Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Documentstage. The optimal pH ranges for coagulation, with aluminum and ferric salts, are 5 to 7 and 5 to8, respectively. At pH values above 7, the removal performance of aluminum-based coagulantsdrops markedly. Feed water pH should be adjusted to the appropriate range, prior to coagulantaddition. Post-filtration pH adjustment may be necessary to optimize corrosion control andcomply with other regulatory requirements.Several batch studies have demonstrated that As (V) removal is positively related tocoagulant dosage. However, specific dose requirements, needed to meet As (V) removalobjectives, were contingent upon the source water quality and pH. Effective coagulant dosageranges were 5-25 mg/L of ferric chloride and as much as 40 mg/L of alum.5.2.2.3 Coagulation Assisted Micro-filtrationCoagulation-assisted micro-filtration uses the same coagulation process described above.However, instead of the granular media filtration step, the water is forced through asemi-permeable membrane by a pressure differential. The membrane retains the As (V) ladenfloc formed in the coagulation step.The use of pre-engineered coagulation assisted microfiltration package plants is arealistic possibility for new installations, where water quality precludes the use of sorptiontreatment. The membrane must be periodically backwashed to dislodge solids and restorehydraulic capacity. Backwash water is typically a high-volume, low solids (less than 1.0%)waste stream. The specific amount of solids will depend on several factors, including coagulanttype, dosage, filter run length and ambient solids concentration.The co-precipitation technique for removal of arsenic has been applied in the followingsystems:• Central Arsenic Removal Plant (ARP) attached with tube wells for piped watersupply,• Arsenic Removal Plant (ARP) attached with Hand Pump, and• Domestic Arsenic Removal Units using Earthen Pots, Plastic Buckets, BucketTreatment Units, Modified BTUs, Stevens Institute Technology, Fill and Draw Units,Naturally Occurring Iron, Chemical Packages, etc.5.2.3 AdsorptionAdsorption technology has been widely used to treat groundwater and drinking watercontaining arsenic. The technology can reduce arsenic concentrations to less than 50 µg/L ingeneral and in some cases even below 10 µg/L. Its effectiveness is sensitive to a variety ofuntreated water contaminants and characteristics. It is used less frequently than precipitation/co-precipitation technologies, and is most commonly used to treat groundwater and drinkingwater.NIH & CGWB 85
Technological Options and Arsenic Removal TechnologiesIn adsorption technology, solutes (contaminants) concentrate at the surface of asorbent, thereby, reducing their concentration in the bulk liquid phase. The adsorption media isusually packed into a column. Contaminants are adsorbed, as the contaminated water is passedthrough the column. When adsorption sites get filled, the column must be regenerated ordisposed of and replaced with new media.Several adsorbents are available for removal of arsenic from water, viz., activated alumina,activated carbon, iron and manganese coated sand, kaolinite clay, hydrated ferric oxide,activated bauxite, titanium oxide, silicium oxide and many natural and synthetic ones. The efficiencyof sorptive media depends on the use of oxidizing agent as aids to sorption of arsenic.Saturation of media, by different contaminants and components of water, takes place at differenttimes of operation, depending on the specific sorption affinity of the medium to thegiven component.Adsorption is probably the most prospective technology for removal of arsenic. It largelydepends on capital cost, operation and maintenance cost, operational procedure and user-friendliness.The following media are commonly used for removal of arsenic throughadsorption technique:i) Activated alumina (AA),ii) Activated Carbon (AC),iii) Iron Based Sorbents (Granular Ferric Hydroxide, Iron Coated Sand, etc.),iv) Indigenous Filters, andv) Cartridge Filters.5.2.3.1 Activated Alumina (AA)Activated Alumina (AA), Al 2O 3, is a porous, granular material having good sorptionproperties. The media, aluminum trioxide, is prepared through the dehydration of aluminum hydroxideat high temperatures. AA grains have a typical diameter of 0.3 to 0.6 mm and a highsurface area for sorption. Activated alumina is commonly used to remove arsenic fromdrinking water and ground water (EPA, 2000a, b). The reported adsorption capacity of AAranges from 0.003 to 0.112 grams of arsenic per gram of AA. It is available in different meshsizes and its particle size affects contaminant removal efficiency. Up to 23,400 bed volumes ofwastewater can be treated before AA requires regeneration or disposal and replacement withnew media.When water passes through a packed column of activated alumina, the impurities includingarsenic present in water are adsorbed on the surfaces of activated alumina grains. Theselectivity of AA towards As (III) is poor, owing to the overall neutral molecular charge at pHlevels below 9.2. Therefore, pre-oxidation of As (III) to As (V) is critical. Several differentstudies have established the optimum pH range as 5.5-6.0, and demonstrated greater than 98%arsenic removal under these conditions. AA column runs operated under acidic pH conditions are 5 to 20 times longer than under natural pH conditions (6.0-9.0).86NIH & 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 Groundwater Arsenic Menace in India

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