An understanding of the nature of wastewater is essential in the design and operation of collection, treatment, and reuse facilities, and in the engineering management of environmental quality. To promote this understanding, the information in this chapter is presented in eight sections dealing with (1)an introduction to the constituents found in wastewater, (2)sampling and analytical procedures, (3) physical characteristics, (4)inorganic nonmetallic constituents, (5) metallic constituents