Bioremediation is a treatment process that uses microorganisms (yeast, fungi, or bacteria) to break down, or degrade, hazardous substances into less toxic or nontoxic substances. Microorganisms eat and digest organic substances for nutrients and energy. The microorganisms break down the organic contaminants into harmless products -- mainly carbon dioxide and water.
The advantages of bioremediation Its cost is very low. It has few side-effects. The remained concentration is low. It can be used in some unique occasions. Soil and underwater can be restored at the same time.
The limitations of bioremediation (1)The microorganism that has active metabolize must exist. (2)These microbe must have a certain degradation speed, and reduce the concentration of pollutants to a certain standard.
(3).These microbe can not produce toxicant. (4).These can’t be substances that restrain the degradation. (5).There must be enough nutrients, oxygen, and proper humidity and temperature. (6).It must cost lower than other technique.
The main problem at present (1).Not all the pollutants can be degraded by microbe. (2).The construction needed can be blocked up by microbe.
(3).The active of microbe is affected by local temperature and other conditions. (4).In some cases, the concentration of remained pollutants is still high after bioremediation.
the Key Points of Bioremediation Technology ( 生物修复的技术要点 )
Part two key points of the bio- remediation nutrient Electron acceptor Cometabolism Bioremediation
Nutrients During Stimulation nutrients, such as nitrates, are added as well as aeration to provide oxygen. This enables the existing microorganism to breakdown the contamination quicker. Biostimulation: the course of enhancing the biodegradation and biotranslation by nutrient addition.
Whether to add nutrient and the effect of nutrient relies on the following factors: 1.amounts of nutrient in polluted environment. 2.ratio of C/N 3.the types of nutrient needed 4.the rate of nutrient adding
Electron acceptor Dissolved oxygen for biodegradation and biotranslation in the aerobic environment is usually restriction factor In the anaerobic environment, nitrate(NO 3 - ), vitriol(SO 4 ² ¯ ), the costliness iron(Fe³ + ) etc play the role of electron acceptors
Co-metabolism substrates It involves the injection of a dilute solution of nutrients such as methane and oxygen into the contaminated groundwater or soil. The microbes that metabolize these nutrients produce enzymes that react with the organic contaminant and degrade it to harmless minerals. Co-metabolism can be defined as: The transformation of an organic compound by a microorganism incapable of using the substrate as a source of energy or of one of its constituent elements.
Enzyme A Substrate Ax- Substrate AProduct BProduct C Enzyme CEnzyme B Product Bx [not metabolized by enzyme C]
Strategies to improve the bioremediation Limiting factor microbe Lack of microorganisms Pollutant Low biodegradability
bioremediation process ( 生物修复工艺 )
There are two general categories of bioremediation: in site and ex site.
In situ bioremediation In situ bioremediation (ISB) is the use of microorganisms to degrade contaminants in place with the goal of obtaining harmless chemicals as end products. Most often in situ bioremediation is applied to the degradation of contaminants in saturated soils and groundwater, although bioremediation in the unsaturated zone can occur.
Advantages of In Situ Bioremediation Contaminants may be completely transformed to innocuous substances (e.g., carbon dioxide, water, ethane). Accelerated ISB can provide volumetric treatment, thereby treating both dissolved and sorbed contaminant. The time required to treat subsurface pollution using in situ bioremediation can often be faster than pump-and- treat processes. In situ bioremediation often costs less than other remedial options. The areal zone of treatment using bioremediation can be larger than with other remedial technologies because the treatment moves with the plume and can reach areas that would otherwise be inaccessible.
Limitations of In Situ Bioremediation Depending on the particular site, some contaminants may not be completely transformed to innocuous products. If biotransformation halts at an intermediate compound, the intermediate may be more toxic and/or mobile than the parent compound. Some contaminants cannot be biodegraded (i.e., they are recalcitrant). When inappropriately applied, injection wells may become clogged from profuse microbial growth resulting from the addition of nutrients, electron donor, and/or electron acceptor. Accelerated In situ bioremediation is difficult to implement in low-permeability aquifers because advective transport of nutrients is limited. Heavy metals and toxic concentrations of organic compounds may inhibit activity of indigenous microorganisms. In situ bioremediation usually requires an acclimated population of microorganisms which may not develop for recent spills or for recalcitrant compounds
Ex site bioremediation Ex site bioremediation is the process of removing contaminated soil or water and treating it elsewhere. Ex site bioremediation is usually done with one of two methods, landfarming or in a bioreactor.
In landfarming,soil is removed and take to another area where it is spread out in thin layers, about 18 inches deep. Then biological materials (microbes) are added and the land is tilled to aerate. Ex site bioreactor processes involve removal of the soil or wastewater to be placed in a reactor. Once in the reactor the microbes are added and the temperature and, in the case of soil, moisture is controlled to obtain maximum degradation.
Summary Bioremediation is a new and promising technology in which the methods and effectiveness are continually being improved. Bioremediation has the ability to remove contaminants form the environment in a manner that is relatively inexpensive and less intrusive to an area then previously used methods. However promising bioremediation is, there are some areas in which it is not useful. As new technologies improve this may change, until then other forms of remediation are still needed.