**2. Fundamental reactions in bioremediation**

Bioremediation can be generally classified into two major types: bioaugmentation and biostimulation. Bioaugmentation is a type of bioremediation that adds microorganisms to enhance degradation of contaminants at contaminated sites. Bioaugmentation involves the delivery of selective and enriched microbial cultures into the subsurface to accelerate biode‐ gradation reactions to achieve rapid and complete degradation of contaminants. The objective of bioaugmentation is to increase the overall degradation rate in cases where the indigenous microbial populations cannot completely degrade a contaminant or degradation rates are too low to meet the remedial goals in an acceptable time frame. The introduced microorganisms augment, but do not replace, the resident microbial population [9-10].

Microorganisms, through their enzymatic pathways, act as biocatalysts and facilitate the progress of biochemical reactions that degrade the targeted contaminants. Bioremediation has been used in the cleanup of organic (e.g., chlorinated solvents, petroleum hydrocarbons, and pesticides), inorganic (e.g., perchlorate and nitrate), metals, and radionuclides contaminated

Because of the role microorganisms play in bioremediation, any factors impacting surviv‐ al and growth of these organisms will impact bioremediation. Although microorganisms have been isolated even under extreme conditions, most of them grow optimally over a narrow physical/chemical/biological range. Thus, to effectively and efficiently cleanup a contaminated site through bioremediation, it is important to achieve optimal biogeochemi‐ cal conditions for microbial communities [4]. The conditions to consider include site hydrogeological characteristics, contaminant concentration, pH, redox potential, nutrients,

In some cases, natural conditions at a contaminated site can provide all essential materials in large enough quantities that bioremediation can occur without human intervention. This process is often referred as intrinsic bioremediation. This is the primary degradation mecha‐ nism behind natural attenuation. However, under the natural conditions of most sites, microorganisms that degrade contaminants may be naturally present in the subsurface, but may not necessarily be there in sufficient quantities required for optimal bioremediation of the site. In these cases, engineered bioremediation (also termed as biostimulation) is needed. Engineered bioremediation relies on accelerating desired biodegradation reactions by encour‐ aging growth of target microorganisms, as well as by optimizing the environment in which the organisms must carry out the detoxification reactions [8]. Engineered bioremediation involves the addition of substrates (electron donors), nutrients, and/or other materials (e.g., pH buffers) into the subsurface to stimulate microbial growth and activity or establish

This chapter focuses on the use of additives to stimulate bioremediation of organic contami‐ nants (e.g., chlorinated solvents, petroleum hydrocarbons, and pesticides). Following the introduction, the second section of the chapter discusses major reaction pathways in biorem‐ diation. The third section focuses on the major types of additives and the fourth section discusses in detail how the additives are implemented. The fifth section deals with potential issues associated with additives use in bioremediation. The final section summarizes the

Bioremediation can be generally classified into two major types: bioaugmentation and biostimulation. Bioaugmentation is a type of bioremediation that adds microorganisms to enhance degradation of contaminants at contaminated sites. Bioaugmentation involves the delivery of selective and enriched microbial cultures into the subsurface to accelerate biode‐ gradation reactions to achieve rapid and complete degradation of contaminants. The objective

sites [1-3].

moisture, and temperature [5-7].

146 Advances in Bioremediation of Wastewater and Polluted Soil

supportive geochemical conditions.

chapter and identifies future research needs.

**2. Fundamental reactions in bioremediation**

Biostimulation, the other type of bioremediation, involves stimulating growth of existing bacteria at sites to enhance degradation of contaminants. This can be done by adding various forms of rate limiting nutrients (such as phosphorus and nitrogen) and electron acceptors (organic carbon), and geochemical conditions modifiers (e.g., oxygen, pH modifiers).

Fundamentally, contaminants at bioremediation sites are degraded through a number of biochemical reactions that include aerobic reactions, anaerobic oxidative reactions, anaerobic reductive reactions, and cometabolic reactions. Aerobic bioremediation uses molecular oxygen (O2) as an electron acceptor in remediation through direct microbial metabolic oxidation of a contaminant. Aerobic bioremediation is most effective in degrading non-halogenated organic compounds (e.g., BTEX, diesel) to carbon dioxide and water [3, 11]. Similar to aerobic biore‐ mediation, anaerobic oxidative bioremediation also relies on direct microbial metabolic oxidation of a contaminant and is an alternative to aerobic bioremediation in anaerobic aquifers [3]. Contaminants that can be anaerobically oxidized include aromatic hydrocarbons, BTEX, fuels, and some chloroethenes [12].

Oxygen needs to be depleted before anaerobic reductive bioremediation can proceed. Anae‐ robic reductive dechlorination may be the most important type of anaerobic reductive bioremediation at contaminated sites. In this reaction, bacteria gain energy and grow as one or more chlorine atoms on chlorinated aliphatic hydrocarbons (CAH) molecule are replaced with hydrogen in an anaerobic environment [13-14]. The chlorinated compound serves as electron acceptor and hydrogen serves as the direct electron donor. Hydrogen in the form of H2 gas used in this reaction is typically supplied by fermentation of organic substrates [15-17].

Cometabolic bioremediation is a reaction in which contaminants are reduced by a non-specific enzyme produced during microbial metabolism of another compound (i.e., the primary substrate) in an anaerobic environment. Cometabolism occurs when microorganisms using one compound as an energy source fortuitously produce an enzyme that chemically trans‐ forms another compound. Organisms thus can degrade a contaminant without gaining any energy from the reaction [3, 15, 18].
