Overview

A biosand filter is an inexpensive water filter that can be made from locally sourced materials. It does not use any electricity and is low maintenance. The container can be made of a variety of materials such as cement, plastic or aluminum. The sand and gravel need to be cleaned and dried three times before being put in the container and it can take up to a month for the biolayer to establish. It uses multiple sand layers of different grain sizes to filter and clean the water. Biosand filters can be made to collect grey water or be connected to a rainwater harvesting system. Grey water is relatively clean water from sinks, shower/tubs, and washing machines (not toilets) that may contain small amounts of household cleaning products, food, and grease from food. If grey water is going to be used then the water should only be used for irrigation or flushing toilets and is NOT safe for consumption. If only rainwater will be used it can be safe for consumption after it is disinfected with: chlorine (liquid or tablet), boiling or setting the water in the sun to disinfect.

How does it work?

The untreated water comes in through the top, using only gravity the water slowly goes through each layer of sand. When the water first enters the container it goes through a diffuser that protects the biolayer from being disrupted. The biolayer is an important feature because the micro-organisms that live there will consume some of the pathogens in the untreated. The fine sand and coarser sand also removes some pathogens and suspended solids. Suspended solids are dirt and other small particles that can make the water look cloudy (also known as turbidity). The gravel at the bottom supports the sand and prevents the sand from getting into the effluent PVC tube. The clean water can then be linked to an irrigation system or collected for another use. There are four ways in which pathogens and suspended solids are dealt with in a Biosand filter:

1. Predation: the micro-organisms in the biolayer will consume some pathogens
2. Trapping: bigger pathogens will get trapped in between the sand and then die due to lack of air and/or nutrients
3. Adsorption: smaller pathogens will get stuck to the sand and then die due to lack of air and/or nutrients
4. Natural Death: as stated above, pathogens can simply die from lack of air and/or nutrients

Benefits

• Low cost to make and uses local materials
• Uses gravity instead of electricity
• Will lower the amount of water needed for irrigation

Limitations

• Depending on the water put into the system it might not be suitable for consumption
• Must be in constant use or else biolayer will dry out and die
• Can take up to a month for biolayer to form

Activities

Below is an easy soda bottle sand filter that can be done in a classroom or at home. It can help students visually see and understand how the water gets cleaned. The soda bottle can be bought or found in a recycle bin. Apart from the bottle, scissors and sharp object such as a nail will be needed to cut the bottle and make holes in the cap. The sand and gravel should be washed beforehand.