Viruses

A virus is an entity that carries the information needed for its replication but does not possess the machinery for such replication (Sterritt and Lester, 1988). Thus, viruses are obligate parasites that require a host in which to live. They are the smallest biological structures known, so they can only be seen with the aid of an electron microscope. Waterborne viral infection is usually indicated by disorders with the nervous system rather than of the gastrointestinal tract. Viruses that are excreted by human beings may become a major health hazard to the public. Waterborne viral pathogens are known to cause poliomyelitis and infectious hepatitis.

Testing for viruses in water is difficult because: (1) they are small, (2) they are of low concentrations in natural waters, (3) there are numerous varieties, (4) they are unstable, and (5) only limited identification methods are available. Because of these testing problems and the uncertainty of viral disinfection, direct recycling of wastewater and the practice of land application of wastewater are areas of concern (Peavy et al., 1987).

7.5.3 Protozoa

Protozoa (singular: protozoan) are mobile, single-celled, completely self-contained organisms that can be free living or parasitic, pathogenic or nonpathogenic, microscopic or macroscopic. Protozoa range in size from two to several hundred microns in length. They are highly adaptable and widely distributed in natural waters, although only a few are parasitic. Most protozoa are harmless; only a few cause illness in humans (e.g., Entamoeba histolytica, which causes an infection of the intestines known as amebiasis). Because aquatic protozoa form cysts during adverse environmental conditions, they are difficult to deactivate by disinfection and must undergo filtration to be removed.

7.5.4 Worms (Helminths)

Worms are the normal inhabitants in organic mud and organic slime. They have aerobic requirements but can metabolize solid organic matter not readily degraded by other microorganisms. Water contamination may result from human and animal waste that contains worms. Worms pose hazards primarily to those persons who come into direct contact with untreated water; thus, swimmers in surface water polluted by sewage or stormwater runoff from cattle feedlots and sewage plant operators are at particular risk.

7.6 CHAPTER REVIEW QUESTIONS

7.1 What are the characteristics or range of characteristics that make water appealing and useful?

7.2 What is the process by which water vapor is emitted by leaves?

7.3 Water we see is known as__

7.4 What is the leading cause of impairment for rivers, lakes, and estuaries?

7.5 All contaminants of water contribute to the__

7.6 The clarity of water is usually measured by its__

7.7 Water has been called the__

7.8 A measure of the ability of water to neutralize acid is referred to as

7.9 A pH value of 7 represents a

7.10 There is no safe level for_exposure.

7.11 What is BOD5?

7.12 Name three sources of wastewater, and give an example of the types of materials associated with each.

7.13 Define organic and inorganic.

7.14 Name the two types of solids based on physical characteristics.

7.15 What is stormwater runoff, and how can it cause problems for the wastewater treatment plant?

7.16 Name three types of wastewater based on the types of waste carried.

7.17 Give three reasons for treating wastewater.

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