BIOSAND FILTER
BIOSAND FILTER
What is it?
The Biosand filter is an innovation on traditional slow sand water filters, having been specifically designed for intermittent use.
Slow sand filtration has been used for centuries
The filter can be produced locally anywhere in the world because it is built using materials that are readily available. It is simply a concrete container, enclosing layers of sand and gravel whose purpose is to eliminate sediments, pathogens and other impurities from the water.
How does it work?
Water is poured into the top of the filter as needed, where a diffuser plate placed above the sand bed dissipates the initial force of the water. Traveling slowly through the sand bed, the water then passes through several layers of gravel and collects in a pipe at the base of the filter. At this point, the water is propelled through plastic piping encased in the concrete exterior, and out of the filter, for the user to collect.
How does it remove contaminants?
As with all slow sand filters, the removal of pathogens occurs in the BioSand filter due to a combination of biological and mechanical processes.
When water is poured into the top of the filter, the organic material it is carrying is trapped at the surface of the fine sand, forming a biological layer or ‘schmutzdecke’.
Over a period of one to three weeks, micro-organisms colonize the schmutzdecke, where organic food and oxygen derived from the water abounds.
Four processes remove pathogens and other contaminants in this filter:
- Predation
- Natural death
- Adsorption
- Mechanical trapping
Predation
The schmutzdecke micro-organisms consume bacteria and other pathogens found in the water, thereby providing highly effective water treatment.
Natural death
Pathogens are removed due to food scarcity and less than optimal temperatures.
Adsorption
Viruses are adsorbed (become attached) to the sand grains. Once attached, they are metabolized by the cells or are inactivated by antiviral chemicals produced by the organisms in the filter. Certain organic compounds are also adsorbed to the sand and thus removed from the water.
Mechanical trapping
Sediments, cysts and worms are removed from the water by becoming trapped in spaces which lie between the sand grains. When precipitated, the filter can remove some inorganic compounds and metals from the water.
How effective is it?
Slow sand filters have been proven to almost entirely remove the disease-causing organisms found in water. The Biosand technological adaptation of slow sand filtration has proven as effective as traditional slow sand filters, in both laboratory and field tests.
In conjunction with the introduction of the technology to communities, the filter has been tested by various government, research, and health institutions, as well as by non-governmental agencies.
Overall, these studies have shown that the Biosand filter removes:
- More than 90% of fecal coliform
- 100% of protozoa and helminths
- 50-90% of organic and inorganic toxicants
- 95-99% of zinc, copper, cadmium and lead
- < 67% of iron and manganese
- <47% of arsenic
- all suspended sediments
To see the results of the studies on the Biosand filter, visit Summary of all lab and field testing
To read anecdotes about the Biosand filter application, visit Anecdotal evidence.
How easy is it to use and maintain?
Operation
Operating the filter is very simple: remove the lid, pour a bucket of water into the filter, and immediately collect the treated water in a container.
Because of its smaller surface area, the filter can produce up to 60 litres/hour.
The following design features of the Biosand filter ensure its ease of use:
- The water needs to be filtered only as required.
- The concrete Biosand filter is devoid of moving parts.
- The concrete container is small, but extremely stable. Therefore, it can be placed anywhere in the home, in the location most convenient to the user.
- The plastic piping is encased in concrete and is not easily damaged.
Between uses, a layer of water (5 – 10 cms deep) is maintained above the sand at all times. It is this design feature that distinguishes the Biosand filter from other slow sand filters and which allows for both small scale construction and for intermittent use. This layer of water is shallow enough that oxygen can diffuse through the whole layer therefore providing the biological layer with enough oxygen to develop.
The biological layer typically takes one to two weeks to develop to maturity in a new filter. Removal efficiency and the subsequent effectiveness of the filter increase throughout this period. Although the filter does remove more than 90% of bacteria, which means the level is often below the infectious dose, it is recommended for people with infants or elderly relatives that they use disinfection (link to that section) after filtering the water.
Maintenance
Continued use of the filter causes the pore openings between the sand grains to become clogged with debris. As a result, the flow rate of water through the filter decreases.
To clean the filter the surface of the sand must be agitated, thereby suspending captured material in the standing layer of water. The dirty water can then simply be removed using a small container. The process can be repeated as many times as necessary to regain the desired flow rate.
After cleaning, a re-establishment of the biological layer takes place, quickly returning removal efficiency to its previous level.
How much does it cost?
Capital
The cost of a concrete filter has varied from US$ 10 to US$ 30, depending upon the country in which it is being manufactured and the amount of paid/volunteer labour available.
The concrete filter utilizes the same technology as the commercial plastic filters produced by Davnor. However, using concrete to build the container costs less for several reasons:
- Cement is readily available in most developing countries.
- People are familiar with the construction techniques used.
- Household labour or volunteer labour can be utilized in the manufacturing process.
- The concrete container is heavy and durable. It does not need to be replaced as often as a plastic container.
- The plastic piping is located inside the filter. Consequently, it is less prone to damage than a plastic filter, which has piping on its exterior.
Operation
As consumables are not required for successful filter operation, the operating costs are negligible.
Advantages and inconvenients
Advantages
- Removes over 90% bacteria, 100% parasites
- Removes turbidity
, some iron, manganese, arsenic and mercury. - Quality of water improves with time
- Sells for US$10-30.
- High flow rate – 60 liters/hour
- No on-going costs – no replaceable parts
- Durable & robust- lasts forever
- Fabricated from local materials
- Opportunity for local businesses
- Water tastes & looks good
- Easy to maintain
Inconvenients
- Heavy – difficult to move
- Biological layer takes 1-2 weeks to develop to maturity
- High turbidity (> 100 NTU) will cause filter to clog and require more maintenance
- Requires that the filter be used periodically on a regular basis
- Cannot remove color or dissolved compounds
What difference can it make to the poor?
Water is a resource vital to life, good health, and the development of human capacity.
Over the past few years, much money has been invested into the drilling of groundwater wells, as groundwater generally requires minimal treatment. However groundwater often contains contaminants which are difficult to remove and the development of affordable treatment processes – of which the Biosand technology is an excellent model – significantly expands source options, including the valuable ability to consider surface water sources, which can be more affordable and convenient to the consumer. Such technology also ensures greater reliablility of water quality within each home. This is crucial, as water contamination in the aquifer, around the wellbore, or in the household is a frequent occurrence. Increasing the volume of clean water supplied by investing in low cost treatment of existing water sources, rather than in searching for additional ones, is both possible and advantageous.
The existence of a piped water supply to one’s home is a convenience that every householder should be entitled to. However, it is not financially feasible to provide this level of service within the foreseeable future to the 1.2 billion people who currently do not have access to clean water. Therefore, it is vital to consider that individual household treatment systems can be just as effective as community-scale systems. Moreover, the particular needs of the poor are such that individual treatment systems are more likely to be socially and financially sustainable as well as more efficient and more easily replicated than community scale systems.
Of all the household treatment systems identified and evaluated by CAWST, the BioSand filter is the most affordable and efficient, as well as being easy for people of the community to use.
How can I obtain a filter?
Country | Organization | Name | Address |
---|---|---|---|
Haiti | Clean Water for Haiti | Tal Woolsey | cleanwaterforhaiti@yahoo.com Tel: 509-547-3210 |
Honduras, Guatemala, Mexico | Friends Who Care | Rod McCaroll | fwc@friendswhocare.ca Tel: 800-728-5690 |
Uganda | ACTS | Geoff Owen | fallfactors@hotmail.com |
Dominican Republic | Asociaci�n de Fabricantes de Filtros | Jos� Rivas | Joseylali31@hotmail.com Tel: 579-7656 Cell: 727-9710 |
For all other countries, please contact Diana Frost at dfrost@cawst.org.
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