Slow sand filter update

It is now March 1 2011, and we are nearing the end of the 4th winter for filter 1, the third winter for filters 2, and 3 and the first winter for the pond filter experiment. February, here was one of the coldest on record. The pond filter and filter 1 froze and ceased operation as of Feb 22. Even though the temperature has been above freezing for several days now, there is still ice in most of the containers. No major damage is obvious – yet. All the filters are functional as of today. Filter 2 and 3 did not freeze during this cold spell. The temperature got down to 19 degrees F here for two consecutive nights and stayed below 32 degrees F at night until the 27th of February. The average temp for the month of February here was below 40 degrees F ( 39 in Seattle ). We had about 6 inches of snow total for the month of February at this location. Driving was EXTREMELY hazardous and it is by the grace of God that we are still unhurt after attempting to drive to work in the hoard of unprepared drivers. Some places less than 20 miles from here at lower elevations got over a foot of snow in one day. Despite all this, very little damage has occurred to any of the filters. Note, however that the below freezing temperatures here do not last very long compared to other places.

In addition to the effects of weather, the filters were basically inactive for 2 weeks while we were in Las Vegas Nevada – where incidentally, water IS a big issue. They get about 4 inches of rain a year there in the valley. I’m not sure rainwater harvesting would even be a viable option without specialized equipment. I did notice lots of water flowing in the huge dry wash near the freeway – 15 I think it is. A visit to the Bureau of Land Management interpretive center at Red Rock Canyon (not too far from Vegas), was interesting and they did have a display about water there. I asked them about rainwater harvesting and they seemed amused at my ignorance. “This is the desert” they said. I should have known – but coming from an area that gets lots of rain I guess I sort of take it for granted – hmmmm; after all my rants about people who take water for granted. . . . If anyone has any information on how to capture rain water in arid environments please feel free to post it here. On the bright side, there was a very large PV array at the interpretive center, and that was encouraging.

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8 Responses to Slow sand filter update

  1. Mark says:

    I’m definitely interested in talking with the author. I have read the information on this page: and I have started reading this blog.

    I recently purchased a home in Northeast Ohio and in the area I live in there are many people that use ponds for sources of water in the home. If they don’t use ponds they use wells with mass amounts of sulfur or cisterns that get filled by truck on a scheduled basis.

    My home has a pond and a filtration system that is very inadequate and simply wrong. Currently water gets pumped from the pond with a floating buoy with a filter attached for particulate. The water leaves the pond underground to the garage. Inside the garage the water is pumped into a 100 gallon drum full of sand. This was their poor attempt at a bio sand filter. The exit on this drum is at the bottom and that means whenever it isn’t filtering water it is dry. This water that has particulate removed is stored in a cement cistern in the ground (1000gal). From that point water is pumped out as required, chlorinated, and put into a 100 gal holding tank for the home. Before it enters the home it goes through a carbon filter.

    I plan on replacing my current drum filter with 3 55gallon barrels designed as slow sand filters described in your website. I also plan on getting a custom pool liner made for my cistern so that I can disinfect the holding tank easier.

    I would love to talk to you in more detail through e-mail or phone (skype). Feel free to contact me.

  2. admin says:

    Some thoughts:

    Your filter set up plan should work quite well, with some caveats. I would encourage the use of a small uv filter at the point of use when the water is to be consumed, as one of the things I have discovered is that the pathogens in water can vary considerably and it never hurts to be sure. The system shown on the website with “filter 1” might be a good start for ideas. The recirculation system keeps water flowing through the filter so it stays “alive” even when no water is being used. The recirculation pump builds up pressure from the main filtered water storage tank, and feeds a small amount back through the filter through a valve hooked to a “Tee”. The other side of the Tee supplies pressurized water at full flow rate, while a separate valve controls the recirculation flow, alternately you could just use a float valve in the slow sand filter.

    Unless you use water continuously, some kind of storage system is a good idea. If you look at the flow rate of filter 1, you will see that it is quite slow. That is because the filter has not been cleaned (wet harrowed) for 3 years. This is on purpose. I want to know how long a filter can go and how clean the water is throughout time. So far, the only disadvantage I have found is a reduced flow rate. I may well be possible to operate these fiters for years without cleaning (wet harrowing) them – I don’t know. 3 or 4 of them in parallel will probably give you all the water you need, and depending on the turbidity of your source water, they may only need cleaning very infrequently. At 10 liters per hour minimum, 4 of them would give you 40 liters per hour. If you choose to wet harrow them often, the flow rate would be much higher. You might want to set up a test filter to see what happens with your water source. Do a few coliform tests at different intervals. In all of the literature I have read about large scale slow sand filters a small test filter is run for 1 season, before building the main system.

    I might add, recently we went through Salem, Oregon where they use slow sand filtration and the water was absolutely wonderful. Too bad more places don’t use it.

    One of the things about using a pond, is that toxic algae may form and a slow sand filter will not remove all of the toxins from all types of algae. I quote a study about that on the website. You might want to check around in your area to be sure there is no toxic algae in the waters surrounding. Here in the pacific northwest part of Washington state toxic algae is an issue and must be considered. Also, be careful of pool liners, Swimming pool liners have embedded chemicals that kill bacteria and should never be used to line water supply systems. Be sure your liner is awwa or ansi approved for potable water systems.

    If you have any questions you can reach me at this email. I must warn you however, that I am about to start another filter project / video to add to the study, and may not be able to answer quickly.

    Be sure to keep this in mind:
    This blog, and the 3 websites associated with it, and any communication with the author do not make water safe to drink. All of the information here and on the websites and any information implied or otherwise inferred through communication with the author does not make water safe to drink. Use the information here at your own risk. The author’s opinions or statements may be in error, as may the comments of others. Use them also at your own risk. Be advised that anything can be in water and every situation will be different. The local health department is the final authority on potable water.

  3. Todd Burleson says:

    Wow, the internet continues to amaze me! Thank you so much for your detailed and very helpful site. I am a resource center teacher and classroom teacher for 20 years. I’ve been involved in a magnificent project called Trout in the Classroom. In fact, this is my second year and our project is going splendidly. You can read about it if you like here: Here’s why I am writing you; we are located just about a half mile from Lake Michigan and we do not appreciate the gift of the great lakes as much as I wish we did. We are currently re-using 30 gallons of water from our trout tank in a vessel that holds the water. What I would like to do now is filter the water so that we can eventually use the water in the trout tank again. I’ve explored many smaller versions of filters, but I haven’t found one that is somewhere between the 55 and a 5 gallon size. I’m wondering if you would have any guidance for me in this area? Part of the idea is that I want to be able to have the students help me build and maintain the filter too.

    I am so grateful for your site.

    Thank you,
    Todd Burleson

  4. admin says:

    Probably, of all the filters, the pond filter would be of most interest to you Todd. I did have fish in the pond for some time and the filter was doing a very good job of keeping the water clean. The pond is filled with lots of stuff, rotting leaves, bird droppings, and many things organic. The filter keeps the water crystal clear and the test came back less that 2 coliform per 100 ml of water. Considering the amount of decaying / decayed material that enters that pond, it is amazing that the filter keeps the water so clean. The container is a 45 gallon barrel and the sand is .45 mm effective size filter sand. This filter could probably be built by elementary school children with some supervision. The baffle is a flat rock just sitting on the surface of the sand. Maintenance could consist of occasionally wet-harrowing and checking the turbidity and water temperature. Whether or not this type of filter would work would be an interesting experiment. So far, it has worked for me here, but there is a lot more testing to be done, and I must find a way to keep the raccoons from eating my fish!

  5. Mark says:

    I thought I would post a follow up, I’m getting my sand from a place nearby called Best Sand in Chardon Ohio. A retired plant manager works with me in my local school district so he hooked me up and that’s where I’ll be getting my sand from.

    This plant manager also was involved in a project: and when I was asking if he knew of a place I could buy the sand I needed he was completely surprised in the similarities of what I was doing and this other organization he was involved in. His plant provided the sand for this organization as well as the city of Cleveland’s water filtration.

    Take a look at the Aqua Clara site and you can see on the front page how they have some kind of pre-made sand filter for use in poor countries.

    The guy I worked with said they use something on the very top layer that is supposed to be better for the biological layer but he couldn’t remember much more than it was a very fine material. Any ideas what they may be using for a better bio layer?

  6. RBE says:

    Dr. Manz of the University of Calgary put out info about the sand he used in his filters. I guess his final design used a 4 layer approach.

    He mentioned something about fine pool filter sand as the top layer I believe. Something with a higher quartz content. Quartz has “electrical” properties that may be of benefit in a filtration application. I didn’t find it in his current info, he redesigned his site and moved things around:

    Good stuff there. He has all the hard technical data about flow rates etc.

  7. Rodolfo says:

    Regarding the design that you propose, why the pipe has a certain
    height? whats the pressure needed for the water can follow up the
    way, and what would be the water pressure at the bottom ?

    i like the design but i would like to test giving a certain height
    to the barrel and putting the pipe at the bottom too,

  8. admin says:

    Rodolfo, thank you for the questions. As I understand it you are asking about the slow sand filters. The output pipe coming from the slow sand filters extends up to 3 cm above the top of the sand. This way the sand is always covered with water. The sand must be under water at all times because the biological activity is aquatic and must be in water to survive. The pressure of the water in the filter caused by its weight pushing down (due to gravity), makes the water in the output pipe rise up to an equal level as the water inside the filter. As more water goes into the filter it forces water out the output pipe until the levels are equal again. (water always seeks its own level). This water seeking its own level in the filter does two things:

    1. It helps to slow down the flow of water through the filter; because if the water flows too fast through the filter it does not get purified.
    2. It maintains a 3 cm water level above the surface of the sand and it is self-regulating.

    In the storage barrels the output could come from the bottom of the barrel and the barrels could be up higher for more pressure. In the slow sand filters, the water could be allowed to flow out the bottom, but the control mechanisms would be expensive and complex to maintain, because the flow in must not be less than the flow out or the sand will be exposed to air and this will kill purification biolayer.

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