August 2014, Vol. 26, No.8

Operator Essentials

What every operator should know about biological respiration

Woodie Mark Muirhead

Respiration is an essential biological process in wastewater treatment. Understanding the different processes of respiration is important for maintaining the proper environmental conditions for nutrient removal in the activated sludge process, as well as for producing desirable products, such as methane, and for preventing undesirable ones, such as hydrogen sulfide, in other treatment processes.




A practical consideration  


Respiration is the way by which microorganisms obtain energy from organic and inorganic substances. The end products of respiration depend on the microorganism and its metabolism. 

Biology textbooks provide a thorough explanation of respiration, including glycolysis, the Krebs cycle, electron transport system, and oxidative phosphorylation. From an operational perspective however, understanding the general terms that define respiration — the compounds that determine the needed environment, and the end-products — can help operators better control the environments needed to accomplish treatment objectives. 

Sources of energy 

There are two general categories of organisms based on their source of energy. Heterotrophic organisms use organic compounds for energy. Autotrophic organisms use inorganic compounds for energy. 

Examples of autotrophic organisms include the following: 


  • Algae get their energy using sunlight to split water molecules.   
  • Nitrifying organisms get their energy from ammonia and nitrite.   
  • Sulfide-oxidizing organisms get their energy from sulfides.   


Heterotrophic organisms are denitrifying organisms, biological phosphorus removal organisms, and those that oxidize carbonaceous biochemical oxygen demand (cBOD). 


Oxidation–reduction is the transfer of electrons from one chemical species (compound or ion) to another. A species is oxidized when it loses electrons, and is reduced when it gains electrons. 

A simple way to understand what is oxidized and what is reduced in biochemical reactions is to follow the fate of hydrogen and oxygen in biochemical reactions. In the aerobic oxidation of glucose (C6H12O6), carbon loses hydrogen and gains oxygen as it is oxidized to carbon dioxide. Oxygen gains hydrogen and is reduced to water (H2O). 


C6H12O6 + 6 O2  6 CO2 + 6 H2O 


The same concept can be used to understand oxidation–reduction during ammonia oxidation: 

NH4+1 + 1.5 O2  2H+1 + H2O + NO2–1 


Nitrogen as ammonia loses hydrogen and gains oxygen as it is oxidized to nitrite. 

Terminal electron acceptor 

During respiration, electrons are transferred to intermediate compounds within the cell. These intermediate compounds eventually must transfer the electrons to an oxidized compound by a process that replenishes the intermediate compound and also moves the electrons outside the cell. The oxidized compound that is last to accept the electrons is called the terminal electron acceptor.  

Because the terminal electron acceptor is an oxidized compound, it becomes reduced when it accepts the electrons. Common oxidized terminal electron acceptors and their reduced products include 


Oxygen (O2)  water (H2O),    

Nitrate (NO3–1)  nitrogen gas (N2) and water (H2O),    

Sulfate (SO4–2)  hydrogen sulfide (H2S) and water (H2O),and 


Carbon dioxide (CO2)  methane gas (CH4) and water (H2O). 

Aerobic respiration 

Organisms that respire aerobically use elemental oxygen (O2) as the terminal electron acceptor. 

Aerobic respiration is the dominant method of respiration in the activated sludge process and includes the following biochemical reactions: 


Aerobic oxidation of organic carbon: 

C6H12O6 + 6 O2  6 CO2 + 6 H2O 



NH4+1 + 1.5 O2  2H+1 + H2O + NO2–1 

NO2–1 + 0.5 O2  NO3–1 


Sulfide oxidation: 

2HS–1 + 4O2  2SO4–2 + 2H+1 


Sulfide oxidation has a minor role in an activated sludge system, but its effect often can be observed as corrosion of concrete above the water line in poorly aerated channels. 


Sulfide oxidizing bacteria attach to the moist, aerobic environment on the surface of the concrete and oxidize the hydrogen sulfide being released from the water in the channel. Sulfuric acid is formed that then dissolves concrete. The same respiratory process can result in corrosion on the crown of pipes in gravity sewers. 

Anoxic respiration 

Organisms that respire anoxically use nitrate and/or nitrite as a terminal electron acceptor. 

Denitrification is considered an anaerobic respiratory process in biological textbooks. The wastewater profession simply has coined the term “anoxic” to differentiate the environment needed to achieve denitrification. Nitrate and/or nitrite are essential for anoxic respiration. As important, oxygen can prevent denitrification since most denitrifying organisms are facultative (see below). 

Anaerobic respiration 

Organisms that respire anaerobically use terminal electronic acceptors other than oxygen, nitrate, or nitrite. These can include sulfate (SO4–2) and carbon dioxide (CO2).  

Hydrogen sulfide is formed when sulfate is used as a terminal electron acceptor in an anaerobic environment, as shown in the following biochemical reaction: 

C6H12O6 + 3SO4–2  3H2S + 6HCO3–1 

Facultative respiration 

Facultative organisms can respire in two or more ways.  

Most denitrifying organism are facultative. If oxygen is present, the organism will use the oxygen instead of nitrite or nitrate. When oxygen is used as the terminal electron acceptor, denitrification will not occur. 


Fermentation is an anaerobic process that results in the incomplete oxidation of organic compounds. Short-chain organic compounds are formed during fermentation.  When fermentation occurs, the organism achieves less energy than if a terminal electron acceptor is available.  

Volatile fatty acids (VFAs) are formed during fermentation. Two of these VFAs, acetate (CH3COO) and propionate (CH3CH2COO), are essential for biological phosphorus removal. 


Though humans are aerobic organisms, our muscle cells can undergo fermentation during strenuous exercise when there is insufficient oxygen for full oxidation. Lactic acid is formed when this occurs. Once these cells again become aerobic, the lactic acid is oxidized to carbon dioxide and water. 

©2014 Water Environment Federation. All rights reserved. 

Woodie Mark Muirhead is an operations and maintenance consultant with WoodMark Consulting (Honolulu).