Waste Treatment

Residual Organics. Microorganisms excel in using organic substances as sources of nutrients and energy. The challenge of wastewater treatment is to remove (1) compounds with a high biochemical oxygen demand, (2) pathogenic organisms and viruses, and (3) a multitude of human-made chemicals. Anaerobic digestion (AD) is commonly used to treat materials with a high content of insoluble organic matter, such as cellulose, and to degrade concentrated industrial wastes, such as those from the food-processing industry. AD is a complex biological process that utilizes a consortium of anaerobic microorganisms to act in concert to hydrolyze complex organics to simple monomers and then to volatile fatty acids (VFAs). These VFAs are ultimately converted to methane and carbon dioxide in the final step in the anaerobic food chain. The degradative and fermentative reactions in the anaerobic treatment processes can be divided into two stages: acid-forming and methane-forming, which involve at least three groups of anaerobic bacteria—acidogenic, syn-trophic, and methanogenic. In the acid-forming stage, complex organic polymers, including carbohydrates, fats, and proteins, are hydrolyzed and fermented to VFAs, alcohols, and ketones by selective anaerobic bacteria. Organic acids and alcohols such as lactic acid, propionic acid, butyric acid, and ethanol are converted to acetic acid and carbon dioxide by acetogenic bacteria in the syntrophic association of, generally, methanogenic bacteria. The acetate and CO2 are finally converted to CH4 by methanogenic bacteria that

Table 4. Novel Oxidoreductase in Clostridium Species

Reduction

Electron donor

Organism

Aldehyde/ketone dehydrogenase Steroids

Methyl ketones Ketones

2-Oxoacid synthase Fatty acids Acetate Linoleic reductase

Linoleic acid Enoate reductase Cinnamic acid Crotonic acid 2-Oxoacid reductase

Phenylpyruvic acid Nitroaryl reductase Chloramphenicol Metronidazole Paro-nitrobenzoate 2-Nitrobenzene Lipoamide hydrogenase NAD/lipoamide

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