Iiipurposes Of Wastewater Treatment

Wastewater is collected in combined and separate sewers from industries, commercial establishments, and households and transported to the treatment plant. The plant effluent is usually disposed of by being discharged into rivers, lakes, or estuaries. See Figure 1.

Wastewater typically contains many different substances, both suspended and dissolved, inorganic and organic. The total amount of organic material is related to the "strength" of the wastewater. This is measured by the biochemical oxygen demand (BOD). Another measure related to "strength" and "quality" of wastewater is the total amount of suspended solids (TSS).

Another group of impurities of major significance in wastewater are plant nutrients, specifically compounds of nitrogen (N) and phosphorus (P). Oil and grease are also common pollutants. The amount of pathogens (disease-causing microorganisms) in wastewater is usually proportional to the concentration of fecal coliform bacteria. BOD, TSS, N, P, and coliform concentration are important parameters of water quality. Actually, wastewater contains so many different substances that it is impractical to identify each substance or microorganism. The approximate composition of an average domestic wastewater is given in Table 1.

Wastewater is treated to meet the minimum level of effluent quality set by the state or the U.S. Environmental Protection Agency (EPA). The purposes of wastewater treatment are to remove most of the suspended and dissolved organic and inorganic material and destroy pathogenic microorganisms. In many cases it is also necessary to remove plant nutrients—nitrogen and phosphorus.

Some pollutants, both inorganic and organic, are resistant to conventional treatment processes. Such contaminants are called "refractory" contaminants. Phosphate, for example, is present in very low concentrations in natural waters but is increased in wastewaters from

Industrial

Combined Domestic wastes wastewater wastewater (usually pretreated)

Industrial

Combined Domestic wastes wastewater wastewater (usually pretreated)

Figure 1 Location of a municipal wastewater treatment plant.

Table 1 Approximate Composition of an Average Domestic Wastewater (mg/L)

Component

Before sedimentation

After sedimentation

Biologically treated

Total solids

800

680

530

Total volatile solids

440

340

220

Suspended solids

240

120

30

Volatile suspended solids

180

100

20

BOD

200

130

30

Ammonia nitrogen as N

15

15

24

Total nitrogen as N

35

30

26

Soluble phosphorus as P

7

7

7

Total phosphorus as P

10

9

8

Source: Adopted with permission from Viessman and Hammer (1993).

Source: Adopted with permission from Viessman and Hammer (1993).

domestic sources due to the use of synthetic detergents and runoff from fertilized fields. Organic nitrogenous compounds decompose to ammonia and oxidize to nitrates during waste treatment. Certain high molecular weight materials (e.g., dyes and surfactants) are not bio-degraded. Also, agricultural land runoff includes pesticides such as chlorinated hydrocarbons.

Excessive concentrations of nutrients often lead to large growths of algae, which in turn become oxygen-demanding materials when they die and settle to the bottom. Conventional waste treatment is only 30-60% effective in removing these nutrients. Advanced waste treatment methods (tertiary treatment) are required for greater nitrogen reduction and phosphorus removal.

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