Clean water: Courtesy sand and microbes

Biosand filter (Source: CAWST)
Biosand filter (Source: CAWST)

Every year, nearly 600,000 children in India die of illnesses associated with unclean drinking water [1]. Inspite of this, 2 out of every 3 households still do not treat their drinking water [2] and half of the rural water supply, where 70 percent of India’s population lives, is routinely contaminated with toxic bacteria.

People are aware of the dangers of contaminated water, but usually do not have the means or convenience to obtain potable water. The fuel needed to boil water may not be affordable, a safe water source may not be an option at all and running a RO (reverse osmosis) system at home, may be both impractical in terms of wastage as well as economics. And then there is the increasing evidence on its effect on health. So most families continue to collect their water from contaminated sources or unreliable tap water, concerned about the safety but unable to do much about it.

A cheap, convenient solution to this nagging everyday problem of unsafe water is the biosand filter, an inexpensive filter to make water safe and potable.

What is a biosand filter?

It is a simple filter that uses sand, gravel and living micro organisms to clean up the water. A compact, concrete or plastic body is filled with layers of prepared sand and gravel. These layers remove the pathogens and suspended solids from the contaminated drinking water. At the top 2 cm of the sand layer, is the bio layer or ‘ schmutzdecke’ (German for dirt cover), which is simply a colony of bacteria and micro organisms, that gobble up most of the pathogens in the water.

How does the filter work?

Water from any source, be it wells, lakes, rivers etc. is poured into the top, and slowly drips through a diffuser plate. This plate is simply a protection measure for the sand and biolayer below, which prevents them from getting damaged. The water flows through the layers, getting filtered on its way down. A tube is embedded at the bottom of the filter, from where gravity pushes the water up the tube. The treated water is then conveniently collected on the outside of the filter.

How is it different from the slow sand filter?

The biosand filter is a slow sand filter, modified for household needs. Both have a bio layer, even though the thickness of the sand layer varies. Water must flow costantly in the slow sand filter, while in the biosand filter, there must be a pause period during which the water sits still, and hence the biosand filter doe not need a constant source of water. 

How much time does it take to clean the water ?

It takes about an hour to obtain 12-18 liters of filtered drinking water. It produces sufficient clean water for a family of 10-12 members, and even for a school of about 70 students.

What are the advantages of using this filter?

  • Compact in size & shape and easily fits on the kitchen slab
  • Needs no power, works under gravity and so a boon for rural areas
  • Water from any source- borwell, pond, open well, river, tap or even rainwater can be purified
  • Very effective in removing turbidity and pathogens
  • Can be locally constructed
  • Has a long lifespan: Concrete filters 30+ years & plastic filters 10+ years, even though lids and diffusers may need to be replaced
  • Simple maintenance to clean sand when the flow rate slows down
  • Ease of use
  • No operating cost

What is the cost of these filters?

Initial purchase cost: In the range of Rs 2200-Rs 2500

Adding the transportation costs, it roughly ends up to a total of Rs 3000/ filter.

Where can I purchase it from ?

These filters can be constructed locally, using the CAWST construction manual. They may also be purchased from the following organisations:

In India, around 37.7 million people are affected by waterborne diseases annually, 1.5 million children are estimated to die of diarrhea alone and 73 million working days are lost due to waterborne disease each year [3]. It’s time we took a look at biosand filters and skew these numbers in our favour.

The FAQ and presentation in Hindi by Lalit Wadher is an added resource that can be used to reach a larger group of people.