Cell Generation Time
Updated: Oct 13, 2023
Bacteria are amazingly complex, even though each is comprised of only a single cell. Bacteria make up about 95 percent of all the microorganisms in activated sludge. As long as enough nutrients are available, bacteria can multiply (cell generation time) very rapidly by splitting into two identical cells. Some can divide in two in only 11 minutes. Many can double in 20 to 30 minutes (Glymph, 2005).
Glymph, Toni. "Wastewater Microbiology: A Handbook for Operators." Denver, Colorado: American Water Works Association, 2005.
The rate at which bacteria reproduce in an activated sludge system is something I am asked about more often than you might expect. Though there is great variability in the rate at which bacterial cells divide, and every wastewater treatment system is different, I thought it might be useful to provide some examples.
If you want more information on wastewater microbiology, I recommend you get both Toni Glymph's book as well as another comprehensive, easy-to-read, reference book by Michael Gerardi.
Gerardi, Michael H. “Wastewater Bacteria.” Hoboken, New Jersey: John Wiley & Sons, Inc., 2006.
The rate at which a bacterial cell reproduces, the cell generation time, by means of cell division (fission), is constantly changing. Reproduction rates differ significantly between (and within) groups of aerobic, facultative, and nitrifying bacteria. In addition, bacterial populations are in a constant state of flux, competing with one another, growing and shrinking as conditions in the bioreactor change.
The most important microorganisms in the activated sludge process are bacteria. Fungi, protozoa, and metazoa are of secondary importance. (Eckenfelder, W. Wesley and Grau, Petr. “Activated Sludge Process Design and Control: Theory and Practice.” Volume 1. Lancaster, Pennsylvania, Technomic Publishing Company, Inc., 1992.)
Aerobic Bacteria: Bacteria that must have molecular (dissolved) oxygen to survive. Nitrifying bacteria are strict aerobic bacteria.
Facultative Bacteria: Bacteria that can use either dissolved oxygen or oxygen obtained from sulfate or nitrate ions. This gives facultative bacteria the ability to live under aerobic or anaerobic conditions. Cell division for facultative bacteria is greater (more efficient) under aerobic conditions.
The majority of the bacteria in activated sludge are facultative—that is, they can live in either the absence or the presence of molecular (dissolved) oxygen. This is critical to the survival of activated sludge during periods of oxygen deficiencies. (Water Pollution Control Federation. “Activated Sludge” Manual of Practice OM-9. 1987.)
Anaerobic Bacteria: Bacteria that do not use free molecular oxygen for the degradation of substrates. These organisms include sulfate-reducing bacteria and methane-forming bacteria that use sulfate and carbon dioxide, respectively (Gerardi).
Floc: Floc is comprised of clumps of bacteria and particles that have come together in the bioreactor to form a cluster. A floc mass is made up of millions of organisms in a good activated sludge, including bacteria, fungi, yeast, protozoa, and worms (Operation of Wastewater Treatment Plants, Volume II, 5th edition. 2001.).
Factors that Influence Cell Generation Time
One key factor is whether we are considering cell generation in a municipal or an industrial wastewater system or some combination of both. For the sake of simplicity, I want to take the position that fewer stressors exist in a municipal wastewater plant compared to an industrial treatment system.
Industrial wastewater treatment plants, for example, are more likely to experience wider swings in the organic load, ammonia load, pH of the wastewater, wastewater temperature, etc. These are called “abiotic factors.” Abiotic factors are the nonliving components or operational conditions in a biological treatment unit that affect the activity and growth of the biomass. Significant abiotic factors in the activated sludge process include alkalinity, ionized ammonia, dissolved oxygen, hydraulic retention time (HRT), nutrients, pH, quantity and types of substrates, sulfate, temperature, toxic wastes, and volatile acids (Gerardi, 2006).
Graphical Display of Cell Generation
Given the condition of optimal abiotic factors in a municipal activated sludge system, cell generation time can be as fast as every 11 minutes. Starting with a single bacterial cell at time 0, when this first cell divides (doubles) 11 minutes later, with each new group of cells continuing to double every 11 minutes, 480 minutes or eight hours later the bacterial population has increased to 17,592,186,044,416 cells!! This is an astounding rate of growth, requiring a much higher sludge wasting rate than is found in most industrial wastewater systems where the cell generation time is longer. A table of the cell production (generation time) using 11 minutes can be found at the bottom of this post.
In contrast, in the more stressed environment of an industrial bioreactor, where less than optimal conditions predominate, cell division can stretch out to every 30 minutes or more. Again, starting with a single bacterial cell at time 0, when this first cell divides (doubles) 30 minutes later, with each new group of cells continuing to double every 30 minutes, 480 minutes or eight hours later the bacterial population has increased to only 65,536 cells.
For nitrifying bacteria, the cell generation time is extremely slow, in the range of 48 to 72 hours under good conditions. In a stressed environment the generation time for nitrifiers can be as long as 15 days. Pay attention to the scale of the X-axis. In the two previous graphs the time was in units of HOURS. For the growth rate of nitrifiers in the graph below the scale is now in units of DAYS. After 30 days of cell division, with cell division occurring every 48 hours, the nitrifying population has only increased to a mere 32,768 bacterial cells!
Recovering from Upset Conditions
The rate at which bacteria reproduce becomes very important when trying to recover from upset conditions generally and inhibitory or toxicity issues specifically. At slower cell generation rates recovery will take longer, perhaps much longer, requiring bioaugmentation to jumpstart biological treatment. In the case of plants that nitrify this becomes extremely critical given the slow cell generation rate for nitrifiers.
Regarding bioaugmentation, this can be accomplished by adding mixed liquor suspended solids (MLSS) or return activated sludge (RAS) with a higher solids concentration, from a nearby wastewater plant. For industrial wastewater systems, due to a complete lack of acclimated microorganisms in the MLSS or RAS from a municipal plant, bioaugmentation using cultured bacteria would be, in my experience, a much better option. For more on bioaugmentation there is another excellent Michael Gerardi textbook I recommend:
Gerardi, Michael. "Wastewater Bioaugmentation and Biostimulation." Lancaster, Pennsylvania: DEStech Publications, Inc. 2016. (http://www.destechpub.com)
I have a blog post on bioaugmentation as well which you can find here:
One final note on cell generation time. In anaerobic digesters methanogens have a very long generation time of 3 to 30 days (Gerardi, 2016).