Bio-Sand Water Filter Description
The bio-sand filter is permanent, affordable, simple to maintain, and can be produced from locally available materials. The efficacy of the filter has been tested extensively and is growing in use in the developing world. It is recognized globally as an effective means of treating water at the household level. When used most effectively it can provide 6 liters of clean drinking water per hour.
The filter itself is simply a concrete container, about 1 meter in height and roughly 1/3 meter in diameter, which holds layers of sand and gravel. Water is poured into the top of the filter, where a diffuser is placed above the sand bed which dissipates the initial force of the water. Traveling slowly through the sand bed, the water then passes through several layers and collects in a tube at the base of the filter. At this point, the water is propelled through plastic tubing encased in the concrete and out of the filter, for the user to collect in a clean water container.
How Does it Work ?
The filter works using a combination of biological and mechanical processes. The microbes which need to be removed are trapped at or very close to the surface of the sand, forming a biological layer. This biological layer matures over one to four weeks, depending on the volume of water put through the filter and the amount of nutrients and micro-organisms in the water.
Viruses are absorbed or become attached to the sand grains and after some time dies or is changed by the other microbes in the filter. Sediments, cysts, and worms are removed from the water by becoming trapped in the spaces between the sand grains.
There is a layer of water which is maintained above the biological layer at all times to allow for the intermittent use of this filter. The filter, when being used to most effectively remove containments, can at its maximum flow rate treat 6 liters of water per hour.
Virus icons by CAWST
How effective is it ?
The Biosand filter 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. Information on the studies conducted can be found on the CAWST web page here
Generally, the filter can be expected to remove:
• Between 85% and 99% of fecal coliforms
• 100% of protozoa and worms
• 50-90% of organic and inorganic toxicants
• Suspended sediments, in all or part
Although the filter does remove most of the bacteria disinfection is recommended as a final step, especially for households with infants or elderly. Recommended disinfection includes boiling, using the sun’s UV rays or addition of a small amount of chlorine bleach to remove any remaining bacteria.
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 be easily removed using a small container.
The process can be repeated as many times as necessary to regain the desired flow rate. This procedure is sometimes called a “swirl and dump” maintenance practice. The need to do this depends on the amount and quality of water being put through the filter. If the water is relatively free of sediment, the filter can likely run for several months without this maintenance procedure.
After cleaning, a re-establishment of the biological layer takes place, quickly returning removal efficiency to its previous level.
How is it maintained ?
• Generally removes over 98.5% bacteria, 100% parasites
• Removes turbidity, some iron, manganese, arsenic
• Quality of water improves with time
• Low cost
• High flow rate – around 6 liters per hour
• No on-going costs – no replaceable parts
• Durable and robust
• Fabricated from local materials
• Opportunity for local businesses
• Water tastes and looks good
• Easy to maintain
• Biological layer takes around 3 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 including salt (same as all other filters)
• Can be difficult to move (weigh 170 lbs / 77 kgs)