Water Sand Filter - A Low-tech Yet Effective Way of Purifying Water

In developing countries, one of the preferred methods of purifying raw water to make it drinkable is the water sand filter, or slow sand filter.

This type of water filter is quite similar to the way that water is naturally purified in the earth. Ground water passes through many layers of sand, gravel and porous rock before it reaches the water table, which is like a massive natural storage tank within the earth’s crust.

In countries that do not have access to high-tech methods of purifying water, sand filter tanks do the job quite well. In fact, this method of water treatment is not limited just to developing countries; it is also currently being used in the UK, to treat all of the water that is supplied to the city of London.

The design of a water sand filter is relatively simple. It consists of a large tank containing a 1-2 metre deep bed of sand. Large water treatment facilities may have a dozen or more of these beds, while a smaller community might only have one or two. The bottom of the bed is made up of a series of drains covered by a base layer of fine pebbles, then another layer of coarse gravel. On top of this are several layers of sand and a thick layer of particularly fine sand at the top. A layer of unfiltered water lies on top, slowly seeping through the layers of sand.

What makes a water sand filter system unique is that it is a completely natural and organic method of purifying water. The secret to its success is in the Schmutzdecke, also known as the hypogeal layer. The Schmutzdecke is a layer of gelatinous biological material that forms on the very top of the sand during the first few weeks of operation. It is made up of a mixture of organic matter including fungi, bacteria, rotifera and protozoa as well as various aquatic insect larvae. As this layer ages, additional life forms develop, including algae and even larger aquatic creatures such as snails and worms.

It may not sound very hygienic, but it is this layer of aquatic life that is responsible for purifying the water. As water passes through this layer, foreign particles are trapped and essentially eaten by all of the little life forms. As the water slowly trickles down through the layers of sand, impurities are left behind, leaving the water between 90% and 99% free from bacteria. Because sand filtration requires no electricity or machinery, its operating costs are low and maintenance is minimal. This makes it ideal for impoverished or remote areas.

There are a few disadvantages to this method of water treatment. One of them however, is the slow speed of the process. Most facilities use holding tanks or reservoirs to store water to avoid running out during times of high demand, but many large cities have opted for faster, more high-tech methods of treating water. Slow sand filtration also requires a large amount of land, which is something that many cities do not have to spare.