Soil Bioremediation

Fundamental Concepts

Bioremediation, in general, involves microorganisms degrading organic COCs. Living organisms obtain energy through a metabolic process. Just like people eat as part of their metabolic process to obtain energy to live, microorganisms must consume and metabolize substances to produce energy to live. Many organic COCs can be “food,” or substrate, for microorganisms. In general, microorganisms in bioremediation are bacteria, which are microscopic single-celled organisms. These organisms undergo metabolism and respiration. When bacteria metabolize organic COCs, they generate waste products. Effective bioremediation requires that those waste products be less toxic than the original COCs.

Bioremediation has been successfully applied to degrade petroleum hydrocarbons, solvents, pesticides, wood preservatives, and other organic compounds. Bioremediation is especially effective for remediating low-level residual contamination. It requires relatively inexpensive materials and usually does not generate residual wastes requiring additional treatment or disposal.

Soil bioremediation can be conducted under aerobic (with oxygen) or anaerobic (absence of oxygen) conditions, but aerobic bioremediation is more popular. The final waste products of complete aerobic biodegradation of hydrocarbons are carbon dioxide, water, and microbial cell mass. In anaerobic bioremediation, organic COCs may be metabolized into methane.

Microorganisms require moisture, oxygen (or absence of oxygen for anaerobic biodegradation), nutrients, and a suitable set of environmental factors to grow. The environmental factors include appropriate pH and temperature, and the absence of toxic conditions. Table 5.1 summarizes the critical conditions needed for microbes to perform bioremediation.

TABLE 5.1

Critical Conditions for Bioremediation (U.S. EPA 1991)

Environmental Factor

Optimum Conditions

Available soil water

25-85% water holding capacity

Oxygen

Aerobic metabolism: > 0.2 mg/L dissolved oxygen

air-filled pore space > 10% by volume

Anaerobic metabolism: oxygen concentration < 1% by volume

Redox potential

Aerobes and facultative anaerobes: > 50 millivolts Anaerobes: < 50 millivolts

Nutrients

Sufficient N, P, and other nutrients

(suggested C:N:P molar ratio of 120:10:1, where moles of C are from the organic chemical and N and P are added nutrients)

pH

5.5—8.5 (for most bacteria)

Temperature

15-45°C (for mesophiles)

Bioremediation can be carried out following two primary processes: biostimulation and bioaugmentation. Bioremediation is considered a destructive technology in that an organic molecule is degraded, or broken down, into smaller compounds. The goal is for a harmful COC molecule to be degraded into less harmful ones. Because the process is done via living microorganisms, it is called biodegradation. We will focus on biostimulation since it is the most often used bioremediation process. But, briefly, bioaugmentation involves the use of microbial cultures that have been specially bred for degradation of the COCs and survival under severe environmental conditions.

Biostimulation is the process of stimulating indigenous (in nature) microorganisms to metabolize COCs. The concept behind biostimulation is that in soil, a large amount and variety of microorganisms exist. Among those microorganisms, some are especially suited to degrade certain organic COCs. If, out of all existing types of microorganisms in the soil, those that can degrade a particular type of COC can be stimulated, then this is the practice of biostimulation.

The stimulants for biostimulation are often substances that promote more efficient respiration in those microorganisms. For example, the most common stimulant for aerobic bioremediation is oxygen. Many organic COCs are biodegraded by aerobic microorganisms. If oxygen is added to the soil, then those aerobic microorganisms will undergo efficient respiration and consume the COCs as part of their metabolic process. Common biostimulants are oxygen and nutrients, primarily ammonium and phosphate, as listed in Table 5.1.

Bioremediation can happen in situ and ex situ. In situ treatment enhances the natural microbial activity of undisturbed soil in place to decompose organic COCs. The most common application of in situ vadose zone bioremediation is bioventing, discussed in Section 5.4.2.

Ex situ soil bioremediation is typically performed using one of three systems: (1) static soil pile, (2) in-vessel (inside a tank), and (3) slurry bioreactor. The static soil pile is the most popular ex situ format. This approach treats soil stockpiled on the site with perforated pipes embedded in the piles as the conduit for biostimulants to be applied. In aerobic bioremediation, often a forced air supply is introduced into the pipes to increase the levels of oxygen in the pile. To minimize fugitive air emissions and potential secondary contamination from leachate, the stockpiles are usually covered on the top and lined at the bottom and an air vacuum is applied to induce the air/oxygen into the piles. A similar process can be carried out in a vessel. In a slurry bioreactor, impacted soil is mixed in a tank with a nutrient solution under controlled operating conditions (i.e., optimal pH, temperature, dissolved oxygen, and mixing).

Cometabolism occurs when microorganisms growing on one compound produce an enzyme that chemically transforms another compound on which they cannot grow. In particular, microorganisms that degrade methane (methanotrophic bacteria) have been found to produce enzymes that can initiate the oxidation of a variety of chlorinated organic compounds under anaerobic conditions.

Many books on microbiology and bioremediation can complement the theory of bioremediation in more depth. The focus of this chapter is the science and engineering principles for applying bioremediation in the field.

 
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