Prior to about 1940, most municipal wastewater was generated from domestic sources. After 1940, as industrial development in the United States grew significantly, increasing amounts of industrial wastewater have been and continue to be discharged into municipal collection systems. The amounts of heavy metals and synthesized organic compounds generated by industrial activities have increased; some 10 000 new organic compounds are added each year. Many of these compounds are now found in the wastewaters.
As technological changes take place in manufacturing, changes also occur in the compounds discharged and the resulting wastewater characteristics. Numerous compounds generated from industrial processes are difficult and costly to treat by conventional wastewater treatment processes. Therefore, effective industrial pretreatment becomes an essential part of an overall water quality management program. Enforcement of an industrial pretreatment program is a daunting task; some of the regulated pollutants still escape to the municipal wastewater collection system and must be treated. In the future with the objective of pollution prevention, every effort should be made by industrial discharges to assess the environmental impacts of any new compounds that may enter the wastewater stream before being approved for use. If a compound cannot be treated effectively with existing technology, it should not be used.
The wastewater from industries varies greatly in both flow and concentration of pollutants. So, it is impossible to assign fixed values to their constituents. In general, industrial wastewaters may contain suspended, colloidal, and dissolved (mineral and organic) solids. In addition, they may be either excessively acidic or alkaline and may contain high or low concentrations of colored matter. These wastes may contain inert, organic, or toxic materials and possibly pathogenic bacteria. These wastes may be discharged into the sewer system provided they have no adverse effect on treatment efficiency or undesirable effects on the sewer system. It may be necessary to pretreat (Section 3.5.2) the wastes prior to release to the municipal system or it is necessary to make a full treatment when the wastes will be discharged directly to surface or ground waters.
The physical and chemical characterization presented below is valid for most wastewaters, both industrial and municipal.
Physical Characteristics
The principal physical characteristics of wastewater include solids content, color, odor, and temperature.
Total Suspended Solids
The total solids in a wastewater consist of the insoluble or total suspended solids and the soluble compounds dissolved in water. The suspended solids content is found by drying and weighing the residue removed by the filtering of the sample. When this residue is ignited the volatile solids are burned off. Volatile solids are presumed to be organic matter, although some organic matter will not burn and some inorganic salts break down at high temperatures. The organic matter consists mainly of proteins, carbohydrates and fats. Between 40 and 65% of the solids in an average wastewater are suspended. Settleable solids, expressed as milligrams per liter (mg/l), are those that are removed by sedimentation. Usually about 60% of the suspended solids in a wastewater are settleable (Crites and Tchbanoglous 1998). Solids may be classified in another way as well: those that are volatilized at a high temperature (600 °C) and those that are not. The former are known as volatile solids, the latter as fixed solids. Usually, volatile solids are organic.
Color
Color is a qualitative characteristic that can be used to assess the general condition of wastewater. Wastewater that is light brown in color is less than six hours old, while a light‐to‐medium grey color is characteristic of wastewaters that have undergone some degree of decomposition or that have been in the collection system for some time. Lastly, if the color is dark grey or black, the wastewater is typically septic, having undergone extensive bacterial decomposition under anaerobic conditions. The blackening of wastewater is often due to the formation of various sulphides, particularly ferrous sulphide. This results when hydrogen sulphide produced under anaerobic conditions combines with divalent metal, such as iron, which may be present. Color is measured by comparison with standards.
Odor
The determination of odor has become increasingly important, as the general public has become more concerned with the proper operation of wastewater treatment facilities. The odor of fresh wastewater is usually not offensive, but a variety of odorous compounds are released when wastewater is decomposed biologically under anaerobic conditions. The different unpleasant odors produced by certain industrial wastewater are presented in Table 3.1.
Temperature
The temperature of wastewater is commonly higher than that of the water supply because warm municipal water has been added. The measurement of temperature is important because most wastewater‐treatment schemes include biological processes that are temperature dependent. The temperature of wastewater will vary from season to season and also with geographic location. In cold regions, the temperature will vary from about 7 to 18 °C, while in warmer regions the temperatures vary from 13 to 24 °C (Crites and Tchobanoglous 1998).
Table 3.1 Unpleasant odors in some industries.
Source: From Eckenfelder (2000).
| Industries | Origin of odors |
| Cement works, Lime Kilns | Dibutyl amines, mercaptans, dibutylsulfide, hydrogen sulfide, sulfur dioxide |
| Food industries | Acetic acid, acetaldehyde |
| Food industries (fish) | Butyl amine, mercaptans, dimethyl sulfide, amines |
| Pharmaceutical industries | Fermentation by‐product produces |
| Pulp and paper industries (kraft) | Total reduced sulfur compounds, TRS: (hydrogen sulfide, mercaptans, methyl disulfide, dimethyl disulfide), sulfur dioxide |
| Rubber industries | Sulfides, mercaptans |
| Textile industriesa | Phenyl mercaptan, phenolic compounds |
| Tomato cannery | Acetic acid, acetaldehyde, thiophenol |
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