In laboratory tests a maximum tolerable microbial population limit in systems is determined. When these data are known in many cases the number of bacteria and other microrganisms needs serious reduction. This can be accomplished by addition of biocides; chemical compounds that are toxic to the present microorganisms. Biocides are usually slug fed to a system to bring about rapid effective population reductions from which the microorganisms cannot easily recover. There are various different biocides, some of which have a wide range of effect on many different kinds of bacteria. They can be divided up into oxidizing agents and non-oxidizing agents.

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