Bioremediation. Features of bioremediation

Bioremediation

The term bioremediation has been made of two parts: “bios” means life and refers to living organisms and “to remediate” that means to solve a problem. “Bioremediate” means to use biological organisms to solve an environmental problem such as contaminated soil or groundwater. Bioremediation is the use of living microorganisms to degrade environmental pollutants or to prevent pollution. In other words, it is a technology for removing pollutants from the environment thus restoring the original natural surroundings and preventing further pollution. It uses naturally occurring bacteria and fungi or plants to degrade or detoxify substances hazardous to human health and/or the environment.

For bioremediation to be effective, microorganisms must enzymatically attack the pollutants and convert them to harmless products. As bioremediation can be effective, only where environmental conditions permit microbial growth and activity, its application often involves the manipulation of environmental parameters to allow microbial growth and degradation to proceed at a faster rate.

Bioremediation could simply be defined as a biological process of the decontamination of contaminated environment. The environment may be either terrestrial, aqueous, or both. However, a more comprehensive definition is presented below:

Bioremediation is a means of cleaning up contaminated environments by exploiting the diverse metabolic abilities of microorganisms to convert contaminants to harmless products by mineralization, generation of carbon (IV) oxide and water, or by conversion into microbial biomass.

A point to emphasize here is that bioremediation and biodegradation should not be confused with each other. Bioremediation as a technique may include biodegradation as only one of the mechanisms involved or applied in the process of bioremediation. A complete biodegradation results in detoxification by mineralizing pollutants to carbon dioxide, water and harmless inorganic salts. Incomplete biodegradation will yield breakdown products, which may or may not be less toxic than the original pollutant and combined alternatives have to be considered, such as dispersion, dilution, biosorption, volatilization and/or the chemical or biochemical stabilization of contaminants.

In general, biodegradation can be defined as the decomposition of substances by microbial activities either by single organisms but most often by microbial consortia. Microorganisms found in soil and water will attempt to utilize any organic substances encountered as sources of energy and carbon by enzymatically breaking them down into simple molecules that can be absorbed. Under suitable environmental conditions all natural organic compounds should be degraded (Fig. 4) and for this reason large-scale deposits of naturally formed organic compounds are rarely observed.

Fig. 4. Natural microbial biodegradation of organic molecules.

Features of bioremediation

The control and optimization of bioremediation processes is a complex system of many aspects. These aspects include the existence of a microbial population capable of degrading the pollutants; the availability of contaminants to the microbial population; the environmental factors (type of soil, temperature, pH, the presence of oxygen or other electron acceptors, and nutrients).

Microbial populations for bioremediation processes

Microorganisms can be isolated from almost any environmental conditions. Microbes will adapt and grow at subzero temperatures, as well as extreme heat, desert conditions, in water, with an excess of oxygen, and in anaerobic conditions, with the presence of hazardous compounds or on any waste stream. The main requirements are an energy source and a carbon source. Because of the adaptability of microbes and other biological systems, these can be used to degrade or remediate environmental hazards. We can subdivide these microorganisms into aerobic (which use free oxygen), anaerobic (which live only in the absence of free oxygen), ligninolytic fungi (they have the ability to degrade an extremely diverse range of persistent or toxic environmental pollutants and methylotrophs (aerobic bacteria that grow utilizing methane for carbon and energy).

For degradation, it is necessary that bacteria and the contaminants be in contact. This is not easily achieved, as neither the microbes nor contaminants are uniformly spread in the soil. Some bacteria are mobile and exhibit a chemotactic response, sensing the contaminant and moving toward it. Other microbes such as fungi grow in a filamentous form toward the contaminant. It is possible to enhance the mobilization of the contaminant utilizing some surfactants such as sodium dodecyl sulphate.

Environmental factors

- Nutrients

Although the microorganisms are present in contaminated soil, they cannot necessarily be there in the numbers required for bioremediation of the site. Their growth and activity must be stimulated. Biostimulation usually involves the addition of nutrients and oxygen to help indigenous microorganisms. These nutrients are the basic building blocks of life and allow microbes to create the necessary enzymes to break down the contaminants. All of them will need nitrogen, phosphorous, and carbon.

Carbon is the most basic element of living forms and is needed in greater quantities than other elements. In addition to hydrogen, oxygen, and nitrogen it constitutes about 95% of the weight of cells. Phosphorous and sulfur contribute with 70% of the remainders. The nutritional requirement of carbon to nitrogen ratio is 10: 1, and carbon to phosphorous is 30: 1.

- Temperature, pH and moisture

Microbial growth and activity are readily affected by pH, temperature, and moisture. Although microorganisms have been also isolated in extreme conditions, most of them grow optimally over a narrow range, so that it is important to achieve optimal conditions.

The prime controlling factors are air (oxygen) availability, moisture content, nutrient levels, matrix pH, and ambient temperature. Usually, for ensuring the greatest efficiency, the ideal range of temperature is 20-30 °C, a pH of 6, 5-7, 5 or 5, 9-9, 0 (dependent on the microbial species involved). Available water is essential for all the living organisms, and irrigation is needed to achieve the optimal moisture level. The amount of available oxygen will determine whether the system is aerobic or anaerobic. Hydrocarbons are readily degraded under aerobic conditions, whereas chlorinate compounds are degraded only in anaerobic ones.

General advantages associated with the use of biological processes for the treatment of hazardous wastes refer to the relatively low costs, simple and well-known technologies, and potential for complete contaminant destruction.

The biotreatment is applied above all in soil bioremediation, wastewater treatment, biotreatment of gaseous streams and solid waste treatment.