Rain water harvesting and slow sand water filters

Filter 5 was frozen solid and not running for 2 weeks in January of this year (2013). Before that for 5 days the filter froze at night, and then ran for 6 hours each day. As of Jan 25 the filter was running 24 hours a day. The samples were taken on the 25 th. The pre-filter water was taken from the water in the top of the filter – the water from the source that has been running through it for 3 days. The post filter water was taken from the output pipe of the filter . The images below confirm that being frozen for 2 weeks causes this filter to produce poor quality water. Another test will be done in 2 weeks, providing the filter does not freeze again.

These images show the reduced abilty to remove pathogens due to the filter being frozen. This test is from filter 5. The colored dots are ecoli, and coliform bacteria. This test was done the same day the filter started flowing again.

These are the results of the most recent tests on filter 5 Feb. 19 2013

All but two of the filters have been frozen solid for 2 weeks (15 days), with the exception of the two filters that purify shallow well water. The water that runs through them has been running continuously and was still running as of last night (Jan 20). These filters are outside where the temps have been 25 degrees F and lower (down to 20 deg. F) for the past 15 days. There has been some warming during the daytime, but not enough to thaw out any of the other filters. The freezing weather started affecting the filters on the 3rd of Jan 2013 – they would freeze at night and run slowly during the day. The two filters (filter 2 and filter 3) that run on shallow well water have been running non-stop, and were not slowed down by the freezing weather (the water underground is about 48 degrees year round). When the freezing weather ends (possibly by next week), we will do a test to determine how well the filters function after being frozen for an extended period of time, after they start flowing again – that may take another week following the end of the freezing weather. As of Jan 25, all of the filters are now thawed out and flowing. The temp got up to 47 degrees F here today.

The filters we work on here in western Washington state have been in operation outdoors through temperatures as low as 15 degrees F (at night) for as long as 1 week, with below freezing (32 degrees F) continuously for 7 days. During these times, the filters did freeze up solid and the water stopped flowing. When they thawed out, they resumed functioning without any major damage, after one modification. At first, the pipes that were glued cracked. After that happening the first year, we started putting the filters together without gluing the pipes. When the filters freeze the pipes just slip apart. There is one exception here. The filters that purify shallow well water did not freeze up as long as they continued to run. This is because the water 10 feet down,here, is way above freezing, and it is warm enough to keep the filters from freezing up, as long as it keeps running. As long as your source water is warm enough to offset the below freezing temps, and your output pipes from the filter don’t freeze up, the filters will keep running, and will purify water at 30 degrees (with somewhat reduced efficiency) . The problem we have here is that the output pipes freeze. That stops the flow and then everything freezes up. Below 15 degrees F – I do not know what would happen. All this said, I must stress that we do not have severely cold winter weather here for extended periods of time (at this specific location) as a general rule; and we have not had the opportunity to test these filters for more that 7 days of continuous below freezing temperatures. If you are in Alaska, the Yukon territory, or north eastern Canada or the northeastern part of the U.S. your winters will most likely be much colder and more severe than here.  Keeping a slow sand water filter running in Siberia, or any mountainous area where there are severe temperatures below freezing will be a real challenge. If the filter can be kept above freezing and water continuously running through the filter it will survive; but that might not be possible. Anyone who has information, or experience,  about running one of these filters in extreme blow freezing temps will hopefully post here.

There are more and more websites and YouTube videos appearing on the internet showing how to “build your own slow sand filter”. Some of the videos have what appear to be very impressive results; but be cautious. The correct functioning of any slow sand filter is totally the responsibility of the owner / operator no matter who designed it, or sold it, or posted it on the internet. There is no way to be 100 percent sure any “do it yourself filter” will work.  There are some things you can do, however, to increase your chances of putting together a functional filter. First of all, do some research on how a slow sand filter works. Any public library will have the information you need. Then look for designs.

The most important question to answer first when deciding which design to use is this: Has the person showing the video, or publishing the website,  provided evidence of epa certified tests on the input and output of the filter that show it is working? Of course, anyone could forge results; but if the name and/or names of the person or persons doing the testing and the name of the lab providing the results are not shown then don’t trust the information. You should be able see the test results with the name of the Laboratory that did the testing and when the testing was done, and who had the testing done.  Another important question to ask is “how long has the filter been in operation?” It takes at least 1 year to adequately test a new slow sand filter design because each situation is different, with different amounts of pollutants in water at different locations. Look for successful tests done in the summer, winter, fall and spring.

It is entirely possible to put a bunch of sand and gravel in a bucket and make it look like polluted water is going in and clean water is coming out. Look for the test results; if there are none, then the video or website is just good entertainment at best. Water can be very clear, and still be highly polluted and dangerously full of disease causing organisms. Combine that with the fact that every situation is different and what works in one environment may not work in another.

Having tests done on the input and output of any filter is the only way to be reasonably sure of the condition of the water output.

The temps have been 28 degrees F at night for the past 4 days now. All the filters have frozen up at night, and thawed out during the day. Last night, Jan 2/3 was the coldest so far, 25 degrees here. (It gets colder here than near Seattle, we are in the foothills at 600 ft. elevation). An attempt will be made today to do a coliform test on at least one of the filters in order to look at how well it works at freezing temps.

Still waiting for the winter to “cut loose on us”. If it does like it did last year, we’re in for it. Last year: 14 inches of snow, and 1/2 inch of ice on top of that. Power out for 3 days. 4 huge trees down here, hundreds of large branches too, much worse in other parts of this area. State of emergency declared by the Gov. .

Slow sand filters are totally sustainable, will work on gravity feed only, take out hydrocarbons, bacteria, viruses, and do not require addeed chemicals to operate. Check out the websites below. And check out the post just before this one if you want to see just how well a slow sand filter removes bacteria.

There are lots of websites, and YouTube videos showing detailed rain water harvesting systems, but very few include any mention of a slow sand filter. For the most part, websites for state governments fail to show any references to slow sand filters. They are so incredibly simple to set up. A container, some sand and drain pipes and mother nature does the rest. This has been happening since life began on earth in natural settings where water is filtered as it flows through sand beds, and more slowly through rock.

Set up a small slow sand filter on just one of your downspouts and let it run for 3 or 4 months and you will have very clean water. Check the websites for details. We have been doing this here for 7 years now – the websites have documentation. At the very least let the water that runs into your rain barrel come through a slow sand filter first, then the water will not go stagnant or be full of nasty bacteria. Check out the websites.

Here are three websites to check out with information about rain water harvesting with a slow sand filter:

Slow sand filter

Roof water harvesting

Biological sand water filters

And here are more:

This is a fantastic site that really shows how effective small slow sand filters can be

And yet another site here

And one more here

Just Google slow sand water filter, or biosand water filter, or biological sand water filter. All you civil engineers out there designing rain water harvesting systems, why not take a look at small slow sand water filters? They work.

Filter 5, the most recent filter set up here in May of this year is now working.  The drainpipes inside the filter at the bottom of the barrel are covered by 4 inches of pea gravel. The sand directly on top of the pea gravel,  on the bottom, is .25 mm effective size with a U.C. of less than 2; and  700 lbs was used. The top sand is .15 mm effective size with a U.C. of less than 2; and 150 lbs was used. The supernatent water depth is approximately 8 inches. The white bucket on top is a 5 gallon reservoir that feeds 1/2 inch pvc drain pipes with small holes drilled in them. The drain pipes are inside the barrel at the very top just under the lid.
This filters roof water from a composition roof. The sand surface area inside at the top of the filter sand is: .2452 sq. m. .  The container depth 40 inches. The filter was started 2012-05-01.

This is a picture of filter 5, after the series of rain storms and wind storms that have passed through here Dec. 2012.

The results of the most recent field test on this filter are shown below. Please note: this is a field test and was not done by an epa certified lab. We did this test here using prepared solution from Micrology Laboratories..

The pink dots are coliform bacteria, the blue/purple dots are ecoli bacteria. “Pre-filter” means the test was done using water taken right out of the top of the filter’s supernatent water source, that is water that will be running through the filter on an ongoing basis. “Post-filter” means the test was done using water taken directly from the output pipe of the filter.

These are the field tests done on filter 5 as of December 21, 2012. The input and output water was tested for the presence of coliform and ecoli bacteria.

This is yet another update on what has been learned regarding the construction and operation of a small slow sand water filter.

These are issues that stand out after experience building, testing, and operating small slow sand filters over the past 6 years:

1. Fine sand (.15 mm effective size) allows for high quality water output both chemically and biologically.

2. Fine sand results in slow output flow rate.

3. Fine sand becomes clogged very quickly if the input water is turbid.

4. Coarse sand (between .35 mm and .50 mm effective size) results in high flow, and lower water output quality.

5. Coarse sand does not clog as quickly as fine sand.

6. Sand used in a slow sand filter will work best if the grains of sand are all very close to the same size (a low uniformity coefficient of 2 or less)

7. Use very coarse sand only at the bottom of the filter (the bottom 5 or 6 inches in a 30 inch tall container) the rest of the sand should all be much finer (smaller grains) sand

8. Roof water quality will vary drastically depending on the time of year, the condition of the roof surface, and the physical location of the roof surface with respect to trees, industry, and wildlife.

9. Some sort of pre-filtering is absolutely necessary if roof water is being filtered by a slow sand filter. A wire screen followed by a first flush diverter is one way to pre-filter roof water. A “settling container” is another way to “pre-filter” roof water.

10. Some sort of re-circulation of output water back through the filter is helpful when filtering roof water.

If roof water is being filtered:
11. Know that water quality will vary considerably
12. Have some sort of pre-filtering in place and,
13. For high water quality; use fine sand and recirculate the output water through the filter at least once
14. Use some sort of secondary purification process such as uv sterilization, boiling, or chlorination

If a slow sand filter is cleaned by “wet harrowing” :
15. be absolutely certain to stop the output flow while the filter is being cleaned by wet harrowing. If the output is not stopped, the contaminated water will flow down into the lower areas of the filter and foul the output. This is not good.

Some of the water quality tests done here on filter 4 and 5 are done. These tests were done with the testing kits available from Micrology Labs. The interpretation of the results of our tests are posted here along with images of the actual results we obtained. Please see the Micrology Labs website for a detailed explanation of the interpretation of these results. There is some math involved. We used 5 ml sample sizes, so the results we obtained by counting must be multiplied by 20 to find the cfu’s per 100 ml. The most important issue here is to note that the results for the pre-filter water showing nearly solid color represent super-highly contaminated water where there are so many colonies forming that it is not possible to count them without magnification. Also note that this is AFTER the water has been initially filtered by a first flush diverter. We have not tested water directly off of the roof surface yet. The number of cfu’s per 100 ml in the super-highly contaminated water turns out to be in the hundreds of thousands of cfu’s per 100 ml. Fortunately, both filters show a log reduction of coliform at slightly over 2 (there are 100 times less coliform bacteria in the output water of each filter.) It is possible to see, however, that the output still contains some coliform bacteria. Note that the ouput of fitler 5 contains no ecoli bacteria (dark blue-greenish colored dots with no pink around them – the blueish stain in the filter 4 output cannot be confirmed as an ecoli colony, although it may be). The filters are working, but we would like to see less bacteria in the output. More tests are upcoming as time permits.

In the images below of the tests of water samples shown please note that these tests are not done by an epa certified laboratory. They are field tests done by a non-professional with some lab experience. Some error is highly likely. What these tests demonstrate is whether or not the filters are starting to remove bacteria from contaminated water. These tests are only a general indication of how well the filters are working. To get a specific accurate number, a test must be done by an epa certified lab, and the sample must be taken by someone with some lab experience and some experience in taking water samples.

The next test will use a 1 ml sample for the pre-filter, and a 5 ml sample for the post filter. We have used the 5 ml sample on both for this test so the images can be compared with validity. If we showed the 1 ml sample size compared to the 5 ml sample size for a visual evaluation, it would be inaccurate and misleading.