Controlling Contamination with Carbon
Everyone wants drinking water that is safe and does not contain harmful chemicals that are detrimental to their health. In the case of water treatment systems that rely on groundwater or well water, there is the possibility of volatile organic chemical (VOC) contamination.
VOCs include dozens of U.S. Environmental Protection Agency (EPA)-listed chemical compounds regulated under the Safe Drinking Water Act that can be harmful to human health. VOCs are organic carbon-containing compounds that evaporate or volatilize easily from water. They can be arranged as coming from several usage groups, such as: fumigants, gasoline hydrocarbons, gasoline oxygenates, organic synthesis compounds, refrigerants, solvents and trihalomethanes (THM). Some of the more common chemicals are vinyl chloride; trichloroethylene; carbon tetrachloride, 1,2; dichlorbenzene, 1,1,1; trichloroethane; and chloroform—a type of THM.
All of these VOCs, except THMs, can get into a water source through leaks, dumping or accidents. A dry cleaner establishment, for example, could leak perchloroethylene from a storage tank into a local water source, or a gasoline station could leak benzene into an aquifer. The EPA has set maximum contaminant levels (MCLs) for VOCs ranging from 2 parts per billion (ppb) for vinyl chloride to 10,000 ppb for total xylenes.
The THMs group is not man-made in the strictest sense, but is created by adding chlorine to water containing natural organic matter. Nearly all municipalities that add chlorine to water will have some level of THMs in the product water. The wave of municipalities switching away from chlorine to chloramine disinfection for potable water is due mostly to efforts to reduce THM formation.
Chloramine is the addition of ammonia to chlorinated water and it is less effective than chlorine, so higher levels must be maintained in the water distribution system. Monochloramine (NH2Cl) is the main type of chloramine formed at the pH level of most water systems (pH>6). The federal MCL for THMs is 80 ppb and some states have lower levels depending on water use.
The good news concerning VOC removal from drinking water is that activated carbon does a good job of adsorbing VOCs from water and protecting human health. Activated carbon is, in fact, the treatment method of choice for most of these VOCs and except for vinyl chloride, it can remove most of them to non-detectable levels as a matter of course.
The Universal Adsorbent
Activated carbon is referred to as the universal adsorbent and there is no filter media more effective in performance and economics in most water treatment cases. The term “activated” refers to the fact that the raw carbonaceous material, such as coal or coconut shell, is processed first by carbonization at approximately 900°F followed by thermal treatment in a furnace or kiln at temperatures in the range of 1,600°F to 1,800°F with steam injection and little oxygen.
This activation process creates the enormous surface area within the carbon particles that can range from 500 m2/g up to 1,200 m2/g for most water treatment carbons. The high surface area with its millions of tiny pores is where the adsorption action occurs.
The most common types of carbon used for water treatment and VOC removal are coal-base (sub-bituminous or bituminous) and coconut shell-base carbon, generally in sizes such as 12 by 40 or 8 by 30 mesh (U.S. Sieve).
One should ask their activated carbon supplier for assistance if designing a carbon system for treating VOCs. The supplier or manufacturer can recommend a specific carbon type and grade depending on the VOCs of concern and other water treatment factors such as pH, temperature and other minerals or compounds in the water.
In the case of a central water treatment plant, granular activated carbon (GAC) made from coal or coconut shell in gravity or pressure filters is used for VOC removal, and these types of filters are usually in series flow and the water between the adsorbers or filters is monitored and tested for VOC content on a regular basis.
Once the leading filter is exhausted for VOC adsorption, the GAC is replaced with virgin GAC and the second filter becomes the lead filter. The spent activated carbon is typically taken away and reprocessed by reactivation at a high temperature and the resulting reactivated carbon product can be sold for other nonpotable water applications.
If a well or small local water system has VOC contamination, it may be economically feasible to have each home or place of business have point-of-use (POU) or point-of-entry (POE) activated-carbon filters in place. VOC concentrations are normally relatively small, in the parts per billion range, and even though the POU or POE filters are small, at less than 1 cu ft of carbon, they can be effective for removing VOCs for many months before carbon replacement is required. Again, these types of filters should be two in series for the best protection and carbon service life.