31 Bioremediation: Processes and Techniques

Dr Kiran Bala

Bioremediation: Processes and Techniques

Introduction

In-situ bioremediation techniques

Ex-situ bioremediation techniques

Features of bioremediation

Limitations of bioremediation

Concluding remarks

 

Introduction:

Bioremediation is a part of environmental technology, which uses living organisms particularly microbes such as bacteria, algae, yeast, fungi etc. to clean our environment. Amongst various microbes, bacteria play an important role in this process. They can digest or break down various toxins such as organic or inorganic, released from various natural/anthropogenic activities. This process of breakage is a usual part of their metabolic activity. Microbes can utilize different grouping of electron donors and acceptors to initiate their metabolism. Additional to these oxidation/reduction reactions, they are certain other strategies for detoxification of their surroundings. Thus an appropriate combination of active microbial community, electron donor / acceptor / pollutant and various additional physical and useful parameters are applied under bioremediation for remediation of polluted site (Cassidy et al. 2015; Garcia-Delgado et al. 2015; Martı´nez-Pascual et al. 2015). The pollutants break down into organic matter and nutrients. Heavy metals are non-biodegradable in nature thus can’t be destroyed by bioremediation technology. However, bacteria can assist in cleaning up of chlorinated pesticides and oil spills. In addition to it, wastewater can also be treated using algae which can take up most of nitrogen and phosphors present in wastewater and helps in cleaning of water.

Microorganism used for such process should be healthy and active i.e. must be present in their active or exponential phase so that they can perform their work efficiently. Different microorganisms have different capability of detoxifying contaminants and toxins. Bioremediation can be achieved in both aerobic and anaerobic environment. In aerobic condition, microbes need oxygen to perform their activity. Oxygen quantity will decide the efficiency of microbes; if sufficient amount of oxygen is present then contaminants and toxins can easily be converted into water and carbon.

Some microbes have capacity to perform in anaerobic conditions i.e. in the absence of oxygen. For example, soil contaminated with different types of chemical compounds can be efficiently decomposed by bacteria under anaerobic conditions, which liberate sufficient amount of energy for the survival of microbes. Bioremediation has been effectively evaluated and used for cleaning up of surface water, soil, groundwater, and sediments etc. It has been evidently established xenobiotic such as nitroglycerine which is an explosive, can also be cleaned up through bioremediation. Bioremediation is a natural attenuation with no or little human efforts. Furthermore, adding of natural or engineered microorganisms can enhance the ideal catalytic abilities (Paul et al. 2005).

Different types of bioremediation (Fig. 1) are described in following section:

1.In situ bioremediation techniques

This involves treatment of polluted substances at polluted site itself. This technique does not disturb soil structure as does not require excavation. Further these procedures seems to be less costly in comparison to ex situ, because no additional cost is involved in excavation procedures but cost of designing and installation of sophisticated instruments on-site for improvement in efficiency increases cost. In situ bioremediation techniques are successfully used for treatment of heavy metals, dyes, hydrocarbons and chlorinated solvents at polluted sites (Folch et al. 2013; Kim et al. 2014; Frascariet al. 2015; Roy et al.). There are certain parameters which affect the performance such as electron acceptor, nutrient availability, moisture content, soil porosity, temperature and pH (Philp and Atlas 2005).

1. a) Bioventing

Bioventing helps with in situ bioremediation of pollutants present in soil by providing enough supply of oxygen to microorganisms involved in converting pollutants into a harmless product (Philp and Atlas 2005; Hohener and Ponsin 2014). Further additional nutrients and moisture help in increased activities of indigenous microbes to enhance bioremediation. Although rate of airflow and air interval are the most important factors of bioventing, still accomplishment depend on number of air injection points for uniform distribution of air.

1.b) Bioslurping

In  this  process,  combination  of  vacuum  enriched  pumping,  soil  vapor  extraction alongwith bioventing is used for remediation of soil and groundwater providing indirect oxygen supply and stimulating the biodegradation of contaminants (Gidarakos and Aivalioti 2007; Kim et al. 2014). This technique can also be used for remediation of semi volatile and volatile organic compounds from contaminated soils. This method is not appropriate for remediation of soil having little permeability but it is less costly due to less amount of groundwater generated from process minimizing cost involved in storage, treatment and disposal.

1.c) Biosparging

Like bioventing, in this technique also air is inoculated into earth subsurface for stimulation of microbial activities to remove pollutants from contaminated sites. But in this technique air is added at saturated zone to result in ascending movement of VOCs to the unsaturated zone to stimulate biodegradation. Main factors i.e. permeability of soil and contaminant biodegradability affects the efficiency of system. Similar to biosparging there is another method identified as in situ air sparging (IAS) leading to pollutant volatilization with high airflow rates, whereas biosparging supports biodegradation. Biosparging is commonly used for treatment of aquifers polluted through petroleum products, particularly kerosene and diesel.

1. d) Phytoremediation

In  this  method,  plants  interact  with  pollutants  for  mitigation  of  noxious  effects. Mechanism of remediation is dependent on characteristics of contaminant i.e. elemental or organic. Elemental contaminants include lethal heavy metals and radionuclides which are typically displaced through transformation, sequestration and extraction mechanisms. Organic pollutants such as hydrocarbons and chlorinated mixtures are mainly removed by the process of degradation, stabilization, rhizoremediation and volatilization. Plants should have some important characteristics to act as phytoremediator i.e. rate of pollutant removal, growth rate of plant, tolerance and survival of plant, fibrous or tap root system on basis of pollutant depth, minimum above ground biomass to be accessible for animal ingestion, plant resistance to diseases and pests etc (Meagher 2000; Kuiper et al. 2004; Lee 2013; Miguel et al. 2013). Plants growing in contaminated site/environment are usually found to be good phytoremediators. Their efficiency can further by enhanced by either biostimulation or bioaugmentation with endogenous or exogenous plant rhizobacteria. Simultaneously some precious metals can also bioaccumulate during remediation which can be recovered afterwards. This process is known as phytomining. Some of advantages of phytoremediation are environmentally friendly, little installation and maintenance expenses, large-scale operation, preservation of soil structure, better soil fertility due to addition of plant organic matter, inhibition of erosion and metal leaching etc.

1 e) Permeable reactive barrier (PRB)

In this method, role of microorganisms is to enhance the process rather than to act as independent technology. Other terms viz. biological PRB, passive bioreactive barrier, bioenhanced PRB are also proposed for this technique. This is an in situ technique mainly uses a semi-permanent or permanent reactive barrier immersed in the path of groundwater polluted with heavy metals and chlorinated compounds. As contaminated water pours via. barrier pollutants get stuck and experience sequence of reactions subsequently cleaning this.

These barriers are typically responsive adequate to capture contaminants, penetrable to only water not contaminants, passive with slight energy involvement, cost effective, and easily available (De Pourcq et al. 2015). Efficiency of PBR depends typically on biogeochemical and hydrogeological conditions, mechanical stability, characteristics of used media, which further is dependent on type of pollutants, environmental conditions etc. (Obiri-Nyarko et al. 2014; Liu et al. 2015).

1.f) Intrinsic bioremediation

This  is  also  acknowledged  as  natural  attenuation  in  which  polluted  sites  are remediated with the help of aerobic and anaerobic microbial activities without any outside force or human involvement. The absence of exterior power makes this method less costly in comparison to additional in situ methods. To maintain the sustainability and continuity, the process need to be monitored frequently also known as monitored natural attenuation. Intrinsic bioremediation sometimes takes lengthier time to reach the marked pollutant level as there is no exterior power combined to accelerate remediation practice. Thus it has been recommended to carry out risk assessment to confirm that less time for remediation is required.

2.Ex situ bioremediation techniques

In  this excavated  samples  from polluted  sites  are  transported  to  another  site  for treatment. Some of the factors, which influence efficiency are cost; intensity, type, degree and depth of pollution; geographical location and geology of the polluted site.

1.a) Biopile

This is a complete treatment technology in which excavated soils are mixed with nutrients to enhance microbial activities and placed on a treatment bed with main components as irrigation, aeration, leachate and nutrient collection systems. Various environmental and physico-chemical parameters viz. heat, moisture, nutrients, pH and oxygen can be controlled for further enhancement of biodegradation (Whelan et al. 2015). Filtering and ventilation of polluted soil, addition of bulking agents like saw dust, straw, wood chips or any other organic materials can further help in enhancement of efficiency. Biopiling can be effectively used to control volatilization of low molecular weight pollutants and can work even under extreme cold environments (Dias et al. 2015; Gomez and Sartaj 2014; Whelan et al. 2015). Additionally, biopile can be used for treatment of huge quantity of contaminated soil in less space in comparison to different ex situ bioremediation methods comprising land farming. Some of major limitations of biofiling are maintenance and operation cost, robust engineering, power fluctuations and unavailability at remote sites which is an essential requirement for even supply of air in heaped soil etc. Excessive air heating can result in drying of soil which will promote volatilization rather than biodegradation due to inhibition of microbial activities.

1.b) Windrows

The efficiency of windrows depends on periodic rotating of heaped contaminated earth with growing degradation activities of native or transient hydrocarbonoclastic bacteria. This periodic rotating of contaminated soil alongwith water addition results in increased ventilation and even delivery of contaminants with speedy bioremediation through the mechanism of assimilation, biotransformation and mineralization (Barr 2002). However, this technique may not be the best option for remediation of soil polluted with toxic volatiles.

1.c) Bioreactor

Bioreactor is a container used for conversion of raw materials to specific products through a chain of biological reactions. Different modes of operation are fed-batch, batch, sequencing batch, continuous and multistage bioreactors. Various process parameters viz. pH, temperature, mixing, rate of aeration, substrate and inoculum concentrations can be controlled effectively in bioreactors which is one of the key benefits of this bioremediation over other ex situ bioremediation techniques. Contaminated samples can be fed into bioreactor either as dry matter or slurry. Optimum growth conditions provided in a bioreactor are needed to maintain natural growth environment. Bioremediation time can be reduced with manipulation of process parameters. Various factors affecting the efficiency of bioremediation are nutrients concentration, bioaugmentation, pollutant availability, contact between microbes and pollutants etc. Water or soil contaminated with VOCs such as BTEX (benzene, toluene, ethylbenzene and xylenes) can also be treated through this technique (Mohan et al. 2007).

1.d) Land farming

 This method is the easiest one with very less expenses and equipment needs. In this method, contaminated soils are typically dug and/or tilled. This can be done in both ex situ or in situ manner depending on depth of pollutant. When treatment of excavated contaminated soil is done on-site it is known as in situ and in another case it is called as ex situ. Like other ex situ methods, land farming also has certain drawbacks such as large area requirement, effect of variable environmental conditions on microbial activities, high cost involved in excavation, and inefficiency in removal of inorganic pollutant (Silva-Castro et al. 2012; Cerqueira et al. 2014). Additionally, land farming is not appropriate for treatment of soil contaminated by harmful/toxic volatile compounds because of design and contaminant removal mechanism i.e. volatilization, mainly in tropical climate locations. Due to these drawbacks and many more make land farming assisted bioremediation more time consuming and less efficient in comparison to other ex situ methods (Khan et al. 2004; Maila and Colete 2004).

Features of Bioremediation

  • This is the most acceptable natural process; for waste treatment and for cleaning of material such as soil, and water etc. In bioremediation, microbes which can degrade the pollutant along with rise in their population at site of contaminant.
  • Bioremediation is an effort less process and can frequently occur on site, without resulting in any main disturbance in regular activities. Since it can be performed at site which eradicates the necessity of transport for carrying huge magnitudes of waste to off site. Simultaneously it can prevent possible threats to human health and environment which arises during transportation.
  • This is a cost effective process as it cost less than other conventional methods.
  • Complete destruction of pollutants can be achieved through bioremediation. Hazardous compounds can be converted into a harmless product.
  • As it is devoid of any hazardous chemicals which make such process environmental friendly. Nutrients added for growth of microbes can be used as fertilizers on lawns and gardens. Bioremediation converts harmful chemicals, wastewater and gases such as carbon dioxide into a harmless product.
  • Limitations of Bioremediation
  • Biodegradable compounds can only be degraded through bioremediation beside which not all compounds are susceptible for complete degradation.
  • Biological processes are highly specific so there might be chance that products of biodegradation may or may not be toxic than the parent compound. Microbes which are in a state of exponential phase having high metabolic process, highly adaptive to environmental growth condition are important site factors required for success of bioremediation.
  • A pilot-scale study of bioremediation is quite a difficult process.
  • Engineered bioremediation technologies are one where research is quite needed to develop the appropriate area for sites which is occupied with composite combinations of contaminants which are not evenly dispersed or degraded in environment. Such pollutants may be present in the form of solids, liquids, or gases.
  • When we talk about additional treatment process, bioremediation is somewhat longer method, excavation and removal of soil or incinerations are some disadvantages of bioremediation.
  • Performance criteria for bioremediation are uncertainty. Besides these there is no accepted definition of “clean”, which can be used for evaluating performance of bioremediation and its difficulty in performing.

Concluding remarks

Main step to an effective bioremediation is site description, which helps in establishment of best appropriate and achievable bioremediation practice i.e. ex situ or in situ. First one i.e. ex situ bioremediation seems to be cost intensive due to added costs for excavation and transportation. But no doubt, these can also be utilized for treatment of varied range of contaminants in a controlled way. On the other hand, in situ practices does not involve additional cost for excavation but there are some other issues related to in-situ such as cost needed for installation of equipment on-site, incapability to efficiently envisage and limit the subsurface of contaminated site. Thus, expenses of remediation should not be the only concern to decide regarding application of particular bioremediation technique. There are other factors too such as type of soil, complexity and type of pollutant, site locality in relation to human occupancy and performance of bioremediation process which needs attention while selecting appropriate method for effective treatment of polluted sites.