By Gary Battenberg
Arsenic (As) is a naturally occurring contaminant found in many groundwater sources. It generally occurs in two forms (valences or oxidation states) and occasionally as organically complexed arsenic. Pentavalent arsenic, indicated as As(V), As (+5), or arsenate and trivalent arsenic, indicated as As(lll), As (+3), or arsenite, are the most common forms encountered either individually or in combination. Although both forms of arsenic are potentially harmful to human health, trivalent arsenic is considered more harmful than pentavalent arsenic. That is why arsenic in any form must be removed from water supplies to prevent ill effects.
Why
It has long been known and confirmed that ill effects result from human exposure to inorganic arsenic (As) through drinking water (and to a lesser extent through various food sources) and organic arsenic exposure from eating seafood. These exposure routes are linked to cancer, skin lesions, hyper-pigmentation (darkening of skin or nails), respiratory ailments (such as coughing and bronchitis), cardiovascular, gastrointestinal and urinary tract ailments, increased risk of high blood pressure and diabetes. Because of many studies by various agencies, US EPA cited the compiled evidence and indicated in their report that lowering the arsenic guideline from 50 to 10 parts per billion (ppb) or µg/L could prevent deaths from bladder, lung and skin cancers and from heart disease. Effective January 23, 2006 all public water systems in the United States had to comply with the 10 µg/L standard issued by the agency.
How
Trivalent arsenic is generally more difficult to remove from drinking water than pentavalent arsenic. Trivalent arsenic can be converted to pentavalent arsenic in the presence of an effective oxidant, such as free available chlorine (FAC). Arsenic in water containing detectable free chlorine or other effective oxidants will be converted to the pentavalent arsenic form. Treatment with chloramines (which is chlorine combined with ammonia) is not sufficient to ensure complete conversion of trivalent arsenic to pentavalent arsenic.
Consumers served by a public water utility may contact their utility to verify whether free-chlorine treatment chemicals are being used to maintain bacteriostasis throughout their distribution system. Those served by a private water supply or water sources that do not have detectable free chlorine residuals should have the water analyzed to determine the form(s) of arsenic present, to ascertain the potential need for oxidation of trivalent arsenic to pentavalent arsenic and appropriate removal method. A total arsenic test conducted by a certified laboratory generally costs approximately $30. It is not always necessary to have a speciation test conducted, where the totals of trivalent and pentavalent arsenic are present. It is best to treat the total arsenic as trivalent and convert it to pentavalent before removal treatment. Local health departments and offices of environmental protection agencies can provide consumers with a list of certified laboratories.
What method(s)
One of the earliest methods for removing arsenic from water was the oxidation method. There were several ways to do this and examples of oxidizing agents include oxygen, ozone, chlorine, peroxide and potassium permanganate. For clarity, it is important to understand that the word ‘agent’ refers to an active substance that produces the oxidation reaction. Let’s look at some examples of applying oxidizing agents to water to aid in lowering the concentration of various contaminants.
Air injection
Perhaps one of the very earliest methods of oxidizing iron, manganese and hydrogen sulfide is air injection. Oxygen (O2) is a good oxidizing agent in and of itself because of its natural oxidation reduction potential (ORP). Oxygen is injected into the water via a Venturi by the applied motive flow and pressure of a water pump. Oxygen being the oxidant and arsenic, iron, manganese, hydrogen sulfide, etc., the ‘reductant,’ brought together forces the reaction previously described.
Before the advent of the hydropneumatic (bladder-type) pressure tank, well-water pressure tanks used an air volume control to maintain the pump pressure differential of approximately 20 psi, which was intended to prevent continuous cycling of the water pump every time a tap was opened. The perceived advantage of the air-head pressure tank not only provided the cushion of air that was compressed during the pump cycle, it also was effective in oxidizing iron, manganese, hydrogen sulfide and arsenic.
This oxidation process was accomplished by a phenomenon called Henry’s Law, which states that “the weight of a gas dissolved (at a given temperature) in a liquid is proportional to the pressure of the gas above the liquid.” Since water does not compress or is only very slightly compressible, the compressed oxygen is infused into the water by the oxidizing demand of those contaminants that can be reduced by the oxidation reaction. Because the available oxygen is diminished between pump cycles, the air-volume control provides atmospheric oxygen (air) replenishment, which maintains the differential pressure, much the same way as the pneumatic (bladder-type) pressure tanks commonly used since the 1960s. The drawback to the air-head pressure tank is the formation of iron bacteria, a gelatinous mass created by bacteria found in water. The bacteria, which thrive on ferrous (soluble) iron, metabolize it and deposit the ferric hydroxide compounds in their life process.
Early treatment options to counter the effects of the iron bacteria included installing a dosing pump between the pump and pressure tank, which fed chlorine (household bleach) into the water every time the pump was initiated to make up the pressure-tank water volume. Addition of chlorine served to control development of bacterial iron and to counter other forms of water contamination that may have entered via atmospheric conditions (such as mold and mildew commonly found in poorly constructed pump houses or damp basement areas). Arsenic levels in a water supply were automatically converted to the pentavalent phase because of the addition of chlorine and thus, filtered out along with the oxidized iron with an activated carbon backwash filter.
Oxidizing filters
Another treatment option is the oxidizing filter. An oxidizing filter is defined as a tank loaded with a chemically reactive media, such as manganese greensand, manganese dioxide (MnO2) or manganese zeolite and regenerated with an oxidizing chemical called potassium permanganate (KMnO4). (Where higher levels of iron [typically three parts per million] or greater are present, manganese greensand has long been used to remove arsenic from the water source.) In fact, US EPA guidelines stipulate that arsenic can be removed when there is sufficient iron (3.0 ppm and greater), that when oxidized will provide the mechanism to capture the pentavalent arsenic, which is subsequently carried to drain during regeneration of the oxidizing media.
In the mid 1920s, many public utilities used manganese greensand filter media to treat groundwater containing iron and or manganese and hydrogen sulfide. They typically used a process called continuous regeneration (CR), where chlorine and potassium permanganate were fed simultaneously into the water flow upstream (before) the water filter. This method provided longer service runs and provided for constant detectable chlorine in the utility service lines serving consumers. Regeneration of the treatment plant filters was delayed until the early hours of the morning when water usage was minimal at most. This tried-and-proven process is still in use today because of the ability to remove iron, manganese, hydrogen sulfide, arsenic and radium from the water supply.
Ozone
Ozone (O3) is a powerful, fast acting oxidizer which has been in use for water treatment since the early 1900s in Europe. Ozone is a highly unstable gas that is formed by an electrical reaction on oxygen molecules. It has been shown to destroy E. coli bacteria up to 3.125 times faster than chlorine and because of its superior oxidizing power, ozone systems require lower concentrations and shorter contact times than if chlorine were used (and there are no chemicals to handle, mix or replenish).
Chlorine typically requires 20 minutes of contact time to effectively oxidize iron, manganese and hydrogen sulfide. Ozone requires as little as 30 seconds to oxidize those same contaminants and, depending on the water temperature, could take as long as three minutes for effective oxidation. Ozone has a very short half-life in water and simply reverts to oxygen when it dissipates. Where arsenic is present in the water, ozone will quickly convert total arsenic to the pentavalent form and activated carbon has been proven to filter out much of the pentavalent arsenic. Another benefit of ozonation is that the ozone creates a biologically activated carbon bed, whereby ozone enrichment of the feed water maintains active microbiological growth for aiding degradation and reduction of organics that have been adsorbed on the surface and in the pore structures of activated carbon.
Modern residential-size ozone systems have seen incredible advancements in recent years that have virtually eliminated external contact tanks and conversion filters. The technology has advanced to the point where the entire process of oxidation, conversion and filtration have all been combined in a single tank with a maximum footprint of 12” x 12”. (This author can remember a time when ozone systems were field-engineered and required a 3’ x 5’ footprint, which was reduced several years later by a skid-mounted, pre-plumbed chassis with a footprint of 24” x 36”.) Shorter installation times, quicker startup and commissioning, as well as easier serviceability and diagnostics, made ozone a more feasible treatment option.
Reverse osmosis and distillation
Drinking water appliances (including reverse osmosis and distillation), which are certified by WQA to NSF/ANSI Standard 58 and NSF/ANSI Standard 62, will typically yield processed water where pentavalent arsenic has been removed by as much as 99.6 percent or more. It is imperative, however, that detectable free chlorine is available to the appliance inlet or on water supplies that have been demonstrated to contain only pentavalent arsenic. Here again, treatment with chloramine is not sufficient to ensure complete conversion of trivalent arsenic to pentavalent arsenic.
When it is uncertain whether the water contains chlorine or chloramine, a call to the local water utility will answer that question. A good visual indication the water is chloraminated is to observe the color of the water when a bathtub is filled. Chlorinated water is usually a light aqua color, whereas water containing chloramine has a yellow cast to it. These types of RO and distillation appliances require routine operational maintenance and replacement of critical components to maintain effective reduction/removal of inorganic contaminants to perform to manufacturers’ specifications. Neglecting or deferring service and maintenance could result in drinking and preparing food and beverages with arsenic contaminated water.
Advanced arsenic selective media
In the late 1990s, a granular ferric oxide media called Bayoxide® E33 was proven to remove up to 99 percent of total arsenic, including both As (+3) and As (+5). For residential and light commercial applications, systems are designed for lead-lag or parallel designs with sample ports for testing of arsenic breakthrough. Treatment vessels with designated media loads are specifically rated for a recommended service flow to prevent arsenic migration and excessive pressure drop in the service plumbing. To help make the arsenic removal consistent, flow controls are installed on the effluent ports of the control valves to prevent both arsenic migration and excessive pressure drop.
In the early 21st century, another arsenic treatment media was introduced. (This nanoparticle-based, arsenic-specific, anion resin selective media exhibits high affinity and capacity for both As (+3) and As (+5)). These systems, too, were set up for a lead-lag or high-flow parallel service with sample valves for testing of the effluent. More recently, advanced technology has seen the production of pour-through paper-type arsenic filters like those used in automatic drip coffee makers. Preliminary testing and validation has shown remarkable potential for complete removal of arsenic from water, so watch for more details on this technology and its various uses in the future.
Particle-bonded media
Another advanced treatment process is particle-bonding, where granular activated carbon (GAC) is combined with different sorbents to treat a specific water contaminant or multiple sorbents to remediate multiple contaminants. Granular activated carbon combined with an arsenic sorbent would provide the steps of dechlorinating the water supply, while simultaneously removing arsenic from a single, multifunctional media.
Choices
There are many choices in arsenic treatment technologies and options available to the consumer whether POE (whole-house) treatment is required or just remediation for drinking water. The important issue is to make sure the products are certified so the consumer has the assurance they will perform to manufacturers’ specifications when properly applied and maintained. There are many more emerging contaminants that require effective remediation. The good news is that the remediation technologies are here now. The technologies presented herein are only a few of those available that have proven track records for effective remediation. Pay attention to the emerging technologies and seek to apply those technologies where they fit into your market(s).
About the author
Gary Battenberg is a Technical Support and Systems Design Specialist with the Fluid System Connectors Division of Parker Hannifin Corporation in Otsego, MI. He has 36 years of experience in the fields of domestic, commercial, industrial, high-purity and sterile water treatment processes. Battenberg has worked in the areas of sales, service, design and manufacturing of water treatment systems and processes utilizing filtration, ion exchange, UV sterilization, reverse osmosis and ozone technologies. He may be reached by phone at (269) 692-6632 or by email, [email protected]