Slow sand filter study: lessons learned
We were recently contacted by a local water district representative regarding setting up some slow sand filters for schools’ irrigation water supply. They wanted a “green” “sustainable” technology to filter water from their rainwater / roofwater harvesting system. In speaking with the representative, I believe I gave the impression that these filters are nearly infallible, and can be operated just like a rapid sand filter, or in a similar fashion to the complicated “under the sink” filtration systems often found in homes. This is only partially true. In some situations, these filters can be very high maintenance devices, and they are not perfect. I will make another attempt in this post to properly explain the small slow sand filters we are working on here.
Let’s summarize with an uncomplicated explanation of how to keep one of these filters working:
Don’t disturb the surface of the sand, unless you intend to “clean” the filter, by “wet harrowing”. Keep water, from the same source, flowing through them all the time. They cannot sit idle for more than 2 days. Keep water over the sand always. Never let the sand become exposed to air, that will kill the biological action. Be aware that these filters will not operate if they are frozen. Remember that contamination can come from anywhere. Know that these filters are good, but not perfect, and there are no absolute guarantees for perfect water output all the time.
More complicated explanation:
Know that these water filters are living mini-ecosystems, and physical filtration systems combined.
Know that rivers, streams, oceans, soil and every other place known to people on the surface of the earth has some sort of life in it, or on it. Water is of main interest to us here.
Know that all water, with the exception of distilled water, or medically sterilized water, contains living micro-organisms. Most of them are harmless to mammals, including people with healthy immune systems. It is because of these micro-organisms, mainly, that a biological sand filter will purify water. A small biological sand filter is very similar, in theory, to a wetland. We are all drinking recycled water. The earth cleans it for us – learn about this and you will have a much better understanding of how these small slow sand filters work.
Know that these filters (and most other slow sand water filters) depend on the microorganisms in the biofilm that forms on and in the sand (most biological action is in the top 3 or 4 inches of sand – but not only there; also throughout the sand bed), and also on physical filtration.
Please; please, do not confuse these filters with rapid sand filters, commercial water filters, or any other non biological means of water purification or filtration, or you will be severely disappointed. These filters are not a “set it and forget it” thing, and they will not work without attention from someone. Depending on the environment in which they work, they may need only occasional attention, or constant attention.
Know that these small slow sand filters are not, I repeat – not – perfect. Let’s go over that:
Know that these filters will improve the quality of water that flows through them, and they are capable of producing very clean water – if monitored
and operated with care. They are also capable of producing highly contaminated water if they are not properly maintained.
Water, containing oxygen, (most water contains oxygen) must flow through these small slow sand filters with only brief interuptions, this means all the time. There can be times of inactivity, but this is limited to one or two days. The longer the filter sits with no water flowing through it, the less effective it will be, and if left sitting idle too long, the filter will “go into a dormant stage”. Think of these filters as living mini-ecosystems, which, in reality, they are. The microscopic organisms in the biofilm on top of the sand must have oxygen and food, or they will either die, or become dormant.
The surface of the sand in these slow sand filters must always, be covered by oxygen containing water – always. If the surface of the sand is allowed to be exposed to open air, the filter biofilm will die quickly, and the filter will need to be restarted run and tested before it can be used again. If the filter sand dries out the entire filter may need to be emptied, the sand washed and sterilized, and the filter then re-filled with sand and restarted.
If one of these filters we describe here goes into a “dormant state” because water is not circulating through; it may be possible to “wake it up” by simply re-starting it, providing the surface of the sand stays under water. This process of “restarting” may take as long as 3 or 4 weeks to produce usable output water, depending on the ambient air temperature.
These filters will freeze up in the winter, and will not function when they are frozen.
Every situation is different. When a filter is set up, it must be tested for at least 2 months, and the site must be evaluated before the filter is put into operation.
These filters do not remove 100 percent of the contamination 100 percent of the time.
More later, as I get time.