Biosand Filter

The biosand filter is an innovation on traditional slow sand water filters (which have been used for community water treatment for hundreds of years CAWST 2009), specifically designed for intermittent or household use. The BSFs was developed by Dr. David Manz in the 1990s at University of Calgary, Canada.

The filter is simple to use and can be produced locally anywhere in the world because it is built using materials that are readily available. Their capital costs depend on the local material and labour costs. However, they require no consumables and the operating costs are negligible.

BSFs consist of a simple container with a lid, enclosing layers of sand and gravel, which traps physically sediments, pathogens and other impurities from the water. A biofilm, which forms as a shallow layer of water, sits atop the sand column and contributes to the elimination of pathogens.

The filter container can be made of concrete, plastic or any other water-proof, rust-proof and non-toxic material. The most widely used version is however the concrete container, approximately 0.9 m tall and with a surface of 0.3 m2 (LANTAGNE et al. 2006). The concrete filter box is cast from a steel mould or made with a pre-fabricated pipe (CAWST 2009). The container is filled with layers of sieved and washed sand and gravel, also referred to as filter media (CAWST 2009). There is a standing water height of 5 cm above the sand layer, which is maintained by adjusting the height of the outlet pipe (LANTAGNE et al. 2006; CAWST 2009). It is this design feature that allows the formation of a biofilm layer and distinguishes the BSF from other slow sand filters, allowing for small-scale construction and intermittent use (see cawst.org). A diffusion layer avoids that water reaches the sand surface too fast, which could disturb the biofilm layer.

The filter operation is very simple. Water is poured onto the top of the filter as needed. Then the water will travel slowly through the sand and gravel bed. At the base of the filter the water is collected in a pipe and is drained through plastic piping out of the filter for be collected and stored in a clean water container. Concrete filters have the outlet pipe embedded in the concrete, protecting it against breaks and leaks (CAWST 2009).

The treated water should be collected by the user in a safe storage container placed on a block or stand, so that the container opening is just under the outlet, minimising the risk for recontamination (CAWST 2009).

Pathogens and suspended solids are removed through a combination of biological and physical processes that take place in the biofilm layer and within the sand layer. These processes include mechanical trapping, predation, adsorption, and natural death (NGAI et al. 2007; EAWAG/SANDEC 2008; CAWST 2009).

• Mechanical trapping and sieving: Suspended solids and pathogens are physically trapped in the spaces between the sand grains.
• Adsorption and attachment: Pathogens become attached to each other (and thus more easily sieved), suspended solids in the water, and the sand grains.
• Predation: Pathogens are consumed by other microorganisms in the biological layer. 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.
• Natural death: Pathogens finish their life cycle or die because there is not enough food or oxygen for them to survive.

The biosand Filter is a proven technology, which removes pathogens such as bacteria, protozoa and helminth. BSFs are also somewhat effective for the removal of virus (CAWST 2009). Physical parameters such as turbidity and iron are also eliminated from drinking water. However, dissolved chemicals (such as organic pesticides or arsenic) are not removed. The treated water generally has an agreeable colour, taste and odour.

The table below shows the biosand filter treatment efficiency in removing pathogens, turbidity and iron (adapted form CAWST 2009).

Health impact studies estimate a 30 to 47% reduction in diarrhoea among all age groups, including children under the age of five, an especially vulnerable population group. Source: SOBSEY (2007); STAUBER et al. (2007)

The flow rate through the filter will slow down over time as the pore openings between the sand grains become clogged. For turbidity levels greater than 50 NTU (Nephelometric Turbidity Units), the water should first be strained through a cloth or sedimented before using the BSF (CAWST 2009). When the flow rate drops to a level that is inadequate for the household use the filter needs to be cleaned. This is done by a simple ‘swirl and dump’ procedure performed on the top of the sand, and only takes a few minutes (CAWST 2009).

The swirl and dump process consist in agitating the surface sand, thereby suspending captured material in the standing layer of the water (see cawst.org). The dirty water is than removed and dumped away. The process can be repeated as many times as necessary to regain the desired flow rate. The need for cleaning 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.

When a BSF is used for the first time, there is no biofilm yet. The biological layer typically takes 20 to 30 days to develop to maturity in a new filter depending on inlet water quality and usage (CAWST 2009). Removal efficiency and the subsequent effectiveness of the filter increase throughout this period. After cleaning, a re-establishment of the biological layer takes place, quickly returning removal efficiency to its previous level.