How Does a Biosand Filter Work?

(updated from a 2013 blog)

While there are a number of water purification technologies in use all over the world, the technology most often implemented by Pure Water for the World is the Hydraid® biosand filter. The Hydraid® biosand filter is intended for point-of-use water filtration and can provide clean water for a family of eight to ten people for more than 20 years.

The greatest benefits of the Hydraid® Biosand filter (pictured right), for the work we do in very rural and remote communities, include:

  • Low in cost
  • Requires no power of any kind
  • No moving parts
  • Simple to use
  • Reliable; requires essentially no maintenance
  • Small enough to fit in any home
  • Lightweight
  • Virtually indestructible. Well suited to a school and/or home environment.
  • Long life (estimated >20 years) with no recurrent expenses.


Slow sand filtration has been in widespread use across Europe and the Americas for over 200 years. On an individual household scale, it is ideally suited for point-of-use filtration by rural, remote populations.

The intermittent slow sand filter shown here (left), the biosand filter, was developed by Dr. David Manz, of the University of Calgary, Alberta in the 1990s and adapted from the traditional slow sand filter. The biosand filter is smaller and modified for intermittent use, making it suitable for households.

The biosand filter has been extensively tested in laboratories at the University of Calgary, MIT and Dartmouth College, and has proven both economical and effective in thousands of field installations.

Initially, the PWW team implemented the original version of the biosand filter that was constructed of cement. It was heavy and cumbersome and logistically very difficult to get to the remote homes of families we serve. After Dr. Manz developed and patented the plastic Hydraid® biosand filter, we tested this version and found it to be much more suitable and effective for the locations in which we typically work.

The Hydraid® biosand filter is based on the concrete filter process but uses a plastic container, making the container itself much lighter and easier to transport. It is about the size of an office water cooler, constructed of medical-grade plastic, and filled with multiple grades of specifically selected and prepared sands and gravels.

Contaminated source water is poured into the top of the container through a diffuser plate that controls the rate of flow and prevents disturbing the valuable upper layer of sand.

Removal of most of the contaminants and pathogens from the source water takes place at the top of the sand, where a biological layer forms after being fed with the organic and inorganic materials that are present in the contaminated water.

This biological layer, or biomass, contains friendly bacteria. The biomass is a hostile environment for the organisms that cause human disease, killing most of the pathogens by temperature, competition for food, and predation (on a very small scale), while mechanically filtering them from the water. Biological destruction and mechanical filtration continue to a lesser extent down through the sand.

Water pressure forces the water through the sand and gravel, up the pipe and out into a clean container. The filtered water is free of contaminants, clear in color, clean in taste and smell, and safe for human consumption.

Many technical reports have been published attesting to the effectiveness of intermittent slow sand filtration systems. These reports confirm that, under optimal operating conditions, the biosand filter is capable of removing: 97% of fecal coliform, 100% of giardia cysts, 99.98% cryptosporidium oocytes, 100% of worms, 99.98% protozoan, 90-99% bacterial, 80-98% E. coli, 100% of parasites, and up to 90% of organic and inorganic toxicants from contaminated water.

Used correctly, in combination with proper hygiene education and training, this simple device can provide sustainable, reliable safe drinking water for a family of 8-10 people for well over 20 years.


To watch a beautiful video created for the Hydraid® biosand filter, click here.

For more detailed information, please see this document (Biosand Fact Sheet), created by our WASH partner, CAWST (Centre for Affordable Water and Sanitation Technology).

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