CHEMICAL CHARACTERISTICS

The chemical characteristics of wastewater of special concern to the Utilitiesman are pH, DO (dissolved oxygen), oxygen demand, nutrients, and toxic substances.

pH

The term pH is used to describe the acid or base properties of water solutions. A scale from

Table 10-4.-Solids of a Typical Domestic Wastewater

0 to 14 has been established where pH value of 7 is neutral. A pH value less than 7 is acidic. A pH value above 7 is alkaline or basic. Table 10-5 lists pH values for some common materials. A pH value less than 7 in the wastewater plant influent may indicate septic conditions of wastewater. The pH values less than 5 and more than 10 usually indicate that industrial wastes exist and are not compatible with biological

wastewater operations. Pretreatment of these wastes at the source is usually required since extreme pH values may damage biological treatment units.

Dissolved oxygen

Dissolved oxygen (DO) in wastewater has a great effect on the characteristics of the water.

Table 10-5.-Common Substance pH Values

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.

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