**1.6. General principles of pathways for aerobic aromatic catabolism**

nitroaromatic compounds [44–47]. The use of hydrogen peroxide in these processes generates toxic intermediates and is therefore not cost-effective [48]. Because of limitations of these

Bioremediation involves biological agents to catalyze the degradation and transformation of recalcitrant molecules to simpler structures. Few common terms used in these processes are

**•** Biodegradation is the breakdown of organic pollutants due to microbial activity. In this process, the microbe feeds on the pollutant to grow. The degradation of contaminant

**•** Biomineralization is the process of complete biodegradation. The organic contaminants are degraded completely through a series of degradation steps and finally converted to inorganic molecules like H2O and CO2. In the process, organic molecule provides both carbon and energy to the microbe, and if organic molecule is nitroaromatic, it provides

**•** Biotransformation is the process where in one organic molecule is modified by the action of biological agents. Sometimes, biotransformation occurs with cometabolism, where a microbe uses a substrate for its growth but transforms another substrate, which is not

Microbes have been isolated from almost all the parts of biosphere. Further, their adoptability for different environmental conditions and ability to utilize even recalcitrant compounds for

**Figure 2.** General principle of aerobic aromatic catabolism in bacteria. The three stages are as follows: the conversion of the growth substrate to catechol (or substituted catechol), then ring cleavage, and finally metabolism of the ring cleav‐ age product to central metabolites by either the *ortho* or *meta* pathways. (Adapted from Williams and Sayers [51])

methods, biodegradation has emerged as a viable alternative.

generates energy and microbe utilizes this energy for its growth.

*1.5.2. Bioremediation*

54 Wastewater Treatment Engineering

defined as follows:

nitrogen as well.

utilized by microbe for its growth.

their foods make them suitable agents for bioremediation.

The pathway for catabolism of aromatic compound basically has three stages (Figure 2) [51– 53]. In the first stage, the substrate undergo changes in its substituent groups by the action of mono- or dioxygenases to form catechols (or substituted catechols). The catechols then serve as substrates for the second stage of catabolism, that is, the ring opening. This process is facilitated by the action of dioxygenases, which breaks carbon–carbon bond by adding molecular oxygen and produce unsaturated aliphatic acid.

**Figure 3.** Central catechol pathway for the aerobic degradation of aromatics.

There are two families of ring opening enzymes, *ortho* or intradioldioxygenases, which produce *cis*,*cis* muconic acid (or its derivative), and *meta* or extradioldioxygenases, which produce 2 hydroxymuconic semialdehyde (or derivative). Both pathways (*ortho* and *meta* cleavage) are shown in Figure 3 and *meta* cleavage pathway for methylcatechols is shown in Figure 4. The third stage of catabolism is the conversion of the ring cleavage products into smaller com‐ pounds that can enter into central metabolic routes.

In general, aromatic compounds are initially catabolized by various pathways (known as peripheral pathways), which converge on a limited number of common intermediates (catechols or its derivatives). These intermediates are further utilized by a small number of common pathways (central pathways).

**Figure 4.** The *meta* cleavage pathway of methylcatechols.
