Microbial Metabolism in Biological Waste Water Treatment

Carbon and Energy Sources for Microbial Growth:

• Organism must have sources of energy, carbon for synthesis of new cellular material, and inorganic elements (nutrients) such as nitrogen, phosphorus, sulfur, potassium, calcium and magnesium;

Carbon Sources:

• Organisms that use organic carbon for formation of new biomass are called heterotrophs; Organisms that derive cell carbon from carbon dioxide are called autotrophs

Energy Sources:

• Energy needed for cell synthesis supplied by light or by chemical oxidation reaction; Those organisms that are able to use light as energy source are called phototrophs; Phototrophic organisms either heterotrophic or autotrophic;
• Organisms that derive energy from chemical reactions are known as chemotrophs; Chemoautotrophs obtain energy from oxidation of reduced inorganic compounds (ammonia, nitrite, ferrous iron and sulfide); Chemoheterotrophs derive their energy from oxidation of organic compounds
• Oxidation‐reduction reactions involve transfer of electrons from electron donor to electron acceptor; Electron donor is oxidized and electron acceptor is reduced; Electron acceptor available within cell during metabolism (endogenous) or it obtained from outside cell (i.e., dissolved oxygen) (exogenous);

Respiratory Metabolism:

• Organisms that generate energy by enzyme‐mediated electron transport to external electron acceptor

Fermentative Metabolism:

Use of internal electron acceptor and is less efficient energy yielding process than respiration

Aerobic:

• When oxygen is used as electron acceptor the reaction is termed aerobic;

Anaerobic:

• When electron acceptors other than oxygen are involved, reaction is considered anaerobic;

Anoxic:

• When nitrite or nitrate is used as electron acceptor, reaction is termed anoxic; Under anoxic conditions nitrite or nitrate reduction to gaseous nitrogen occurs, also referred to as biological denitrification
• Organisms that can only meet their energy needs with oxygen are called obligate aerobes
• Bacteria that can use oxygen or nitrite/nitrate as electron acceptor in absence of oxygen are called facultative aerobes
• Organisms that generate energy by fermentation and that can exist only in environment devoid of oxygen are obligate anaerobes
• Organisms having ability to grow in either presence or absence of oxygen are facultative anaerobes