Rainwater harvesting, for the average homeowner, can be a sustainable solution to water conservation. It has been made legal in Washington state, and changes are happening elsewhere, for the good. Adding a filter to your rain water harvesting system is a good idea. A slow sand water filter is a sustainable solution.
I want to write mostly about slow sand filters here. These filters are often referred to by different names: Bio sand filter, biological sand filter, and Biological sand water filter. All this nomenclature actually refers to the same concept: water purification through biological means. These filters are actually mini-ecosystems, not unlike a wetland.
Let me digress a bit here. Think about this: all the potable water we have on this earth is recycled water. We drink the same water that was here when the first people walked the earth. We depend on the ecosystem of this earth to clean our water for us. Thankfully we are trying to clean up polluted water sources because we all care about this planet and each other.
Now back to slow sand filters. A brief simplified explanation of a small slow sand filter (a biological sand filter) follows. Picture a container, preferably a 55 gallon recycled food grade barrel, filled with clean fine grained sand and having gravel and drainage pipes at the bottom. The sand covers up about 2 inches of 3/4 inch pea gravel at the very bottom. In that gravel is a drain pipe system with holes drilled in the pipes small enough to keep the gravel out but let the water through. This drain pipe assembly is connected to a vertical output pipe. The output pipe runs up to just above the surface of the sand in the barrel. The surface of the sand is about 6 inches below the top of the barrel. Water, 6 inches deep, always covers the sand and flows through the sand bed to the bottom through the drain pipes with the help of water pressure created by gravity. Purified water flows out the top of the output pipe. Yes, this is true it works. After about 3 to 4 weeks of water flowing through the sand, a natural biological layer of micro-organisms forms on the top of the sand and in the sand. (All water with the exception of distilled water or sterilized water has micro-organisms in it.) In this biological layer, pathogens (bad bacteria and viruses) are either killed or eaten by friendly micro-organisms. (this same thing, on a much larger scale, also happens in wetlands, by the way). Petroleum derivatives are biodegraded, and heavy metals are significantly reduced, in much the same way as they are in nature. The key factor here is that the water flows slowly through the sand; slowly enough for the good bugs to eat and kill the bad bugs. Water will flow out of one of these filters slowly (about 35 liters an hour average), but will be very, very clean. Using a slow sand filter such as this is ideal for purifying roof water. Just set the filter slightly higher than the storage and every thing works with gravity.
Now, what can this “purified” water be used for? This is a bit tricky. These filters do not automatically make potable water, that is very important to understand. However, they do clean up the water enough to allow it to be used for nearly everything except drinking without creating a mosquito infested smelly rain barrel full of foul smelling water in the summer. The water that flows through these filters does not go stagnant if it sits in a storage barrel during the summer in the hottest months. I should mention that in remote areas outside of the U.S., small slow sand filters do provide potable water. It is important to note, however, that those filters have been professionally designed and tested at the locations where they are used, and the owners fully understand how to use them, and how to properly maintain them.
To summarize: Slow sand water filtration works. And can work on a small scale for the individual. A functioning filter can be put together by just about anyone. The degree of sophistication is only limited by the ingenuity of the owner / operator. The website I have linked to here has all the information needed to understand how put one of these filters together. The reason these filters work is because of what exists on this planet. It is there for all of us. Be cautioned, however, the water from any filter, slow sand filters included, must be tested and approved by a knowledgeable person before it is ever used as a potable water source.