Microbial Remediation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 133

5.2 General Considerations for Microbial Remediation of Pesticides
Bioremediation of a contaminated site depends on several factors: (i) types and amounts
of harmful chemicals present, (ii) size and depth of the polluted area, (iii) type of soil and
the conditions present, and (iv) whether the remediation process occurs above ground or
underground. These factors vary from site to site. It can take a few months or even several
years for microbes for the partial or complete degradation of harmful chemicals to clean
up the site (US EPA 2001). A flowchart outlining the strategy involved in bioremediation
activities is illustrated in Figure 5.1 (modified from EPA 2005). Bioremediation strategy
employs modification of the existing environment to encourage the growth and reproduc-
tion of natural or exogenous microorganisms. To grow and reproduce, these microorgan-
isms require a source of energy (i.e., electron donor) and a means of extracting this energy
from the electron donor via an appropriate electron acceptor as the following (Equation for
heterotrophic organisms) (US EPA 1998):

Microbes + Electron Donor (Energy and Carbon Source) + Nutrients + Electron Acceptor →
More Microbes + Oxidized (Aerobic) or Reduced (Anaerobic) End Products

Biological systems generally treat contaminated media by using waste contaminants
of concern as the electron donor and supply microorganisms with the required electron
acceptors and nutrients. The anaerobic chlorination of chlorinated solvents is the excep-
tion. The chlorinated solvents serve as the electron acceptor under highly reducing condi-
tions. Generally, the limiting factor in full-scale, engineered biological treatment systems is
the rate of transfer of the electron acceptor to the reaction site. Electron acceptors are those
chemicals that can be used by biological systems to extract energy from electron donors
for microbial cell growth and replication. The main electron acceptors of interest include
oxygen, nitrate, sulfate, iron, manganese, carbon dioxide, and organic carbon. If oxygen is

Determine nature and extent of site contamination

Off-site On-site

Licensed
bioremediation
facility

Assess site suitability,
including environment and
sensitive receptors

Prepare management plans
for process

Construct biopile(s) or
landfarm and monitor

Determine fate of soil

FIGURE 5.1
Strategic flowchart for bioremediation activities.