**2. Microbial degradation of nitroaromatics**

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‐

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

pounds that can enter into central metabolic routes.

common pathways (central pathways).

56 Wastewater Treatment Engineering

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

When a nitroaromatic compound is exposed to the environment, its biodegradation takes place either by anaerobic route or by aerobic route. Different strategies applied for the degradation of nitroaromatic compounds by bacterial strains are shown in Figure 5.

**Figure 5.** Different strategies of microbial remediation of nitroaromatics. (Adapted from Kulkarni and Chaudhari [49])
