Arsenic is one of the most toxic metals in the environment and is of great interest in environmental studies. Arsenic is the root cause of many diseases and disorders.
Technologies
The technologies under review perform most effectively when treating arsenic in the form of As(V). As (III) may be converted through pre-oxidation to As(V). Data on oxidants indicate that chlorine, ferric chloride, and potassium permanganate are effective in oxidizing As(III) to As(V). Pre-oxidation with chlorine may create undesirable concentrations of disinfection by-products. Ozone and hydrogen peroxide should oxidize As(III) to As(V), but no data are available on performance.
Coagulation/Filtration (C/F), is an effective treatment process for removal of As(V) according to laboratory and pilot-plant tests. The type of coagulant and dosage used affects the efficiency of the process. Within either high or low pH ranges, the efficiency of C/F is significantly reduced. Alum performance is slightly lower than ferric sulfate. Other coagulants were also less effective than ferric sulfate. Disposal of the arsenic-contaminated coagulation sludge may be a concern especially if nearby landfills are unwilling to accept such a sludge.
Lime Softening (LS) operated within the optimum pH range of greater than 10.5 is likely to provide a high percentage of As removal for influent concentrations of 50 µg/L. However, it may be difficult to reduce consistently to 1 µg/L by LS alone. Systems using LS may require secondary treatment to meet that goal.
Activated Alumina(AA) is effective in treating water with high total dissolved solids (TDS). However, selenium, fluoride, chloride, and sulfate, if present at high levels, may compete for adsorption sites. AA is highly selective towards As(V); and this strong attraction results in regeneration problems, possibly resulting in 5 to 10 percent loss of adsorptive capacity for each run. Application of point-of-use treatment devices would need to consider regeneration and replacement.
Ion Exchange (IE) can effectively remove arsenic. However, sulfate, TDS, selenium, fluoride, and nitrate compete with arsenic and can affect run length. Passage through a series of columns could improve removal and decrease regeneration frequency. Suspended solids and precipitated iron can cause clogging of the IE bed. Systems containing high levels of these constituents may require pretreatment.
Reverse Osmosis (RO) provided removal efficiencies of greater than 95 percent when operating pressure is at ideal psi. If RO is used by small systems in the western U. S., 60% water recovery will lead to an increased need for raw water. The water recovery is the volume of water produced by the process divided by the influent stream (product water/influent stream). Discharge of reject water or brine may also be a concern. If RO is used by small systems in the western U. S., water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.
Electrodialysis Reversal (EDR) is expected to achieve removal efficiencies of 80 percent. One study demonstrated arsenic removal to 3 µg/L from an influent concentration of 21 µg/L.
Nanofiltration (NF) was capable of arsenic removals of over 90%. The recoveries ranged between 15 to 20%. A recent study showed that the removal efficiency dropped significantly during pilot-scale tests where the process was operated at more realistic recoveries. If nanofiltration is used by small systems in the western U. S., water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.
Point of Use/Point of Entry (POU/POE)The 1996 SDWA amendments specifically identify Point-of-Use (POU) and Point-of-Entry (POE) devices as options that can be used for compliance with NPDWRs. POU and POE devices can be effective and affordable compliance options for small systems in meeting a new arsenic MCL. A Federal Register notice is being prepared by EPA to delete the prohibition {§141.101} on the use of POU devices as compliance technologies. Because of this prohibition, few field studies exist on the application of POU and POE devices. One such case study was performed by EPA, in conjunction with the Village of San Ysidro, in New Mexico (Rogers 1990). The study was performed to determine if POU Reverse Osmosis (RO) units could satisfactorily function in lieu of central treatment to remove arsenic and fluoride from the drinking water supply of a small rural community of approximately 200 people. A RO unit, a common type of POU device, is a membrane system that rejects compounds based on their molecular properties and characteristics of the reverse osmosis membrane. The RO units removed 86% of the total arsenic.
Prospective Technologies
Ion Exchange with Brine Recycle. Research recently completed by the University of Houston (Clifford) at McFarland, CA and Albuquerque, NM has shown that ion exchange treatment can reduce arsenic (V) levels to below 2 µg/L even with sulfate levels as high as 200 mg/L. Sulfate does impact run length, however; the higher the sulfate concentration, the shorter the run length to arsenic breakthrough. The research also showed the brine regeneration solution could be reused as many as 20 times with no impact on arsenic removal provided that some salt was added to the solution to provide adequate chloride levels for regeneration. Brine recycle reduces the amount of waste for disposal and the cost of operation.
Iron (Addition) Coagulation with Direct Filtration. The University of Houston (Clifford) recently completed pilot studies at Albuquerque, NM on iron addition (coagulation) followed by direct filtration (microfiltration system) resulting in arsenic (V) being consistently removed to below 2 µg/L. Critical operating parameters are iron dose, mixing energy, detention time, and pH.
Conventional Iron/Manganese (Fe/Mn) Removal Processes. Iron coagulation/filtration and iron addition with direct filtration methods are effective for arsenic (V) removal. Source waters containing naturally occurring iron and/or manganese and arsenic can be treated for arsenic removal by using conventional Fe/Mn removal processes. These processes can significantly reduce the arsenic by removing the iron and manganese from the source water based upon the same mechanisms that occur with the iron addition methods. The addition of iron may be required if the concentration of naturally occurring iron/manganese is not sufficient to achieved the required arsenic removal level.
(Source: http://www.eng-consult.com/arsenic/treat1.htm)
Some people who drink water containing arsenic well in excess of the MCL for many years could experience skin damage or problems with their circulatory system, and may have an increased risk of getting cancer.
The following treatment method(s) have proven to be effective for removing arsenic to below 0.010 mg/L or 10 ppb: adsorption media, ion exchange, coagulation/filtration, oxidation/filtration, and point-of-use or point-of-entry treatment using activated alumina or reverse osmosis.
http://www.epa.gov/nrmrl/wswrd/dw/arsenic/pdfs/arsenic_report.pdf
arsenic below 0.010 mg/L can be effectively removed by adsorption media, ion exchange, coagulation, oxidation, and point-of-use or point-of-entry treatment using activated alumina or reverse osmosis.
Thanks. It has become one of the major environmental problems for people worldwide to be exposed to high arsenic concentrations through contaminated drinking water, and even the long-term intake of small doses of arsenic has a carcinogenic effect. As an efficient and economic approach for the purification of arsenic-containing water, the adsorbents in adsorption processes have been widely studied. Among a variety of adsorbents reported, the metal oxide heterostructures with high surface area and specific affinity for arsenic adsorption from aqueous systems have demonstrated a promising performance in practical applications. This review paper aims to summarize briefly the metal oxide heterostructures in arsenic removal from contaminated water, so as to provide efficient, economic, and robust solutions for water purification.http://www.hindawi.com/journals/jnm/2014/793610/
Technologies to remove As from water:
The technologies under review perform most effectively when treating arsenic in the form of As(V). As (III) may be converted through pre-oxidation to As(V). Data on oxidants indicate that chlorine, ferric chloride, and potassium permanganate are effective in oxidizing As(III) to As(V). Pre-oxidation with chlorine may create undesirable concentrations of disinfection by-products. Ozone and hydrogen peroxide should oxidize As(III) to As(V), but no data are available on performance.
Coagulation/Filtration (C/F), is an effective treatment process for removal of As(V) according to laboratory and pilot-plant tests. The type of coagulant and dosage used affects the efficiency of the process. Within either high or low pH ranges, the efficiency of C/F is significantly reduced. Alum performance is slightly lower than ferric sulfate. Other coagulants were also less effective than ferric sulfate. Disposal of the arsenic-contaminated coagulation sludge may be a concern especially if nearby landfills are unwilling to accept such a sludge.
Lime Softening (LS) operated within the optimum pH range of greater than 10.5 is likely to provide a high percentage of As removal for influent concentrations of 50 µg/L. However, it may be difficult to reduce consistently to 1 µg/L by LS alone. Systems using LS may require secondary treatment to meet that goal.
Activated Alumina(AA) is effective in treating water with high total dissolved solids (TDS). However, selenium, fluoride, chloride, and sulfate, if present at high levels, may compete for adsorption sites. AA is highly selective towards As(V); and this strong attraction results in regeneration problems, possibly resulting in 5 to 10 percent loss of adsorptive capacity for each run. Application of point-of-use treatment devices would need to consider regeneration and replacement.
Ion Exchange (IE) can effectively remove arsenic. However, sulfate, TDS, selenium, fluoride, and nitrate compete with arsenic and can affect run length. Passage through a series of columns could improve removal and decrease regeneration frequency. Suspended solids and precipitated iron can cause clogging of the IE bed. Systems containing high levels of these constituents may require pretreatment.
Reverse Osmosis (RO) provided removal efficiencies of greater than 95 percent when operating pressure is at ideal psi. If RO is used by small systems in the western U. S., 60% water recovery will lead to an increased need for raw water. The water recovery is the volume of water produced by the process divided by the influent stream (product water/influent stream). Discharge of reject water or brine may also be a concern. If RO is used by small systems in the western U. S., water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.
Electrodialysis Reversal (EDR) is expected to achieve removal efficiencies of 80 percent. One study demonstrated arsenic removal to 3 µg/L from an influent concentration of 21 µg/L.
Nanofiltration (NF) was capable of arsenic removals of over 90%. The recoveries ranged between 15 to 20%. A recent study showed that the removal efficiency dropped significantly during pilot-scale tests where the process was operated at more realistic recoveries. If nanofiltration is used by small systems in the western U. S., water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.
Point of Use/Point of Entry (POU/POE)The 1996 SDWA amendments specifically identify Point-of-Use (POU) and Point-of-Entry (POE) devices as options that can be used for compliance with NPDWRs. POU and POE devices can be effective and affordable compliance options for small systems in meeting a new arsenic MCL. A Federal Register notice is being prepared by EPA to delete the prohibition {§141.101} on the use of POU devices as compliance technologies. Because of this prohibition, few field studies exist on the application of POU and POE devices. One such case study was performed by EPA, in conjunction with the Village of San Ysidro, in New Mexico (Rogers 1990). The study was performed to determine if POU Reverse Osmosis (RO) units could satisfactorily function in lieu of central treatment to remove arsenic and fluoride from the drinking water supply of a small rural community of approximately 200 people. A RO unit, a common type of POU device, is a membrane system that rejects compounds based on their molecular properties and characteristics of the reverse osmosis membrane. The RO units removed 86% of the total arsenic.
Much efforts are being made towards arsenic removal from water sources. The extensive formation of microbial mats formed by filamentous iron oxidising bacteria like Gallionela and Leptothrix has been found to be an excellent substrate for adsorbing arsenic, phosphate from water. Further developments towards utilisation of these mats at industry scale can help effectively remove arsenic from water.
The link below discusses a recent method -
"a team at Lawrence Berkeley National Laboratory are developing a patented technology that uses electricity to remove arsenic from water, making it safe to consume. Citing the invention's efficiency and affordability, Luminous Water Technologies in India has deployed the technology to villages in India and Bangladesh."
http://www.sfgate.com/business/article/Arsenic-free-water-aided-by-Bay-Area-team-s-5302126.php
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