Bacteria need a supply of substrate or food, predominantly carbon, for building new cellular material plus a supply of growth energy for carrying out that work. The bacteria utilizing organic material, or BOD5, in sewage obtain their growth energy by oxidizing about one third of the organics to inorganic products such as water, carbon dioxide, and ammonia. With this energy they convert the other two thirds to new bacterial cells. Bacteria which obtain their carbon for cellular synthesis from organic substrate are called heterotrophs.
At the same time, the bacteria suffer a continuous loss of mass due to oxidative mechanisms such as maintenance of cellular components and functions, death and reuse of bacterial cells and predation by higher organisms. Thus in the absence of a BOD supply, the mass of solids declines. These mechanisms causing loss of mass result in a base oxygen demand known as endogenous respiration. The growth stages for bacteria are shown in Figure 1, as a function of the sludge age or mean cell residence time (MCRT). Bacterial growth is portrayed as a function of the food supply in Figure 2.
Figure 1: Bacterial Rate of Growth as a Function of MCRT
For each growth phase identified in Figure 1, a brief explanation of that phase is as follows:
Figure 2: Bacterial Growth as a Function of Food
Bacteria in municipal wastewater can divide (fission) as frequently as every 11 minutes. Given this rate of reproduction, after just 11 hours a single cell has become 1,152,921,504,606,850,000 new cells.
Bacterial cell division in a more complex industrial environment occurs at a slower rate, dividing on the order of every 30 minutes. Given this rate of reproduction, a single bacteria cell after 11 hours has become 4,194,304 new cells. And after 24 hours, that single cell has given birth to 281,474,976,710,656 new cells.
The activated sludge process is a living biological process consisting of many different types of microorganisms. In a volume of just one cubic inch, an MLSS sample can contain as many as 9 trillion bacterial cells.
There really is a point to all of this: The microorganisms in a biological reactor, an aeration tank, are reproducing at a rate far greater than is needed to provide treatment. As a result, there is always a need to waste sludge from the system.
The various components of the mixed liquor suspended solids, the biomass, is as follows: