Design Example

Classification of Micro organisms
Growth Pattern of Micro organisms
Biomass Growth Rate
Maintenance as Endogenous Respiration
Growth Yield

Classification of Micro organisms

  1. Nutritional Requirements: On the basis of chemical form of carbon required, microorganisms are classified as
    1. Autotrophic: organisms that use CO2 or HCO3- as their sole source of carbon.
    2. Heterotrophic: organisms that use carbon from organic compounds.
  2. Energy Requirements: On the basis of energy source required, microorganisms are classified as
    1. Phototrophs: organisms that use light as their energy source.
    2. Chemotrophs: organisms that employ oxidation-reduction reactions to provide energy. They are further classified on the basis of chemical compounds oxidized (i.e., electron donor)
      1. Chemoorganotrophs: Organisms that use complex organic molecules as their electron donor.
      2. Chemoautotrophs: Organisms that use simple inorganic molecules such as hydrogen sulfide or ammonia as their electron donor.
  3. Temperature Range: On the basis of temperature range within which they can proliferate, microorganisms are classified as
    1. Psychrophilic: organisms whose growth is optimum within 15 to 30°C.
    2. Mesophilic: organisms whose growth is optimum within 30 to 45°C.
    3. Thermophilic: organisms whose growth is optimum within 45 to 70°C.
  4. Oxygen Requirements: On the basis of oxygen requirement microorganisms are classified as
    1. Aerobes: organisms that use molecular oxygen as electron acceptor.
    2. Anaerobes: organisms that use some molecule other than molecular oxygen as electron acceptor.
    3. Facultative organisms : organisms that can use either molecular oxygen or some other chemical compound as electron acceptor.

Growth Pattern of Micro organisms

When a small number of viable bacterial cells are placed in a close vessel containing excessive food supply in a suitable environment, conditions are established in which unrestricted growth takes place. However, growth of an organism do not go on indefinitely, and after a characteristic size is reached, the cell divides due to hereditary and internal limitations. The growth rate may follow a pattern similar to as shown in figure:

The curve shown may be divided into six well defined phases:

  1. Lag Phase:adaptation to new environment, long generation time and null growth rate.
  2. Accelaration phase: decreasing generation time and increasing growth rate.
  3. Exponential phase: minimal and constant generation time, maximal and constant specific growth rate and maximum rate of substrate conversion.
  4. Declining growth phase: increasing generation time and decreasing specific growth rate due to gradual decrease in substrate concentration and increased accumulation of toxic metabolites.
  5. Stationary phase: exaustion of nutrients, high concentration of toxic metabolites, and cells in a state of suspended animation.
  6. Endogenous phase: endogenous metabolism, high death rate and cell lysis.

Biomass Growth Rate

The most widely used expression for the growth rate of micro organisms is given by Monod:

Total rate of microbial growth,dx = mmXS
                                          dt    Ks+ S


mm= maximum specific growth rate

X = micro organism concentration

S = substrate concentration

Ks= substrate concentration at one half the maximum growth rate

Similarly, rate of substrate utilization,

     dS = k X S
     dt    Ks+ S


k = maximum specific substrate utilization rate

Maintenance as Endogenous Respiration

Net growth rate of micro organisms is computed by subtracting from the total growth rate, the rate of micro organisms endogenously decayed to satisfy maintenance energy requirement. Therefore,

Net rate of microbial growth = mmX S - kdX
                                          Ks+ S


kd = endogenous decay coefficient

Growth Yield

Growth yield is defined as the incremental increase in biomass which results from the utilization of the incremental amount of substrate. The maximum specific growth rate is given by: mm =Y.k

where, Y is the maximum yield coefficient and is defined as the ratio of maximum mass of cells formed to the mass of substrate utilized. The coefficients Y, kd, k and Ks are designated as kinetic coefficients. The values of kinetic coefficients depend upon the nature of wastewater and operational and environmental conditions in biological reactor. The biological reactors can be completely mixed flow or plug flow reactor with or without recycle.