Filtration

Filtration is the second step in a good water treatment process, and can include household technologies as well as a process of straining.

Straining

Straining is simply running water through a cloth. Laboratory studies supported by the National Science Foundation in the U.S. showed that by folding an sari cloth 8 to 10 times, 99% of cholera bacteria were eliminated. The incidence of cholera was 50% less in villages that used sari straining.

Household Technologies

Water filtration through simple household filters, such as ceramic, fiber and sand filters, will remove a large proportion of solid matter and micro organisms. These filters can be constructed with locally available materials and are easy to operate and maintain. To ensure longer filter life and sustained particle removal efficiency, a sedimentation step should be used.

Two types of household filtration systems which are low cost, simple to construct, and easy to maintain include:

Several other filtration techniques that are more complex and costly include rapid sand filters, Reverse Osmosis and carbon filters. While these technologies are highly effective for treating water, they are often too costly for the poor, and depending on the local water source, may not be appropriate for the conditions. The Virginia Cooperative Extension and Wikipedia have good resources on filter technologies.

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, enclosing layers of sand and gravel which 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: Biosand Filter

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.

Here is a list of recommendations to help ensure the highest probability of success when implementing a biosand filter program.

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. Traveling 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:

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.
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.
Predation: The microorganisms within the "schmutzdecke" or biological layer consume bacteria and other pathogens found in the water, thereby providing highly effective water treatment.
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). [ Download Executive Summary or Report ]

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. [Download Summary]

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 about the biosand filter, download CAWST's biosand filter manual.

Technology updates on the biosand filter are also available.

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Ceramic Filter

Ceramic filters are an age-old technology that seem to be making a comeback. There are many variations of the ceramic filter: some are made entirely from ceramics such as the Potters for Peace filters, some have a ceramic pot hanging in a plastic container such as Filter Pure (Agua Pure), while others use plastic buckets and a ceramic candle (Ketadyne). Frequently, a colloidal silver coating is added to the ceramic filter.

The following information is based on the Potters for Peace ceramic filter, sometimes known as the Filtron or Rabbit Filter, as it is an appropriate technology for poor communities.

How Does it Work? Ceramic Filter

The ceramic filter comes in two separate parts: a ceramic pot and a plastic container that the pot rests inside. The water is added into the pot, passes through the pores in a ceramic and falls into the plastic holding container.

Removal of Contaminants

The ceramic for the filter is constructed such that there are very small pores in the clay/ceramic. When water is poured through, contaminants are mechanically trapped in the pore spaces.

The ceramic filter has a colloidal silver coating. In its ionic colloid state, silver is recognized as a germicide, or in some cases as a bacteriostatic. It is believed that silver:

Disables the enzyme that pathogenic bacteria and fungi use for oxygen metabolism, thus suffocating them.
Destroys pathogens with an electric charge, causing their internal protoplast to collapse.
Renders pathogens unable to reproduce.
Kills parasites while in their egg stage.

Effectiveness

If properly constructed and operated, a ceramic filter can be very effective in producing good quality water. Ceramic filters with the silver coating have consistently tested to 99-100% removal of bacteria and parasites. These filters also eliminate the suspended sediments in the water, including any organic compounds that are adsorbed to the particles.

A 2007 report on the Use of Ceramic Water Filters in Cambodia by the Water and Sanitation Program shows how effective ceramic filters are at improving water quality and health.

Operation & Maintenance

Operation

Operating a ceramic filter is very simple. You simply remove the lid, pour the water into the ceramic pot and collect the percolating water in the lower plastic container.
Ceramic filters typically produce 1-3 litres of water per hour.

The following features of the ceramic filter ensure its ease of use:

Water is filtered as needed
The filter is small and relatively lightweight; it can be placed anywhere in the home, in the location most convenient to the user
The silver coating ensures disinfection therefore disinfection is not required

Maintenance

Continued use of the filter causes the pore openings in the ceramic to become clogged with debris. As a result, the flow rate of water through the filter decreases.
To clean the filter, scrub the surface of the ceramic filter with a sturdy brush and flush with water.

Cost

Capital: The cost of a Potters for Peace ceramic filter is estimated at between US$ 3-7, depending upon the country in which it is being manufactured. Certain economic advantages exist for this type of filter:

Ceramic is produced locally in most developing countries
Household labour or volunteer labour can be utilized in the manufacturing process
Due to the colloidal silver coating, there is no need for additional costs for disinfection

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

Advantages and Limitations

Advantages

Removes most pathogens and suspended solids
Inexpensive (US$ 3-7)
Supports local craftsmen
Sustainable, minimal outside resources are required
Water tastes good
Easy to use

Limitations

Low flow rate (1-3 liters/hr)
Fragile, easy to crack, allowing bad water through undetected cracks
Turbid water plugs filter
Cleaning results in removal of ceramic layer; over time, filter will need replacing
Quality conrol difficult to ensure
Dissolved compounds are not removed (same as other filters)

More Info

Purifier has additional information on the ceramic filter.

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