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By Eweis, Ergas, Chang, and Schroeder
Published by McGraw-Hill Series in Water Resources and Environmental Engineering
These University of California, Davis, professors have provided a most useful addition to remediation study resources with this McGraw-Hill text. They designed it for use in an introductory course on remediation of soil and groundwater contaminated by organic compounds. Petroleum compounds are a prime example of those organics. The text has been “vetted” by environmental engineering students at Davis over a five-year period.
The NST/Engineers, Inc. reviewers consider knowledge of bioremediation principles to be a requirement of students in environmental science or engineering. This book is an excellent choice as a study text or as a reference for practicing engineers. Problems, discussion questions, and references follow each chapter. Sample problems are worked-out within the chapters.
Following an introductory overview, the authors cover background material in five chapters. The soil environment is explained. A triangular soil classification graph displays the classes of soil texture, rated according to the proportions present of sand, clay, and silt. Additionally, the authors treat the effects of soil gases, moisture, porosity and other factors on the fate (destruction or preservation) and transport of contaminants through soil bodies. Methods are presented for calculating the distribution of organics between soils and liquids, and between liquids and vapors. The use of Henry’s Law is featured for the latter.
Incidentally, a recent reference that will be helpful in providing measured data and best estimates for transport and physical properties of representative organics is “Chemical Properties Handbook” by Carl L. Yaws McGraw-Hill, 1999.
Two chapters, “Microbial Ecology” and “Metabolism and Energy Production”, provide basic information on types of microorganisms, stimulating cell growth, and maintaining cell health. This emphasis on biological basics highlights the greater contemporary importance of biological than chemical and other methods of destruction of organic contaminants in soils. Attention is focused on bacteria, the most abundant group of microorganisms in soil and groundwater; also the group that plays the major role in biodegrading toxic and otherwise offensive organic contaminants. A chapter is devoted to the chemical reactions occurring during biodegradation, such as oxidation, hydrolysis, bond rupture, and dehalogenation.
In the final four chapters, the authors explain the engineering tasks involved in applying the fundamental principles covered in the earlier chapters. They describe the characteristics of in-situ treatment, and then solid-phase, slurry-phase, and vapor-phase bioremediation.
In-situ treatment can be highly favored over removal of soil or water to an off-site location for treatment because of economic, schedule, logistic, and other constraints. Providing balance, the inherent difficulties of in-situ work are also covered. There are always some uncertainties about: subsurface soil structure, water bodies, and contaminant distribution. Factors involved in the major remediation methods of: “pump, treat, and injection”, “air sparging”, and “bioventing” are covered.
A chapter on “Solid Phase Bioremediation” covers the landfarming ( spreading soil to be treated no more than a few feet deep over large land areas) and composting options. In composting systems, various types of covered piles of contaminated soil are maintained with control of nutrients, moisture, oxygen content, and temperature. “Slurry Phase Bioremediation” is something like stirred-tank reactor processing in the chemical industry with microbes replacing catalysts. Microbes and their nutrients, and catalysts and their supports, must be carefully chosen. With proper operation to prevent microbe death and catalyst poisoning or deactivation, both materials may be used in the next batch or in continuous operation. The reactors are large, enclosed, storage tanks provided with feed and discharge ports, agitation, and temperature control. Slurry phase work is also shown to be amenable to existing uncovered, lagoons when contaminant volatilization is not a problem. Floating agitators have been used in those cases.
The last chapter describes, and provides designs and parameters for, “Vapor Phase Biological Treatment”. VOCs are almost always present in the off-gases of soil or groundwater treatment. The authors summarize the pros and cons of currently used vapor-phase pollution control technology, such as adsorption and thermal and catalytic oxidation. They then present the case for “biofilters” or “biotrickling” filters. The former has microbes kept alive with nutrients and supported on various kinds of media. Off-gas vapors are blown in at the tower bottom, pass through the media, and exit at the top. The latter differs in that water spray is added at the top and nutrient solution is collected at the bottom and is recycled to the top. In either case, operation must control plugging of the media and thereby prevent a high vapor delta P through the column.
A useful Appendix table provides properties, including Henry’s Law constant at 20 deg.C, for about 80 organics currently found in hazardous wastes.
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