Rainwater harvesting with Sustainable technology:
A look at the design, construction and operation of a small scale slow sand water filter. (Building a small slow sand water filter for individual use)


> Introduction
Water Test Results Water color improvement Slow sand filter FAQ Literature Cited Contact the author Terms of Use / Copyright information External links: Slow Sand filter Blog Roof water harvesting summary More info on rainwater harvesting and filter construction Biosand filter info and sustainability focus First posting 20080917 Page Updated 20120901 Website Updated 20151018 
Introduction
The slow sand filter projects documented on this website have taken place in the pacific northwest area of Washington state, USA. The first filter project (filter one) was started in early 2007. All of the projects are ongoing and will continue as long as the author is able. The water quality tests that have been done by epa certified testing laboratories are noted as such. The preliminary field tests are done onsite and are also noted as such. The researcher has a four year degree from the University of Washington (class of 2006) in environmental studies plus 35 years of experience in nonacademic work. Chemicals used for drinking water treatment such as chlorine, or ozone that produce toxic cancer causing byproducts are not used in these filters. The longest running slow sand water filter project this site documents has been ongoing for seven years (it was started in the summer of 2007). It will continue as long as possible. Keep in mind that "Backwashing" a biological sand filter will destroy it.; and that a biological sand filter is constantly full of water up to and covering the top surface of the sand inside by at least 1 inch or more  the sand must not be exposed to air. Know that the content of this site will change considerably as this study continues. Rain barrels are now legal in Washington state! For over 90 years it has been illegal to catch rainwater in Washington state.^{note 2} As of October 9 2009, catching roof water is no longer a crime! The department of Ecology in Washington state (the D.O.E.) has finally clarified the law so as to allow individuals to catch rainwater for their own use. (from this preceding link, click on the links at the D.O.E. site on the upper right hand side of their page that say "new" highlighted in yellow.) So far work has been done on Five separate small slow sand filters:


Notes: Average flow rates: filter 1: 12 liters per hour filter 2: 35 liters per hour filter 3: 36 liters per hour filter 4: 45 liters per hour filter 5: 12 liters per hour Originally, flow measurements were taken by recording the number of minutes required to fill a 12 litre container. Most recently (20101219) we are using a 4 litre container, as it is more indicative of instantaneous flow volume when measuring the flow from a filter that is not fed by a steady flow of water. The 4 litre container was tested against the 12 liter container using filter 2 which has a steady flow (its input is controlled by a float valve and pressurized water). The 4 litre container was found to result in flow measurement precision and accuracy equal to the 12 litre container when the flow volume totals were averaged. 1 cubic meter = 1000 litres 1 litre = .001 cubic meter We use flow rate per unit area per unit of time because we are interested in quantifying how fast the water flows past a given level in a given area. Simply measuring liters per hour does not give the full story. For example; 20 liters per hour from a 25 cm diameter container does not allow the same sand particle contact time as 20 liters per hour from a 75 cm diameter container, and sand particle contact time is critical for allowing maximum purification. flow rate should be between .1 and .4 meters per hour (ideally between .1 and .3 meters per hour for the best results This is how to figure the flow rate by just measuring how many litres flow per hour: flow rate in meters per 1 hour = [(liters per 1hr)÷1000]÷(area of sand bed surface in square meters) Explanation: Here we will use X to mean multiply; and / to mean "per" as in per hour; and the ÷ to mean divide L means litres and m means meters In this formula, a cubic meter is represented by meters cubed or just: m^{3} (in other words meters X meters X meters or think of length X width X height the way we find volume) And the area of the sand surface is represented by meters squared or just m^{2} meters X meters (length X width the way we find area) Now, know that 1 cubic meter = 1000 Litres, or more simply: m^{3} = 1000L Divide litres by 1000 to get cubic meters; or multiply litres by .001 to get cubic meters If m^{3} = 1000L, then cubic meters/hour = Litres/hour ÷1000 (the per hour thing is there because we need to have a concept of motion over a time period since it is 1 hour it does not change the result division by 1 or multiplication by 1 is identity) or more simply: m^{3} per hour = (L per hour)÷1000 Now; m/hr = [(L/1hr)÷1000]÷m^{2} or just m/1hr = (m^{3}/1hr)÷m^{2} (the m^{3} and m^{2} cancel out into just m so we end up with m/1hr) Now, the above translated into language: flow rate in meters per hour = volume in cubic meters per hour ÷ sand surface area in square meters and finally back to where we started: flow rate in meters per 1 hour = [(liters per 1hr)÷1000]÷(area of sand bed surface in square meters) A simplified version of the above is: For the shorter large barrel with a sand surface area of .26791 sq meters (23 inch diameter): flow rate in meters per 1 hour = (53.75) ÷ (time in seconds to fill a 4 litre container) For the taller smaller diameter barrel with a sand surface area of .2452 sq meters (22 inch diameter): flow rate in meters per 1 hour = (58.73) ÷ (time in seconds to fill a 4 litre container) Page design by Perpetual PC's This work is licensed under Creative Commons Attribution Share Alike 3.0. terms of use 