Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision DocumentArsenic removal is accomplished by continuously passing water, under pressure throughone or more columns, packed with exchange resin. As (V) can be removed by the use ofstrong-base anion exchange resin, in either chloride or hydroxide form. These resins areinsensitive to pH in the range 6.5 to 9.0 (EPA, 2000a, b ).Different categories of synthetic resins can be used for arsenic removal. The process issimilar to that of adsorption; just the medium is a synthetic resin of more well defined ionexchange capacity. Few manufacturers have developed synthetic resins suitable for arsenicremoval. However, the resins need to be replenished after use and renewal interval is dependenton the quantity of arsenic in water. The hand pump attached arsenic removal plants working withion exchange principle need meticulous attention for operation as well as for regular backwashing.The arsenic removal capacity is dependent on sulfate and nitrate contents of raw wateras sulfate and nitrate are exchanged before arsenic. The ion exchange process is less dependenton pH of water. The efficiency of ion exchange process is radically improved by pre-oxidation ofAs (III) to As (V) but the excess of oxidant often needs to be removed before the ion exchangein order to avoid the damage of sensitive resins. Development of ion specific resin for exclusiveremoval of arsenic can make the process very attractive.The exchange affinity of various ions is a function of the net surface charge. Therefore,the efficiency of the ion exchange process for As (V) removal depends strongly on the solutionpH and the concentration of other anions, most notably sulfates and nitrates. High levels of totaldissolved solids (TDS) can adversely affect the performance of an Ion Exchange system. Ingeneral, the Ion Exchange process is not an economically viable treatment technology if sourcewater contains over 500 mg/L of TDS (Wang et al., 2000) or over 50 mg/L of sulfate. Thepresence of suspended solids in the feed water could gradually plug the media, therebyincreasing head loss and necessitating more frequent backwashing. Therefore, pre-filtration isrecommended if the source water turbidity exceeds 0.3 NTU.Tetrahedron ion exchange resin filter tested under rapid assessment program inBangladesh (BAMWSP, DFID and Water Aid, 2000) showed promising results in arsenicremoval. The system needs pre-oxidation of arsenite by sodium hypochloride. The residualchlorine helps to minimize bacterial growth in the media. The saturated resin requiresregeneration by re-circulating sodium chloride (NaCl) solution. The liquid wastes, rich in salt andarsenic, produced during regeneration require special treatment. Some other ion exchange resinshave also been tried in Bangladesh but sufficient field test results are not available on theperformance of those resins.5.2.5 Membrane TechnologyMembrane technology can remove a wide range of contaminants from water. Thistechnology typically can reduce arsenic concentrations to less than 50 µg/L and in some cases tobelow 10 µg/L. However, its effectiveness is sensitive to a variety of untreated waterNIH & CGWB 91
Technological Options and Arsenic Removal Technologiescontaminants and characteristics. It also produces a larger volume of residuals and tends to bemore expensive than other arsenic treatment technologies. Therefore, it is used less frequentlythan precipitation/co precipitation, adsorption and ion exchange.Membrane techniques are capable of removing all kinds of dissolved solids includingarsenic from water. They can address numerous water quality problems while maintainingsimplicity and ease of operation. In this process, water is allowed to pass through special filtermedia which physically retain the impurities present in water. The water, for treatment bymembrane techniques, should be free from suspended solids and the arsenic in water shall be inpentavalent form. Most membranes, however, can not withstand oxidizing agent.There are four types of membrane processes: microfiltration (MF), ultrafiltration (UF),nanofiltration (NF) and reverse osmosis (RO). All the four processes are pressure-driven andare categorized by the size of the particles that can pass through the membranes or by themolecular weight cut off (i.e., pore size) of the membrane (EPA, 2000a, b). The force requiredto drive fluid across the membrane depends on the pore size; NF and RO require a relatively highpressure (50 to 150 psi), while MF and UF require lower pressure (5 to 100 psi). The lowpressure processes primarily remove contaminants through physical sieving and the highpressure processes through chemical diffusion across the permeable membrane (EPA, 2000a, b).Because arsenic species dissolved in water tend to have relatively low molecular weights,only NF and RO membrane processes are likely to effectively treat dissolved arsenic (EPA,2000a, b). MF has been used with precipitation/co precipitation to remove solids containingarsenic. MF generates two treatment residuals from the influent waste stream: a treatedeffluent (permeate) and a rejected waste stream of concentrated contaminants (reject).RO is a high pressure process that primarily removes smaller ions typically associatedwith total dissolved solids. The molecular weight cut off for RO membranes ranges from 1 to20,000, which is a significantly lower cut off than that for NF membranes. The molecular weightcut off for NF membranes ranges from approximately 150 to 20,000. NF is a high pressureprocess that primarily removes larger divalent ions associated with hardness (for example,calcium [Ca], and magnesium [Mg] but not monovalent salts (for example, sodium [Na] andchlorine [Cl]). NF is slightly less efficient than RO in removing dissolved arsenic from water(EPA, 2000a,b).Reverse Osmosis (RO) units can be used as stand-alone arsenic treatment under mostwater quality conditions. Most RO membranes are made of cellulose acetate or polyamidecomposites cast into a thin film. The semi-permeable (non-porous) membrane is then constructedinto a cartridge called an RO module, typically either hollow-fiber or spiral-wound. It is a pressure-drivenmembrane separation process capable of removing dissolved solutes from water bymeans of particle size, dielectric characteristics and hydrophilicity/hydrophobicity. Reverse osmosisis capable of achieving over 97% removal of As (V) and 92% removal of As (III) in a92NIH & CGWB
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ACKNOWLEDGEMENTThe Government of In
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nearly 33 villages in 4 districts i
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(iii) Whether ex-situ removal techn
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6.4 Experiences during Arsenic Remo
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List of FiguresFigure no. Figure ca
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List of TablesTable no. Table Capti
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Mitigation and Remedy of Groundwate
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Mitigation and Remedy of Groundwater Arsenic Menace in India

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