemicals out of the slow sand filter. A first flush diverter (sometimes called a "roof washer") is a device that manages the flow of rainwater into a harvesting system. It diverts the first flow of runoff from a roof, after a dry spell, away from a roof water harvesting system. There are many different designs, but the idea is the same - prevent water that contains dissolved pollutants such as: dust, vehicle exhaust particles, animal droppings, and particulate matter due to poor air quality from entering the water storage system. The runoff from the first rain after several months of a dry spell will contain concentrated highly toxic materials including harmful bacteria. A diverter usually has a "temporary storage chamber" that fills with the most polluted water and then seals itself off to allow the fresh rainwater to flow into the storage system while slowly emptying this "temporary storage chamber" so it will be ready for the next rain event.  back to top

18. Can I purify water from a creek, stream or pond using a slow sand filter?
Possibly, but again use extreme CAUTION: Know the exact origin and path of the water in the creek or pond and always ALWAYS have the water tested pre-filter and post-filter, before consuming any. Also, be aware that many racoons carry roundworms (Baylisascaris procyonis) that produce millions of tiny eggs which are deadly if ingested by humans.43, 44, 45 (see faq 16) These eggs can survive for years in harsh environments. Although infection is rare, it has happened and there is no cure. The eggs are approximately 50 microns in size, so most commercial water filters will remove them easily. A properly operating slow sand filter should be able to remove them. Also, be aware that urban and suburban areas have chemical pollutants, too numerous to mention here, that flow into creeks and ponds; and many of these substances can be very harmful. If you are in a rural setting and you are sure that the creek or pond is not anywhere near populated areas your worst problem will probably be biological contamination, and the slow sand filter will be able to purify the water if the turbidity level is not too high. A slow sand filter can remove 99.9 percent of harmful organisms from water; however algae will quickly impede the flow of water through a slow sand filter. A slow sand filter will not remove all poisons produced by toxic algae; and high concentrations of chemical contamination cannot be completely removed by a slow sand filter.   back to top

19. What is "wet harrowing"?
This is one way to "clean" a slow sand filter. When the flow rate slows down too much, the water immediately over the top surface of the sand (the top of the filter where the unfiltered water flows in) is gently agitated to stir up the biolayer. This breaks up the overly thick covering that has built up on the top of the sand. It is important to stop the output flow while this is done. Water must not be allowed to flow down through the filter while the top surface is being agitated because there will be contamination seeping too far down into the sand bed. The resulting cloudy water is drained off until it runs clear from the wet harrow drain, and the filter is then allowed to run for a week to re-establish the biolayer before the filter is put back into service .  back to top

20. Will a slow sand filter remove industrial pollution?
A slow sand filter will remove at least half and up to 90 percent of some chemical pollution at levels up to 100 times greater than recommended maximum allowable levels 12. A properly functioning slow sand filter is capable of removing Diesel contaminants present in water from less than 1 mg/L down to less than .07 mg/L, and heavy oil contaminants from less than 1 mg/L down to less than .095 mg/L. Tests for the presence of chemical pollutants are very expensive.  back to top

21. What is the difference between a biosand filter and a slow sand filter?
Nomenclature - mainly. The name "Biosand Filter" refers to an invention by Dr. David Manz. Typically a biosand filter is smaller than a slow sand filter and may operate intermittently. A slow sand filter usually operates continuously and can be quite large. Slow sand filters are typically used in municipal water supply systems, whereas biosand filters are usually for individual use. The biological action is similar in both types of filters. A slow sand filter can be large supplying thousands of gallons per day, or small supplying 5 or 6 gallons per day.   back to top

22. Can I purify roof water with a slow sand filter?
A slow sand filter can purify water harvested from a roof, but the water should not be consumed unless the roof is made of metal with a non-toxic baked on enamel finish; and a first flush diverter is used. In some cases a composition roof may be able to provide water suitable for consumption - see faq 16 - but use extreme caution. Without a first flush diverter, or some sort of prefiltering, water from a roof can clog a slow sand filter within hours; especially if there are trees and wildlife near the roof. Gutters must be kept free from inorganic debris and/or small pieces of roofing litter that result from normal wear; and free from leaves, and other organic material. If the filter plugs up and the flow is significantly reduced, cleaning will be necessary (see faq number 19) and the filter output will be unusable for at least a week.   back to top

23. Where can I get plans for a slow sand filter?
Search on the internet under "slow sand water filter" or "Biosand Water filter" or look at the drawing on this website. Also read about their theory of operation - it is not that complicated and does not take a college degree to understand how to put one together and operate it successfully.   back to top

24. Can I safely water my vegetable garden with roof water that has been purified by a slow sand filter?
Probably, depending on what type of roof you have (read faq number 16 above) and what wildlife is present in your area. As for chemicals that leach from a roof surface, the key concept here is "bioaccumulation". Plants that are regrown each year from seed won't accumulate much poison, unless you have an unusually polluted environment. Fruit trees or perennials like berries may accumulate poisons over time. Use a first flush diverter and have a metal roof with a non-toxic coating to get the purest water. Any other type of roof (excluding possibly some types of tile roofs) will add toxins in proportion to their age and manufacturing. It is also important to note that many carcinogens can be present in extremely small concentrations and only a lab can detect them - and as they build up over time (months or years) in tissue they become quite toxic. The toxins in roofing are varied and expensive to identify. There are coatings available that will vastly improve the quality of water taken from a roof (see number FAQ 16 above). Also, if you have a "carport" or separate garage, just install suitable metal roofing on that building and your filtered water will be ok to water a garden. If you intend to have a new roof put on your house have a non-toxic metal roof installed - they will last 50 years or longer, stand up way better in a storm than composition or cedar shakes, will not poison your yard, will give you non-toxic rain water, can be recycled, they are fireproof, and can be installed over old composition roofing. From a biological standpoint, more caution is advised. Racoons carry parasites in their digestive systems (Baylisascaris procyonis - sometimes called racoon roundworms) that are usually harmless to the racoons. These roundworms each produce millions of eggs which can persist in the environment for years and can be deadly to humans. Rooftops may be contaminated by these parasites' eggs if racoons' latrines are present. If there is a raccoon latrine on the roof, then water may pick up these microscopic eggs and deposit them in rainbarrels. Raccoons infected with Baylisascaris procyonis have been found in the Middle Atlantic, Midwest, and Northeast regions of the United States and are quite common in California and Georgia. Human cases have been found in California, Oregon, New York, Pennsylvania, Illinois, Michigan, and Minnesota, and a possible occurrence in Missouri.63 Ingestion of the eggs of Baylisascaris procyonis will cause severe illness and there is no cure. Young children are particularly susceptible. THIS PROBLEM IS RARE, ACTUALLY QUITE RARE; however, the consequences warrant extreme caution. If you have ANY raccoons anywhere near your roof you must assume your roof is contaminated until you have it checked by a professional. This study has not tested for the removal of these parasites' eggs and although the eggs are 50 microns in diameter,45 a slow sand filter may not remove them unless it is operating correctly. Although simple precautions will prevent infection, (leave racoons alone, always wash your hands after working in the garden, and don't let children get near contaminated areas) watering a vegetable garden with unfiltered water from a rainbarrel that collects water from a roof that harbors a raccoon latrine, may result in severe infections. DO NOT EVER WATER VEGETABLES WITH UNFILTERED OR UNTREATED WATER STRAIGHT FROM A RAIN BARREL - NO MATTER WHAT KIND OF ROOF YOU HAVE. 43, 44, 46   back to top

25.  Can I use a slow sand filter to purify tap water?
Maybe. It will depend on how much bacteria is in the tap water, and how much chlorine is present in the tap water. If your water comes from a public water supply system, it is unlikely that there will be enough living "good" organisms to start a biological layer in a sand filter ( that is assuming your public water supply is completely filtered and purified ). There must be living microbes in the water to form the biological layer. The physical action of the sand will do some filtering, but a 10 stage carbon filter will be much better at removing harmful chemicals from your tap water. If, however, there are even a small number of living microbes in your tap water, it is likely that a functional biological layer will eventually form; but it may take several months for the layer to mature. If you are on a non-public water system ( a private well ) or you get your water from an open stream or irrigation ditch, then a slow sand filter will probably work.  back to top

26.  I have seen videos of slow sand filters in operation that show very dirty water being poured in and clean looking water flowing out. Is this realistic?
Yes, but with several caveats. Too much turbid water in will clog the filter very quickly. A slow sand filter gets better at filtering as the biofilm (schmutzdecke) matures and will do a very good job of filtering out more than just pathogens , however at the expense of flow rate. Water typically takes some time (depending on the size of the filter) to pass through a slow sand filter. The water that is poured in is not the same water that immediately flows out. It can take anywhere from minutes (inefficient purification) to hours (much more efficient purification) for water to flow completely through a slow sand filter. Also a small charcoal filter on the output will help eliminate odors and colors from the output water.  back to top

27.  How is a slow sand filter "cleaned"?
Cleaning methods include wet harrowing, (described above in faq 19) or physically removing the top 2 or 3 centimeters of sand (usually done on larger filters with the water level significantly reduced). DO NOT FORCEFULLY "BACKWASH" A SLOW SAND WATER FILTER ( force water to flow backwards - up through the sand from the bottom drain at any pressure - high or low ). There are some filter designs that use a cleaning method similar to backwashing,62 (p.7-9) but the flow does not change the sand layers and the design has been carefully engineered, tested and shown to be functional. Simply hooking up a pressurized water source to the bottom of a biological sand water filter and forcing water backwards through the filter ( as shown on several youtube videos) is not the same thing and will most likely destroy the filter. In a biological sand filter (slow sand water filter), the top few centimeters of sand contain living organisms and a biological mat. The biological mat thickens and small particles are trapped in and on top of this mat. This is what slows down the flow. If you backwash the filter you will mix all the debris throughout the filter as gravity allows the material from the top to settle down throughout the depth of the sand and this will plug the filter for good. Unlike a rapid sand filter, backwashing disrupts and may very well destroy the biological component and the physical structure of the lower layers of a biological sand filter. There is considerable biological activity below the top layer that does not significantly impede the flow of water, yet is part of the biological filtration process.33 (p 17, 93) If you are using a small slow sand filter with different sand sizes in layers, backwashing may mix the sand layers and significanlty change the operating parameters of your filter. It is possible to allow water to flow down through the filter more rapidly by opening a valve at the bottom of the filter (utilizing only the pressure due to gravity) to wash out turbidity (when the filter is put into operation for the first time after it is built) without seriously disrupting the filter strata. When this is done care must be taken to be certain the surface of the sand on top is not exposed to air.   back to top

28.  What is the difference between rapid sand filtration and slow sand filtration (biological sand filtration)?
Rapid sand filtration does not intentionally use a biological process, (some beneficial bacteria may grow between backwashing events 33 (p 18)) its design uses strictly physical filtration. Very few disease causing organisms are inactivated. Viruses are not removed. Water, that has had coagulant added, is forced through coarse sand under high pressure resulting from depth of water and gravity or added external pressure. These filters require cleaning every several days, or more often, by "backwashing" - forcing purified water or air through them from the bottom drain pipes in a reverse direction of normal flow. Chemicals such as chlorine or ozone must be added to the water to kill harmful bacteria and viruses. Large volumes of purified water are required for backwashing, and large volumes of toxic sludge are produced as a result of daily cleaning events called "backwashing". Rapid sand filters are not practical as a water purification method for individual use. Rapid sand filtration is not sustainable technology - it requires continuous significant energy and chemical input and is a highly complex man-made technological process which requires constant monitoring. Rapid sand filters can supply very large volumes of water in a timely fashion typically needed by large cities with populations in the millions of people. To discuss this and read more follow this link to the slow sand filter blog

Biological sand filtration takes place through biological action, and some physical action. It is much slower than rapid sand filtration and requires more area and much finer sand in layers of progressively coarser sand towards the bottom of the filter. Layers of "good" organisms form in the sand. The thickest is in the top 2 or 3 cm of sand, however beneficial organisms are present at depths of up to 40 cm.33 (p 22) The organisms in the top 2 or 3 cm form a mat of organic substance called a "schmutzdecke" which physically filters water while the bacteria in this schmutzdecke, along with the other bacteria present at lower depths in the sand, actually consume and / or inactivate up to 99.9999 percent of all pathogens (including viruses) present in the water that passes through the filter. Even though some slow sand filters have been designed to be cleaned using water forced through the top 20 cm of sand, this is NOT backwashing - the bottom layers are left undisturbed. These biological filters will be ruined if they are backwashed. They are referred to as: slow sand filters, or biosand filters or biological sand filters. These filters are sustainable technology. They require no chemical input and very little energy input and do not require constant monitoring. They are the single most effective filter for removing cryptosporidium (Beaver Fever) cysts from water (rapid sand filtration combined with chemical disinfection is not as effective - it takes dangerously toxic amounts of chlorine or ozone to kill the spores). The material resulting from cleaning is biologically non-toxic and can be used for fertilizer. If wet-harrowing is used, the sand rarely requires replacement.33 (p 17, 93) 25(p4), 26(p103-104), 27(p268-273)

29.   What is COD (Chemical Oxygen Demand)?
Chemical Oxygen Demand is a measurement of the amount of organic material in water.32

30.   Will a slow sand filter remove poisons in water from toxic algae blooms?
In some cases, yes; but this is still being studied and there are differing varieties toxic algae found in most parts of the world. There are 7 different toxins associated with toxic algae blooms: Microcystins, Nodularin, Anatoxins, Saxitoxins, Cylindrospermopsins, Lyngbyatoxin a, and Aplysiatoxins. Slow sand filters have been shown to remove (biodegrade) Microcystins. Some algae (cyanobacteria) are only toxic at certian times and others are highly toxic. Only highly skilled persons with access to a laboratory can positively identify toxins, and toxic algae types. DO NOT EVER USE WATER FROM A SOURCE THAT CONTAINS ANY VISABLE SIGNS OF ANY TYPE OF ALGAE, UNLESS YOU ARE ABSOLUTELY CERTAIN THAT IT IS NOT TOXIC. DO NOT GUESS. YOU HAVE BEEN WARNED. Toxic algae is seldom present in sub-surface fresh water, or in fresh water that is moving.34,35,36,37,38,39

31.   How fast does water move through a slow sand filter?
A slow sand filter is a dynamic device and will change over time. The amount of time it takes water to flow through a slow sand filter will depend on the size of the sand, the depth of the sand, how long the filter has been in operation after initial start up or after the most recent cleaning, the depth of water above the surface of the sand, the area of the sand surface, and the turbidity of the input water. The filters described in this study have flow volumes that vary from 3 litres per hour ( after 1 year of running without wet harrowing ) to 45 litres per hour ( after 5 months of operation without wet harrowing ). The sand in these filters described here typically holds 50 percent water by volume - approximately 100 litres. So the simple math tells us that it takes anywhere from about 2 hours (almost too fast) to about 30 hours ( more than slow enough ) for the water to flow through the filters described here. This is approximate time. More water on top of the sand will increase the rate of flow and less water will decrease the rate of flow.

The way many studies refer to the flow rate is in meters per hour (m/hr) (1 cubic meter = 1000 litres ) rather than liters per hour (L/hr):
(cubic meters of water flow) divided by (unit time in hours or seconds) divided by (square meters of filter bed area) 47 (p 5)
or:   m3 / 1hr / m2
or:   (doing the math)   m / hr
or meters per hour.
Acceptable rates are between .1 and .3 m/hr
The above described filter with 45 litres per hour throughput has a "flow rate" of .184 m / hr

An explanation of the mathematics and theory to accurately describe the rate of flow at any given point in a slow sand filter is beyond the scope of this faq. Wikipedia has information on Darcy's law which describes the flow of water through sand and might be a good place to start. Oklahoma state university has more detailed information about Darcy's law Keep in mind the different layers in a slow sand filter have differing resistances to the flow of water.


32.   Will a slow sand filter work at near freezing temperatures?
Yes; under certain conditions. If the surface of the sand does not freeze, and the water continues to flow through the filter. In the study summarized on this website, the most recent test was done when the average ambient temperature was 32 degrees. At this temperature the slow sand filter was able to remove 99.9996 percent of the coliform bacteria from the input water. A log reduction of 3.477 ( from 30,000 cfu to 10 cfu ). If water stops flowing, and / or the surface of the sand freezes then the filter stops functioning.

33.   I have built a slow sand filter; now, how do I test it?
After your filter has had water running through it for at least 3 weeks (about 20 days) the output water should be running perfectly clear. Have an epa certified laboratory test an output water sample for coliform bacteria and, at the same time have an input water sample tested for coliform bacteria. It is critical that the water used as input on an ongoing basis be tested at the same time as the output water is tested. If the input water shows no coliform bacteria and the output water has coliform bacteria in it the filter is not working and has contamination somewhere. If the input water shows coliform bacteria and the output also shows coliform bacteria the filter may or may not be working. If the coliform bacteria numbers are less in the output, than in the input water the filter probably needs more time to develop the schmutzdecke (biofilm). Allow more running time and then do another test. If the input water shows coliform bacteria, and the output shows no coliform bacteria, then the filter is working. There should be a considerable difference between the numbers of coliform in the input water and the numbers of coliform in the output water. The filters described on this site have less than 1 coliform per 100 mililiter in their output water given an input up into the hundreds, or even tens of thousands of coliform colonies per 100 mililiter, and this is the desired result. If you are filtering roof water from a composition (tar shingle) roof you will also need to have a sample tested for PAH's and TPH's (Polycyclic aromatic hydrocarbons) and (Total Petroleum Hydrocarbons). Also have a sample tested to determine if the water meets your local county and / or state requirements for your planned use of the water. Here, the recommended tests check for: fluoride, Sodium, Nitrite, Nitrate, Silver, Arsenic, Barium, Cadmium, Chromium, Mercury, Lead, Selenium, and coliforms. Know that contamination can come from anywhere and the operation of a filter is totally dependent on the owner - any filter can become toxic if not properly maintained. The key is to know what might be in your source water, then you will know what else to check for. Most importantly, be absolutely certain about the condition of your source water. Where does it come from? Is it rainwater, roofwater, creek water, lake water, well water, or just surface runoff? The contamination in these sources will vary considerably, and it is not possible to know what contaminants may be in a particular sample of water without knowing all of the environmental conditions that influence the water. For example, creek water could have anything in it from upstream - anything. Lake water also is subject to just about any contamination known to exist. Rainwater, and or roofwater are probably the best choice. Well water can also be a good choice, but be careful of shallow wells near industrial areas or agricultural areas, or large residential areas. Roofing material made from petrochemicals (bitumens or simply "tar") sometimes called "composition" roofing will leach PAH's into water runoff. If this water just sits in a rainbarrel, there is not enough bacteria, and no sunlight to help it biodegrade. Bacilli in the soil can and do break down petrochemicals into elements.

34.   What does "effective size for filtration" and/or "effective size" mean in reference to sand for a slow sand water filter?
People who operate machinery to prepare sand to be used in water filters use these phrases to indicate how the sand is sorted by the size of the individual grains, and how the sand reacts to water passing through it. A smaller number indicates finer grains of sand that will cause water to flow more slowly than sand with larger grains. The sand with larger grains will have a higher number referring to the effective size. Typically, .15 mm effective size sand for filtration is about as fine as can be used and still maintain a usable flow of water. An effective size of between .15mm and .25 mm is probably ideal for a small slow sand water filter. The effective size is determined by each manufacturer and my vary slightly. The effective size is not exactly the same thing as the uniformity coefficient (see faq number 13 ).

35.   Can I use beach sand from the ocean shore in a slow sand filter?
The short answer is NO. . . . however, there may be exceptions to this. If ALL the salt is removed from the sand, and the beach is not subject to public access, and is not exposed to surface water contamination, and is not an environmentally sensitive area, and the sand is washed, dried, and sterilized before use, and is sifted to insure proper size the sand may be used. From a practical standpoint this makes "beach" sand a very poor choice.

36.   Can I use beach sand found at a lake or river in a slow sand filter?
The short anser here is NO. . . . However, if you have permission from the landowner and if the area is not an environmentally senstive area, and if there is no chemical or biological contamination in the water or the sand, and the sand is washed and sterilzed before use, and sifted to insure proper size, this sand may be used. In an emergency situation this may be the best option. Technically, the sand would probably allow a biofilm to form, which would, in theory, "work" to filter water.



Note: Websites describing biological sand filters / slow sand filters have been appearing with instructions recommending BACKWASHING a biological sand filter when the flow rate slows down. FORCEFULLY BACKWASHING A BIOLOGICAL SLOW SAND FILTER WILL DESTROY IT, AND MOST LIKELY CAUSE IT TO PRODUCE TOXIC WATER OUPUT. These well meaning people have confused rapid sand filtration with biological sand filtration , and backwashing with reverse flow. In a biological sand filter, the flow rate is slowed by a buildup of material on the TOP FEW CENTIMETERS of sand. Simply gently agitating this layer and DRAINING OFF the cloudy water will restore the flow rate. In some filter designs, water is allowed to flow "backwards" (at nearly same rate as it flows in normal operation ) through a biological sand filter without "fluidizing" the sand bed, but this is not the same thing as backwashing. Backwashing fluidizes the sand bed, reverse flow does not. 52 (p 5-11).


Think of this: whatever is on your roof will wash off in the rain and onto and into the soil near your house. If your garden is near any downspout and if there is any lead, arsenic, copper, PAH contamination, or chromium on your roof the plants in your garden will be able to take those chemicals into their roots, and dust from the soil will be on the leaves. 48, 49, 50 The more time that passes the more concentrated the chemicals will become in the soil around your house. ALLOWING THE DOWNSPOUTS TO DRAIN INTO THE COUNTY OR CITY WASTEWATER SYSTEM IS ABSOLUTELY UNACCEPTABLE - PERIOD. The combination of a first flush diverter and a slow sand filter will take a considerable amount of that pollution out of the water that runs off of your roof - but not all of it; and some of the chemicals from your roof water runoff may biodegrade. Also, whatever pollution that is in the rain (and it can be a considerable amount depending on where you are located) that falls on your vegetalble garden will accumulate in the soil. In addition to this the dust that builds up on your roof in a dry spell mixed with runoff will be very toxic and extremely concentrated in water from the first rain event following the dry spell. If you use a diverter (which can be built for under 75 dollars), much of this toxic liquid will be held back, and more of the toxic chemicals will be removed by the slow sand filter.

note 2-F: The information used in this FAQ was taken from experience working with small slow sand filters and from information noted by following the Literature cited link at the top left of this page.back to top




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