The Biosand Filters


Biosand filter*

The Biosand filter is an innovation on traditional slow sand water filters, specifically designed for intermittent or household use. 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 that encloses layers of sand and gravel. The sand and gravel trap and eliminate sediments, pathogens and other impurities from the water.

Plastic can also be used to manufacture the container. However, using concrete has several advantages:biosandfilter

  • Cement can be easily acquired in most developing countries.
  • People are familiar with the construction techniques.
  • 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 spout piping is located inside the filter. Consequently, it is less prone to damage than a plastic filter, which has piping on its exterior.
  • Plastic fabrication usually requires the importation of raw material or the finished product. Depending on your views of international trade, this can be an advantage or disadvantage.
  • The life expectancy of a plastic model is substantially less than a concrete model. The ultimate disposal of plastic is a concern for the environment.
  • Encouraging and empowering local people to completely manage their own project increases local sustainability and motivates others to take action for safer water. This process can only be facilitated if the product supply is managed locally. In most cases, if a plastic model is used, there will always be a dependence on outside manufacturers and distributors to supply filters at a higher cost.

    That being said, there are a number of organizations that are pursuing the fabrication of a plastic filter.

     

    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. Travelling slowly through the sand bed, the water then passes through a bed of prepared sand media 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. It should be used with a safe water container.

    Removal of Contaminants

    As with all slow sand filters, a combination of biological and mechanical processes removes pathogens in the water. 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."  This biological layer matures over one to three weeks, depending on volume of water put through the filter and the amount of nutrients and micro-organisms in the water.

    Four processes remove pathogens and other contaminants in the Biosand filter:

    1. Mechanical trapping: Sediments, cysts and worms are removed from the water by becoming trapped in the spaces between the sand grains. Over time the spaces become smaller, enabling the filter to trap smaller particles sooner in the sand bed. The filter can remove some inorganic compounds and metals if they are attached to other materials or each other.
    2. Adsorption or attachment: Viruses are adsorbed or 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 in the sand and thus removed from the water.
    3. Predation: The microorganisms within the "schmutzdecke" or biological layer consume bacteria and other pathogens found in the water, thereby providing highly effective water treatment.
    4. Natural death: Food scarcity, less than optimal temperatures and a relatively short life span will cause pathogens to die off and become nutrients for other micro-organisms.

      Effectiveness

      Slow sand filters have been proven to almost entirely remove the disease-causing organisms found in water. The Biosand filter adaptation 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. CAWST carried out water analyses of 107 long term Biosand filter users in Haiti in 2005 and found average removal effectiveness of 98.5% (measuring E. coli, the indicator bacteria recommended by the WHO).

       

      The filter has also been proven to remove:

      • More than 96% of fecal coliforms
      • 100% of protozoa and helminthes
      • 50-90% of organic and inorganic toxicants
      • Greater than 75% of iron and manganese
      • Suspended sediments, in all or part

        CAWST has prepared a summary of laboratory and field studies for the Biosand filter. The body of evidence shows the success of biosand filter implementation since its inception and outlines further challenges to implementation. 

         

         

        Operation and Maintenance

        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. The filter can produce up to 36 litres/hour.

        The following design features of the Biosand filter ensure its ease of use:

        • Water can be filtered as required
        • The concrete Biosand filter is devoid of moving parts
        • The concrete container is small, but extremely stable; 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 (5cm deep) is maintained above the sand at all times. It is this design feature that distinguishes the Biosand filter from other slow sand filters, allowing for small scale construction and intermittent use. The layer of water is shallow enough for oxygen to diffuse through, providing the biological layer with enough oxygen to develop.

          The biological layer typically takes three 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 98% of bacteria, below the infectious dose, disinfection is recommended as a final step for households with infants or the elderly.

          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 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 clean  (turbidity less than 30 NTU), 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.

          Cost

          Capital: The cost of a concrete filter varies from US$ 10-30, depending on the local material and labour cost.

          Operation: As consumables are not required for successful filter operation, the operating costs are negligible.

          Advantages & Limitations

          Advantages

          • Removes over 98.5% bacteria, 100% parasites
          • Removes turbidity, some iron, manganese, arsenic
          • Quality of water improves with time
          • Sells for US$10-30
          • High flow rate – up to 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 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 (same as all other filters)
            • Can be difficult to move (weigh 170 lbs / 77 kgs)

              *For more information please contact CAWST, the Centre for Affordable Water and Sanitation Technology

              http://www.cawst.org/