Note: It is important for wastewater operators to understand the self-purification process in streams because, as stated earlier, in a sense, wastewater treatment is a stream in a box.
Self- or natural purification refers to the ability of a stream or river (given enough time and distance) to purify itself. For example, when waste-water is discharged to a body of moving water, natural processes occur that will remove some forms of pollution from the water (see Figure 8.16). This process has been ongoing since time immemorial. It is only when the stream becomes overloaded with pollution that the natural cleaning action is retarded. When wastes were less complex than they are today, natural processes could remove the majority of pollutants; however, with increasing population levels (more and larger settlements along rivers and streams), the natural process has much more difficulty doing so.
As shown in Figure 8.16, the natural process consists of four zones or stages (although it is sometimes difficult to distinguish when one zone ends and the next begins). Let's take a look at each of these zones and how the self-purification process actually works.
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