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Optimizing water resources in arsenic treatment processes
Arsenic is a semi-metal element. It is a tasteless and odorless mineral. It enters drinking water supplies from agricultural and industrial practices or from natural deposits in the earth, and enters natural waters through electronics production wastes, runoff from glass production wastes, runoff from orchards and erosion of natural deposits.
It is widely believed that naturally occurring arsenic dissolves out of certain rock formations when groundwater levels drop significantly. Some industries in the U.S. release thousands of pounds of arsenic into the environment every year. Once released, arsenic remains in the environment for a long time. It is removed from the air by rain, snow and gradual settling. Once on the ground or in surface water, it can slowly enter groundwater.
High arsenic levels in wells can originate from certain arsenic-containing fertilizers used in the past or from industrial waste. Its presence also may indicate improper well construction or overuse of chemical fertilizers or herbicides in the past.
The non-cancerous effects of arsenic include blindness, numbness in feet and hands, partial paralysis, diarrhea, vomiting, nausea, stomach pain and discoloration, and thickening of the skin. Arsenic has been linked to the occurrence of cancer of the prostate, liver, nasal passages, kidney, skin, lungs and bladder.
Arsenic in the U.S.
The arsenic standard for drinking water—set by the U.S. Environmental Protection Agency (EPA)—is 10 ppb to protect consumers served by public water systems from the effects of long-term exposure. This standard went into effect in January 2006.
Groundwater sources tend to more often demonstrate significant arsenic levels than surface water sources. The demand on groundwater from private water wells and municipal systems can cause water levels to drop and release arsenic from rock formations. Western states have more systems with arsenic levels greater than the 10-ppb standard than other areas of the U.S. Parts of the Midwest and New England also have some systems with current arsenic levels greater than 10 ppb. There are more systems in the Midwest and New England, however, that demonstrate arsenic levels that range from 2 to 10 ppb than in any other area of the U.S.
Arsenic occurs naturally in animals, plants, air, water, soil and rocks. It also can be released into the environment through natural activities such as forest fires, erosion, volcanic action or human activity. Ninety percent of the industrial arsenic in the U.S. currently is used as wood preservative, but it also is used in semi-conductors, soap, drugs, metals, dyes and paints. Animal feeding operations and certain fertilizers also can result in high arsenic levels in the environment. Industrial practices such as coal burning, mining and copper smelting contribute to arsenic in the environment as well.
The most common valence states of arsenic in water are arsenic (V), or arsenate, which is more widespread in aerobic surface waters, and arsenic (III), or arsenite, which is more likely to occur in anaerobic groundwater. In the pH range of 4 to 10, the predominant arsenic (III) compound is neutral in charge, while the arsenic (V) species are negatively charged. Removal efficiencies for arsenic (III) are poor compared with removal of arsenic (V) due to the negative charge.
The best method for removing arsenic from raw water—and the methodology that performs the most efficiently—is to treat it in the form of arsenic (V), because arsenic (III) can be converted through pre-oxidation to arsenic (V). Potassium permanganate, ferric chloride and chlorine are effective in oxidizing arsenic (III) to arsenic (V) for treatment. Pre-oxidation with chlorine, however, may create undesirable concentrations of disinfection byproducts. Ozone and hydrogen peroxide also will oxidize arsenic (III) to arsenic (V).
Coagulation and filtration are effective treatment processes for removal of arsenic (V), according to pilot plant and laboratory tests. However, the type of coagulant and dosage used affect the efficiency of the process. Within either high or low pH ranges, the efficiency of coagulation and filtration significantly is reduced. Alum performance is slightly lower than ferric sulfate. Other coagulants also are less effective than ferric sulfate, and the disposal of the arsenic-contaminated coagulation sludge can be a concern, especially if nearby landfills are unwilling to accept it.
Lime softening processes operated at a pH range of greater than 10.5 are likely to provide a high percentage of arsenic removal for influent concentrations of 50 ppb. It may be difficult, however, to reduce arsenic concentrations consistently to 1 ppb using lime softening alone. Systems using lime softening may require secondary treatment to meet the 1 ppb goal.
Coagulation, filtration and lime softening are not appropriate for most small systems because of the variability in process performance, need for well-trained operators and high cost. These processes may have difficulty consistently meeting the 10-ppb maximum contaminant level.
Ion exchange can be used as a polishing step in removing arsenic. Disposal of the sludge when utilizing this method, however, can be a problem.
Evaluating Alternative Technologies
Activated alumina is effective in removing arsenic from raw water with high total dissolved solids (TDS). Selenium, fluoride, chloride and sulfate, if present at high levels, can compete for adsorption sites and decrease efficiency. Activated alumina is highly selective for arsenic (V). This strong attraction results in regeneration problems, causing a 5% to 10% loss of adsorptive capacity for each treatment run. Application of point-of-use treatment devices would need to consider regeneration and replacement during implementation. Activated alumina is a viable method of removing arsenic, but there is a lack of available F-1 alumina, and testing of substitutes has not demonstrated similar results. Also, chemical handling requirements may make the process too complex and dangerous for small systems to implement. Activated alumina may not be efficient in the long term because it seems to lose significant adsorptive capacity with each regeneration cycle, and highly concentrated waste streams and disposal of brine can be problematic.
Ion exchange can effectively remove arsenic. Nitrate, fluoride, selenium, TDS and sulfate, however, compete with arsenic, which can reduce treatment run times. Passage through a series of columns can decrease regeneration frequency and improve removal. Precipitated iron and suspended solids can cause clogging of the ion exchange bed. Systems containing high levels of iron and suspended solids may require pretreatment to remove them from the raw water before the ion exchange unit. Ion exchange produces a highly concentrated waste stream, and disposal of the brine can be a problem. Brine recycling might reduce this impact.
Sulfate levels also affect treatment run times. This option, however, is recommended as a best available technology primarily for small groundwater systems with low sulfate and TDS and as a polishing step after filtration for low-level arsenic treatment options.
Removal efficiencies of greater than 95% can be achieved with reverse osmosis (RO), provided the operating pressure is ideal. If RO is used by small systems in the western U.S., then 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. Discharge of reject water or brine also can be a concern.
If RO is used by small systems in the western U.S., water recovery likely will need to be optimized because of the scarcity of water resources in the region. Optimizing water recovery can lead to increased costs for arsenic removal. RO and nanofiltration (NF) also can result in product water that would require extensive corrosion control, and the ability to blend product water with raw water would be limited. Water rejection, which is about 20% to 25% of influent, can be an issue in water-scarce regions as well.
Electrodialysis reversal is expected to achieve removal efficiencies of 80%. One study has demonstrated arsenic removal to 3 ppb from an influent concentration of 21 ppb. Electrodialysis reversal has a water rejection of 20% to 25% of influent, and it also can be an issue in water-scarce regions. This process may not be competitive with respect to costs and process efficiency when compared with RO and NF, even though it is easier to operate.
NF is capable of arsenic removal of more than 90%. Recovery rates range between 15% and 20%. Studies have demonstrated that removal efficiency drops significantly when the process is operated at more realistic recovery rates. For large treatment plants, a large body of water would likely be needed to discharge the contaminated brine stream from RO and NF technologies. Inland treatment plants possibly would need some pretreatment prior to discharge, or they would need to discharge to a sanitary sewer because of the increase in salinity. Discharge to sanitary sewers may require pretreatment in order to remove high arsenic levels. The waste stream produced by ion exchange and activated alumina technologies is highly concentrated brine with high TDS. These brine streams may require some pretreatment prior to discharge to a receiving body of water or a sanitary sewer.
Once arsenic is identified in the water supply, it can be removed efficiently. Technology allows water treatment processes to reduce arsenic levels to
1 ppb. Waste streams from treatment processes are hazardous, and they require pretreatment before their discharge to the environment. Optimizing water resources must be a priority in the utilization of arsenic removal treatment processes.