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[[Image:CAWST_Kachan.png|thumb|right|150px| Cross-section through a Kachan Arsenic Filter]]
The Kanchan Arsenic Filter (KAF) is an adaptation of the biosand filter (BSF). The KAF has been designed to remove arsenic from drinking water, in addition to providing microbiological water treatment. Arsenic removal is achieved by incorporating a layer of rusty nails in the diffuser basin of the filter.
The filter container can be constructed out of concrete or plastic. The container is about 0.9 m tall and either 0.3 m square or 0.3 m in diameter.
The container is filled with layers of sieved and washed sand and gravel (also referred to as filter media). There is a standing water height of 5 cm above the sand layer.
Similar to in slow sand filters, a biological layer of microorganisms (also known as the biolayer or schmutzedecke) develops at the sand surface, which contributes to the water treatment.
The diffuser basin is filled with 5 to 6 kg of non-galvanized iron nails for arsenic removal. A layer of bricks on top of the nails prevents displacement of the nails when water is poured into the filter.
Arsenic from the water is rapidly adsorbed onto the rust on the iron nails. The rust and arsenic flake off the nails, and are caught in the sand filter and retained. This is a very tight bond; re-suspension of arsenic into the water, or re-mobilization of the arsenic from the waste produced from cleaning the filter has shown to be negligible.
In addition, pathogens, iron and suspended material are removed from water through a combination of biological and physical processes. These occur both in both the biolayer and within the sand bed. These processes include: mechanical trapping, adsorption/attraction, predation and natural death.
==History and social context==
==Suitable conditions ==
It is suitable to process batches of 12-18 litres. If two batches a day are filtered, this provides 24-36 litres of clean water.
{{Potential_Treatment_Capacity_table
}}
==Technical specification==
Contaminated water is poured into the top of the filter 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.
The recommended flow rate for the concrete KAF is 0.6 L/minute measured when the inlet reservoir is full of water. If the flow rate is much faster, the filter may become less efficient at removing pathogens. If the flow rate is much slower, the user may become impatient and not use the filter even though the filter is working well at removing pathogens. Since the flow rate is controlled by the size of the sand grains, it is very important to select, sieve and wash the sand properly.
The KAF filter requires maintenance when treated water should be collected by the flow rate drops to user in a safe storage container placed on a level block or stand, so that is inadequate for the household use. This container opening is done by a simple ‘swirl and dump’ procedure performed on just under the top of outlet, minimizing the sand, and only takes a few minutes. The outlet should also be cleaned regularly using soap and water or a chlorine solutionrisk for recontamination.
====Treatment efficiencyEfficiency====
{{Treatment_Efficiency
|extra_field=Arsenic
The best performance requires a consistent water source; switching sources may decrease treatment efficiency.
Normal cleaning will reduce filter efficiency until the disturbed biolayer re-establishes itself. Appearance and odour of treated water is generally improved. The treatment cannot remove pesticides or fertilizers (organic chemicals) nor remove salt, hardness, and scale (dissolved compounds). It does not provide residual protection to minimize recontamination. ==Operating criteria==*'''Flow rate:''' <0.6 litres/minute. This is the maximum recommended flow rate; the actual flow rate will fluctuate over the filter cleaning cycle and between filters.*'''Batch volume:''' 12-18 litres*'''Daily water supply:''' 24-36 litres. This is based on 2 batches per day to ensure effective arsenic removal. A pause period is needed between uses to allow time for the microorganisms in the biolayer to consume pathogens in the water. The recommended pause period is 6 to 12 hours with a minimum of 1 hour and maximum of 48 hours.
==Robustness==Maintenance====
There are no moving or mechanical parts to break. In concrete models, the piping is embedded in concrete, protecting it against breaks and leaks. Concrete has been shown to last in excess of 30 years.
Concrete filters are heavy (70 – 75 kg for thin wall version and 135 kg for heavy wall version). Poor transportation of filters can lead to cracking and/or breakage. Cracks can sometimes be repaired. Filters should not be moved after installation.
=====Worldwide producers=====Free mold designs are available from CAWST =====Local production=Manufacturing====
Local production of filters is most common. Molds can be borrowed, rented, bought or constructed locally. Filters can be constructed at a central production facility, or in the community. Filter sand and gravel can be prepared (sieved and washed) on-site or nearby.
*Steel mold
*Sand, gravel, and cement
*Facilities: Workshop space for filter construction
A skilled welder required to fabricate molds. Anyone can be trained to construct and install the filter. Individual householders can assist in constructing their own filters.
Working with cement and heavy molds is potentially hazardous and adequate safety precautions should be used. Concrete filters are heavy and difficult to move and transport.
==Maintenance requirements==Estimated Lifespan====Maintenance is required when the flow rate drops to a level that The estimated lifespan is inadequate for the household use. Swirl and dump maintenance for the top layer of sand is simpleunlimited, takes a few minutes and can be done by household usersexisting filters are still performing satisfactorily after 10+ years. The frequency of swirl Lids and dump depends on turbidity of inlet waterdiffusers may need replacement. The outlet, lid and diffuser should Nails need to be cleaned on a regular basisreplaced every 2-3 years to ensure effective arsenic removal. ====Suppliers====Free mold designs are available from CAWST
==Cost==
Note: Program, transportation and education costs are not included.
==OtherCountry experiences==Sand and iron nail selection and preparation are critical to ensure flow rate and treatment. Filters should not be moved after installation. ==Manuals==
==Movies==
==External linksLinks==
*[http://www.cawst.org CAWST (Centre for Affordable Water and Sanitation Technology)]
*[http://www.enpho.org/drinking_water_quality.htm Drinking water quality information from Environment and Public Health Organization (ENPHO)]
* <cite id=Uy08> Uy, D., Chea, S., Mao, S., Ngai, T. and T. Mahin (2008). Kanchan Arsenic Filter - Evaluation of Applicability to Cambodia - Phase I Technical Report. Cambodian Ministry of Rural Development and the Institute of Technology of Cambodia.</cite>
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