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The Physical & Biological Treatment of Contaminated Water
by Laura Walbrink
Because it is essential for survival, the purity of groundwater is a chemical and biological issue that directly affects the quality of human lives. The enactment of the Clean Air Act, which forced large companies to reduce pollutant emissions, inspired more research into treating and extracting groundwater pollution. Two professors in the University of Minnesota's Department of Civil Engineering, Dr. Michael Semmens and Dr. Daryl Dwyer, are each researching the treatment of contaminated water in different ways.
Dr. Semmens is using physical methods to make an existing hollow-fiber membrane pure oxygen aeration system viable for large volumes of contaminated water. Dr. Dwyer, who is taking a more biological approach, is testing synthetic bacteria to determine if they can neutralize pollutants, a process known as bioremediation.
The process of aeration (adding oxygen to water) has long been used to treat contaminated wastewater because bacteria, which need oxygen for respiration, effectively break down the pollutants. Unfor-tunately, the traditional method of aeration, which is similar to that used to aerate an aquarium, is expensive and energy-inefficient. Therefore, it represents a high proportion of operation costs at waste treatment facilities.
Additionally, negative environmental consequences result from the bubbles formed by the diffusion of oxygen into water. Pollutants in the water are released into the bubbles, which reach the surface of the water and break, thus transferring the pollutants into the air. Odors are a common result of this process. Although aeration effectively removes contaminants from wastewater, it can also transfer them to the air, which creates another environmental problem. If wastewater contains volatile organic compounds (VOCs), such as solvents and gasoline, then aeration causes the VOCs to be released into the bubbles and then into the air rather than be treated by the bacteria. The organic content of the water is reduced, but the pollutants are transferred into the air. In addition to Clean Air Act regulations restricting VOC emissions, the presence of VOCs in air is undesirable for health reasons. VOCs also impact air quality and contribute to the depletion of the ozone layer.
The goal is to find a more environmentally-friendly and cost-effective way of putting oxygen into water. One such alternative is adding pure oxygen to water. Using pure oxygen rather than air requires approximately one-fifth of the volume of air because oxygen is only twenty percent of air by volume. The amount of VOCs released into the air is directly proportional to the volume of oxygen pushed through the water. Thus, substituting pure oxygen for air in the aeration process can theoretically reduce VOC emissions and odor problems by approximately eighty percent.
Despite these benefits, adding pure oxygen to water is not always a viable or necessary alternative to using air for aeration purposes. First, producing pure oxygen is expensive, whereas air is free. At small companies, the problem of VOC emissions may be relatively minor, so the traditional aeration method may make more financial sense. Because of the high cost of pure oxygen, even the most effective pure oxygen aeration system will be more expensive than one using air.
In some instances, however, the environmental benefits outweigh the additional cost. In Japan, for example, where space is at a premium, treatment systems are housed in the basements of buildings. These "package plants" are so common that they are sold in stores. A compact system that can handle a high rate of water flow is essential. The system must also reduce or eliminate odor problems because of its proximity to the residents of the building. Europe is encountering similar problems related to population densities and resolving them using the pure oxygen aeration method.
Dr. Semmens's research focuses on increasing the rate of water flow and improving the performance of the pure oxygen system using a hollow-fiber membrane aerator. The system suppresses the formation of bubbles by pushing pure oxygen through hollow fibers. The aerator contains a number of these gas-permeable fibers, each of which is sealed at the end and contains pure oxygen under pressure.
The oxygen does not escape the walls of the fiber; rather, it remains at the surface of the membrane. When water contacts the surface of the fiber, oxygen is dissolved directly into the water without forming bubbles. The membrane of the fiber allows high oxygen permeation, and the transfer occurs rapidly enough so as to make the presence of the membrane virtually unnoticeable.
The fibers, which are made from microporous polypropylene membrane, measure between 200 and 400 micrometers in diameter. A small aerator can hold a proportionally large surface area of membrane because the diameter of the fibers is so minuscule. The equation for oxygen transfer is:
dC/dt=KLa(C*-C)
KLa represents the overall oxygen transfer coefficient; C* is the saturation concentration, and C is the actual concentration of dissolved oxygen in the water. Optimal oxygen transfer occurs when KLa and C* are large.
The rate at which water flows past the fibers and the direction of the fibers determines the value of KL. More effective contact is attained if water strikes the surface of the fibers at an angle perpendicular to the direction of the fibers than if water flows parallel to the fibers. Specially coated membranes can increase the value of C*. When pure oxygen is used, the membranes can handle pressures of up to 60 psi.
The hollow-fiber membrane aeration system works well with small volumes of water, but it is not as successful with large volumes of water such as those encountered at wastewater treatment facilities. Al-though pure oxygen is inserted at 4 atm, twenty times its normal pressure, using large volumes of water results in a pressure drop across the fiber modules, which requires a great deal of energy to resolve. Dr. Semmens, who has researched since the late 1980s the adaptation of the pure oxygen aeration system to treat large environmental systems, added that the method is not currently suitable for environmental applications. Solids, sediment, bacteria, and other contaminants in the water can block up the small tubes.
As these problems are solved, hollow-fiber membrane aeration using pure oxygen will become more viable. With the increase in population and environmental restrictions and regulations, more companies, cities, and countries are searching for energy- and space-efficient alternatives to traditional methods of decontaminating water. For them, the research that Dr. Semmens is conducting and the resulting technology may prove to be very beneficial.
Dr. Dwyer, whose research is centered around biological rather than physical methods, is studying the bioremediation of aquifers. In bioremediation, microorganisms re-move or degrade contaminants in water. In the past, bioremediation relied on natural bacteria to treat hazardous waste, but inoculation has recently emerged as a supplement to this standard method of removing contaminants. Inoculation uses synthetic bacteria, grown in labs for the purpose of treating waste, to degrade chemicals in water.
Dr. Dwyer has researched this topic since 1989. He began his study at the German Institute for Biotechnology and works in cooperation with the U.S. Geological Survey (USGS). Field work is conducted at the USGS Cape Cod Groundwater Contamination Studies site, chosen for its proximity to a military base. The groundwater there, which is contaminated by pollutants from the military site, is buried beneath layers of sand. The bacteria are grown in large quantities at the Bioprocess Technology Institute, which is located in the Twin Cities, and then centrifuged from 200 L to 1L. After being concentrated, the bacteria are transported to the Cape Cod site. Inserting the bacteria into the ground involves a process very similar to that of digging a well. A drilling rig digs holes about 50 feet deep, and the bacteria are then inserted into the aquifer.
Within a short period of time, the inserted bacteria are recaptured for testing. Success is indicated by the survival of the transported microorganisms. Their survival rate is determined by counting the number of live bacteria and dividing that figure by the original number of bacteria If they live, they have broken down the contaminants; if they die, they are not robust enough to degrade the pollutants. Back in the lab, the microorganisms are tested for their degradative ability. Simulated aquifers are used to collect this data.
Three synthetic strains of bacteria are commonly used to combat their pollutant counterparts at the Cape Cod site. Pseudomonas cepacia G4 counters trichloroethylene, a degreasing agent used for a multitude of purposes, including dry cleaning. Pseudomonas putida breaks down phenol, and Pseudomonas sp strain B13 degrades 3-chlorobenzoate and 4-methylbenzoate.
Using bioremediation to treat contaminated sites like Cape Cod is an attractive method for several reasons. Bioremediation, like other methods, helps to clean the environment, and using bacteria is cheap and environmentally safe. One alternative is to bulldoze the site, pump the water out, and then treat it, but this damages the environment and is time-consuming and expensive. Bioremediation through inoculation of synthetic bacteria has thus far proved to be largely effective.
Drs. Dwyer and Semmens are approaching a similar problem using different methods--physical and biological, respectively--but their research into the treatment of groundwater has important and useful applications. As the United States and other countries grow more concerned about environmental waste and contamination, efficient processes for the removal and treatment of pollutants in water and air become increasingly necessary. The population density and the consumption of natural resources are increasing, but environmental engineering promises a cleaner, healthier future.
