Bioremediation is defined as the process where by organic wastes are biologically degraded under controlled conditions to an innocuous state, or to levels below concentration limits established by regulatory authorities. Simply, it is the use of living organisms, primarily microorganisms, to degrade the environmental contaminants into less toxic forms.
Bioremediation has its own limitations. Some contaminants, such as chlorinated organic or high aromatic hydrocarbons, are resistant to microbial attack. They are degraded either slowly or not at all. The various microorganisms involved in the biodegradation process is classified as follows.
- Aerobic micro organism: Pseudomonas, Sphingomonas, Rhodococcus and Mycobacterium.
- The anaerobic microorganism is one which lives in the absence of oxygen.
- ligninolytic fungi: Phanaerochaete chrysosporium
- Methylotrophs uses methane as carbon and energy.
Factors affecting bioremediation
The existence of a microbial population:
Availability of contaminants:
Type of Soil:
Removal of contaminant from clay soil is more cumbersome procedure compared to sandy soil
Moderate temperature favors microbial growth thereby facilitate degradation process. Higher temperature kills the micro organism and low temperature deactivates the enzyme. Hence, these temperatures are not good for degradation process.
Neutral and alkaline reaction favors bacterial population which helps degradation in comparison to the acidic soil reaction which favors fungus population.
Presence of oxygen and nutrients:
Oxygen is essential for respiration and nutrients are essential for the metabolism in microbes. As a result, poor oxygen and nutrient status of soil slow down the degradation process.
Approaches to bioremediation
In situ Bioremediation
Bioaugmentation techniques involve the addition of microorganisms that degrade pollutants. It involves both indigenous as well as exogenous microorganisms. Sometimes, exogenous population can’t able to compete with indigenous microbes. In most soils indigenous microorganisms degrades the pollutant effectively under good management.
Ex situ Bioremediation
Ex-situ techniques are those techniques that are applied to soil and groundwater which are removed from the site and dumped elsewhere. The various ex situ measures are described hereunder.
- Land farming is a technique which contaminated soil is excavated and spread over a prepared bed and periodically tilled until pollutants are degraded. The goal is to facilitate the aerobic degradation of contaminants. In general, the practice is limited to the treatment of superficial 10-35 cm of soil. It reduces monitoring and maintenance as well as clean-up cost.
- Composting technique involves combining of contaminated soil with organic amendments. These organic materials are used as energy source by microbes.
- Biopiles are the hybrid farming and composting. In this engineered cells are constructed as aerated composted piles. Biopiles provide a favorable environment for indigenous aerobic and anaerobic microorganisms. It controls the physical losses of the contaminants by leaching and volatilization.
- Bioreactors: Bio reactors are used for ex situ treatment of contaminated soil or water. It involves the processing of contaminated solid material or water through an engineered containment system.
Advantages of bioremediation
- It is natural process and perceived as an acceptable waste treatment process.
- Microbial population sustains naturally using contaminant and decreases pollution.
- The output residues are harmless products (Carbon dioxide or CO2, water, and cell biomass).
- It is useful for complete destruction of wide variety of contaminants.
- This process transforms the hazardous compounds into harmless products.
- The complete destruction of target pollutants is possible.
- Bioremediation can be carried out without disrupting normal activities.
- The potential threats to human health and the environment are less
- Bioremediation is less expensive.
Disadvantages of Bioremediation
- 1. Bioremediation is often highly specific and limited to biodegradable compounds
- In some cases, the end products may be more persistent or toxic than the parent compound.
- The presence of metabolically capable microbial populations, environmental conditions, and levels of nutrients determines the extent of degradation which is normally not controllable.
- It is difficult to extrapolate pilot scale studies to full-scale field operations.
- Research is needed to develop bioremediation technologies for different sites.
- Bioremediation takes longer time than other treatment.
- It requires pretreatment such as excavation, washing or physical extraction before being placed in a bioreactor.