About the hard water filter

Executive summary

A biological sand filter (BSF) is a simple home water treatment device, which is more innovative than traditional slow sand filters specially designed for intermittent (irregular) use. This filter consists of a cement or plastic container filled with sand and gravel specially selected and prepared for this purpose. As the water flows through the filter, the physical filtration removes pathogens, iron, turbidity, and manganese from the drinking water. A shallow layer of water resides on top of the sand and a biofilm is formed. The biofilm contributes to the removal of pathogens due to predation and competition for food between harmless microorganisms present in the biosphere and harmful organisms in the water.


  • High removal of pathogens.
  • Remove turbidity, color, odor, and iron (water tastes and feels good)
  • Relatively high flow rates (over 30 liters per hour) can be achieved.
  • One time set-up with few maintenance requirements and minimal operating costs.
  • Long life.
  • It can be manufactured from locally available materials, generating an opportunity for local companies.
  • Easy to operate and maintain.


  • The biological layer takes from 20 to 30 days from formation to maturity.
  • Reduced rate of virus inactivation.
  • High turbidity (greater than 50 NTU) will cause filter clogging and require further maintenance.
  • The filter is required to be used at regular intervals.
  • It is not able to remove dissolved compounds.
  • It may be difficult to move or move (due to weight).
  • Lack of residual protection (risk of recontamination).


The bio-sand filter is more innovative than traditional slow sand water filters (which have been used to treat community water for hundreds of years (Concessional Water and Sanitation Technology Report 2009), and are specially designed for intermittent or home use. This filter was invented. By Dr. David Mannes in 1990 at the University of Calgary, Canada The filter is easy to use and can be produced locally anywhere in the world because it is built with materials that are readily available. The capital costs of the filters depend on local material costs and labor costs. However, they are It does not require consumables and the operating costs are negligible.

The filter consists of a simple container with a lid, comprising layers of sand and gravel, which physically trap sediments, pathogens, and other impurities from the water. The biofilm, which forms as a thin layer over the sand, sits on top of the sand column and contributes to the removal of pathogens.

How it works?

The filter container can be made of concrete, plastic, or any non-toxic water-proof and rust-proof material. However, the most widely used version is the concrete vessel, approximately 0.9 meters long and a surface area of ​​0.3 square meters (LANTAGNE 2006) ). The concrete filter box is either a steel mold or a prefabricated tube (CAWST 2009a). The container is filled with permeable and washed layers of sand and gravel, also known as a filter medium (CAWST 2009a). Above the sand layer lies an anchored layer of water 5 cm high, which is maintained by adjusting the height of the outlet pipe (LANTAGNE 2006; CAWST 2009a). This is a design feature that allows the formation of the biofilm (fixed biofilm) and distinguishes the filter from other slow sand filters, allowing its creation on a small scale, and intermittent use (http://www.cawst.org/en/resources/biosand- filter). There is a spreading layer avoiding water from reaching the sand surface too quickly, which can disrupt and disturb the biofilm.

The filter operation process is very simple. Water is poured over the top of the filter as needed. Then the water moves slowly through the sand and gravel layer. At the base of the filter, the water is collected in a tube and discharged through the plastic piping system outside the filter to be collected and stored in the clean water container. Concrete filters have a tubular outlet and are an integral part of the concrete, protecting it from breakage and leaks (CAWST 2009a).

Treated water should be collected by the user in a safe storage container placed on a rack or tray, with the container opened only under the outlet, to reduce the risk of re-contamination (CAWST 2009a).

Pathogens and suspended solids are removed through a combination of biological and physical processes that occur in the biofilm and across the sand layer. These processes include mechanical retention, predation, adsorption and natural death

(NGAI et al. 2007; EAWAG / SANDEC 2008; CAWST 2009a).

Mechanical retention and screening: suspended solids and pathogens are physically trapped in the spaces between sand grains.

  • Absorption and adhesion: Pathogenic organisms adhere to each other (and thus sift more easily), to suspended solids in water, and to grains of sand.

Predation: The pathogen is consumed by other microorganisms in the biological layer. This biological layer is formed and ripens within one to three weeks, depending on the volume of water placed through the filter and the number of nutrients (nutrients) and microorganisms in the water.

Natural death: Pathogens end their life cycle or die because there is not enough food or oxygen for them to survive.


A bio-sand filter is a proven (proven) technology, which removes pathogens such as bacteria, protozoa, and parasitic worms. The filter is also moderately effective for virus removal (CAWST 2009a). Physical factors such as turbidity and iron are also removed from drinking water. However, dissolved chemicals (such as organic pesticides or arsenic) are not removed. The treated water usually has an acceptable color, taste, and aroma.

Operating and Maintenance

The flow rate through the filter becomes slow over time as the pore openings between the sand grains become clogged. For turbidity levels greater than 50 NTU (turbidity measurement unit), the water must first be filtered through a cloth or settler before using the filter (CWAST 2009a).

When the flow rate drops to a level insufficient for home use, the filter needs to be cleaned. This is done by “vortexing and emptying,” a simple procedure applied to the top of the sand, and takes only a few minutes (CWAST 2009a). The vortex and emptying process consists of agitating and agitating the surface sand, thus suspending the trapped material over the upper anchorage water layer. The unclean water is then removed and discarded away. This process can be repeated multiple times to obtain the desired flow rate (http://www.cawst.org/en/resources/biosand-filter). The need for cleaning depends on the amount and quality of water that was put into the filter. If the water is relatively clean (turbidity is less than 30 units (NTU), a filter likely to have run for several months can operate without this maintenance (http://www.cawst.org/en/resources/biosand-filter).

When the filter is used for the first time, the biofilm has not yet been formed. The biofilm usually takes 20-30 days to complete formation and maturity in a new filter depending on the quality of the water entering the filter and the type of use (Soft Water and Sanitation Technology Report 2009; http://www.cawst.org/en/resources/biosand- filter). The removal efficiency and effectiveness of the filter increase throughout this period. After cleaning, the re-establishment and rearrangement of the biological layer occurs, and the removal efficiency quickly returns to its previous level.

The application

The filter is suitable for treating water at home, school, or at the community level. The filter can treat polluted surface water or groundwater efficiently and directly since it also removes turbidity and iron. However, it is recommended not to use, moreover, water with turbidity of more than 50 NTU units. Dissolved chemicals (such as organic pesticides or arsenic) are not removed.

Chlorinated water (chlorinated) is not applied in this filter because chlorine kills the micro-organisms present in the biofilm, which leads to a decrease in the efficiency of removing pathogenic organisms. However, the water can be treated with chlorine after filtration in order to improve safety (safety Water for elderly groups or children in the family / community.

The filter should only be constructed by trained technicians. Although the construction and installation appear to be very simple, improper design and construction of the filter can lead to poor filter performance however, materials are usually locally available and construction by trained local personnel may creates local job opportunities.

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