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BioSand Water Filter

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Biosand Filter Overview

2.1 What is the Biosand Filter?

The biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community water treatment for almost 200 hundred years. The biosand filter is smaller and adapted for intermittent use, making it suitable for households. The filter container can be made of concrete or plastic and is filled with layers of specially selected and prepared sand and gravel.

2.2 History of the Biosand Filter

Dr. David Manz developed the household biosand filter in the 1990s at the University of Calgary, Canada. Dr Manz has trained many organizations on the design, construction, installation, operation and maintenance of the biosand filter. He also co-founded CAWST in 2001 to provide the professional services needed for the humanitarian distribution of the filter in developing countries. As of June 2009, CAWST estimates that over 200,000 biosand filters have been implemented in more than 70 countries around the world.

2.3 Biosand Filter Components

 BioSand Filter
BioSand Filter Components

  1. Lid – Tightly fitting lid prevents contamination and unwanted pests.
  2. Diffuser – Prevents disturbing the filtration sand layer and protects the biolayer when water is poured into the filter.
  3. Filtration Sand Layer – Removes pathogens and suspended solids.
  4. Outlet Tube – Required to conduct water from the base to the outside of the filter.
  5. Filter Body – Holds the sand and gravel layers.
  6. Separating Gravel Layer – Supports the filtration sand and prevents it from going into the drainage layer and outlet tube.
  7. Drainage Gravel Layer – Supports the separating gravel layer and helps water to flow into the outlet tube.

2.4 How Does the Biosand Filter Work?

The biosand filter has five distinct zones: 1) inlet reservoir zone, 2) standing water zone, 3) biological zone, 4) non-biological zone, and 5) gravel zone.

  1. Inlet Reservoir Zone - Where water is poured into the filter. 
  2. Standing Water Zone – This water keeps the sand wet while letting oxygen pass to the biolayer.
  3. Biological Zone – Develops at the top 5-10 cm (2-4”) of the sand surface. The filtration sand removes pathogens, suspended particles and other contaminants. As in slow sand filters, a biological layer of microorganisms (also known as the biolayer or schmutzedecke) develops at the top 1-2 cm (0.4-0.8”) of the sand surface.
  4. Non-Biological Zone – Contains virtually no living microorganisms due to the lack of nutrients and oxygen.
  5. Gravel Zone – Holds the sand in place and protects the outlet tube from clogging. 

Pathogens and suspended solids are removed through a combination of biological and physical processes that take place in the biolayer and within the sand layer. These processes include: mechanical trapping, predation, adsorption, and natural death.

  • Mechanical trapping. Suspended solids and pathogens are physically trapped in the spaces between the sand grains.
  • Predation. Pathogens are consumed by other microorganisms in the biolayer.
  • Adsorption. Pathogens become attached to each other, suspended solids in the water, and the sand grains.
  • Natural death. Pathogens finish their life cycle or die because there is not enough food or oxygen for them to survive.
  • Contaminated water is poured into the reservoir on an intermittent basis. The water slowly passes through the diffuser and percolates down through the biolayer, sand and gravel. Treated water naturally flows from the outlet tube.

During the Run

The high water level pushes the water through the diffuser and filter (also called the hydraulic head). The water level in the reservoir goes down as it flows evenly through the sand. The flow rate will slow down over time because there is less pressure to force the water through the filter.

The inlet water contains dissolved oxygen, nutrients and contaminants. It provides the oxygen required by the microorganisms in the biolayer.

Larger suspended particles and pathogens are trapped in the top of the sand and they partially plug the pore spaces between the sand grains. This also causes the flow rate to slow down.

Pause Period

The water finally stops flowing. The standing water layer will be at the same height as the end of the outlet tube. Some oxygen from the air diffuses through the standing water to the biolayer.

The pause period allows time for microorganisms in the biolayer to consume the pathogens and nutrients in the water. The flow rate through the filter is restored as they are consumed. If the pause period is too long, the biolayer will eventually consume all of the pathogens and nutrients and eventually die off. This will reduce the removal efficiency of the filter when it is used again. The pause period should be a minimum of 1 hour after the water has stopped flowing up to a maximum of 48 hours.

Pathogens in the non-biological zone die off due to the lack of nutrients and oxygen.

2.5 How Well Does the Biosand Filter Work?

Water naturally contains many living things. Some are harmless and others can make people sick. Living things that cause disease are also known as pathogens. They are sometimes called other names, such as microorganisms, microbes or bugs, depending on the local language and country. There are four different categories of pathogens that are shown in Table 1: bacteria, viruses, protozoa and helminths.

The physical characteristics of drinking water are usually things that we can measure with our senses: turbidity, colour, taste, smell and temperature. Turbid water looks cloudy, dirty or muddy. Turbidity is caused by sand, silt and clay that are floating in the water. Drinking turbid water will not make people sick by itself. However, viruses, parasites and some bacteria can sometimes attach themselves to the suspended solids in water. This means that turbid water usually has more pathogens so drinking it increases the chances of becoming sick.

The following Table 1 shows the biosand filter treatment efficiency in removing pathogens and turbidity.

  1. Buzunis (1995)
  2. Baumgartner (2006)
  3. Stauber et al. (2006)
  4. Palmateer et al. (1997)
  5. Not researched. However, helminths are too large to pass between the sand, up to 100% removal efficiency is assumed
  6. Earwaker (2006)
  7. Duke & Baker (2005)
  8. Ngai et al. (2004)

Health impact studies estimate a 30-47% reduction in diarrhea among all age groups, including children under the age of five, an especially vulnerable population (Sobsey, 2007; Stauber, 2007).

Source:

BioSand Filter Manual
Center for Affordable Water and Sanitation Technology (CAWST)