Bacteria - These small, single-celled organisms are found in large numbers in polluted water. Phase contrast microscopes are used to identify most bacteria in wastewater.
Protozoa - These single-celled organisms are typically much larger than bacteria and are typically found in large numbers in wastewater, many being quite mobile. Protozoa have received increased attention because of their connection with waterborne diseases.
Metazoa - Multi-celled organisms that are larger and more complex than Protozoa. Rotifers are included in this group and are fairly common in wastewater treatment processes, as they play an active role in the breakdown of organic wastes.
Viruses - These are much smaller than bacteria and can not be seen with a standard microscope. Viruses are parasites that can reproduce within a host cell. Many diseases caused by viruses can be transmitted through polluted waters.
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