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Technology Options for Utilisation of Arsenic Contaminated Ground Water

Author: 
K.K.Srivastava, A. K.Chattopadhyay, B.C. Mehta
Source: 
Bhujal News Quarterly Journal, April-Sept, 2009

INTRODUCTION


A ‘SAFE’ water supply can be defined as one that provides water that is free from chemicals injurious to human body including Arsenic and biological contamination. The onus of supplying safe drinking water meeting primary health based standards lies with the water management authorities. The contamination of ground water by geogenic leached out Arsenic has assumed an alarming proportion in several countries including India and Bangladesh. It has been proved that continued and prolonged ingestion even at 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 and its removal from ground water by different technological options, millions of people continue to suffer particularly in the developing countries.

The most important remedial action is prevention of further exposure by providing safe drinking water. The cost and difficulty of reducing arsenic in drinking water increases as the targeted concentration lowers. In India earlier the desirable limit of Arsenic was 0.05 mg/L which has been reduced to 0.01 mg/L by BIS).It also varies with the arsenic concentration in the source water ,the chemical matrix of the water including interfering solutes, availability of alternative sources of low arsenic water ,mitigation technologies and amount of water to be treated .

The control of arsenic in drinking water is more complex where drinking water is obtained from many individual sources (such as hand pumps and wells) as is common in rural India. Low Arsenic water is only needed for drinking and cooking. Arsenic rich water can be used to limited extent for laundry and bathing.

TECHNOLOGY OPTIONS FOR THE SAFE WATER SUPPLY IN THE ARSENIC AFFECTED AREA


The technology options for the safe water supply in the arsenic affected area could be one 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 net work system after purification by conventional method of treatment viz. coagulation, flocculation, rapid sand filtration and disinfection. Horizontal roughing filter with slow sand filter may also be adopted using pond water.

ii) Tapping Alternate Aquifer For Arsenic Free Ground Water<>br> Installation of deep tube well is one of the best options depending upon the feasibility as Arsenic contamination has been found mainly in shallow aquifers.

This will depend upon the local geo hydrological condition as for safe arsenic free supply; the upper arseniferous aquifer has to be cement sealed if proper impervious layer is available to prevent percolation of arsenic contaminated ground water from the top aquifer, as concerns about cross-examination of the deeper aquifers by arsenic seeping from shallow aquifers remain significantly important. The isotopic studies carried out in West Bengal by CGWB with BARC has proved that there is a wide difference between the age of shallow contaminated ground water and deeper arsenic free aquifers.

The two options are being adopted wherever possible notwithstanding the huge financial requirements for the river water passed piped water scheme or deep tube wells.

(iii) Rain Water Harvesting
Rainwater harvesting may be adopted if an appropriate roof is available to facilitate collection with introduction of line filter and intermittent disinfection. Rainwater harvesting can also be utilized in some areas where sufficient rainfall is available for most the time and the subsurface geology is suitable for the same. Rain water harvesting can potentially suffer from microbiological contamination and may require some 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 developed technologies for removal of arsenic, the community at large and water supply professional 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 following points:

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 drinking water . The principle of such processes for on surface and in-situ removal include Coprecipitation, ion exchange, adsorption, membrane separation ,bioremediation and oxidation of Arsenic ( III ) and Iron (II ). There are considerable applications of theses methodologies in up gradation of water quality. In this connection, it would be appropriate to explore thoroughly all the possible methods for removal of arsenic from drinking water and to arrive at An appropriate technology which could be effectively used for up gradation of water quality in Arsenic affected areas besides, appropriate technology must be economically viable and socially acceptable.

The following are the different techniques available for removal of Arsenic from drinking water.

(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 , H2 AsO 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. The chemical behaviours of the two forms are different and as such during removal of arsenic concentration each redox species need to be estimated. Different studies indicate 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 effective removal of arsenic from water requires complete oxidation of As(III). The redox reaction is,

H3 AsO4 + 2 H + 2e = H3 AsO3 + H2O E0 = 0.56 V

Accordingly selection of appropriate oxidizing agent is very important. The following criteria 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 very slow(Clifford et.al 1983)
(b) Powdered active carbon as dissolved oxygen catalytic oxidation: The process require very high quantum of powdered active carbon need to be removed.
(c) UV radiation: Requires high pressure mercury lamp the process is quite fast organic compounds if present in water may get oxidized. Application of the process in domestic unit as well as community models is not feasible in rural Area
(d) Chemicals: Free chlorine, Hypochlorite, Bleaching powder, , Permanganate and hydrogen peroxide can be used. Bleaching powder solution or sodium hypochlorite could be used for oxidation which is readily available..Potassium permanganate is very effective for oxidation of arsenite but it may develop some faint colour.(Viraghvan and Pokhrel,2006)
(e) Ozone: Ozone dose of approximately2000 micro gram /L is suitable for 70mg/L of Arsenic prior to filtration (Kim and Nriag 1999). Application of Ozone would be Costly
(f) Sunlight: In the presence of sunlight and natural occurring high absorbing minerals 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 and colloids in the water . Arsenic removal by metal ions is the best known and most frequent.As such for the removal of arsenic from water Aluminium or ferric salts are added. Both metal salts undergo hydrolysis to various products, but can be reduced to very low residual if the poorly soluble hydroxides are formed at the proper pH and can be filtered off completely. For removal of Arsenic (V) ferric salt is slightly more effective than aluminium salt. While the arsenic removal efficiency with application of aluminium salt is 90 to 95 %, whereas with ferric salts it may be 95 to 99%.(Heringet.al. 1997)

(iii) Ion Exchange
Ion exchange resins can be used to remove As (V). During flow through resin As (III) is passed through column of anion exchange resin whereas As(V) is found effectively on resin. It works best when As (III) got oxidized to As (V) and performs simultaneous removal of arsenic,iron and bacteria from water.The process is normally used for removal of specific undesirable cation or anion from water. As the resin becomes exhausted ,it needs to be regenerated. The arsenic exchange and regeneration equations 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 of iron present in water. The arsenic removal capacity is dependent on sulphate and nitrate contents of raw water as sulphate and nitrate are exchanged before arsenic.The efficiency improves by pre oxidation As (III) to As(V) and process is less dependent on pH of water.

(iv) Adsorption
Effective arsenic removal could be obtained by using activated alumina. As (V) is adsorbed 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 through activated alumina bed . The best removal is possible between pH 5.5 to 9.0. (Gifford et.al.1983)The mechanism which is one of the exchanges of contaminants anions for surface hydroxide on aluminum is generally called adsorption. The typical activated alumina used in water treatment is 0.3 to 0.6 mm size. These are mixture of amorphous and gamma aluminium oxide prepared by low temperature 9300-600 oC ) dehydration of Al (OH) 3. By using the model of hydroxylated alumina surface subject to protonation and deportation. The following legend –exchange reaction can be written to arsenic adsorption in acid solution alumina exhaustion in which Al represents the alumina surface 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 below pH 8.5 ,a typical pH corresponding to zero point of charge( ZPC) below which the alumina surface has a net positive charge. Above pHzpc alumina is predominantly a cation 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. The performance of activated carbon is not that satisfactory as the regeneration of the bed is very difficult.

Granulated ferric hydroxide has been used widely but iron should be preferably removed before subjecting water to ferric hydroxide treatment .Granular ferric hydroxide is prepared from ferric chloride solution by neutralization and precipitation with 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 no drying procedure is included in its preparation, all the pores are completely filled with water, leading to a high density of available adsorption sites And thus to a high adsorption capacity. Phosphate competes strongly with arsenic. So its presence in raw water may reduce the arsenate adsorption capacity.

The residue is a solid waste with an arsenic content of 1-10 g/kg. The spent adsorption being non toxic and its volume being small its disposal is less problematic .Both domestic and community based plants are available.

A highly efficient process technology for simultaneous removal of arsenic and iron in ground water has been designed by C.G.C.R.I. based on the principle of adsorption using suspended media in colloidal form and efficient cross-flow microfiltration by ceramic membranes.

A simpler and less expensive form of arsenic removal using 3 pitchers containing cast iron filling and sand in the first pitcher and wood activated carbon and sand in the second has been developed known as Sono arsenic filter. Plastic buckets can also be used .

(v) Reverse Osmosis & Electro dialysis Both reverse osmosis and electro dialysis process can be used but it has been found that 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) has the potential to remediate the ground water loaded with arsenic./ This sulphate reducing technology takes advantage of anaerobic heterotrophic bacteria already present, though it requires nutrients to stimulate metabolism. Soluble organic carbon is required for the purpose.

(vii) In situ remediation
The subsurface removal of arsenic has been practiced in some countries. It is usually linked 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 ground water are oxidized. The oxidation zone created by aerated water boosts the activity of the arsenic oxidizing microorganism which can oxidize arsenic fromAs (III) to As(V).The oxidation can be brought about by the injection of air or an oxidizing agent such as hydrogen peroxide. In Vyredox method for removal of iron a ring of wells injects aerated water around a central supply well . The iron precipitates, thus arsenic also in outer part of the aquifer furthest from the supply well . Clogging of the aquifer is generally not a problem in the life time of the plant. In-situ oxidation of arsenic and iron in the aquifer has also been tried. The aerated tube well water is stored in a tank and released back in to the aquifers through the tube wellby opening a valve in a pipe connecting the water tank to the tube well pipe under the pump head. The dissolved oxygen in water oxidized arsenite to less mobile arsenate and also the ferrous iron in the 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 of water during collection and subsequent storage in houses may cause a reduction in arsenic concentration in stored water. The concentration can be reduced to even zero by passive sedimentation.The use of naturally occurring iron precipitates in ground water which helps in removing arsenic by adsorption. Although no good correlation between concentration of dissolved iron and arsenic has been derived. Iron and arsenic have been found to co exist in ground water. Arsenic reduction by sedimentation appears to be 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 reduce arsenic content of the drinking water Weglin et.al. 2000. Ultra violet radiation can catalyze 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, ferric chloride,FeCl3 and ferric sulphate Fe2(SO4)3.7 are effective in removing arsenic from water. H2O (x) Iron Coated Sand
Iron coated sand based treatment units for removal of arsenic has been attempted. It is prepared as suggested by Joshi and Choudhuri1996. The iron content of iron coated sand was found to be 25 mg/g of sand. Raw water having 300 micro gram /L of arsenic when filtered through iron coated sand becomes essentially arsenic free. As such iron coated 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 tried but the maximum capacity has been found about 2.6 mmol/kg Feldspar has also used for removal of arsenic. These studies indicate the possible application of natural oxides for removal of arsenic but their small specific surface are limits their capacity of adsorption.

(xi) Low pressure Nanofiltration
Nano filtration membrane process for the treatment of arsenic contaminated water applying low pressure has been found suitable. This works better with As (V) hence pre oxidation of As (III) is recommended(Oh et.al.2000).BARC has also developed ultrafiltration (UF) based membrane technology for water decontamination .

Issues related to management of the arsenic contaminated waste generated by these technologies has not fully resolved yet. It may be understood that no process can make arsenic zero in the environment and requires extensive research to tackle the sludge produced.

CONCLUSION


The supply of safe and arsenic free water to the population in the affected area is a serious challenge to the planners and the water management people specially for those who are responsible for community water supply.

It is important to point out each of the safe water options have some challenges in its implementation on a large scale. The capital costs and the cost associated with effective operation and maintenance of each option has to be carefully weighed. Water supply experts recently recommended that piped water supply should be deemed as the eventual target, but any of the other options can be utilized locally in accordance with the 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 Andolon 177-188
• Clifford,D.A. e.al.(1983) “Arsenic (III)/Arsenic (V) sepration by chloride Ion Exchange Resins XI AWWA Water Quality Technology Conference,Norflok December,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-A review Water Science and Technology 44,89-98
• Kim,M.J. and Nriagu,J (1999) “ Oxidation of Arsenite in ground water using ozone and oxygen” Science and Total Environment,247,pp.71-79.
• Oh,J.I. et.al (2000) Modelling of arsenic rejection considering affinity and steric hindrance 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 a modified 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)

Blocks Locations of Arsenic in ground water K.K.Srivastava, A. K.Chattopadhyay, B.C. Mehta - Central Ground Water Board, ER, Kolkata

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