Treatment choices available to the water treatment professionals
The dissolution of ammonium, magnesium, potassium, or sodium salts into the groundwater pose a threat to drinking water supplies.
Ammonium perchlorate is used in the manufacture of rocket and missile propellant, road flares and fireworks. It serves as a source of oxygen for solid rocket propellant. Perchlorate salts are found in agricultural fertilizers, air bags and other industrial processes. The dissolution of these ammonium, magnesium, potassium, or sodium salts into the groundwater pose a threat to drinking water supplies. The perchlorate ion (ClO4) is very stable once in solution.
Types of Perchlorate Salts
- Ammonium perchlorate;
- Potassium perchlorate;
- Magnesium perchlorate; and
- Sodium perchlorate.
It has been estimated that between 22 to 35 states have perchlorate in their water supplies, with more than 11 million people consuming water with greater than 4 parts per billion (ppb).
Health experts say that perchlorate cannot be ingested through the skin, so the emphasis is on treatment of water used for drinking and food preparation.
Perchlorate can reduce the thyroid gland’s ability to produce enough hormone, by preventing iodide uptake into the gland. Tumors may also develop as a result of changes in the thyroid hormone levels. At greatest risk are expectant mothers, developing fetuses and infants.
The U.S. Environmental Protection Agency (EPA) has suggested a 1 ppb level maximum in public water supplies. The state of California has set a public health goal of 6 ppb.
Recently, the National Academy of Sciences (NAS) issued a report stating that perchlorate is safe at levels about 20 times higher than the EPA’s recommendations. This has generated a lot of controversy on the topic, including a challenge by the National Resources Defense Council.
Treatment is complicated for the municipality or water utility because the perchlorate ion does not respond to the classic techniques such as filtration or coagulation. The perchlorate compound is structured with four oxygen atoms surrounding a central chlorine atom. It can persist for years in the environment because it does not react with other compounds present in the water.
There are two categories of treatment technologies:
1) Destructive Processes.
- Biological Reduction;
- Chemical Reduction; and
- Electrochemical Reduction.
2) Physical Removal Processes.
- Anion Exchange; and
- Membrane Filtration (RO).
The destructive processes are used on large scale and are not appropriate for the POU/POE market. Anion exchange appears to work well on both large and small scale.
It will be some time before the EPA and the NAS come to agreement on a “safe” and acceptable maximum level of perchlorate. In the meantime, the technology exists now for the water treatment dealer to provide drinking water virtually free of perchlorates.
Anion exchange resins exhibit very strong affinities to the perchlorate ion. They can be used on a disposable or a regenerable basis. Type 1 or type 2 strong base anion resins have a higher affinity for the perchlorate ion than the nitrate or sulfate ions. This can lead to a long service run for perchlorate removal, but in cases where there is also nitrate present, the potential for nitrate dumping exists.
The standard SBA resins can be expected to remove perchlorates with capacities of between 2,000 to 10,000 bed volumes. The expected life of the media in a once through application can be months under this scenario.
“Targeted treatment” by the use of selective anion resins have helped to lower the overall treatment cost of perchlorate removal.
A fair number of selective anion resins have been synthesized, but only two are available commercially, the tributylamine and triethylamine types. Each of the two types of selective resins has its own advantages, depending on the application. They both show very low affinities for sulfate and therefore practically eliminate the potential for nitrate dumping by sulfate. The triethlyamine structure, because of its smaller size, yields a resin with a higher operating capacity than the tributylamine (TBA) type. However, the TBA may provide lower chemical operating costs in large systems when regenerant use is minimized through brine reclamation schemes. The TBA resin shows the best promise as a selective resin for perchlorate removal.
The TBA resin has shown throughput capacities more than two orders of magnitude higher than the standard anion resin. For point-of-use, this could result in a cartridge-type installation that would only need changing out once or twice a year.
Low pressure RO systems, typical of an under-the-sink units, can provide about 95% rejection of perchlorates. Ion exchange polishing might be considered post-RO to achieve very low effluent levels. The combination of these units may be necessary to get the effluent consistently below 4 ppb and to get such a system NSF qualified.
To get the most benefit from the available capacity of the selective resin, a lead-lag arrangement should be used. The typical setup has an A unit and a B unit in series. The A unit functions as the workhorse and is removing the brunt of the loading, and the B unit is acting as a polisher. When the A unit is exhausted, the B unit is moved into the first position and a fresh unit is placed in the polishing position.
The lead-lag arrangement is usually looked upon favorably by the governing health bodies (state, county or local officials) because of the degree of safety it incorporates. If, for some reason, there is a shock to the system that might prematurely exhaust the primary unit, the polisher is there to catch the mishap.
As with any long life media that is expected to be in service for a number of months, there is a potential for microbial growth. Installation of disinfection equipment such as UV should be considered downstream.
It will be some time before the governing bodies come to agreement to what the actual maximum contaminant level of perchlorate in drinking water should be. But right now, you have the means to economically provide your customers drinking water that is virtually free of perchlorate (technically below the 4 ppb detection threshold).
The water dealer can begin by finding if there are any contaminated water sources in the region. The EPA has prepared a fairly comprehensive mapping of releases in the U.S. which are featured at www.denix.osd.mil/denix/Public/Library/Water/Perchlorate/releases.html.
Anticipated throughputs of the selective media used in perchlorate removal systems can be calculated by your ion exchange resin provider. Parameters to include for testing customers’ waters are alkalinity, sulfate, chloride, TDS, pH and perchlorate.
The detection limit right now for reporting purposes is 4 ppb. Analysis of perchlorate by EPA Method 314 uses Ion Chromatography (IC), where an aqueous sample is filtered through a 0.45 micron filter and then introduced into the IC. The perchlorate is then detected by a conductivity detector. The adoption of any new maximum perchlorate level in drinking water must take into account the feasibility and cost of testing techniques.