Wastewater Microorganisms

Although most organisms in biological wastewater treatment plants are microscopic in size, there are some organisms such as bristleworms and insect larvae that are macroscopic in size. Macroscopic organisms can be observed with the naked eye— that is, without the use of a light microscope. Microscopic organisms can only be observed with the use of a light microscope. Of the microscopic organisms the bacteria (singular: bacterium) are the most important in wastewater treatment plants and can be seen with the light microscope only under highest magnification. Several groups of microorganisms such as protozoa and some metazoa possess large and more complex cells that can be observed easily with the light microscope without the use of highest magnification. Compared to other organisms, microorganisms have relatively simple structures.

All living cells can be classified as procaryotic or eucaryotic (Table 1.1). Pro-caryotic cells lack a nucleus and other membrane-bound structures, while eucary-otic cells possess these structures (Figure 1.1). The nucleus is the primary membrane-bound structure in eucaryotic cells. It regulates cellular activity and contains the genetic information. Examples of membrane-bound structures or organelles found in eucaryotic cells include the golgi apparatus (which regulates cellular metabolism) and lysomes (which contain hydrolytic enzymes).

Based upon cellular structure and function, microorganisms are commonly classified as eucaryotes and procaryotes. The procaryotes consist of (1) eubacteria or "true" bacteria and (2) archaebacteria or "ancient" bacteria (Table 1.2). The eubac-teria and archaebacteria are the most important microorganisms in biological, wastewater treatment plants. Together, these two procaryotes commonly are referred to as bacteria.

TABLE 1.1 Major Differences between Procaryotic Organisms and Eucaryotic Organisms


Procaryotic Organism

Eucaryotic Organism

Genetic material



Not contained in a membrane




FIGURE 1.1 Procaryotic and eucaryotic cells. The procaryotic cell (a) contains no membrane-bound organelles such as the nucleus, golgi body, and lysosome that are found in the eucaryotic cell (b).

TABLE 1.2 Classification of Microorganisms in Wastewater Treatment Plants


Cell Structure






Bristleworms, flatworms, free-living

nematodes, waterbears







Unicellular with unique

Halophiles, methanogens,

cellular chemistry


There are four important eucaryotic organisms in the activated sludge process. These organisms are fungi, protozoa, rotifers, and nematodes.These free-living (non-disease-causing) eucaryotes enter wastewater treatment plants through inflow and infiltration (I/I) as soil and water organisms.

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