Biosand filters  are a simple water and sanitation technology that is active around the world.  They are an innovation on the old slow sand filters. There are roughly 650,000 biosand filters active in 55 developing countries. They are perfect for household water treatment because of how well they work and they are easy to make. Also, they don't need much maintenance compared to other types of water purification methods.  

There are two main types of biosand filters. There are concrete biosand filters, which have a container made of concrete and there are also plastic containers. Both are equally fine, but concrete is much more durable, easier to produce, and are therefore more common. Sand and gravel make up most of the filters. When dirty water is poured at the top of the filter, the water cleaning process begins.

The water first goes through a diffuser plate with holes in it. This plate spreads the water evenly and protects the sand below it. The high water level pushes the water down through the filter. Below the diffuser plate is a biolayer. This layer has good bacteria that eat disease causing germs in the water. This biolayer lays right on top of the sand and acts as the first line of defense for cleaning.

When the water reaches the sand, the germs start to become trapped, absorbed and die. As the water goes down to the bottom of the container, it meets two layers of gravel. These gravel layers support each other and the sand and make sure no sand is in the water. The drinking water then collects at the bottom and pushes up to the outlet tube through gravity.

Biosand filters have been studied on how well they kill germs. Many studies have shown that biosand filters can remove the following:

* 100% of worms and their eggs 

* 99.8% of protozoa

* 97-99% of bacteria

* 95% of viruses

These findings show that the water quality from biosand filters is high quality. The filters can also remove turbidity and heavy metals like iron. They can't remove dissolved chemicals like salt, arsenic or fluoride though. Also, the filters do not work well in cold climates. The freezing conditions slow down the flow rate, meaning the filters need a constant flow of water when its cold. Also, less bacteria dies based on how cold it is. Besides that, the water always comes out clean and odourless.

As a unit, biosand filters are sustainable because they do not use electricity or fuels to work. The method that cleans the filter is the 'swirl and dump' method. It's as easy as swirling the top layer of sand and dumping any dirty debris. This fixes the flow rate of the water and only takes a few minutes to do. A biosand filter can last a lifetime if it’s maintained right as none of the parts ever need to be replaced.

Overall, biosand filters are an affordable water and sanitation method for people in developing countries. You can learn more about this type of water filter and how they work from GROSCHE