The International Information Center for Geotechnical Engineers

Bioremediation - Introduction

Introduction

Bioremediation is a term that refers to a number of remediation technologies for treatment of both soil and groundwater using microorganisms (USEPA, 2012). Bioremediation is typically used to treat sites contaminated with organic substances (USEPA, 2001a), but it can also be used to immobilize inorganic contaminants such as heavy metals, although this is a developing area (Sharma & Reddy, 2004). Of sites where bioremediation is used 21% are considered petroleum sites and 31% are wood preserving sites, 19% are landfill sites and 13% are underground storage tanks (USEPA, 2001a). The remaining 16% of sites are less common and include fire training areas, vehicle maintainance areas, pesticide manufacturing sites and spills (USEPA, 2001a). The most common organic contaminants typically include polycyclic aromatic hydrocarbons (PAHs) (e.g. benzene, toluene, ethylbenze and xylenes (BTEX)), polychlorinated biphenyls (PCBs), pesticides and herbicides (Sharma & Reddy, 2004), (USEPA, 2001a).

Fundamentally, bioremediation uses microbes (e.g. bacteria, yeast, and fungi) to ‘digest’ toxic organic contaminants (Sharma & Reddy, 2004), such as oil, producing non-toxic products such as water and carbon dioxide (USEPA, 2001b). The process of breaking down organic contaminants with microorganisms is referred to as biodegradation. This can occur in the presence of oxygen or without oxygen, known as aerobic and anaerobic conditions, respectively. This is shown diagrammatically in Figure 1 and using a simplified equation in Equation 1 below.

 1

Figure 1 – Diagram representing basic concept of bioremediation (USEPA, 2001b)

 

eq1

 

Equation 1 – Simplified chemical equation of aerobic bioremediation (Sharma & Reddy, 2004)

 

This is simplification and the actual process is much more complex. Sometimes the ‘native’ population of microorganisms, known as indigenous microorganisms, may not be able to biodegrade the contaminant in the soil. In this case ‘foreign’ exogenous microorganisms must be introduced to the soil (Sharma & Reddy, 2004). The addition of exogenous microorganisms to the soil is known as bioaugmentation. For the microorganisms to survive in the ground, a delicate balance of a number of parameters including temperature, pH, moisture content, oxygen concentration and nutrients in the soil must be achieved (Sharma & Reddy, 2004) and (USEPA, 2012). Bioremediation is most efficient (Sharma & Reddy, 2004):

  • In the temperature range of 15 to 45 degrees.
  • When pH is around 7 (can work with pH between 5.5 and 8.5).
  • At moisture levels of 40-80% of field capacity.
  • At oxygen concentrations of >2 mg/L (aerobic) and <2mg/L (anaerobic).
  • When nutrients including Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorous are in abundance.

 

If this balance is not present in the soil it can be achieved by adding amendments to the soil in a process known as biostimulation (USEPA, 2012). Amendments such as H2O2 (hydrogen peroxide) or Oxygen-Releasing-Compounds (ORCs) are typically pumped or injected into the ground. Figure 2 shows a schematic of the balance of nutrients for microorganisms.

2

Figure 2 – Schematic of microorganism requirements (USEPA, 2012)

If the adequate conditions in the soil cannot be achieved easily, then ‘in-ground’ or in-situ treatment may not be possible. Instead the soil may be excavated and treated ‘above-ground’ or ex-situ (USEPA, 2012). Typically groundwater contamination is treated in-situ. There are many different treatment methods available and these include (USEPA 2001a) (Sharma & Reddy, 2004).

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