Back to Spring 2010 Newsletter
by Katherine Cai, Department of Chemical Engineering, Arizona State University
Water quality is one of the more commonly overlooked environmental and health issues. However, there are a plethora of pollutants that plague drinking water. Trichloroethylene (TCE) is a ubiquitous groundwater contaminant. It is a chlorinated and colorless man-made compound that has been increasingly found in groundwater due to its release in air emissions from metal degreasing plants as well as its use as an industrial solvent. Wastewater from metal, paint, electrical and rubber processing industries often contain TCE. The Toxics Release Inventory recorded a total of 320 million pounds of TCE releases in land and water in 1991 alone. Since it is such a highly toxic substance, it has become a major environmental concern.
According to the U.S. Environmental Protection Agency between 9 and 34 percent of U.S. water is contaminated with TCE. The contamination is located at over 380 different Superfund sites, uncontrolled hazardous waste sites targeted by the federal government for clean-up. In the National Toxicology Program’s 11th Report on Carcinogens, TCE was reasonably anticipated as a human carcinogen. People who are exposed to TCE or drinking water containing an excess of TCE experience many health problems including liver damage and central nervous system depression.
In order to ensure safety, governmental organizations have implemented some basic standards. EPA has set maximum contaminant levels for TCE in drinking water of 5 parts per billion parts water, or 0.005 mg/L, and has developed regulations for working with and disposing of TCE. In addition, the Occupational Safety and Health Administration has set a maximum exposure limit of 100 parts per million parts of air for a standards 40-hour work week.
There are many current analytical methods used to detect TCE. The EPA, as well as the National Institute for Occupational Safety and Health, has identified a number of approved techniques that can be used for a variety of samples. This includes water and soil samples that can be measured either in situ, at the site of contamination, or in a laboratory. Gas chromatography, especially using the headspace gas above the surface of liquid samples, is very common. Gas chromatograms have very good detection limits for TCE, generally with a lower limit of 1 μg/L for water and 1 μg/kg for soil. They can also be used in conjunction with mass spectrometers to offer even higher accuracy and lower detection limits.
Newer developments for in situ sampling and analysis are the membrane interface probe and the halogen specific probe. These use permeable membranes that, when heated, cause different volatile organic compounds or halogens to move across the membrane. At the surface, the probe uses either an ion trap mass spectrometer or downhole analyzer respectively to determine relative TCE concentrations.
Beginning in 1989, the National Primary Drinking Water Regulations began regulating and ensuring drinking water standards for TCE. The EPA now requires all water suppliers to take water samples every three months to check for TCE. If TCE is present, EPA has approved using packed tower aeration, a filter system with reverse osmosis distillation, to remove the compound from the water. However, there has been some recent groundbreaking research that shows new possible techniques for in situ treatment.
At the Biodesign Institute at Arizona State University, the Center for Environmental Biotechnology under the direction of Bruce Rittmann, has been researching a drinking water technology called the Membrane Biofilm Reactor (MbfR) that biologically degrades TCE. Jinwook Chung, with advisement from Dr. Rittmann and Dr. Rosa Krajmalnik Brown, from this lab published the only reported study of MbfR use, “Bioreduction of Trichloroethene Using a Hydrogen Based Membrane Biofilm Reactor” in Environmental Science Technology 2008.
The MBfR is a glass structure that contains a bundle of hollow fibers pressurized with H2, which functions as an electron donor. This gas is delivered through a bubbleless gas transfer membrane to a biofilm, a group of microorganisms forming a web structure, on the wall of these fiber membranes. Water is pumped through the reactor and the microorganisms in the biofilm oxidize the H2 and reduce the TCE to the nontoxic compound ethene. While there are many different communities of microorganisms on the fibers, the MBfR study shows that the bacteria Dehalococcoides is a part of this atutotrophic biofilm community that is capable of dechlorinating TCE. While many different bacteria can remove halogens from substances, Dehalococcoides is the only identified bacteria that is capable of removing the chlorine from 1,2 Dicholoroethene (1,2 DCE) and vinyl chloride in the final steps of dechlorination. The use of bacteria to remove TCE is very promising for future methods of water treatment.
This is a very important development because the hydrogen based membrane biofilm reactors create a natural system to remove contaminants. Bioremediation using bacteria is a biological process which is safer, cheaper, and cleaner than using a chemical or physical process to separate and remove TCE from water completely. Biological processes require nominal addition of chemicals, leaving the water cleaner for human consumption. MBfRs support efficient clean up strategies, using natural resources and alternatives for in situ purification technology.
Water quality is a timeless issue with the innumerous bacteria, parasites, and compounds that contaminate drinking water. TCE is one of the less recognized contaminants, but it still poses a major problem in terms of the health and well-being of mankind. Fortunately, EPA has already begun implementing the measures necessary to ensure clean water, and state-of-the-art research is leading the world to better alternatives to effectively clean water with lower projected costs and health risks.