Bioremediation

Contamination of groundwater with toxic and carcinogenic compounds is a serious concern for public health and environmental quality. This problem is commonly manifested as a contaminant plume migrating in the direction of groundwater flow from a point source. Containment of the contaminant plume is important for preventing further migration and localizing the plume for in situ or ex situ remediation. Current containment methods include sheet pilings and grout curtains. These abiotic barriers require extensive physical manipulation of the site (e.g. excavation and back-filling) and are expensive to construct. An alternative approach, biobarrier technology, involves the use of microbial biomass produced in situ to manipulate groundwater flow (Figure 1). Biobarriers promise to be more cost effective and cause less surface disruption then conventional barrier technologies. Furthermore, containment using biobarriers can be combined with in situ biodegradation or biosequestration. This chapter will review published research that relates to biobarrier formation and present results from a mesocosm test of biobarrier longevity. These results demonstrate the effectiveness of microbial barriers for manipulation of hydraulics in mesoscale porous medium reactors.

Field and laboratory studies have shown that by increasing intervals of contaminant contact with soils and sediments longer periods of time are required to biodegrade an equivalent amount of contaminant, and the fraction of the total contaminant mass supplied to the system that cannot be biodegraded is increased. These observations have been referred to as the effect of “aging,” the time interval of contaminant contact with soil or sediment, on “bioavailability.”

The high cost of remediation has driven interest in developing a better understanding of bioremediation and the application of bioremediation technologies. The success of any bioremediation technology depends on a number of factors including site characteristics, environmental factors (e.g., temperature, pH, electron acceptor, nutrients), the nature of the contamination, whether appropriate biodegradative genes are present, and the bioavailability of the contaminants to degrading microorganisms within the site. To date, biodegradation research has focused primarily on the impact of environmental factors and biodegradative capacity. However, while inherent degradative capability and suitable environmental factors are certainly necessary, the importance of bioavailability to biodegradation also needs to be addressed. This is especially true for subsurface environments, where bioavailability may often control the occurrence and rate of biodegradation.

Monitoring the environment for synthetic and naturally occurring chemical and biological toxicants, pathogenic microorganisms, and soil microflora used in bioremediation are diverse aspects of the general rubric known as environmental monitoring and diagnostics. Food and animal feeds need to be screened for disease vectors, both natural and intentionally produced. Meat and milk are often contaminated with antibiotics and growth hormones. Ground water is often polluted with industrial wastes and agricultural pesticides. The viability of pollutant-degrading microorganisms and the efficacy of bioremediation processes need to be ascertained.

Bioremediation technology is analogous to all other technologies that gradually become accepted by society. Bioremediation is based on fundamental scientific and engineering principles that are translated into applications that reliably serve the public good. All technologies (from airplanes to automobiles to electric lights to personal computers) traverse various developmental stages — each of unpredictable duration, that include conception, proof of concept, prototype development, scale up, design refinements, applications testing, and marketing (among others). Implicit in the title of this chapter is the fact that many applications of bioremediation technology are still in their infancy. Bioremediation still needs quality control — this technology still needs to define its boundaries between promise and reality.

Industry, agriculture, sewage treatment and mining operations all combine to produce enormous amounts of metal (and other inorganic) ion contaminated wastewaters. Nuclear fuel cycle waste adds an additional and unique burden of radioactive metals. Unlike organic chemicals, metals persists in the environment indefinitely, posing threats to all living organisms which are exposed to them (Volesky and Holan, 1995). Control of such discharges has only been energetically regulated by governments in the past two or three decades. As a result, many toxic inorganics have accumulated in soils, sediments and impoundments throughout the world. Metal bearing wastewaters may be of known and predictable composition if derived from a single industry, e.g., electroplating wastewaters, or in other cases are a heterogeneous mix of many dissolved metal ions and organics at various pHs, with salts, colloidal and particulate matter present as well. The approach by many governmental regulators is now to prevent further deterioration of the environment by intercepting toxic metals before they are discharged. It is recognized that new, hopefully more cost-effective, technologies are needed to replace those borrowed primarily from the unit operations of the chemical industry which rely on a mixture of physical and chemical processes (Table I) to render the contaminants less toxic or more easily handled (Crusberg et al., 1994; Gretsky, 1994). Unfortunately, the form of the converted metal is itself often in need of careful and expensive disposal, and conventional treatment becomes less efficient and more expensive when metal ion concentrations fall in the 1–100 mg/l range. Table II provides a listing of discharge limits of metal finishing wastewaters in the U.S., and as such represent goals that must be met by any new technology or existing technologies.

The term biosorption describes the passive (i.e. not metabolically mediated) binding of heavy metals by dead or living biomass. The term bioaccumulation, on the other hand, refers to active processes which require the metabolic activity of living organisms. Biosorption can be employed to eliminate heavy metals from industrial effluents (e.g. from the mining or electroplating industry) or to recover precious metals from processing solutions or from the seawater. The metal laden biosorbent possesses a metal concentration that can be 1000 times higher than that in the liquid phase. Thus, biosorption can serve to reduce the waste volume. Instead of heaving to deal with a large volume of liquid waste, only a small volume of solid waste could be disposed of by incineration or deposition in landfills. An alternative and preferred way of dealing with the metal laden biosorbent is, however, to desorb the metal from the biomass. Thereby the biosorbent is regenerated and a highly concentrated metal solution is obtained. This solution can either be treated by precipitation which, however, generates toxic sludge. The preferable option is to recover the metal from this concentrate by electro-winning This leads to a “zero discharge” technology. A scheme of such a biosorption process involving adsorption and desorption columns is presented in Figure 1.