According to Crawford (2010), bioremediation has been defined as any process that uses microorganisms, fungi, green plants or their enzymes to return altered environments by contaminants to its original condition. In a brief approach, bioremediation is divided into two forms. The first form is in situ remediation. In situ literally means &on site& and within the bioremediation scope, in situ means that the bioremediation process has taken place at the site of contamination without the translocation of the polluted materials for further treatment elsewhere. Ex situ remediation involves performing the remediation activity off the area of contamination by moving the polluted materials. The first remediation technique is rhizofiltration, which has been illustrated by Singh (2008),atype of phytoremediation (use of special plant roots to remediate contaminated water through absorption of the pollutants) and it is mainly an in situ method. The contaminants are removed by getting absorbed through the root systems of plants into the stems and leaves. The plants are then removed and replaced to continue the cycle until better levels are achieved. Rhizofiltration is mostly aimed at removing heavy metal contaminants and rhizofiltration may be applicable to the treatment of both surface and groundwater, industrial and residential effluents, agricultural runoffs and diluted sludges among the many contaminants. Rhizofiltration reduces cost of processing as it is done on site, but it has its own limitations and one being that the filtration will not go further than where the roots end and this drawback could make the process to take years to reach a regulatory level.Bio-sparging is an in situ remediation technique, used to treat saturated soils and groundwater contaminated by volatile organic compounds (VOCs) like petroleumhydrocarbons. Bio-sparging involves the injection of a hydrocarbon-free gaseous medium into the ground through a system of wells, air compressors and vacuum pumps drilled on the surface and directly below the point of contamination and it involves injecting a gaseous nedium below groundwater or soil. On the surface, the air compressor pumps air underground through the wells and as air and vapor are mixed together, they are pulled out to the surface through a system of vacuum pumps going up where the contaminants are extracted. The major drawback with Bio-sparging is that it is extremely expensive to set up and run but, this process helps in having a fast clean up.The third technique is bio-factor. Crawford (2010) explains that it is an ex situ technique as it involves the process of  pumping soil and water from a contaminated plume and processing it through a specially engineered containment system or vessel designed to support a biological environment. In a bio-factor, the degradation  is greater than in situ because the contained environment is more controllable and predictable. The best example of a bio-reactor is the treatment of sewage and wastewater. In these systems, there is a continous  presence  of a free-flowing, chemically inert medium which acts as a breeding ground for the bacteria that break down the raw sewage. The bioreactors usually have separate tanks and a machines to speed the separation of water and biosolids. Through the bioreactor process, the contaminated water is filtered and treated until it is fit for reuse and the the biosolids can be collected used as fertilizer since most bioreactors are used for sewage works . Bioaugmentation  is the addition of bacteria and other growth factors  required to enhance the rate of   degradation   of a   contaminant . It is an in situ technique as bioaugmentation usually requires studying the  indigenous   varieties present in the location to determine if   biostimulation   is possible. If the indigenous variety do not have the   metabolic   capability to perform the remediation process,   exogenous   varieties with such sophisticated pathways are introduced. The advantage of bioaugmentation is that it is one of the most versatile techniques as it can be used in many situations including soil contaminations and petroleum spills, but even though it is highly effective, if the polluted location is not carefully studied, it becomes hard to get rid of the contaminationFinally,  bioventing is is a process which works in the same way as a Bio-sparging only that in bioventing its only the sil and not the groundwater being dealt with. The degrading medium is supplied in a well used and is introduced into the  contaminated soil and later on vacuum pumps uptake the contaminants up to the surface for treatment. This is the most common method of dealing with underground soil contaminations as it is highly effective.

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