Introduction

Microbial bioremediation is the biodegradation process of pollutants using micro-organisms or their derivatives like enzymes and bio-surfactants to remediate the contaminated environment. Microbes degrade or transform the contaminants with or without change in the metabolic process. In this process, native or non-native microbes are used to degrade the pollutant. PCBs- polychlorinated biphenyls, PAH- polyaromatic hydrocarbons, metals, and radionuclides such pollutants are degraded using various microbes. This process is natural, low cost, and does not pollute the environment. It can be used in both in-situ (onsite) and ex-situ (off-site) environments. Some of the drawbacks are also noticed, such as climate manipulating the degradation process. It is a slow process and also produces some undesired substance, so that needs the expertise to handle such a situation. Microbial bioremediation is categorized into two categories that depend on where it can execute.

In-Situ bioremediation

Bioaugmentation:

Bioaugmentation is a part of the intrinsic bioremediation process that takes place onsite. In this mechanism, the addition of microorganisms contaminated either native, exotic, or genetically engineered to degrade the pollutant present there are either organic or inorganic. The native microbes of the contaminated sites are isolated to enhance such a mechanism, genetically modified, cultured, and sent back to the site. This type of method confirms the remediation and eliminates the chlorinated compounds.

Biosparging:

This method of remediation resembles bio-venting; the air is introduced into the subsurface soil to stimulate microbiota activities, which can remove the pollutant. The effectiveness of this method relies on the permeability of soil and biodegradability of pollutants. This approach is used to treat contaminated aquifers with petroleum products.

Bio venting:

In this process, proper oxygen or air is provided into the soil or water in the saturated zone to stimulate bacterial growth, which degrades the contaminants. It is an aerobic process; air or oxygen is injected through a well that degrades the contaminant and releases it into vapor form. It is best for petroleum contamination.

Bio attenuation:

It is also referred to as the natural attenuation process that is used to eliminate pollutants from the environment by using native, exotic, or genetically modified microbes. This process uses biological (aerobic or anaerobic), physical (diffusion, advection dilution, volatilization), and chemical (complexation or ion exchange). Pollutants are naturally remediated by microbes, which are present in the environment. The pollutant present in the soil or water is consumed and degraded by microbes and produces a harmless substance; these substances are attached to the soil, restricting movement. When they dissolve in water, it gets diluted and reduces the concentration. Some substances like solvent and oil evaporate, and sunlight destroys them easily. The use of bio-stimulation can enhance this method.

http://www.oil-gasportal.com/in-situ-remediation-of-soil-sediments-and-groundwater-contaminated-by-hazardous-substances/?print=print

Ex-situ bioremediation

Biopiling:

In this technique, the pollution is dug out and created a pile on the ground, and required nutrients are added with aeration that stimulates the microbial activity. Biopiling consists of irrigation, nutrients, leachate collection, and a treatment bed system. This ex-situ method easily controls the pH, temperature, humidity, and aeration. This method is used to remove low volatile molecular weight pollutants.   

Land Farming:

It is an ex-situ process in which a polluted soil site is regularly dug out and tilled, and the treatment site regulates the bioremediation naturally by the aerobic process. It is effortless, low capital cost, and can be helpful for the large polluted areas with a low investment of energy and environmental impact.

Compositing:  

It is the off-site process of remediation in which mainly organic wastes are treated with the help of microbes,  in which large complex organic substances are converted into simple substances by fungi or bacteria they feed on them and convert into microbial biomass. The waste is converted into compost, which can be used as fertilizer and employed in crop fields. One of the best methods is the bioremediation process without producing any harmful substance.

Bioreactor:

It is a vessel or machine in which raw waste material is converted into the desired specific product via the biological process. This vessel provides an optimum environment for the microbes, working effectively and converting the waste into a harmless substance. Bioreactors are of many types: sequence batch, fed-batch, multistage, and continuous. This vessel provides flexibility and maximizes biodegradation.  

https://link.springer.com/chapter/10.1007/978-981-13-2420-8_2

Microorganisms Used in Bioremediation

Aerobic:

In this category of microbial bioremediation, those microorganisms are used, which can degrade the pollutant in the presence of oxygen or require oxygen to biodegrade the pollutant. Microbes are aerobic  Mycobacterium, Sphingomonas,  Pseudomonas Rhodococcus, Nocardia, Flavobacterium. These microbes are used to degrade hydrocarbons, polyaromatic compounds, pesticides, and alkanes.

Anaerobic:

In this type, microbes do not require oxygen or aeration to degrade the pollutant. This degradation process works in the absence of oxygen. If oxygen is provided, microbes’ growth halts, and the pollutant degradation process stops simultaneously—microbes used in Dehalococcoides and Dehalobacter, halo respiring bacteria (HRBs) sulfate reducers. Pollutants degraded by these microbes are polychlorinated biphenyls, chloroform, trichloroethylene, and chlorinated aromatic compounds.  

Common Contaminants

Heavy Metals:

These are those metals that come with pesticides or mining. Heavy metals cause various diseases if they enter the food chain like itai-itai, minimata, etc. Heavy metals are those which have a high density that is greater than water at a temperature of 4 degrees Celsius, for example, zinc, cadmium, mercury, chromium, lead, arsenic, nickel, gold, silver.

Halogenated Organic Compounds:

These compounds have halogen elements in their structure, like polychlorinated biphenyl, chloroform, and chlorinated aromatic compounds. These halogenated organic pollutants are harmful and lead to cancer.

Aromatic Compounds:

These are compounds that have an aromatic ring-like structure, for example, naphthalene, polyaromatic compounds,  toluene, paraffin, and many more. Such compounds come from petroleum and oil refineries that cause soil and water pollution, and these pollutants are recalcitrant.