Difference between revisions of "Arsenic filter"

Revision as of 02:29, 6 July 2012

Arsenic removal system: oxidation method.
Photo: Ngai.

Magc-Alcan Filter (left) and Nirmal Filter (right): adsorption method. Photo: Ngai.

There are at least five ways to remove arsenic from drinking water. The adsorption method, the coagulation-flocculation method, the surface complexation method, a Kanchan arsenic filter, or removal by oxidation.

Adsorption: Magc-Alcan filter and Shapla filter
The Magc-Alcan is a two bucket filter. The buckets are in series and both filled with an American-made activated alumina media. The media has been developed by MAGC Technologies and Alcanof US; and it is produced by thermal dehydration (at 250-1150°C) of an aluminium hydroxide. The Magc-Alcan filter removes arsenic by adsorption (adhesion or sticking together) of the arsenic to the enhanced activated alumina which is porous and has a high surface area. Removal rates can be sensitive to varying pH levels, so additional equipment may be required to control pH levels.

A similar filter called “Nirmal Filter” also exists in India. It uses arsenic adsorption on an Indian-made activated alumina and it is followed by filtration through a ceramic candle. It is less expensive than a Magc-Alcan filter ($10-15 capital cost) but it needs to be regenerated every 6 months. Efficiency: 80-90% arsenic removal.

The Shapla filter is an earthen household arsenic removal technology developed by International Development Enterprises (IDE), Bangladesh. It is based on adsorption (adhesion or sticking together) of the arsenic to the iron on the coated brick chips, which works as well as iron coated sand. The bricks are coated by treatment with a ferrous sulphate solution (Ahmed, 2005). The filter can hold up to 30 litres of water. As water passes through the filter, arsenic from the water is rapidly adsorbed by the iron on the brick chips. The media (20 kg) filters 4,000 litres of arsenic-contaminated water reducing arsenic concentrations to undetectable levels and supplying an average family with 25-32 litres of safe drinking water per day.

Coagulation-Flocculation: Bucket Treatment Unit (BTU) and 2-Kolshi filter
The Bucket Treatment Unit has been developed within the DPHE-Danida project. It consists of oxidation (loss of electrons of As(III) to form As(V)) and coagulation (clumping of particles to promote sedimentation) processes using potassium permanganate and aluminum sulphate respectively. The water is then filtered through cloth into a second bucket that contains a sand filter. The BTU can be constructed from locally available materials.

2-Kolshi is an arsenic removal method using two buckets. The first step is a coagulation process using a specific amount of iron sulphate and enough sodium hypochlorite to create a distinctive chlorine odour upon stirring in a normal bucket of water, followed by a filtration in a ceramic filter. The 2-Kolshi technique oxidizes As(III) to As(V) by stirring the water, co-precipitates As5+ with iron chloride and ash, and then it filters the water to remove the formed particles.

Coagulation/filtration is a traditional treatment process that adds a chemical coagulant (typically iron sulfate or iron chloride) to contaminated water. The coagulant modifies the physical or chemical properties of dissolved or suspended contaminants so that they settle from solution by gravity or can be removed by filtration. As part of the coagulation process, arsenic is co-precipitated with the iron. The stirring process helps to build the flocs into larger particles.

Surface complexation: Sono filter
The Sono Filter is a three bucket system with a composite iron matrix (CIM) as the active arsenic removal component. It creates surface complexation of arsenic on the CIM followed by a filtration. The filter is manufactured from indigenous materials and it works without chemical treatment, without regeneration, and without producing toxic waste. It is efficient to remove arsenic and 22 other heavy metals as well as bacteria.

Kanchan Arsenic filter
The Kanchan Arsenic Filter (KAF) is an adaptation of the biosand filter. 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.

Passive Oxidation
Passive oxidation is based on oxidation mechanisms of iron compounds that naturally reduce the arsenic content of groundwater. Naturally occurring dissolved Fe(OH)2 in the groundwater undergoes a natural chemical process called oxidation (when an element loses electrons) to produce a solid form or precipitate of Fe(OH)3 which attracts arsenic to stick to it, a process called adsorption (Ahmed, 2002). This adsorption produces a co-precipitate of Fe(OH)3 and arsenic that settles to the bottom of the container thereby removing the arsenic from the water. This technology does not require chemicals, but rather is based on the passive oxidation process and subsequent sedimentation.

Passive oxidation is seen as an easy technology, because of the natural habits of rural people to store their water in pitchers, before they drink it. Nevertheless, its performance at removing arsenic at safe levels has not been proven.

Construction, operations and maintenance

Adsorption: Magc-Alcan filter

• Place two buckets (with taps) filled with activated alumina media in series using a stand.
• Place a safe water container after the second tap.
• Pour water in the top bucket with all of the taps open.

Adsorption: Shapla filter

• Pour the water into the filter and allow it to pass through the filter medium.
• The used filter media is non-toxic and can be disposed of safely without danger to the environment or human health.
• The media container is re-useable and easily maintained.

Coagulation-Flocculation: Bucket Treatment Unit (BTU) The unit consists of a two bucket system. The upper bucket is filled with raw water. Chemicals are added and the water is stirred fast for approximately 25 seconds with a wooden spoon provided with the unit.
Recommended chemical doses are (Tanhura et al.):

• 200 mg/L aluminum sulphate
• 2 mg/L of potassium permanganate

The mixture is then allowed to settle for 1-2 hours. The tap is then turned on so that the water can pass through a cloth screen and a sand filter in the lower bucket.

Coagulation-Flocculation: 2-Kolshi filter

• Add ENPHO chemical (iron sulphate, sodium hypochlorite and ash) to a bucket of water
• Stir water for a few minutes
• Let settle for one hour
• Pass the water through a ceramic filter

Surface complexation: Sono filter
Each bucket has different media and functions:

• Top bucket: 3 kg iron filings and 2 kg coarse sand
• Middle bucket: 2 kg fine sand, 1 kg of wood charcoal and brick chips
• Bottom bucket: water collection container

The water first flows through the coarse river sand and a composite iron matrix (CIM). The water flows into a second bucket where it again filters through coarse river sand, then wood charcoal to remove organics, and finally through fine river sand and brick chips to remove fine particles and stabilize water flow.

Kanchan arsenic 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. As 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.

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 biolayer is the key pathogen removing component of the filter. Without it, the filter is significantly less effective. It may take up to 30 days to establish the biolayer depending on inlet water quality and frequency of use. The water from the filter can be used during the first few weeks while the biolayer is being established, but disinfection is recommended during this time, as during regular on-going use.

Much more information and details on the Kanchan arsenic filter: [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CFYQFjAB&url=http%3A%2F%2Fwww.cawst.org%2Fassets%2FFile%2FHWTS_Fact_Sheet_KAF.pdf&ei=gz72T6esPOmy2QXl8-ToBg&usg=AFQjCNHExV2LsQy4IgoZzMR2avHnREOnsg&sig2=ClJFwW7vj9UkNrgU-4udPA Household Water Treatment and Safe Storage Factsheet: Kanchan Arsenic Filter.]

Passive Oxidation

Costs

Adsorption: Magc-Alcan filter

Adsorption: Shapla filter

Coagulation-Flocculation: Bucket Treatment Unit (BTU)

Coagulation-Flocculation: 2-Kolshi filter

Surface complexation: Sono filter

Acknowledgements

• Household Water Treatment for Arsenic Removal Fact Sheet: Adsorption. CAWST.
• Household Water Treatment for Arsenic Removal Fact Sheet: Coagulation-Flocculation. CAWST.
• Household Water Treatment for Arsenic Removal Fact Sheet: Surface Complexation. CAWST.

• Household Water Treatment Fact Sheets. CAWST.