Grantee Research Project Results

2000 Progress Report: Role of Microbial Metabolism and Cometabolism in Treating Mixtures of Biodegradable and Nonbiodegradable Chemicals in Granular Activated Carbon Columns

EPA Grant Number: R826170
Title: Role of Microbial Metabolism and Cometabolism in Treating Mixtures of Biodegradable and Nonbiodegradable Chemicals in Granular Activated Carbon Columns
Investigators: Speitel, Gerald E.
Institution: The University of Texas at Austin
EPA Project Officer: Aja, Hayley
Project Period: December 1, 1997 through November 30, 2000
Project Period Covered by this Report: December 1, 1999 through November 30, 2000
Project Amount: $304,688
RFA: Exploratory Research - Environmental Engineering (1997) RFA Text |  Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management

Objective:

Granular activated carbon (GAC) is used widely to treat water contaminated with synthetic organic chemicals (SOCs). Practically no information is available on combining adsorption and biodegradation to treat mixtures of biodegradable and nonbiodegradable SOCs, a very common problem. Biodegradation can increase the GAC service life and improve process performance relative to adsorption alone. This research seeks to: (1) develop a better understanding of the effect of biodegradation on the service life of GAC columns, (2) identify conditions where metabolism of SOCs is advantageous, and (3) identify conditions where cometabolism of SOCs is advantageous.

Progress Summary:

This past year's work focused on GAC column experiments with two-component mixtures of biodegradable and nonbiodegradable SOCs. In particular, the effect of biological activity in the exhausted GAC zone of a column was studied in detail, as this portion of the GAC column is where biodegradation is likely to have the largest impact on GAC service life. To this end, two series of experiments were completed encompassing a range of empty bed contact times (EBCTs) and SOC concentrations and adsorbabilities. Two biodegradable chemicals were studied: benzene, which is weakly adsorbed, and toluene, which is moderately adsorbed. Likewise two nonbiodegradable chemicals were studied: carbon tetrachloride, which is weakly adsorbed, and tetrachloroethylene (PCE), which is moderately adsorbed. Experiments were run with either the weakly or moderately adsorbed pairs of SOCs. Experiments were run with chemicals of similar adsorbability because computer simulation models suggest that the greatest benefits from establishing biological activity in GAC columns are likely to result when the biodegradable and nonbiodegradable chemicals have similar adsorption characteristics. Experimental measurements included influent and effluent concentrations by gas chromatography (GC), chemical loadings on the GAC at the beginning and end of experiments via solvent extraction, and bioregeneration via ¹⁴C-radiochemical techniques.

Significant bioregeneration of the GAC occurred in experiments with benzene and toluene as the biodegradable chemicals. As expected, liquid phase concentrations of both chemicals were, in general, driven to low levels. Occasionally, biodegradation was inhibited by low dissolved oxygen levels, which resulted from vigorous biodegradation of adsorbed benzene or toluene (i.e., bioregeneration). Bioregeneration of the GAC led to additional adsorption capacity for the nonbiodegradable SOCs, carbon tetrachloride, or PCE. For example, in experiments with toluene and PCE in exhausted GAC columns, the PCE loading after 500 hours of operation was 1.5 to 2 times the initial loading at the influent end of the columns. This increased loading resulted from bioregeneration of the GAC and the resulting reduced competition between PCE and toluene for adsorption sites.

The extent of bioregeneration is the key factor in realizing increased GAC capacity for nonbiodegradable SOCs. This research shows clear trends with respect to bioregeneration: (1) a shorter EBCT results in a smaller percent bioregeneration, and (2) a lower influent concentration results in a smaller percent bioregeneration. Both these findings have been suggested by previous research with single component systems. A shorter EBCT implies a higher liquid phase concentration of the biodegradable SOC, which in turn provides a smaller driving force for desorption and subsequent biodegradation of the sorbed SOC. The concentration dependence of bioregeneration is probably related to two phenomena. First, as concentration decreases the SOC loading on the GAC surface decreases, which means that the chemicals on average are attached to higher energy adsorption sites. Desorption is more difficult from such sites and in the extreme may not occur at all (i.e., irreversible adsorption). Second, lower concentrations inherently have a lower potential for establishing large concentration gradients within the GAC. Beyond the initial period of bioregeneration, the process tends to be diffusion limited, so the smaller concentration gradients associated with lower concentrations yield both lower diffusion and bioregeneration rates.

The extent of bioregeneration ranged from 30 to 45 percent in this research, which is larger than was seen in most previous research with single chemical systems. The larger extent of bioregeneration probably results from both the higher concentrations used in this research and the lower adsorbability of benzene and toluene in comparison to the various phenolics used in other research. Obviously, the greater the extent of bioregeneration, the greater the expected additional capacity for nonbiodegradable SOCs. Therefore, biological activity on GAC is likely to yield greater benefits with respect to nonbiodegradable SOC removal for moderately adsorbable chemicals, and these benefits will increase with increasing concentration. Biodegradable chemicals of low adsorbability also yield a large extent of bioregeneration, but biodegradation of such chemicals does not provide as much additional adsorption capacity for nonbiodegradable SOCs. Presumably, the improvement is not as great simply because these chemicals do not have as large an affinity for the GAC surface, so their biodegradation does not re-open as many adsorption sites.

Future Activities:

Experiments over the next year will focus on cometabolism, as outlined below:

• Estimation of adsorption and biodegradation parameters for the biodegradable and cometabolizable chemical. This study involves individual adsorption isotherms for each chemical as well a competitive isotherm for the pair of biodegradable and traditionally nonbiodegradable chemicals, a GAC desorption kinetic study for the biodegradable chemical, and 14C-radiochemical biodegradation experiments to measure both metabolism and cometabolism kinetic parameters. The SOC combination to be studied is expected to be toluene and trichloroethylene (TCE, the cometabolite).

• GAC column experiments. An increase in the GAC service life will result from a combination of (1) decreased competition for GAC adsorption sites through metabolism of the biodegradable SOC and (2) reduced demand for GAC adsorption sites by the "nonbiodegradable" SOC because a portion is cometabolized. It will be necessary to distinguish between the relative contributions of these two phenomena in order to definitively understand the significance of cometabolism to process performance. This will be done by using radiolabeled SOCs to track metabolism and cometabolism through the concentration of radiolabeled carbon dioxide that is produced as a result of biodegradation. In general, two GAC columns will be run simultaneously. In one column, the SOC undergoing cometabolism will be radiolabeled, while in the other column, the SOC undergoing metabolism will be radiolabeled. Thus, the biodegradation rates for each chemical will be measured in parallel experiments. These experiments will involve the use of one biodegradable SOC and one traditionally nonbiodegradable SOC, as well as bacteria acclimated to the biodegradable SOC. In exhausted GAC experiments, the GAC is pre-saturated to observe the effects of bioregeneration on exhausted GAC. The largest effect of cometabolism on process performance should be observed in these experiments. Besides GC and radioactive analyses, toluene dioxygenase enzyme (the enzyme produced in the degradation of toluene that also degrades TCE) production will be measured spectrophotometrically by observing the enzymatic change of indole to indigo over time.
  A virgin GAC experiment will be run to culminate the cometabolism study. Two columns will be run simultaneously, one sterile and one with bacteria, so that the increase in service life due to metabolism and cometabolism can be fully appreciated. A small amount of carbon will be used so that the experiment can be run in a reasonable amount of time.

Journal Articles:

No journal articles submitted with this report: View all 9 publications for this project

Supplemental Keywords:

water, adsorption, bioregeneration, bioremediation, innovative technology, synthetic organic chemicals., RFA, Scientific Discipline, Water, Ecological Risk Assessment, Environmental Chemistry, Wastewater, Bioremediation, Environmental Engineering, gas chromatography, cometabolism, kinetic studies, water quality, bioregeneration, granular activated carbon, biodegradation, fate and transport, water treatment, mathematical modeling

Relevant Websites:

http://www.ce.utexas.edu/prof/speitel/home.html

Progress and Final Reports:

Original Abstract

1998 Progress Report

1999 Progress Report

Final Report