Environmental Pathology and Health
Effects of Arsenic
Poisoning: An Introduction and Overview
Chronic Arsenic Poisoning: History, Study and Remediation
Arsenic Removal Arsenal
Who is Affected
Environmental Pathology and Health Effects of Arsenic Poisoning: An Introduction and Overview
Jose A. Centeno1, Florabel G. Mullick1, Leonor Martinez1, Herman Gibb2, David Longfellow3 and Claudia Thompson4
1Department of Environmental and Toxicologic Pathology, International Tissue and Tumor Repository on Chronic Arseniasis, Division of Biophysical Toxicology, Armed Forces Institute of Pathology, Washington, D.C. 20306-6000; 2National Center for Environmental Assessment, U.S. Environmental Protection Agency, Washington, D.C. 3Chemical and Physical Carcinogenesis Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD; 4National Institute of Environmental Health Sciences, Research Triangle Park, NC.
Arsenic is a ubiquitous element in the earth’s crust. It is transported in the environment mainly by water, although other natural and anthropogenic sources of exposure to arsenic including burning of arsenic-rich coal, mining and smelter activities which are of increasing concern. While a wealth of epidemiologic studies have confirmed the carcinogenicity of inhaled and ingested arsenic, the pathological characteristics of arsenic-induced cancers have never been examined extensively. Moreover, recent studies appear also to suggest that the health effects of arsenic are systemic and may involve multiple organs.1-3 In nearly all cases where internal cancers are attributed to arsenic exposure, there has been cutaneous evidence of arsenic adverse effects in the form of arsenical keratosis, hyperpigmentation, and multiple cutaneous malignancies.4 The aim of this short communication is to provide an overview of arsenic health effects, and to discuss with examples, our recent studies on the environmental pathology of arsenic poisoning including a histopathological description of arsenic-induced lesions. The data were derived from the International Tissue and Tumor Repository on Chronic Arsenosis.5,*
II. Arsenic Poisoning: Adverse Health Effects
Skin. Epidemiological and clinical studies reported in the medical literature have confirmed the role of arsenic in the induction of cancers of the skin. Arsenic-induced skin lesions may include keratosis, squamous cell carcinoma and basal cell carcinoma. Arsenical keratosis in its fully developed form is a well established clinical syndrome, characterized by several specific pathological features, including hyperkeratosis, parakeratosis, arsenical pigmentation, and squamous cell carcinoma in situ (indistinguishable from Bowen's disease). Within the spectrum of keratotic lesion, arsenical keratosis may be differentiated from the more commonly diagnosed actinic keratosis by the absence of epidermal atrophy and basophilic degeneration of the upper dermis. All arsenical skin changes, including keratoses, tend to occur in non-exposed sites with an absence of dermal solar elastosis noted histologically. The lesions are normally most pronounced on the feet and hands, although they can occur on the trunk and other areas of the extremities.
Squamous cell carcinoma in situ is the most common form of skin cancer induced by arsenic, which may develop from two to 20 years after exposure. Bowen's Disease is an intraepidermal squamous cell carcinoma, referred to as squamous cell carcinoma in situ. It is considered a precancerous dermatosis, in the same group as leukoplakia, senile keratosis, and xeroderma pigmentosum. Histologically, Bowen's Disease presents as intraepithelial atypism, with marked variation in cell and nuclear size and shape. Multinucleated giant cells, and numerous mitotic figures are observed throughout all levels of the epidermis.
Arsenic-induced skin lesions assume protean forms. Besides keratotic lesions and skin cancers, pigmentation disorders represent another characteristic manifestation of arsenic exposure. The pigmentation may present as hyper- or hypopigmentation. Hyperpigmentation is reported to be one of the most common skin changes seen in people chronically exposed to arsenic. It most often occurs in the trunk, but may be more accentuated in areas that are more heavily pigmented such as the groin and areola. Histologic examination reveals increased melanin pigment in melanocytes in the basal cell of the epidermis extending up to the granular cell layer. Signs of arsenic pigmentation may herald the later development of skin cancer. In one study of patients showing signs of arsenical pigmentation, nearly 90% developed skin cancer. Hypopigmentation occurs as well and may show a characteristic "rain drop" pattern.
Internal Lesions. In addition to skin lesions, including skin cancer, epidemiological studies have provided suggestive evidence linking arsenic exposure to various internal cancers, including angiosarcoma of the liver (see Figure 1), lung cancer, and bladder cancer. In the majority of cases in which the internal cancer is ascribed to arsenic exposure, some dermatological hallmark of arsenic poisoning is identified.1 Gastrointestinal manifestations have also been reported due to chronic arsenic exposure and includes noncirrhotic portal hypertension (NCPH),6 hepatic or splenic enlargement, hepatocellular carcinoma (see Figure 2),5 and liver angiosarcoma.7 NCPH is a rare, but relatively specific effect that may occur after years of arsenic ingestion at concentrations of 0.01 mg/kg/d. Recent case reports indicate a possible relationship between arsenic exposure and the occurrence of hepatocellular carcinoma; however, epidemiologic studies have not as yet confirmed this association. The increased incidence of hepatocellular carcinoma in arsenic-exposed endemic areas of Taiwan may have an arsenic etiology in addition to a viral causation. The association between arsenic exposure and angiosarcomas of the liver has also been reported. However, most of the published literature has consisted of case reports rather than population-based epidemiological studies.
Figures 1 and 2 present microscopic views of an angiosarcoma and hepatocellular carcinoma, respectively (Reference 5. Centeno JA, et al. Arsenic-Induced Lesions, 2000).
Non-Cancer Effects of Chronic Arsenic Poisoning. In addition to internal cancers, recent published studies have suggested an association between arsenic exposure and an increased risk for a variety of non-cancer effects. These include peripheral vascular disease, cardiovascular disease, diabetes, neurological effects, chronic lung diseases (shortness of breath and chest signs), diminished hearing, and cerebrovascular disease.1-3, 8-10 It is quite apparent that the hazardous effects of arsenic are multi-organ related with extensive system pathology.
Reproductive effects of arsenic in humans has not been extensively investigated. Evidence from both animal and human studies suggests reproductive toxicity from arsenic, but data in humans is still sparse, and the results from laboratory experiments in animals are not conclusive. The evidence from a few human studies suggests that arsenic exposure may increase the incidence of pre-clampsia in pregnant women, decrease birth weight of newborn infants and increase in the risk of malformations and stillbirths, as well as that of spontaneous abortions.11,12 Although recent laboratory studies suggest an increase in malformations and stillbirths in animals,13,14 the effects of arsenic from drinking water in human reproduction have not been adequately studied. In order to assess the potential effects of arsenic in human reproduction, a properly designed epidemiological study in a large enough population is necessary.
In conclusion, the evidence for a casual relationship between cancers of the skin and arsenic exposure is strong and indisputable. Arsenic-induced skin cancers are predictable from exposure biomarkers of the skin, including hyperkeratosis, and hyper- or hypopigmentation. Cancers of the internal organs do not have such distinct exposure biomarkers and thus their association with a particular etiologic agent cannot be established with the same degree of confidence. Nevertheless, chronic arsenic exposure represents a significant risk factor for future development of liver cancer.
*Part of this work has been published in "Arsenic-Induced Lesions" (April 2000), Armed Forces Institute of Pathology, ISBN:1-881041-68-9.
1. Tsai S-M, Wang T-N, Ko Y-C. Mortality for certain diseases in areas with high levels of arsenic in drinking water. Arch Environ Health 1999;54:186-193.
2. Lai MS, Hsueh YM, Chen CJ, et al. Ingested inorganic arsenic and prevalence of diabetes Mellitus. Am J Epidemiol 1994;139:484-492.
3. Chen CJ, Chiou HY, Chiang MH, et al. Dose-response relationship between ischemic heart disease mortality and long-term arsenic exposure. Arterioscler Thromb Vasc Biol 1996; 16:504-510.
4. Maloney M. Arsenic in dermatology. Dermatol Surg 1996; 22:301-304.
5. Centeno JA, Martinez L, Ladich ER, Page NP, Mullick FG, Ishak KG, et al. Arsenic-Induced Lesions. Armed Forces Institute of Pathology, Washington D.C. (April 2000), pp 1-46. ISBN: 1-881041-68-9.
6. Nevens F, Fevery J, van Stenbergen W, et al. Arsenic and non-cirrhotic portal hypertension: A report of eight cases. J Hepatol 1990;11:80-85.
7. Neshiwat LF, Friedland ML, Schorr-Lesnick B, Felman S, Glucksman WJ, Russo RD. Hepatic Angiosarcoma. Am J Med 1992;93:219-222.
8. Chen CJ, Hsueh YM, Lai MS, et al. Increased prevalence of hypertension and long-term arsenic exposure. Hyppertension 1995; 25:53-60.
9. Wu MM, Kuo TL, Hwang YH, et al. Dose-response relation between arsenic concentration in well water and mortality from cancers and vascular diseases. Am J Epidemiol 1989;130:1123-1132.
10. Guha Mazumder DN, De BK, Santra A, Dasgupta J, et al. Chronic arsenic toxicity: epidemiology, natural history, and treatment. In Arsenic Exposure and Health Effects (ed. By Chappell WR, Abernathy CO, Calderon RL) 1999 Elsevier Science B.V. pp 335-347.
11. Stein Z, et al. Spontaneous abortions as a screening device: the effect of fetal survival on the incidence of birth defects. Am J. Epidemiol 1975;102:275.
12. Nordstrom S. et al. Occupational and environmental risks in an around a smelter in northern Sweden: III. Frequencies of spontaneous abortion. Hereditas 1978;88:51.
13. Leonard A, Lauwerys RR. Carcinogenicity, teratogenicity, and mutagenicity of arsenic. Mutation Research 1980;75:49.
14. Hodd RD, et al. Effects in the mouse and rot of prenatal to arsenic. Environ Health Perspec 1977;19:219.
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Arsenic Links. Retrieved from:
ToxFAQs for Arsenic This site on arsenic, in a "frequently asked questions" format, was produced by the Agency for Toxic Substances and Disease Registry (ATSDR), a division of the U.S. Department of Health and Human Services. The mission of the ATSDR is "to prevent exposure and adverse human health effects and diminished quality of life associated with exposure to hazardous substances from waste sites, unplanned releases, and other sources of pollution present in the environment." The FAQ is one in a series of summaries about hazardous substances and their health effects. Site includes phone contacts for additional information and a listing of related resources.
Arsenic rulemaking: EPA Office of Ground Water and Drinking Water Describes the history of the United States Environmental Protection Agency's rulemaking efforts related to federal standards for arsenic in public drinking water supplies. The revised standard was required by the Safe Drinking Water Act. Includes fact sheets on arsenic, news releases from the EPA and links to support documents including scientific reviews by the independent expert panels convened by the National Academy of Sciences, the National Drinking Water Advisory Council and the EPA Science Advisory on the politics of arsenic regulation. Good reading for informed citizens. Requires Adobe Acrobat to view PDF files.
Arsenic in Drinking Water FAQ A brief summary on arsenic and its health effects produced by the National Resource Defense Council, an environmental advocacy group. It includes a FAQ on arsenic, including advice on filters that can be used in the home, and links to related pages on drinking water quality, water pollution and a broad spectrum of environmental issues including exposure to toxic chemicals.
Arsenic in Ground Water of the US A site detailing the occurrence of arsenic in ground water in the United States maintained by the U.S. Geological Survey (USGS), an agency under the US Department of the Interior. The USGS researches conditions involving the country's natural resources. The major feature of the site is a map showing the location and extent of arsenic in ground water across the country, along with related reports and analyses and a fact sheet on interpreting the information. The site also includes basic information on arsenic and an extensive set of arsenic links.
Harvard Arsenic Project Maintained by Harvard University's Richard Wilson, this comprehensive site focuses on the public health aspect of acute arsenic poisoning from drinking water, with an emphasis on the catastrophic problem of arsenic poisoning in Bangladesh. Includes numerous links to online articles, scientific and technical papers, an extensive bibliography, information on instruments for measuring and speciation of arsenic, a photo gallery, lists of conferences and a multi-national viewpoint on the problem.
WebElements Periodic Table: Arsenic Includes extensive information on the chemical properties of arsenic, from the simple to the complex. Designed for students and for curious, somewhat science-savvy citizens. Includes geology, bond enthalpies, and uses, and details properties, compounds, and interdisciplinary value.
USGS Arsenic Studies Group The site, produced by the U.S. Geological Survey, is intended to increase communication among scientists working on arsenic and to make the work of USGS scientists more widely available to others working on arsenic studies. The site includes descriptions of ongoing studies with contact information, a bibliography on arsenic research, and a list of symposia sites with arsenic research abstracts and papers somewhat technical information.
Arsenic Case Study — Agency for Toxic Substance and Disease Registry (ATSDR) This ATSDR case study, while written for primary care providers and therefore full of medical terminology, provides comprehensive information on risk, exposure, fate, effects, and treatment of arsenic. Set in the Northwest, the case study follows the diagnosis and treatment of a carpenter exposed to arsenic via a variety of pathways. Some of the terminology may impede understanding for those without a medical degree, but most of the information is simple and direct. Providers may use this site as continuing education credit, authorized by AMA, AAFP, ACEP, and AOA.
West Bengal and Bangladesh Arsenic Crisis Information Centre The Arsenic Crisis Information Centre, or ACIC, contains several valuable resources — a monthly newsletter, presentations, news articles, international conference information, and a rather good links page. Also set up are three discussion groups over Yahoo Servers. Several international groups comprise the ACIC, although the site is unclear about how they are organized.
The London Arsenic Group This group of British geologists and mineralogists focuses on the source, fate, and transport of arsenic in the environment. This page contains good general information on arsenic, as well as specifics relevant to the Bangladeshi crisis. Publications from the popular press as well as papers, posters, and conference presentations from the group members are all located here. Of interest is the “Layperson’s guide to the way groundwater in Bangladesh has become polluted by arsenic,” located on the main page.
Fifth International Conference on Arsenic Exposure and Health Effects The most interesting segment of this web site for the July, 2002 conference is the 4th Conference proceedings and abstracts/posters. The 5th Conference web site has not yet been updated with the posters and papers presented there — however, it serves as a directory to find posters and abstracts from many other conferences. These papers deal with arsenic exposure, bioavailability, toxicokinetics, dose-response relationships, health effects, EPA's arsenic risk assessment and MCL, abatement and control strategies.
Health Effects and Geochemistry of Arsenic and Lead — Columbia University’s NIEHS Superfund Basic Research Program. Columbia University’s Superfund Program focuses on health problems arising from arsenic and lead in soil and drinking water. Consisting of four biomedical projects and three geological projects, this research takes place in four Superfund sites and has connections to the ongoing problems in Bangladesh. The web site outlines all these projects and their related cores, as well as presenting all the publications from the group.
Arsenic and Human Health — National Library of Medicine. A very comprehensive links page. Divisions include Government Information, Current Interest, and some directly from the National Library. The latter are probably the most informative, as they are directly from MedLine. The Current Concerns section includes CCA-pressure treated wood and munitions cleanup along with groundwater arsenic.
Arsenic, King of Poisons — Dr. Anil Aggrawal’s Forensic Articles Dr. Aggrawal writes the series "Poison Sleuths for the Science Reporter," a monthly science magazine published by the National Institute of Science Communication in New Delhi. This article, published in February 1997, gives a good, simple overview of arsenic poisoning and how it is diagnosed, in a conversational format between a forensic pathologist and a visitor to his lab. Because this site is located on tripod.com, it does have a number of extra popup windows, so be advised. (There is a computerized version of “Did you ever know that you’re my hero” playing continuously as well.)
Arsenic in Pressure-treated Wood — Environmental Working Group The EWG is a Washington, DC-based environmental advocacy group that produces reports, original analysis and critiques of government data and other studies. This page contains three reports on pressure-treated wood developed by the EWG as well as an arsenic-test kit ordering link.
Asia Arsenic Network A group founded in 1994 to help people suffering from the widespread arsenic poisoning in Asia publishes this web site. Each major site is listed and a description given, along with research aimed at a better understanding of arsenic and its remediation.
Wisconsin’s Arsenic in Drinking Water and Ground Water — Wisconsin Department of Natural Resources. The aquifer supplying water to much of Wisconsin is embedded in a natural sandstone, which happens to contain arsenic. This Wisconsin Arsenic page, therefore, has maps of potentially high arsenic sites, test kits and laboratory information, and a hefty supply of recommendations, articles, and studies, to keep citizens well-informed.
Arsenic Health Effects Research Program — University of California at Berkeley This page showcases the broad epidemiological studies of the UC-Berkeley research team, specifically studying the relationship of arsenic exposure to cancer. Much of their research has taken place in Argentina and Chile, although recent work in the South-western United States is published here as well.
Arsenic in Drinking Water 2001 Update — National Academies Press This is an open-book formatted publication from the National Academies Press. While it cannot be downloaded in its entirety (only page by page), it is possible to read the entire update on screen. The most important feature of the online format is the impressive search capability. Related books are accessible and quite interesting as well.
Chronic Arsenic Poisoning: History, Study and Remediation (from: http://phys4.harvard.edu/~wilson/arsenic/arsenic_project_introduction.html)
Multiple skin cancers
Environmental Protection Management and Problem Solving for Environmental Professionals: Arsenic Removal
Arsenic Removal Arsenal
The U.S. Environmental Protection Agency (EPA) has historically regulated arsenic at 50 parts per billion (ppb), but the agency will lower the maximum contaminant level (MCL) for arsenic to 10 ppb by 2006. Some states are even setting their own limits well below this level. EPA estimates one in 20 (or roughly 4,000) of the 74,000 U.S. systems that must comply with the new standard will need to install additional treatment processes or adopt different measures to meet the requirements. Nearly 97 percent of those are small systems serving communities of fewer than 10,000 people.
According to EPA, the new arsenic standard will increase the average American's water bill by $32 annually. Those living in smaller communities may see a substantially greater hike, ranging from $58 to $327 per year. EPA predicts the new regulation will cost local communities approximately $200 million annually.
The new arsenic standard will not protect private well owners, and EPA is not sure how many of the 2.5 million private well users consume water with arsenic that exceeds the new MCL. Installing a well treatment system that serves 500 people is approximated to cost $160,000 and $27,000 in capital and annual operating costs, respectively.
According to EPA, the new arsenic standard will increase the average American's water bill by $32 annually.
Some public works are adopting treatment avoidance measures as the most economical way to comply with the upcoming arsenic regulation. Such practices include abandoning existing water supplies with high arsenic concentrations in favor of new water supplies that have lower arsenic concentrations. Another solution includes blocking the flow of water from zone(s) within an aquifer that have higher arsenic concentrations, reducing the overall arsenic concentration produced from the well to, in some cases, below acceptable levels. Where these treatment avoidance strategies work, the capital cost and operating costs are lower than the cost of removal treatment. However, implementing treatment avoidance strategies is not a viable option for all public works.
Luckily, several treatment options for arsenic removal exist. Pairing the appropriate removal method with an existing system depends on several factors including:
Arsenic Chemistry and Removal Mechanisms
Arsenic exists in two valence states: arsenite (As III) and arsenate (As V). Arsenite has a neutral charge, so it is not readily removed by most of the treatment processes. Arsenate, on the other hand, has a negative charge and is more thoroughly removed by the treatment processes described in this article.
Waters with a high arsenite concentration usually need to be pre-oxidized. This converts the hard-to-treat arsenite into the more easily removable arsenate. Chlorine, a preferred pre-oxidant, also provides disinfection capacity. As the majority of arsenic-bearing water supplies are groundwaters with low influent dissolved organic carbon levels, chlorinated organic disinfection byproducts are not of significant concern. Potassium permanganate is another oxidant that may be used to oxidize other present contaminants such as manganese.
For water supplies with naturally occurring iron, the pre-oxidation process using chlorine or potassium permanganate will oxidize soluble iron as well as arsenite. The resulting iron floc provides a suitable particle for co-precipitation of arsenic. If the concentration of naturally occurring iron is insufficient for adequate arsenic removal, additional coagulant can be added after the oxidation process.
Coagulants such as ferric or aluminum salts are fed into the water to co-precipitate arsenic so that it can be removed through settling and/or filtration. Ferric coagulants tend to be more effective at arsenic removal and, more importantly, are less pH-sensitive than alum. Alum and aluminum salt coagulants can be effective in binding arsenic, but the process is highly pH-sensitive, with optimum pH often found in the 6.0 to 6.5 range. If aluminum salt coagulant is used, acid dosing may be needed to achieve this optimum pH range. Ferric coagulants are effective over a much wider pH range and are therefore most commonly used for arsenic co-precipitation.
Several studies investigating the stability of sludges derived from ferric coagulants have determined that the sludges are stable, based on the toxic characteristics leaching procedure (TCLP) protocol. The results, which may need to be duplicated on a site-by-site basis, indicate that ferric sludges from arsenic removal processes would be classified as nonhazardous waste under the Resource Conservation Recovery Act and therefore can be disposed of in sanitary landfills. Public works are encouraged to contact their local regulatory approval agency for guidelines on disposing of arsenic residuals.
Iron-based Adsorption Media
Stringent U.S. and European arsenic regulations have prompted manufacturers to develop and promote more iron-based adsorption media. These specialized media operate similarly to granular activated carbon contactors. The media is placed in a pressurized treatment vessel in a fixed bed adsorber. Raw water passes through the media that adsorbs the arsenic, which means that the arsenic adheres to the surface of the media. Backwash is performed infrequently to prevent compaction and to remove any particulate that may be present in the supply. This process requires minimal operator attention, compared to other arsenic removal processes.
Variants of iron-based media include granular ferric hydroxide, granular ferric oxide, iron hydroxide-coated sand, metallic iron (referred to as zero valent iron), sulfur/iron mixtures (referred to as sulfur-modified iron) and many others. Several of the ferric-based materials can sufficiently adsorb arsenite, making pre-oxidation unnecessary. Many of these materials are at the field-trial stage, but others are already being used in full-scale applications throughout Europe and the United States.
Some public works are adopting treatment avoidance measures as the most economical way to comply with the upcoming arsenic regulation.
Sorption processes with single-use media provide a simple treatment solution for small installations and wellhead applications that have low or moderate arsenic concentrations with no treatment process in place. Water supplies that have iron or manganese should consider using alternative treatment processes that specifically remove these contaminants as well as arsenic.
The primary limitations -- effective media lifetime and consequent media replacement cost -- affect applications at higher flow rates and at higher concentrations. Most of the residuals in this process are solid waste, which alleviates concerns about disposing of liquid waste. And, as the spent ferric sorbents have passed TCLP testing and therefore are classified as nonhazardous waste, spent media may be sent to sanitary landfills.
Who is Affected?
Empty bed contact time, loading rate and media bed lifetime capacity are key design parameters for iron-based sorbents. Bed lifetime depends on arsenic concentration, competing ion concentrations, arsenic species and pH, and is specific to a particular type of media. Operating conditions may also influence the observed lifetime since mixing the bed during backwash can cause it to behave as a stirred tank batch adsorber rather than as a fixed bed adsorber. Most iron-based media have capacity to remove other contaminants such as selenium, vanadium, antimony and others.
A broad series of processes are included in the filtration category for arsenic removal. These processes generally oxidize arsenite (if required), co-precipitate the arsenic with iron or aluminum salts and filter the resulting solids. A pressure filter is the most common filtration device for groundwater supplies, as it eliminates the need to re-pump the water after treatment. Gravity filtration may be more economical for larger public works or for those with a groundwater storage tank. The filtration process simply separates the formed solids from the water. A wide range of products is available to best meet specific project requirements.
For project planning purposes, the filtration processes described below can readily be bench-and pilot-scale tested to determine removal efficiency and overall process design.
The filtration processes require regular backwashing that generates a solids-laden liquid waste. In cases where disposal of backwash waste to sewer is impractical, the management of the backwash waste stream proves a major disadvantage. This process is best applied in areas where it is easy to dispose of liquid waste.
Commonly used to remove iron and manganese, oxidation/filtration also effectively removes arsenic. The process minimizes chemical costs since naturally occurring iron is used for co-precipitation. At moderate levels of arsenic contamination, even a relatively low iron concentration [0.3 milligrams (mg) or higher] adsorbs enough arsenic to adequately remove the contaminant. However, to be effective, the iron content of the water must be stable or at least remain above the required threshold for arsenic removal.
The backwash waste produced contains highly stable iron-arsenic sludge that can typically be disposed of in a landfill.
This process is similar to the oxidation/filtration removal process except the coagulant is added after oxidation to increase arsenic removal efficiency. Laboratory, pilot-plant tests and full-scale operating plants have shown coagulation and filtration to be an effective treatment process for arsenate, iron and manganese removal. Pre-oxidation is necessary for arsenite-laden water supplies.
Naturally occurring iron helps remove arsenic and, as a result, impacts the amount of coagulant used. Similarly, water conditions may affect the process reaction time, and additional detention prior to filtration may be required. Ferric salt such as ferric chloride or ferric sulfate is the most common coagulant selected.
As with oxidation/filtration, the backwash waste produced contains highly stable iron-arsenic sludge that can typically be disposed of as nonhazardous waste in a sanitary landfill.
This coagulation/filtration treatment process uses a microfiltration membrane system instead of a granular media filter. Ferric coagulant is typically used. Pre-oxidation may be required, depending on the arsenic species. The process also removes waterborne pathogens such as cryptosporidium and giardia.
Oxidant-tolerant microfiltration membranes are necessary because of the pre-oxidation requirement. The key economic driver in using a microfiltration filter is design flux, which is the flow rate per unit membrane area. The flux dictates the size of the equipment, and is sensitive to the ferric solids load. Depending on the water chemistry, it may be worthwhile to reduce the ferric dose by adjusting the pH, thus optimizing the membrane area requirement.
Coagulant-assisted microfiltration systems require periodic backwash and produce sludge as waste similar to granular media filtration.
With high influent arsenic concentration, the coagulant dose necessary for adequate arsenic removal results in impracticably short filter run times. Elevated levels of other contaminants such as iron and manganese also contribute to increasing solids load. Adding sedimentation before filtration reduces solids load, which extends filter operation and minimizes backwash events.
Capable of handling a higher influent content of contaminants and a wider range of water conditions, the coagulation/sedimentation/filtration process is appropriate for existing surface water treatment plants requiring arsenic removal or for large groundwater systems opting for centralized treatment.
Arsenic exists in two valence states: arsenite (As III) and arsenate (As V).
Robinson and Leible have reported excellent arsenic removal in such systems, specifically citing a Microfloc® Tricon installation that treats 9 million gallons per day (mgd) of groundwater by pre-oxidation, coagulation, adsorption clarification and filtration (1999). They report reducing influent arsenic of 150 ppb to 6 ppb in the treated water.
Despite increasing the process complexity, the sedimentation step improves the overall performance. Additionally, this system increases detention time during which reactions occur.
Both the sedimentation and filtration sections generate sludge waste streams sufficiently stable for landfill disposal.
Under appropriate conditions, activated aluminais an effective arsenic sorbent that can be operated in either a single-use or regenerated mode. Sorption capacity is pH-dependent, with slightly acidic conditions generally favored. Many anionic species such as selenium, fluoride, chloride and sulfate compete with arsenic for adsorption sites and, if present at high levels, will reduce the economic effectiveness of the process. Pre-oxidation and pH adjustment are normally required to maximize removal and sorption capacity. While activated alumina can be regenerated, significant loss of capacity (of 10 percent or more) can occur on each cycle, leading to future replacement of the media.
This process may be more appropriate for use in larger water treatment applications, due to the incomplete nature of regeneration and the problems of handling and disposing of chemical streams. Smaller sites may prefer to treat activated alumina as a once-through, disposable sorbent. Waste produced by the activated alumina process will consist of concentrated arsenic in solution.
Pre-oxidation of arsenite to arsenate followed by the exchange of anions (ions with negative charges) is normally required to maximize arsenic removal. Anion exchange can effectively remove arsenic, generally using strong base anion exchange resins in the chloride form. Sulfate and other anions compete with arsenic and can greatly reduce the operating cycle if present in elevated concentrations.
Pilot-testing indicates that the brine regeneration solution can be reused as many as 20 times with no impact on arsenic removal, provided that some salt is added to the solution to provide adequate chloride levels for regeneration. Regenerate reuse reduces the amount of waste for disposal, but increases the ultimate arsenic concentration of the spent brine.
Spent brine disposal options are critical in assessing the viability of ion exchange for specific treatment situations. The brine can be treated with ferric coagulant to remove the arsenic from the liquid waste. Smaller sites may not prefer this treatment process, due to regeneration and the handling of chemical waste streams.
Reverse Osmosis and Nanofiltration
These high-pressure membrane systems provide effective removal of arsenic and many other dissolved species such as calcium, magnesium, nitrates and sulfates. These technologies are ideal for point-of-use and point-of-entry applications at low flow rates, particularly when arsenic is just one of several water quality parameters requiring treatment.
For sites with several water quality issues to address, particularly if these are related to dissolved solids, reverse osmosis or nanofiltration provide complete treatment in a single process step. If a high concentration of undissolved solids is present in the supply, pretreatment for removal may be required.
Larger flows lose 15 percent to 30 percent of feed flow as reject and require 50 pounds per square inch (psi) to 150 psi operating pressure even for modern high-efficiency, low-pressure reverse osmosis operating on low total dissolved solids feedwater. While the reject flow is high, the concentration of arsenic in the liquid waste is lower than in many of the other processes. This is an important consideration when disposing of the liquid waste to a sanitary sewer, as some wastewater treatment plants prefer an increased flow with lower concentration versus small batch flows with higher concentrations.
Waste produced by the reverse osmosis and nanofiltration systems will consist of concentrated arsenic in solution.
Lime is added to water to raise the alkalinity and, when combined with media filtration, effectively removes arsenic through adsorption-co-precipitation. Able to handle a wide range of contaminants and water conditions, lime softening applies to groundwater and surface water supplies with high hardness, iron, manganese and arsenic. Existing plants that use lime softening should be able to simply increase pH in the reaction basin or make other process modifications to meet arsenic removal standards.
The lime softening process is unlikely to be chosen solely for arsenic removal, due to operational complexities and the cost of equipment and operation. However, the softening stage decreases solids load, which increases the filter run time between backwashes and provides the added benefit of reducing hardness. The softening basins and filter backwash waste produce sludge waste.
Arsenic is a growing problem in communities great and small, urban and rural. Newly discovered contaminated sites appear in the news every day.
The good news is there are many arsenic treatment removal options available. Industry professionals are becoming more educated about the issue and are sharing that knowledge with community leaders and public works.
Contaminated sites should be evaluated individually to determine which solution best meets their needs.
Who is Affected?
This article originally appeared in the February 2004 issue Environmental Protection, Vol. 15, No.2.
Darin St. Germain, PE, is vice president of sales and marketing for conventional products at USFilter Memcor Products in Ames, Iowa. He can be reached at StgermainD@usfilter.com or (515) 268.8546.
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