DIY Slow sand filters: More lessons learned

The most recent slow sand filter set up here (filter 5) has failed miserably. I’ve done everything right – or so I thought. There is a diverter in the line that supplies (supposedly) water with very low turbidity to the input of the filter. I’ve tested the output water and found it to be quite free of coliform, and ecoli bacteria. I’ve used the same design as the other filters here. I’m still checking this out, but for now it looks like the main reasons for failure are my own mistakes being specifically, lack of proper maintenance, and failing to be aware of obvious major changes in the surrounding area.


This filter had been working quite well; up until about 4 weeks ago – several weeks after we had the large fir tree right above it trimmed. The tree has shed needles and they became trapped in the gutter. Then, we had some very warm sunny weather, and this gave all that junk in the gutter time to decompose slightly with plenty of anaerobic action.  Then the weather turned cool, and the rains returned with a vengeance. The rotting organic material built up there and then flowed into the diverter, plugging it up and causing very turbid contaminated water to flow into the filter.  I thought the filter should be able to handle that. Wrong, very very wrong. The filter quickly plugged up and the flow was, for all intents and purposes, completely stopped. I tried a wet-harrow cleaning. It did not work, and the output became horrifically foul. I had to remove the top 3 inches of sand and replace it, and drain off all the water on top, which was filled with very nasty stuff. This is a complete failure. It is a good thing we were NOT using this filter to supply drinking water.

Why did this happen? I still do not have all the answers, but I can tell you the mistakes I made (mostly breaking my own rules I rant about all the time):

1. I did not monitor the diverter. The slow drip plugged up and the diverter stopped working, spewing excessive decomposed organic material directly into the filter.  The diverter design needs modifications for working in a highly contaminated environment.

2. When I wet-harrowed the filter, I forgot to shut off the output, as a result nasty water flowed down into the lower sand layer.

3. I did not keep a close watch on the gutter after the work was done on the tree above it. I should have known there would be issues with lots of debris falling from the tree’s branches.

4. The top layer of sand is .15 mm effective size. That is too fine for the location. It should be .25 mm at the smallest. Also, a roughing filter should be used in this location because of the excessive amounts of debris in the area.

5. I wrongly assumed that what worked well 200 feet away, would work right under the tree, too. (This is a huge tree – huge: 4 feet in diameter at waist height, and over 125 feet tall) There is lots of stuff in that tree. I failed to take that issue into consideration.

These filters work, and they work well – but they do have limitations; and each situation is different. Be sure to check out a potential location carefully. Do a “pilot test” for 1 year if possible, to determine the operating situation. When wet harrowing one of these filters, be sure to completely shut off the output flow until finished with the wet-harrowing, and the water on top of the sand is clear.

This all does not mean that slow sand water filters are not good. This post, and this entire blog is here for the purpose of learning. So far, these filters have worked well, if properly maintained and understood. All the other filters here are working without fail. It does look as though this failure is due to my own negligence. . .   but there may be other reasons?

Update; October 5, 2013:

After 5 months of testing and observing this filter, and filter 4 which also failed; it has been determined that maintaining a constant, consistent flow of water through the filter is the single most important aspect of operation. The failure to do this was the main reason for the failure.


More info is coming as I investigate this mess. If anyone has suggestions, please do post them here in the comments section.

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