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Introduction

Arsenic has been long known as a poison. Exposure to arsenic via drinking water initially causes skin diseases such as pigmentation (dark and light spots on the skin) and arsenicosis (hardening of skin on hands and feets). Later, cancer of the skin, lungs, bladder, and kidney may occur . Unfortunately, there is no cure for these diseases. Both the World Health Organization (WHO) and the U.S. Environmental Protection Agency (USEPA) have classified arsenic as a carcinogen. In some parts of Bangladesh, West Bengal, and Nepal, the arsenic level in the groundwater can be over 100 times higher than the WHO and EPA guidelines of 10 ug/L.

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Numerous arsenic removal technologies have been developed by universities, government organizations, groups, and the private sector. These technologies can be classified into 9 general categories based on the dominant removal process (although sometimes, a given technology may make use of multiple treatment processes).
These processes are:
1. Oxidation
2. Coagulation/Co-Precipitation
3. Sedimentation
4. Filtration
5. Adsorption
6. Ion Exchange
7. Membrane/ Reverse Osmosis
8. Biological
9. Other

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Oxidation

Of the two predominant forms of arsenic in water, arsenate and arsenite, most treatment processes are effective at removing arsenate, but not arsenite, since arsenite is typically non-charged below pH 9.2. Therefore, treatment for the removal of arsenic often includes an oxidation step to convert arsenite to arsenate.

Oxidation can be simply the addition of oxygen to a compound, or more generally, any reaction involving the loss of electrons from an atom. Aeration, the supplying
of air, oxidizes arsenic, converting arsenite to arsenate, and the iron that co-occurs. This is precipitated as FeAsO4. Arsenic can also be oxidized by a number of other chemicals including chlorine, hypochlorite, ozone, permanganate, hydrogen peroxide and Fenton’s reagent (H2O2/Fe2+). Photochemical oxidization proceeds from the reaction of radiant energy and a chemical system.

Oxidation alone does not remove arsenic from solution but must be combined with an arsenic removal process.

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Coagulation/ Precipitation

Coagulation encompasses all reactions, mechanisms and results in the overall process of particle growth (floc formation) and particle aggregation within a water
being treated, including in situ coagulant formation, chemical particle destabilization and physical inter-particle contacts. Coagulation involves the removal of colloidal (0.001 - 100 microns) and settleable (> 100 microns) particles. However the term also commonly refers to the removal of dissolved ions (< 0.001 microns), which is actually precipitation.

Chemical precipitation is the process by which dissolved ions in solution form an insoluble solid via a chemical reaction. For example, naturally occurring dissolved iron in groundwater, when exposed to oxygen, forms a precipitate. Co-precipitation occurs when an inorganic contaminant forms an insoluble complex with the
coagulant. Both the valence of the inorganic contaminant and the pH of the solution are important removal by co-precipitation.

There are 4 types of co-precipitation:

1. Inclusion: mechanical entrapment of a portion of the solution surrounding the growing particle. Typically, this only is significant for large crystals.
2. Adsorption: the attachment of an impurity onto the surface of a particle or precipitate. This typoe of co-precipitation is generally not important
   if the particle size is large, because large particles have very small surface areas in proporation to the amount of precipitate they contain.
   Adsorption may be a major means of contaminant removal if the particles are small.
3. Occlusion: A contaminant is trapped in the interior of a particle of precipitate. This type of co-precipitation occurs by adsorption of the
   contaminant onto the surface of a growing particle, followed by further growth of the particle to enclose the adsorbed contaminant.
4. Solid-solution formation: another type of occlusion where a particle of precipitate becomes contaminated with a different type of particle that
   precipitates under similar conditions and is formed from ions whose sizes are nearly equal to those of the original precipitate.

Coagulation converts soluble arsenic into insoluble reaction products, allowing separation by sedimentation and/or filtration. Factors affecting arsenic removal by
coagulation/precipitation include coagulant type and dose, mixing time and speed, pH, arsenic oxidation state and concentration, presence of inorganic solutes.

Three mechanisms are potentially involved in arsenic removal:

• Precipitation: the formation of insoluble compounds Al(AsO4) or Fe(AsO4)
• Co-precipitation: incorporation of soluble arsenic species into the metal hydroxide floc
• Adsorption: the electrostatic binding of soluble arsenic to the external surfaces of the insoluble metal hydroxides.

Direct precipitation plays the least important role in arsenic removal, however, co-precipitation and adsorption are both active arsenic removal mechanisms (Johnston and Heijnen, 2001)

Soon Kyu (Jeff) Hwang evaluated the arsenic removal performance of ENPHO's Two-Kolshi Arsenic Removal System, which is based on coagulation and filtration processes. For more information, please see Jeff's thesis (2002).

Georges Tabbal compared technical performance and social acceptance of three arsenic removal technologies in Nepal, including the Two-Kolshi System. For more information, please see Georges' thesis (2003).

Source: Johnston, R. and Heijnen, H. 2001. “Safe Water Technology for Arsenic Removal.” In: Ahmed, M.F. et. al. [Eds]. Technologies for Arsenic Removal from Drinking Water. Bangladesh University of Engineering and Technology, Dhaka. Bangladesh

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Sedimentation

Sedimentation is the gravity separation of solids from liquid by settling. It is generally used in conjunction with precipitation or coagulation.

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Filtration

Conventional filtration is the separation of solid particles from water by passing the solution through a medium. Particles are removed during filtration as a result of
any one or combination of mechanisms: mechanical straining, sedimentation, flocculation, adsorption and/or biological metabolism (AWWA, 1999). The filter medium may be of various materials, for example, sand, anthracite coal, activated carbon, cloth, paper, that retains the solid on its surface and allows the water to pass through.

Common particulates removed by filtration include silt, clay, colloidal and precipitated natural organic matter, naturally-occurring iron and manganese precipitates, precipitates from metal salt or polymer coagulation, microorganisms. Filters may be classified in various ways, according to the type of granular medium used, by the hydraulic system (e.g. gravity, up-flow, etc.), rate of filtration, and/or by the location of particle accumulation (e.g. cake filtration, depth filtration).

Soon Kyu (Jeff) Hwang evaluated the arsenic removal performance of ENPHO's Two-Kolshi Arsenic Removal System, which is based on coagulation and filtration processes. For more information, please see Jeff's thesis (2002).

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Adsorption

Adsorption is the accumulation of materials at an interface, the liquid/solid boundary layer. It is a mass transfer process where a substance is transferred from the liquid phase to the surface of a solid and becomes bound by chemical or physical forces. Adsorption can take place on suspended particles, as part of the process of coagulation/co-precipitation, or on fixed media. Since adsorption is a surface phenomenon, the greater the surface area of the medium, the greater it’s capacity to accumulate material. Each adsorbent medium has different associated properties, performances and costs. Arsenic is adsorbed onto the surface of various granular, activated, clay and celluosic adsorbents, including:

• oxides (e.g. hydrated ferric oxide, activated alumina oxide, manganese oxide, titanium oxide, silicium oxide)
• iron oxide-coated or MnO2-coated sand
• clay minerals (e.g. kaolinite, bentonite, Bijoypur clay)
• bauxite, hematite, feldspar
• synthetic anion exchange resins
• chitin and chitosan
• bone char
• cellulose materials (sawdust, newspaper pulp).

Jessica Hurd evaluated the arsenic removal performance of three adsorption media, including iron filings (3-Kolshi Filter and Jerry Can Filter) and activated alumina (Apryon Filter). For more information, please see Jessica's thesis (2001).

Barika Poole evaluated the arsenic adsorption performance of several types of iron oxide-coated sands. For more information, please see Barika's thesis (2001).

Tommy Ngai evaluated the arsenic adsorption performance of an activated-alumina-based manganese oxide media. For more information, please see Tommy's thesis (2001).

Tommy Ngai and Sophie Walewijk also designed and evaluated the Arsenic Biosand Filter (ABF). The dominant arsenic removal process for this filter is adsoprtion to iron nails. Refer to their report for more details (2003).

Georges Tabbal compared technical performance and social acceptance of three arsenic removal technologies in Nepal, including the Two-Kolshi System and the ABF. For more information, please see Georges' thesis (2003).

Also refer to the World Bank Project section for additional information on the ABF.

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Ion Exchange

Ion exchange is the reversible interchange of ions between the solid and the liquid phase where there is no permanent change in the structure of the solid. Developed for large-scale applications, ion exchange is probably not appropriate for small hand?pumped wells, but could potentially be used on a village scale in Bangladesh.

Synthetic ion exchange resins are based on a cross-linked polymer matrix, typically composed of polystyrene cross-linked with vinylbenzene. Charged functional groups are attached to the matrix through covalent bonding and fall into four groups (Clifford, 1999).

• Strongly acidic
• Weakly acidic
• Strongly basic
• Weakly basic

Various strong base anion exchange resins are commercially available that can effectively remove arsenate from water, producing effluents with less than 1 ug/L arsenic. Arsenite, being uncharged, is not removed, unless an oxidation step to convert arsenite to arsenate is included in the process.

Conventional sulfate-selective resins are particularly suited for arsenate removal. Nitrate-selective resins also remove arsenic, but arsenic breakthrough occurs earlier. Ion exchangers are typically down-flow, packed bed columns with ion exchange resin beads pre-saturated with an exchangeable ion. Source water is passed through the packed bed until the appearance of the unwanted contaminant in the effluent. At this stage, the ion exchange media is reactivated with a regenerant solution and rinsed with water in preparation for another treatment cycle. Both the redox potential and pH are important factors with regard to arsenic removal by ion exchange.

Tommy Ngai used Bio-Rad Laboratories AG1-X8 Strong Ion Exchange Resin to separate arsenite from arsenate in arsenic-containing groundwater in Nepal. Of the 40+ tubewell water surveyed, arsenite was found to be the dominant inorganic species, representating approximately 79% of all inorganic arsenic. For more information, please refer to Tommy's thesis (2001).

Source: M. Davis and D. Cornwell, Introduction to Environmental Engineering. McGraw-Hill. New York, 1991. p.277.
Source: Clifford, D. 1999. “Ion Exchange and Inorganic Adsorption. In: Letterman {Ed]. Water Quality and Treatment, 5th Edition. American Water Works Association, McGraw Hill, New York.

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Membrane/ Reverse Osmosis

Membrane separation uses semi-permeable membranes that are selectively permeable to water and certain solutes to separate impurities from water. Membranes are able to remove many different kinds of dissolved solids, including arsenic, from water. However, they are usually expensive and therefore are typically considered in applications such as desalination, brackish water conversion and for removal of specific ions, such as arsenic, that are difficult to remove by other means.

There are many different membrane alternatives including microfiltration, reverse osmosis, electrodialysis, ultrafiltration and nanofiltration. Membrane process treatment performance is dependent on the quality of the feed water and the desired quality of the product water. Generally the more contaminated the feed water and the higher the desired product water quality, the greater the likelihood of membrane fouling caused by particulate matter, scaling and biofouling.

Source: S. Kawamura. Integrated Design of Water Treatment Facilities. John Wiley & Sons Inc. New York, 1991. p.556.

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Biological

Biological treatment transforms, stabilizes and/or removes arsenic by means of microorganisms. Microorganisms, primarily certain specific bacteria, accomplish this by oxidation/reduction, mineralization, detoxification or methylation. Critical factors include energy and carbon source; aerobic, anoxic or anaerobic conditions; temperature;pH.

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Other

Other methods in providing arsenic-free drinking include:

• Dugwell
• Deeper tubewell
• Pond
• Rainwater Harvesting
• Solar Distillation

Solar distillation uses the sun's energy to evaporate water, which then recondenses. The process of evaporation and recondensation separates all chemicals, including arsenic, from the water. In Bangladesh, where solar energy is plentiful, this approach may be especially suited for application in crisis areas, and, if cost-effective approaches can be developed, in rural areas generally.

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Evaluation of 8 arsenic removal technologies in Nepal (Abstract)

MIT Nepal Water Project:Development of Arsenic Remediation Technologies from 1999-2003

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