Wastewater that has DO is called aerobic or fresh. Aerobic raw sewage is usually gray in color and has a musty odor.

Wastewater that has no DO is called anaerobic or septic. Anaerobic raw sewage is usually black and has an offensive hydrogen sulfide or rotten egg odor.

Oxygen Demand

Oxygen demand is the amount of oxygen used by bacteria and other wastewater organisms as they feed upon the organic solids in the wastewater. Chemical tests such as the BOD (biochemical oxygen demand), the COD (chemical oxygen demand), the ODI (instantaneous oxygen demand or oxygen demand index), and the TOC (total organic carbon) measure the "strength" of sewage. These tests are discussed in detail later in this chapter. It is important that organic wastes be removed to protect the receiving body of water into which the wastewater plant is discharging. Sludge deposits, odors, and fish kills may occur if removal is not adequate.

Nutrients

Nutrients are life-supporting nitrogen and phosphorus. They stimulate excessive growths of algae and other aquatic plant life. They are always present in domestic wastewaters and are not removed during conventional primary and secondary treatment. Removal is accomplished by processes in addition to normal wastewater treatment or tertiary treatment, when specific reuse requirements require it.

Toxic Chemicals

Most military and industrial installations use various types of toxic chemicals, the discharges of which can be harmful to wastewater treatment processes. These toxic chemicals should be pretreated or removed before the waste-water enters the collecting system. Table 10-6

Table 10-6. - Chemicals and Discharges Commonly Found at Military Installations