An Evaluation of Activated Carbon for Drinking Water Treatment

This chapter contains the findings of the Subcommittee on Adsorption of the National Research Council's Safe Drinking Water Committee, which studied the efficacy of granular activated carbon (GAC) and related adsorbents in the treatment of drinking water. Some attention is given to an examination of the potential health effects related to the use of these adsorbents, but detailed toxicological and epidemiological implications resulting from the presence of organic compounds in drinking water are considered in separate chapters of Drinking Water and Health, Volume 3. The development of standards for GAC and the economic aspects of its use was not a part of this study.

The subcommittee defined ''activated carbon" as a family of carbonaceous substances that are characterized primarily by their surface area, pore size distribution, and sorptive and catalytic properties. Different raw materials and manufacturing processes produce final products with different adsorption characteristics.

The use of GAC under specified conditions was proposed by the U.S. Environmental Protection Agency (EPA) as the option of choice for the control of "synthetic organic chemicals" in drinking water. During the subcommittee's study, the EPA held hearings and received written comments regarding this treatment.

The subcommittee reviewed the pertinent literature and rigorously assessed the scientific data base. Its scope of work included a review of work on:

• adsorption efficiency
• microbial activity on adsorbents
• physiochemical interactions
• regeneration of adsorbents
• analytical methods to monitor adsorption processes

The subcommittee considered the ability of adsorbents to remove organic compounds of concern to health and the possible products of the adsorption process. A large and diverse segment of the scientific literature, particularly that concerning recent European experience, was scrutinized. Studies that met established criteria for quality assurance and completeness of data were used as primary sources by the subcommittee. Where possible, stress was placed on studies of chemicals at nanogram to microgram per liter concentrations, which are typically found in drinking water. The subcommittee was confronted by a continual flow of new data and the need for postulation and interpretation. To ensure a thorough review of each topic, the data for each type of adsorbent were considered and reported separately.

Carbon and other adsorbents in various forms have been used for the treatment of water and as detoxifying pharmaceutical agents in medicine for many centuries. There has been an uninterrupted use of carbonaceous adsorbents since biblical times (Old Testament, Num. 19:9; Maimonides, 1185) and there have been marked changes in the nature of the adsorbent since that time (Kunin, 1974a,b).

During the twentieth century, GAC and powdered activated carbon (PAC) have been used in the United States to control taste and odors in drinking water (U.S. Environmental Protection Agency, 1978a). During the past 20 yr, research on the use of adsorbents to treat drinking water has emphasized the removal of specific organics. The removal of organic compounds from drinking water has been based primarily on the measurement of organic matter as measured by carbon chloroform extract (CCE), total organic carbon (TOC), or other group parameters. However, it has long been recognized that these group parameters provide only estimates of performance for target compounds. Studies beginning with those of Middleton and Rosen (1956) began to identify the specific organic compounds in drinking water and their removal by the carbon adsorption.

Over 700 volatile organic compounds have been identified in drinking water (U.S. Environmental Protection Agency, 1978c). These compounds make up only a small fraction of the total organic matter (National Academy of Sciences, 1977). Approximately 90% of the volatile organic compounds that can be analyzed by gas chromatography have been analyzed, but this represents no more than 10% by weight of the total organic material. Only 5%-10% of the nonvolatile organic compounds that comprise the remaining 90% of the total organic matter have been identified.

The EPA (1978c) has categorized the organic compounds in drinking water into five different classes. Each class has distinctly different characteristics of concern to those involved in water treatment.

Class I: organic compounds that cause taste and odor and/or color problems;

Class II: synthetic organic chemicals that are present in source waters from upstream discharges or runoff;

Class III: organic compounds (precursors) that react with disinfectants to produce "disinfection by-products";

Class IV: organic chemicals that are the disinfection by-products themselves; and

Class V: natural (non-Class III) organic compounds of little direct toxicological importance.

Today there are GAC beds in U.S. water treatment plants for removal of Class I compounds. Consideration is being given to the use of GAC for removal of Class II, III, and IV compounds as data become available. Class V compounds are of interest because they may compete for adsorption sites, thereby lessening the removal of other compounds.

This report identifies the compounds that may be removed and/or added to drinking water by the adsorption process with its attendant chemical and microbial processes. It focuses on recently published lists of organic chemicals of concern to health (Interagency Regulatory Liaison Group, 1978; National Academy of Sciences, 1977, 1979; National Cancer Institute, 1978).

Each section deals with complex subjects in which there are uncertainties, inconclusive or incomplete data, and, thus, conflicting opinions. The length of each section represents only the number of studies reviewed and does not reflect the relative importance of the subjects.