Water supplies meeting treatment requirements

Water Treatment & Arsenic

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Arsenic

 

Lake Superior

Summary

• Arsenic is a naturally occurring semi-metal which can contaminate water supplies naturally or from industry or agriculture, leading to negative health effects
• The current Maximum Contaminant Level for arsenic is 10 parts per billion for both the U. S. and Canada
• Approved methods for arsenic treatment include adsorption, ion exchange, and coagulation with filtration but in some cases, pretreatment is required

Where does it come from?

Arsenic is a naturally occurring semi-metal element which can enter water supplies from both natural and human-made sources. [1,2] Arsenic is found naturally in soils and rocks but is also a pollutant from industry and agriculture. [1,2] Wood preservation, pesticide use, and byproducts from mining and coal burning can contribute to arsenic pollution which enters groundwater over time through deposition from precipitation. [1,2,4] Arsenic may enter wells from heavy fertilizer or pesticide use in the past or from the improper construction of a well.[2] Arsenic from any combination of these sources can contaminate drinking water sources where it can lead to negative health effects. [1,3]

How is it treated to meet applicable standards?

The current standard for arsenic is 10 parts per billion (ppb) for both the U. S. EPA and Health Canada.[1,6] Arsenic cannot be removed by boiling or disinfection and filters must be certified to remove arsenic. [2,3] The EPA list of approved methods for arsenic treatment include adsorption, ion exchange, and coagulation with filtration.[5] These methods are dependent on chemical properties of the source water and the arsenic state/form, so testing is needed prior to treatment and, in some cases, pretreatment or pre-oxidation is required.

Adsorption removes arsenic by passing untreated pressurized water through granular media. These media include titanium-based, activated alumina, zirconium-based, and iron-based, all of which adsorb negatively charged arsenic ions. These systems are lower cost, effective, and require minimal operator attention, however spent media needs to be disposed of as waste.

Ion exchange treats pressurized water through columns packed with anion exchange resin, which removes arsenic ions. The efficiency and cost of this system depends on the concentration of other anions (such as nitrates and sulfates) and level of total dissolved solids. Monitoring the effluent stream is needed to check for loss of capacity or resin fouling. The resin can be regenerated or reconditioned, but the waste materials from this process need to be handled properly, increasing the price.

In coagulation, arsenic is precipitated out of solution using coagulates such as aluminum and ferric salts, which hydrolyze to form aluminum and iron hydroxide particulates, respectively. These are then filtered out of the drinking water using media filters or micro-filters. The cost and efficiency of this technique is dependent on the pH, type of coagulate, and mixing intensity, though optimized systems can remove up to 90% of inorganic arsenic. Spent filter backwash water is created as liquid waste, which can be disposed of through indirect discharge or dewatered into sludge for municipal waste disposal depending on toxicity.

Reduced-form arsenic is often found in groundwater and must be oxidized with a chemical oxidizer for optimal removal. Arsenic can then be removed by adsorption or coprecipitation for filtration. Additionally, point-of-use techniques may be used at the point of a household sink to treat drinking water using reverse osmosis or activated alumina. These point of use systems do not necessarily address pre-oxidation and their price may be prohibitive for larger scale use.

References

 

  1. EPA, Chemical Contaminant Rules (2021). Accessible online: https://www.epa.gov/dwreginfo/chemical-contaminant-rules
  2. CDC, Arsenic and Drinking Water from Private Wells (2015). Accessible online: https://www.cdc.gov/healthywater/drinking/private/wells/disease/arsenic.html
  3. Costello, M. How to Protect Yourself and Your Family From Arsenic in Your Drinking Water (2021). NFS. Accessible online: https://www.nsf.org/blog/consumer/arsenic-drinking-water
  4. Maine Division of Environmental and Community Health, Arsenic (2022). Accessible online: https://www.maine.gov/dhhs/mecdc/environmental-health/dwp/consumers/faq.shtml
  5. EPA, Arsenic in Drinking Water (2015). Accessible online: https://cfpub.epa.gov/safewater/arsenic/arsenictradeshow/arsenic.cfm?action=Treatment
  6. Government of Canada, Guidelines for Canadian Drinking Water Quality – Summary Table (2020). Accessible online: https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/water-quality/guidelines-canadian-drinking-water-quality-summary-table.html