The best thing about using slow sand water filtration is the fact that it utilizes biological processes in non-pressurized systems in order to produce clean, palatable water.
There are no chemicals and no electricity used in the process, so it is very green.
In addition, they are so simple to construct that it can be done as a do-it-yourself project and only require a small bit of operator training to operate and maintain once they are installed.
Slow sand filters have been used successfully in countries, such as in isolated areas of Afghanistan, where water quality is low.
This can be the simplest, least expensive, and most efficient method of water treatment for many poor areas of the world.
Slow sand filters are typically employed to clean surface water.
They do require a large space in which to operate.
These rectangular or cylindrical filters are generally 1 to 2 yards deep and operate very slowly in order for the biological processes to work their magic.
They offer a constant flow of clean water that is stored in tanks for when it is needed.
Although these filters are primarily used in developing countries, they are sometimes found in very modern regions of the world, such as London.
Larger facilities that serve more people can have as many as 12 sand beds operating at the same time.
Each bed lies on a series of herringbone drains covered with layers of pebbles and coarse gravel.
Layers of sand are put onto the bedrock, and then the rest of the space is filled with very fine sand.
In fact, the majority of the sand filter is made up of fine sand.
The bed is then topped by a layer of raw, unfiltered water.
Within the first 10 days of operation, a gelatinous layer forms in the fine sand.
This layer is made up of protozoa, fungi, rotifera, bacteria, and miscellaneous larvae of aquatic insects.
As the layer continues to age, it spawns more algae as well as larger organisms, such as snails and worms.
It is this layer that filters the clean water and makes it safe to drink.
The process actually can reduce bacteria in the water by as much as 90 to 99%.
When the filtering layer becomes too full of contaminants, it needs to be overhauled in order to keep a constant flow of water running.
In some cases, the gelatinous layer is completely removed and then more water is introduced so that the layer will start forming again.
This takes a considerable amount of time Wet harrowing is another, quicker method which requires lowering the water level until it's just above the gelatinous layer and then stirring.
This process causes contaminants to become suspended in the water where they can be washed away before new water is added.
In the United States, slow sand filters aren't being used as widely as they were in the past.
Mainly this is due to the fact that they require so much land on which to operate, and as cities expand, they can no longer devote all of this land to water filtration.
Slow sand filtration is, however, still one of the most effective ways on earth for the purification of water.