Microorganisms have several critical functions in wastewater treatment processes. The microbial component of the aquatic ecosystem provides the purifying capacity of natural waters in which microorganisms respond to the rise of organic pollutant concentration by increased growth and metabolism. The self-regulated purification mechanism in natural waters subscribes to the same fundamental principles of biological pollutant abatement as those used in biological wastewater treatment processes. In addition to the abundant food for microorganisms, food and agricultural wastewaters also contain microorganisms themselves. With a controlled and suitable environment for optimal growth and metabolism of microorganisms, all the organic matters that exist in the wastewater streams can be biodegraded.

Microorganisms utilize organic materials for the production of energy by cellular respiration and for the synthesis of cellular material such as proteins for maintenance and production of new cells. The utilization of organic matters by microorganisms can be summarized with an equation of the overall reaction of wastewater treatment(Equation 2.1):

Organic matter + O2 + NH2+4 + P b New cells + CO2 + H2O

where P stands for phosphate.

In wastewater treatment, bacteria are the ones responsible for degradation of organic matters. Other microorganisms such as fungi, algae, protozoa, and higher organisms also contribute to the biotransformation of organic materials in wastewaters.

A basic understanding of the key biological organisms, microbial organisms or otherwise—bacteria, fungi, algae, protozoa, and metazoan (crustaceans)—is essential in development of strategies for food and agricultural wastewater treatment and utilization. With the exception of wastewater streams right off certain food processing operations such as retort, food and agricultural wastewater provides an ideal growth media where diverse communities of microorganisms can thrive. These organisms play an important role in all stages of biological wastewater treatment, and they also have some influences on sludge formulation and characteristics. It is without exaggeration to say that a biological waste-water treatment process could not even survive if not for the existence of mixed communities of ravenous microorganisms feeding on organic matters and each other. The exact compositions of these communities will depend on the outcome of competition of food supplies and environmental conditions such as pH and temperature. The information of mi-crobial communities at each stage of biological wastewater treatment is important for designers and planners of food and agricultural wastewater management.

Although living things, as we know since our grammar-school days, are classified as in the plant kingdom or animal kingdom, microorganisms belong to a unique kingdom called Protista. Mixed communities of microorganisms have plantlike constituents such as algae and animallike inhabitants such as protozoa. These are simple life forms with various structures of their cells.

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