Bio-Sand Water Filter Description

Bio-sand filtration is a simple, cheap and effective means of removing disease-causing micro-organisms from contaminated water. It is a modification of traditional slow Filtration Technology that has been around for over 150 years. While the traditional slow-sand filter technology requires a continuous flow of water, this unique design was developed specifically for intermittent use in the households of developing countries where a continuous supply of water is usually difficult to maintain. It can be constructed out of low cost materials that are available world wide. This filter has no moving parts and is very low maintenance. Bio-sand filters have been shown to almost entirely remove harmful microbes found in water. This filter design has been introduced in more than 30 countries, by a variety of organizations and people.

So How Does it Work?

The filter itself is simply a concrete container, about 1 meter in height and roughly 1/3 meter is diameter, which holds layers of sand and gravel.

Water is poured into the top of the filter, where a diffuser plate placed above the sand bed, dissipates the initial force of the water. Traveling slowly through the sand bed, the water than passes through several layers 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 and out of the filter, for the user to collect in a clean water container.

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 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 die or are 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 layer of water which is maintained above the biological layer at all times to allow for intermittent use of this filter. The filter, when being used to most effectively remove containments, can at it’s maximum flow rate treat 36 liters of water per hour.

So 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 (see our Links page). We do not currently have any significant data to show the effectiveness of the filters in Tanzania.

Generally the filter can be expected to remove:
• Between 85% and 99 %of fecal coliforms
• 100% of protozoa and helminthes
• 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

So How is it maintained?

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 call 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.

Advantages & Limitations

Advantages
• 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 36 liters per hour (0.6L per minute)
• 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

Limitations
• 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)