|Roofwater harvesting using slow sand filtration|
The information presented here is focused on safely harvesting rainwater from a roof using a diverter and a biological sand filter. Harvesting rainwater in the USA is just now beginning to become more widely accepted. In Washington state, as of 2009; and Utah, as of 2010, rainwater harvesting for personal use is now legal. (exact wording on Utah's change is here). Colorado has started to come around but not completely; there are still severe limitations there. In other places here in the U.S., this outdated, narrow-minded attitude of exclusive govenment sanctioned "water ownership" as applied to rain, is also rapidly changing. A list of states' rainwater harveting laws is here. The laws are varied, complicated and far beyond the scope of this website. Be sure to check this out before you set up rain barrels. Purifying that water from a rain barrel may be of interest to more people than in years past here in Washington state; and hopefully this will set an example for people everywhere.
March 23, 2013: Filter 4 is still functional, after 25 days of being frozen solid in January of this year. It did take about 3 weeks for the output to clear up.
April 16, 2012: the water output clarity has improved on filter 4.
February 8, 2012 1:30 pm pst:
The new filter (Filter 4) described on the system overview page is working.
Water is necessary for life. People need water every day. The water cycle provides rain water that has been purified due to evaporation. Although it is true that air pollution threatens the purity of rainwater; rain is still cleaner than surface water. Collecting water from rooftops is becoming popular in "developed" countries and is now finally being recognized as a viable source of water in an increasing number of places in the U.S. .
Common sense tells us that there are a lot of nasty things on a roof surface that must be removed from the water it provides to make it usable and safe. Water from rainbarrels can be used for utility purposes such as irrigating decoratve plants or washing out gutters; and this can usually be done safely without extensive filtering; however, if the water is to be used for other purposes, it MUST have adequate filtering and purification. A biological sand filter otherwise known as a slow sand filter, can be a key part of this filtering and purification. This website will focus on explaining how to purify roof water using the sustainable technology of slow sand filtration. We will look at two seperate systems. The system with Filter 1 has been in operation for five years. The system with Filter 4 is the newest addition to the study. Filter 4 has been in operation for 7 and a half months (as of July 30 2012). It has been tested and shown to remove all coliform bacteria from contaminated input roof water.
Slow sand filters do not need electricity, petroleum products, or man made chemicals to function. They can be built with recycled materials and can last indefinitely. They produce no toxic inorganic chemical waste and require only intermittent maintainence. They function with the aid of gravity and naturally occurring microbiological life, much in the same way that water is purified by a wetland near a deep sandy riverbank.
A slow sand water filter (essentially the same thing as a biological sand filter) works like this:
Water is added and flows through a container filled with sand and gravel. The sand is about 3 feet deep with 4 inches of gravel below the sand covering drain pipes. Water slowly flows through the sand and gravel to the drain pipes and then is allowed to flow into a storage container. The water must flow very slowly through the filter. A slow sand filter in a 55 gallon barrel will typically supply about 10 gallons per hour. The top surface of the sand must always be covered by at least 1/2 inch of water. After about 3 weeks, as the water flows through the sand and gravel, a layer of living microscopic organisms forms in the top few inches of the sand. This happens because almost all water (except distilled water) contains at least some aerobic bacteria, plankton and algae that stay alive by consuming bad bacteria, and protozoa in the water. This layer is not visible until it becomes very thick. Because of tiny electrical charges (called van der Waals force), on particles and bacteria in the sand and water, and because of the layer of organisms and organic material on the top few centimeters of sand; bacteria and harmful organisms are trapped and consumed as water flows down through the sand. As gravity causes the water flowing down to compact the sand; the filter's ability to remove bad bacteria and small particles below the first few inches of sand is increased because the effective distance between the grains of sand, or porosity, decreases and some additional "good" organisms grow. The water that flows out of the filter is 99.99 percent free of bacteria; in other words, the output water is purified; not just sterilzed. The water must flow slowly enough for the good bacteria to consume the bad bacteria, so some kind of flow regulation is necessary. Most small slow sand filters have an output pipe running up to just above the surface of the sand. This design functions as a regulator and causes the water to flow slowly enough to allow the bacteria to work. A slow sand filter will also remove some chemical contaminants from water, and as the filter matures it becomes more effective at removing contaminants. The sand does not need to be replaced or removed and no chemicals are needed for the filter to remove bad bacteria. The filter is cleaned by gently agitating the water over the surface of the sand just enough to produce cloudy water. The cloudy water is then drained off. This is called "wet harrowing". This is all that needs to be done. DO NOT BACKWASH A SLOW SAND FILTER, YOU WILL RUIN IT - all the natural biological layers will be destroyed and the filter will become ineffective. Backwashing is for rapid sand filters and they are very different.
With proper filtration and purification, rain water collected from a roof is quite usable. There are only a few exceptions listed below.
Roofing material to avoid:
Cedar shakes with preservative added (most cedar shakes fall into this catagory)
Composition roofing with moss killer applied or embedded in it
Composition roofing applied before 1980 that may contain asbestos
Roofing material that may be acceptable:
Composition roofing applied after 1980
Cedar shakes without preservative added
Roofing material that is acceptable and most recommended:
Galvalume or Zincalume (metal roofing with non-toxic baked on enamel finish)
The majority of roofing material applied today is free from asbestos, and not all cedar shake roofing contains added poisons, although cedar does contain naturally occuring tannins and oils that will make the water non-potable. For the majority of homeowners, rainwater harvesting is not only feasable, but also is very helpful to the environment by preventing excessive runoff from entering and overloading city and county storm drains.
Metal roofing with the brand name Galvalume, or Zincalume is the best choice for rainwater harvesting from a roof. The coating is non-toxic and provides a smooth surface which is most easily washed off by rain. Tile roofing is also used with good results. Composition roofing with moss killer ( copper granules ) embedded in it or moss killer ( zinc strips that form zinc oxide) see question 8 at this page on it is not a good choice for rainwater harvesting and should not be used. Cedar shake roofing with preservatives added (copper compounds: copper chromium arsenate or CCA - a mixture of arsenic pentoxide, chromic acid, and copper or cupric oxide - highly toxic) is also a very poor choice and will not produce safe water. Old composition roofing with asbestos in it is also a bad choice and may produce runoff that contains asbestos fibers. The best thing do do if you are in doubt is to have runoff from your roof tested for these chemicals.
Although not commonly used for rainwater collection, composition roofing can produce quite pure water if proper filtering is used and no poisons are used or embedded in the material. A slow sand filter will remove hydrocarbons from the water as well as harmful pathogens and dissolved lead from drain vents. The slow sand filter described here filters water from a composition roof. It has been tested and has been shown to remove hydrocarbons and produce water that passes EPA and county standards for drinking water purity . . . These are some exceptional claims, but read on for the details.
There are three topics of interest here:
1.) A roof washer (sometimes called a first flush diverter) with screens in the input
2.) Keeping gutters and roof surfaces reasonably clean
3.) Understanding the operation of a slow sand water filter ( sometimes referred to as a Bio sand water filter).
Once a person has these concepts understood, harvesting rain water from a roof will become second nature.
A first flush diverter allows a set amount of runoff to carry dust, bacteria, and petroleum hydrocarbons off of the roof surface and away from the water filter and storage system. Since water is a very good solvent , most pollutants are washed off the roof fairly quickly during a rain event. About 1 gallon per 100 square feet of roof surface is the recommended amount of runoff to be diverted before allowing water from the roof to flow into the water collection system under ideal conditions. Experience on this project with composition roofing in a forested area (in northwest Washington state) has shown it is considerably more. A minimum diversion of 5 gallons per 100 square feet in the summer and 2 gallons per 100 square feet in the rainy season (fall, winter, and spring) is necessary to keep out excessively contaminated water.
A considerable amount of petroleum hydrocarbon and chemical contamination comes from small pieces of roofing material that break off as compostion roofing ages. This shows up as "sand" like material in the gutters. If gutters are cleaned out every four or five months this contamination and bacterial contamination from leaves and other organic material can be kept to a minimum.
With a properly functioning diverter having screens on the input and reasonably clean gutters, fairly clean water can be stored for non-potable uses such as watering lawns and decorative plants and washing out gutters. Be cautioned, however. Depending on the geographical location and local conditions, water harvested from a roof may contain harmful bacteria and must never be used for vegetable gardens or any other purpose where the water will be consumed directly or indirectly; unless carefull attention is payed to biological and chemical water quality. This is not the end of the story. A biological sand water filter will take out all of the harmful bacteria and most of the harmful chemicals. The water will then be safe for irrigating vegetable gardens. Use caution however: the filters described on this site are NOT intended for the production of potable water, even though they will, under ideal circumstances, produce very pure water that exceeds epa standards for potable water. Know that the operation of a slow sand filter is totally dependent on the owner / operator and contamination can come from anywhere. Do not drink or otherwise directly consume water from any of the filters described as built on this site. Also, know that roof water quality will very drastically depending on the location and time of year. You have been advised.
Note: It has been brought to my attention more than once, that there are locations where people use roofwater for all their needs without using a slow sand filter and notice no illness or undesireable water conditions. This site has its focus on rainwater harvested from a roof that has high potential for considerable biological contamination. Water harvested in open areas where there are extended hot dry conditions and no overhanging trees may not need as much filtering as water harvested in a forested area or any area where there is abundant wildlife capable of accessing roof surfaces.