Oxidations associated with chemical disinfection are considered in a different presentation. Some of the chlorine chemistry of disinfection applies to industrial waste treatment. Although a relatively low concentration of chlorine is used for disinfecting drinking water, it may be used in large amounts as an oxidizing agent. The purpose of oxidation or reduction is destruction of an undesirable species. Sometimes the reaction product is also objectionable but less so than the original pollutant.
Elements have an oxidation state of zero.
When electrons leave an element, it acquires plus charge and is in an oxidized state. Reduction is gaining of electrons while changing to a more negative charge. When several atoms are present in a compound, we use rules that are somewhat arbitrary to assign oxidation state. When the entire group has a charge, the assigned oxidation states must sum to that charge. The convention keeps track of overall oxidation and reduction quite nicely although it distorts the concepts of reactivity.