Arsenic in Drinking Water
Arsenic is a semi-metal element in the periodic table. It is odorless and tasteless. It enters drinking water supplies from natural deposits in the earth or from agricultural and industrial practices. The health effects of Arsenic exposure can be severe, and include stomach pain, nausea, diarrhea and vomiting; thickening and discoloration of the skin; numbness in hands and feet; partial paralysis; and blindness. Prolonged exposure or exposure to large doses can be fatal. Arsenic has also been linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate.
The Environmental Protection Agency (EPA) has established a Maximum Contaminant Level (MCL) for arsenic in drinking water at 10 ppb (parts per billion/micrograms per liter, µg/L).
Treatment Technologies for Arsenic
EPA identifies the following treatment technologies as Best Available Technologies for removing Arsenic from drinking water:
- Anionic Exchange
Anionic exchange is a physical/chemical process by which ions are exchanged between a resin bed and the water passing through. Ion exchange systems are used to soften water, remove iron and manganese, and to lower nitrate and arsenic levels. The contaminants that can be removed is determined by the composition of the resin bed used.
Typical Anionic Exchange Systems
An anionic exchange system works by passing water through the resin bed, which is “charged” with chloride ions from dissolved salt. The arsenic molecules knock the chloride ions off the resin and take their place. This process continues until all of the sites on the resin are full. The resin is then back washed with water that is super saturated with dissolved salt. The trillions of chlorine ions in this back wash water overwhelms the arsenic molecules, forcing them into the back wash water and out of the system in waste water. New chlorine ions then take the place of the arsenic molecules, causing the resin to be “recharged”, starting the process over again. These systems would typically be “point of entry” systems, treating al of the water coming into the home.
Pros and Cons of Anionic Exchange
Advantages of Anionic Exchange Treatment:
- Anionic exchange can be very effective at removing arsenic from water if conditions are just right.
- Anionic exchange requires very little maintenance, only the addition of salt every few weeks. It's very easy for a homeowner to use.
- Systems are typically installed to treat an entire house.
- Systems are typically priced within reach of most Maine homeowners.
Disadvantages of Anionic Exchange Treatment:
- Other constituents in water can compete with resin sites, reducing the effectiveness of the system. EPA recommends the influent water have less than 500 mg/l total dissolved solids and less than 25 mg/l sulfate.
- Treated water can have a very low pH and high levels of chloride, which will cause the water to very corrosive, with high copper and/or lead levels resulting.
- If the system fails, all of the arsenic on the resin can be released at once, causing a large concentration in the "treated" water.
- Reverse Osmosis
The most cost effective method for removing arsenic from a domestic water supply appears to be reverse osmosis (RO). RO can be thought of as atomic scale filtration. It works by squeezing water through a special membrane. The membrane has microscopic holes that are specially sized to allow relatively small water molecules to pass through, while trapping larger inorganic elements like lead, iron, chromium, and arsenic. Studies have shown RO to be up to 95% effective at removing the most common form of arsenic routinely found in Maine well water. RO requires very little regular maintenance, no chemical addition, is very reliable, and installation is fairly straightforward.
Typical Reverse Osmosis Systems
The RO system most homeowners have installed is a point of use system, designed to treat only a small amount of water daily. It is usually located near the kitchen sink, and is capable of producing two or three gallons of treated water per day. It consists of a particulate pre-filter that removes sand and grit, the membrane where the reverse osmosis occurs, and an activated carbon polishing filter to aid in taste and odor control (pictured at right). Treated water is stored in a small tank, and is accessed through a faucet (picture above left) located next to the regular kitchen faucet. The cost of these systems is usually between $800 and $1,500. Costs can vary depending on water chemistry.
Pros and Cons of Reverse Osmosis
With every water treatment technology there is an immediate benefit, like the removal of arsenic or other contaminants, and usually some side effects that may not be acceptable for all people. These side effects are normally more of an aesthetic problem than they are health concerns, but need to be carefully considered during the process of evaluating what treatment method to use.
Advantages of Reverse Osmosis Treatment:
- RO is very effective at removing inorganic materials (minerals) like iron, chromium, lead, manganese and arsenic from water. In fact, RO is usually about 95% effective at removing the kind of arsenic encountered most of the time in Maine groundwater (+5 valence state).
- RO requires very little maintenance, and no addition of chemicals. It's very easy for a homeowner to use.
- Systems can be installed that treat an entire house, or just one faucet.
- Smaller systems are priced within reach of most Maine homeowners.
Disadvantages of Reverse Osmosis Treatment:
- The inorganic materials removed are the same ones that impart taste in drinking water. Without them, some people find RO treated water bland.
- Smaller RO units designed as point of use systems produce only a few gallons of treated water per day, so the supply of drinkable water is limited in quantity, and is normally found only in the kitchen area of a home.
- Larger systems can treat all the water used in a home, but at a greatly increased cost.
- If you have +3 Arsenic you’ll need to pre-oxidize to change your arsenic to +5 type, which will add to the cost.
- If you have significant amounts of iron or manganese in your water you’ll likely require additional treatment equipment to remove those prior to RO treatment.
- Iron Oxide Filters
A relatively new and promising method for reducing arsenic levels in drinking water is the use of iron oxide filters. Like activated carbon, these granular filters have large amounts of surface area and an affinity for arsenic to adhere to its surface. Although these filters are fairly new to the home treatment market, the principals behind them have been used by public water suppliers for many years.
Typical Iron Oxide Systems
Iron oxide media can be housed in small inline filter cartages (point of use systems) or larger tanks similar to the ones used for ion exchange systems for whole house treatment. These filters can also be used to improve the performance of reverse osmosis systems that are not effectively removing trivalent arsenic. Filters can be designed for almost any size application, from single-family homes to larger commercial operations. The media can be disposed of as non-hazardous waste.
Pros and Cons of Iron Oxide Filters
Advantages of Iron Oxide Filters:
- Effective for both trivalent and pentavalent arsenic removal.
- Can be used as point of use (one location) or point of entry (whole house).
- Removes other inorganic contaminants.
- Simple to use and install.
- Disposable as non-hazardous waste.
- Reasonably priced treatment option for arsenic removal.
Disadvantages of Iron Oxide Filters:
- Media must be replaced on a regular basis.
- The presence of iron, manganese, sulfate, silica or organic carbon can reduce its effectiveness.
- Activated Alumina
- Modified Coagulation/Filtration
- Modified Lime Softening
- Electrodialysis Reversal
The Drinking Water Program recommends that water systems and homeowners seek advice from a water treatment professional to determine the most effective treatment based on the characteristics of their specific water system. Public water systems should contact their Public Water System Inspector for approval before installing or making changes to any treatment in their system.
Arsenic Treatment Checklist
- Have I had my water tested at a state certified laboratory?
- Have I done a retest to confirm the results?
- Have I also tested for iron and manganese? They are very common in Maine and can foul RO systems. They may also require removal.
- Have I contacted a water treatment professional to aid in the analysis and the selection of an adequate solution?
- Do I need whole house treatment or is point of use treatment adequate?
- Have I checked references (other homes that he/she has worked in) for the water treatment professional I've selected?
- Are there other problems with my drinking water system that should also be addressed?