INTRODUCTION
A ‘SAFE’ water supply can be defined as one that provides water that is free fromchemicals injurious to human body including Arsenic and biological contamination. Theonus of supplying safe drinking water meeting primary health based standards lieswith the water management authorities. The contamination of ground water by geogenicleached out Arsenic has assumed an alarming proportion in several countries includingIndia and Bangladesh. It has been proved that continued and prolonged ingestion evenat a very low level (WHO limit is 0.01 mg/L) can lead to serious arsenic related diseases.Though literature abounds in occurrence of ground water contamination by Arsenic andits removal from ground water by different technological options, millions of peoplecontinue to suffer particularly in the developing countries.
The most important remedial action is prevention of further exposure by providing safedrinking water. The cost and difficulty of reducing arsenic in drinking water increasesas the targeted concentration lowers. In India earlier the desirable limit of Arsenic was0.05 mg/L which has been reduced to 0.01 mg/L by BIS).It also varies with the arsenicconcentration in the source water ,the chemical matrix of the water includinginterfering solutes, availability of alternative sources of low arsenic water ,mitigationtechnologies and amount of water to be treated .
The control of arsenic in drinking water is more complex where drinking water isobtained from many individual sources (such as hand pumps and wells) as is commonin rural India. Low Arsenic water is only needed for drinking and cooking. Arsenic richwater can be used to limited extent for laundry and bathing.
TECHNOLOGY OPTIONS FOR THE SAFE WATER SUPPLY IN THE ARSENICAFFECTED AREA
The technology options for the safe water supply in the arsenic affected area could beone of the following
(i) Using surface water sources such as ponds, dug wells, rivers etc.
(ii) Tapping alternate aquifer for arsenic free ground water.
(iii) Artificial recharge
(iv) Removal of Arsenic from ground water
i) Supply of Surface Water
Supply of surface water sources such as ponds, dug wells, rivers etc. through pipe network system after purification by conventional method of treatment viz. coagulation,flocculation, rapid sand filtration and disinfection. Horizontal roughing filter with slowsand filter may also be adopted using pond water.
ii) Tapping Alternate Aquifer For Arsenic Free Ground Waterbr>Installation of deep tube well is one of the best options depending upon the feasibility asArsenic contamination has been found mainly in shallow aquifers.
This will depend upon the local geo hydrological condition as for safe arsenic freesupply; the upper arseniferous aquifer has to be cement sealed if proper imperviouslayer is available to prevent percolation of arsenic contaminated ground water from thetop aquifer, as concerns about cross-examination of the deeper aquifers by arsenicseeping from shallow aquifers remain significantly important. The isotopic studiescarried out in West Bengal by CGWB with BARC has proved that there is a widedifference between the age of shallow contaminated ground water and deeper arsenicfree aquifers.
The two options are being adopted wherever possible notwithstanding the hugefinancial requirements for the river water passed piped water scheme or deep tubewells.
(iii) Rain Water Harvesting
Rainwater harvesting may be adopted if an appropriate roof is available to facilitatecollection with introduction of line filter and intermittent disinfection. Rainwaterharvesting can also be utilized in some areas where sufficient rainfall is available formost the time and the subsurface geology is suitable for the same. Rain waterharvesting can potentially suffer from microbiological contamination and may requiresome treatment to ensure acceptable quality.
(iv) Removal of Arsenic from Ground Water
The treatment of tube water for removal of Arsenic has not been applied in a big way.Though the number of Indian institutes and organization from abroad has developedtechnologies for removal of arsenic, the community at large and water supplyprofessional are not adequately informed for the same.
SUITABILITY OF ARSENIC REMOVAL PLANTS
Before attempting to use any of Arsenic removal plants ,one has to ensure the followingpoints:
1. The plant should have high efficiency as far as removal of Arsenic is concerned
2. The technology to be used should be safe
3. It should be cost effective
4. It should produce minimum residual mass.
5. Sufficient life operation and
6. It must be users friendly.
TECHNOLOGIES AVAILABLE FOR REMOVAL OF ARSENIC
There are several technologies available by which Arsenic is removed from drinkingwater . The principle of such processes for on surface and in-situ removal include Coprecipitation, ion exchange, adsorption, membrane separation ,bioremediation andoxidation of Arsenic ( III ) and Iron (II ). There are considerable applications of thesesmethodologies in up gradation of water quality. In this connection, it would beappropriate to explore thoroughly all the possible methods for removal of arsenic fromdrinking water and to arrive at An appropriate technology which could be effectivelyused for up gradation of water quality in Arsenic affected areas besides, appropriatetechnology must be economically viable and socially acceptable.
The following are the different techniques available for removal of Arsenic from drinkingwater.
(i) Oxidation of Arsenic (III)
(ii) Coagulation-flocculation-Sedimentation-filtration (Co-precipitation)
(iii) Ion exchange
(iv) Adsorption on different media
(v) Reverse Osmosis and electro dialysis
(vi) Bioremediation and
(vii) In- situ remediation
(viii) Passive sedimentation
(ix) Solar oxidation
(x) Iron coated sand
(xi) Low pressure Nanofiltration
(i) Oxidation of Arsenic (III)
The common valency of Arsenic in raw water sources are + 3 (Arsenite) and +5(Arsenate) as are evident in the inorganic hydrolysis species such as H3 AsO3 , H2AsO 3, HAsO3-2, and AsO3-3 and H3 AsO4, H2 AsO4, H2 AsO4-2 and AsO4-3.
In geogenic Arsenic the above mentioned 2 valance forms mainly concern. Thechemical behaviours of the two forms are different and as such during removal ofarsenic concentration each redox species need to be estimated. Different studiesindicate that Arsenic (III) can not be removed from water effectively.
Oxidation of As(III) by dissolved oxygen in water is a very slow process . But effectiveremoval of arsenic from water requires complete oxidation of As(III). The redoxreaction is,
H3 AsO4 + 2 H + 2e = H3 AsO3 + H2O E0 = 0.56 V
Accordingly selection of appropriate oxidizing agent is very important. The followingcriteria are required to be considered for selection of appropriate oxidizing agent.
(i) Residual effect
(ii) Oxidation by product
(iii) Oxidation of other in organic and organic constituents
(iv) Reaction kinetics
The following oxidizing agent could be used for conversion of arsenite to arsenate
(a) Oxygen : Aeration process may help in oxidizing arsenite but the process is veryslow(Clifford et.al 1983)
(b) Powdered active carbon as dissolved oxygen catalytic oxidation: The processrequire very high quantum of powdered active carbon need to be removed.
(c) UV radiation: Requires high pressure mercury lamp the process is quite fastorganic compounds if present in water may get oxidized. Application of theprocess in domestic unit as well as community models is not feasible in ruralArea
(d) Chemicals: Free chlorine, Hypochlorite, Bleaching powder, , Permanganate andhydrogen peroxide can be used. Bleaching powder solution or sodiumhypochlorite could be used for oxidation which is readily available..Potassiumpermanganate is very effective for oxidation of arsenite but it may develop somefaint colour.(Viraghvan and Pokhrel,2006)
(e) Ozone: Ozone dose of approximately2000 micro gram /L is suitable for 70mg/Lof Arsenic prior to filtration (Kim and Nriag 1999). Application of Ozone would beCostly
(f) Sunlight: In the presence of sunlight and natural occurring high absorbingminerals the rate of oxidation of Arsenic(III) by oxygen can be increased.
(ii) Coagulation-Flocculation-Sedimentation & Filtration (co precipitation)
In water treatment aluminum or ferric salts are used for coagulation of particles andcolloids in the water . Arsenic removal by metal ions is the best known and mostfrequent.As such for the removal of arsenic from water Aluminium or ferric salts areadded. Both metal salts undergo hydrolysis to various products, but can be reduced tovery low residual if the poorly soluble hydroxides are formed at the proper pH and canbe filtered off completely. For removal of Arsenic (V) ferric salt is slightly more effectivethan aluminium salt. While the arsenic removal efficiency with application ofaluminium salt is 90 to 95 %, whereas with ferric salts it may be 95 to99%.(Heringet.al. 1997)
(iii) Ion Exchange
Ion exchange resins can be used to remove As (V). During flow through resin As (III) ispassed through column of anion exchange resin whereas As(V) is found effectively onresin. It works best when As (III) got oxidized to As (V) and performs simultaneousremoval of arsenic,iron and bacteria from water.The process is normally used forremoval of specific undesirable cation or anion from water. As the resin becomesexhausted ,it needs to be regenerated. The arsenic exchange and regenerationequations with common salt solution as regeneration agents are as follows
Arsenic exchange
2R-Cl + HAsO4- = R2 HAsO4 + 2 Cl-
Regeneration
R2 HAsO4 + 2 Cl- + 2Na+ = 2R-Cl + HAsO4- + 2 Na+
The frequency of regeneration or replacement of resin depends upon the quantum ofiron present in water. The arsenic removal capacity is dependent on sulphate andnitrate contents of raw water as sulphate and nitrate are exchanged before arsenic.Theefficiency improves by pre oxidation As (III) to As(V) and process is less dependent onpH of water.
(iv) Adsorption
Effective arsenic removal could be obtained by using activated alumina. As (V) isadsorbed effectively by activated alumina whereas arsenic (III) remains unabsorbed.However if iron is present in ground water along with arsenic which is very common,than considerable amount of arsenic (III) may be removed during filtration throughactivated alumina bed . The best removal is possible between pH 5.5 to 9.0. (Giffordet.al.1983)The mechanism which is one of the exchanges of contaminants anions forsurface hydroxide on aluminum is generally called adsorption. The typical activatedalumina used in water treatment is 0.3 to 0.6 mm size. These are mixture of amorphousand gamma aluminium oxide prepared by low temperature 9300-600 oC ) dehydrationof Al (OH) 3. By using the model of hydroxylated alumina surface subject to protonationand deportation. The following legend –exchange reaction can be written to arsenicadsorption in acid solution alumina exhaustion in which Al represents the aluminasurface and over bar denotes solid surface
Al.OH + H+ + H2AsO4 Al. H2AsO4 + HOH
The equation for arsenic desorption by hydroxide, Alumina regeneration is
Al. H2AsO4 + OH- Al.OH + H2AsO4
Activated alumina processes are sensitive to pH and anions are best adsorbed belowpH 8.5 ,a typical pH corresponding to zero point of charge( ZPC) below which thealumina surface has a net positive charge. Above pHzpc alumina is predominantly acation exchanger.
Cement based stabilization is suitable for the disposal of arsenic containing sludge.Attempt have been made to stabilize arsenic laden sludge with cement and sand.Activated carbon can adsorb arsenic if water is passed through fixed bed. Theperformance of activated carbon is not that satisfactory as the regeneration of the bed isvery difficult.
Granulated ferric hydroxide has been used widely but iron should be preferablyremoved before subjecting water to ferric hydroxide treatment .Granular ferrichydroxide is prepared from ferric chloride solution by neutralization and precipitationwith sodium hydroxide. It is poorly crystallized - FeOOH with a specific surface of 250-300 m2/g and a porosity of 75-80%. The grain size ranges from 0.2 to 2.0 mm. As nodrying procedure is included in its preparation, all the pores are completely filled withwater, leading to a high density of available adsorption sites And thus to a highadsorption capacity. Phosphate competes strongly with arsenic. So its presence in rawwater may reduce the arsenate adsorption capacity.
The residue is a solid waste with an arsenic content of 1-10 g/kg. The spent adsorptionbeing non toxic and its volume being small its disposal is less problematic .Bothdomestic and community based plants are available.
A highly efficient process technology for simultaneous removal of arsenic and iron inground water has been designed by C.G.C.R.I. based on the principle of adsorptionusing suspended media in colloidal form and efficient cross-flow microfiltration byceramic membranes.
A simpler and less expensive form of arsenic removal using 3 pitchers containing castiron filling and sand in the first pitcher and wood activated carbon and sand in thesecond has been developed known as Sono arsenic filter. Plastic buckets can also beused .
(v) Reverse Osmosis & Electro dialysisBoth reverse osmosis and electro dialysis process can be used but it has been foundthat As (V) is effectively removed (95-98%) while As (III) is only partially separated (46-75%) due to neutral form of As (III) as H3 AsO3.
(vi) Bioremediation
Artificial stimulation of metabolism of indigenous sulphate reducing bacteria (SRB) hasthe potential to remediate the ground water loaded with arsenic./ This sulphatereducing technology takes advantage of anaerobic heterotrophic bacteria alreadypresent, though it requires nutrients to stimulate metabolism. Soluble organic carbon isrequired for the purpose.
(vii) In situ remediation
The subsurface removal of arsenic has been practiced in some countries. It is usuallylinked to artificial recharge. This relies on the strong adsorption of As especially As (V)by iron (III) oxides that are formed when reduced near neutral sediments and groundwater are oxidized. The oxidation zone created by aerated water boosts the activity ofthe arsenic oxidizing microorganism which can oxidize arsenic fromAs (III) to As(V).Theoxidation can be brought about by the injection of air or an oxidizing agent such ashydrogen peroxide. In Vyredox method for removal of iron a ring of wells injects aeratedwater around a central supply well . The iron precipitates, thus arsenic also in outerpart of the aquifer furthest from the supply well . Clogging of the aquifer is generally nota problem in the life time of the plant. In-situ oxidation of arsenic and iron in theaquifer has also been tried. The aerated tube well water is stored in a tank andreleased back in to the aquifers through the tube wellby opening a valve in a pipeconnecting the water tank to the tube well pipe under the pump head. The dissolvedoxygen in water oxidized arsenite to less mobile arsenate and also the ferrous iron inthe aquifer to ferric iron ,resulting in reduction of arsenic and iron both.
(viii) Passive sedimentation
It is one of the easiest method for removal of Arsenic in ground water. Oxidation ofwater during collection and subsequent storage in houses may cause a reduction inarsenic concentration in stored water. The concentration can be reduced to even zero bypassive sedimentation.The use of naturally occurring iron precipitates in ground waterwhich helps in removing arsenic by adsorption. Although no good correlation betweenconcentration of dissolved iron and arsenic has been derived. Iron and arsenic havebeen found to co exist in ground water. Arsenic reduction by sedimentation appears tobe dependent on water quality particularly the presence of ferrous iron. Ahmed et.al.2000.
(ix) SOLAR Oxidation
SORAS is a simple method of solar oxidation of Arsenic in transparent bottles to reducearsenic content of the drinking water Weglin et.al. 2000. Ultra violet radiation cancatalyze the process of oxidation of arsenite in presence of other oxidants like oxygen.
Water treatment with coagulants such as aluminium alum Al2(SO4)3.18 H2O, ferricchloride,FeCl3 and ferric sulphate Fe2(SO4)3.7 are effective in removing arsenic fromwater. H2O(x) Iron Coated Sand
Iron coated sand based treatment units for removal of arsenic has been attempted. It isprepared as suggested by Joshi and Choudhuri1996. The iron content of iron coatedsand was found to be 25 mg/g of sand. Raw water having 300 micro gram /L of arsenicwhen filtered through iron coated sand becomes essentially arsenic free. As such ironcoated sand is equally effective in removing both As (III) and As(V).(Jiang,2001)
The removal of As (III) by haemetite ( particle size around 200mm) has also been triedbut the maximum capacity has been found about 2.6 mmol/kg Feldspar has also usedfor removal of arsenic. These studies indicate the possible application of natural oxidesfor removal of arsenic but their small specific surface are limits their capacity ofadsorption.
(xi) Low pressure Nanofiltration
Nano filtration membrane process for the treatment of arsenic contaminated waterapplying low pressure has been found suitable. This works better with As (V) hence preoxidation of As (III) is recommended(Oh et.al.2000).BARC has also developedultrafiltration (UF) based membrane technology for water decontamination .
Issues related to management of the arsenic contaminated waste generated by thesetechnologies has not fully resolved yet. It may be understood that no process can makearsenic zero in the environment and requires extensive research to tackle the sludgeproduced.
CONCLUSION
The supply of safe and arsenic free water to the population in the affected area is aserious challenge to the planners and the water management people specially for thosewho are responsible for community water supply.
It is important to point out each of the safe water options have some challenges in itsimplementation on a large scale. The capital costs and the cost associated with effectiveoperation and maintenance of each option has to be carefully weighed. Water supplyexperts recently recommended that piped water supply should be deemed as theeventual target, but any of the other options can be utilized locally in accordance withthe persisting local conditions.
REFERENCES
• Ahmed M.F, et.al. (2000) An overview of arsenic removal technologies in BUET,Bangladesh Environment-2000, M.F. Ahmed (Ed.) Bangladesh Poribesh Andolon177-188
• Clifford,D.A. e.al.(1983) “Arsenic (III)/Arsenic (V) sepration by chloride IonExchange Resins XI AWWA Water Quality Technology Conference,NorflokDecember,223-236
• Hering,J.G. et.al. (1997) Arsenic removal from drinking water during coagulation ,J.of Environmental Engineering 8,pp800-807
• Jiang,J.Q. (2001) Removing arsenic from ground water for the developing world-Areview Water Science and Technology 44,89-98
• Kim,M.J. and Nriagu,J (1999) “ Oxidation of Arsenite in ground water using ozoneand oxygen” Science and Total Environment,247,pp.71-79.
• Oh,J.I. et.al (2000) Modelling of arsenic rejection considering affinity and sterichindrance effect in nanofiltration membranes, Water Science and Technology,42,3-4:173-180
• Viraghvan,T., and Pokhrel,S.(2006) Arsenic removal from an aqueous solution by amodified Fungal Biomass ,Water Research40,pp549-552
• Wagelin,Met.al. (2000 SORAS –a simple arsenic removal process(http//phys4.harvard.edu/-wilson/mitigation/SORAS_Paper.html)
K.K.Srivastava, A. K.Chattopadhyay, B.C. Mehta - Central Ground Water Board, ER, KolkataPosted by
