Adsorptive media for arsenic removal

Oct. 13, 2010

It offers advantages, but other water parameters must be known.

Deciding how to solve an arsenic (As) problem in well water can be a difficult task for a homeowner. Several arsenic treatment technologies are now available, and most of them can effectively reduce the arsenic concentration in drinking water to a level below the US EPA regulated maximum contaminant level (MCL) of 10 µg/L (parts per billion, or ppb).

Choosing a technology that is both effective and economical depends upon multiple factors which can make it a challenge for the water treatment specialist.

However, arsenic treatment using point-of-entry (POE) and point-of-use (POU) units based on a fixed bed of arsenic-selective adsorptive media can be an attractive option for a private well owner. Benefits include simple installation, minimal maintenance and small size.

Adsorption advantages

POE and POU devices based on adsorption technology offer some advantages in various arsenic-removal situations. For instance, those designed around a hybrid hydrous iron oxide/polymer media do not generate liquid wastes at the customer’s home/well site because the systems do not require backwashing or regeneration, and they do not produce a reject stream. This is an advantage over devices based on adsorptive granular media. The latter tend to produce fines during use due to their poor mechanical strength, which means they must be fitted with a backwash capability.

Anion exchange resins require regular regeneration on-site because of low arsenic selectivity, resulting in significant volumes of brine for disposal to sewer or septic. Reverse osmosis (RO) systems, though effective at reducing the concentration of many contaminants, generate a reject waste stream which can be as high as 75 percent of the processed water.

Oxidation/filtration processes, e.g., a manganese greensand, may be an option for small systems if the raw water has sufficient amounts of soluble iron, but these can also be cumbersome to maintain in a customer’s home and produce secondary waste volumes.

With POU/POE systems based on adsorptive media, you should consider some rough guidelines to determine whether such a system is a viable option for treating arsenic at a private well. Following are key aspects that must be considered.

What is the water quality?

While total arsenic concentration in the raw water feed is the most important water-quality parameter, other water constituents can interfere with arsenic adsorption, and the level of these components should be known.

The most significant factors affecting arsenic removal from water by any adsorptive media are the levels of silica and phosphate, the pH and the prevalent oxidation state (species) of arsenic — As(V) or As(III). Speciation of arsenic is important because the adsorptive capacity for As(V) is significantly greater than for As(III). For a POU/POE system containing a hybrid adsorptive media to be able to operate and perform as it has been designed, the above water-quality parameters should fall below the limits given for the parameters in the accompanying table (sidebar: “Optimum water quality parameters for an adsorption POU/POE system”).

The system will still remove arsenic if some concentrations of the competing components are higher than the recommended limits for optimum performance, but if that is the case, the effective operational life of the device can significantly decrease.

Pretreatment strategies

If the maximum advised limits are exceeded, pretreatment may need to be considered to remove or minimize the effect of the interfering component.

This may include oxidizing the As(III) species to As(V), lowering the pH to improve arsenic capacity or removing the excess iron and manganese by oxidation/filtration. The US EPA recommends that minimums of a 20:1 iron-to-arsenic ratio and a pH less than 7.5 would be required for an iron oxidation process to be sufficient to reduce the arsenic below the MCL.

Each of the interfering chemicals has a different way of affecting the arsenic removal. Silica may polymerize on the media and physically coat and block the arsenic adsorption sites. Phosphate has an aqueous chemistry that is similar to arsenate and competes for the same adsorption sites as arsenic. Thus, high levels of phosphate will reduce the media’s arsenic capacity.

The arsenic capacity of an iron oxide media is higher at low pH values due to the increased positive surface charge of the media, which is attractive to the negatively charged As(V) species prevailing at a pH of less than 7.5. Therefore, lowering the pH of the water will improve the capacity.

Determining the species of arsenic is important if the presence of As(III) is suspected. This test is done most accurately in the field at the well site, because As(III) can rapidly oxidize to As(V) unless samples are carefully preserved. If As(III) concentration is higher than 10 µg/L, oxidation of As(III) to As(V) will also result in better performance.

Water use, system sizing

The POU/POE devices designed around a rugged hybrid iron oxide/polymer media (e.g., LayneRT, ArsenXnp) offer easy operation and simple maintenance. With such POE systems, sizing is based upon an assumption that a single-family home uses an average of 300 gallons of water per day (gpd), or 109,500 gallons annually.

Based on performance data from existing POE systems using a hybrid media, a tank containing 1 cubic foot of media can treat the water for a whole household for about 1.5 to three years [assuming an influent arsenic As(V) concentration less than 50 ppb and a pH below 8].

The POE systems are designed for flow rates of up to 10 gallons per minute (gpm) with two columns operating in a lead-lag configuration to ensure constant compliance with the arsenic limit. The rapid arsenic adsorption kinetics of the media allow for a relatively short empty bed contact time (EBCT) of 0.75 minutes per column, or 1.5 minutes for the entire two-column POE system.

The average indoor daily water use per person at a single faucet is 10.9 gpd, which is equivalent to 4,000 gallons per year. Small, single-column POU systems containing about 4 liters (0.14 cubic feet) of hybrid arsenic removal media are designed to treat an intermittent flow of cold water at a sink. At a maximum flow rate of 2 gpm, these devices can remove arsenic without replacement or maintenance for over two years under ideal water conditions.

Treating problem waters

Occasionally the levels of the interfering components in the water are higher than the maximum limits given in the accompanying table. In such circumstances, a POU/POE system based on adsorption may not be an economical or a viable choice. Following are rough guidelines for choosing pretreatment or an alternative treatment for problem water:

• If the total arsenic concentration is greater than 0.3 mg/L (parts per million, or ppm, with 0.3 ppm equivalent to 300 ppb) and all other parameters are below recommended levels, adsorptive POE/POU is viable. However, due to rapid exhaustion of the media, frequent change-outs of tanks may render the technology expensive.

In some cases, iron coagulation/filtration may be a more economical option to treat high-level arsenic waters, particularly if some iron is already present in the raw water.

• When As(III) is present in levels higher than 0.010 mg/L (10 ppb), oxidation prior to the adsorptive media is recommended to improve media life.

• If the arsenic level is low but phosphate and/or silica are higher than the limits in the table shown here, the most economic way to treat may be iron precipitation/filtration.

Considering cost

In selecting a suitable arsenic treatment, the economics of the technology are important, and this becomes particularly pronounced when considering the treatment of problem water.

It is paramount to characterize the water chemistry prior to making a decision, because a single technology is not suitable for all waters. Operation and maintenance costs (including potential pre-treatment) and waste disposal can quickly add to the initial capital cost if careful thought is not given to the effects of the water quality parameters.

In general, as water treatment professionals know, the further the water quality is removed from ideal conditions, the higher the treatment cost for a small system.

Teresia Möller, is a senior scientist for SolmeteX – A Division of Layne Christensen, based in Northborough, MA. SolmeteX is a producer of high-performance water treatment technologies.

Paul Sylvester obtained his bachelor’s of science in geochemistry and Ph.D. in inorganic chemistry at the University of Reading in the United Kingdom and is currently manager of research and development at SolmeteX.

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