Figure 1 shows the schematic of a conventional wastewater treatment plant using primary treatment. Raw wastewater is introduced either to the screen or to the comminutor. The grit channel removes the larger particles from the screened sewage, and the primary clarifier removes the larger particles of organic matter as well as inorganic matter that escapes removal by the grit channel. Primary treated sewage is then introduced to a secondary treatment process train downstream (not shown) where the colloidal and dissolved organic matter are degraded by microorganisms.
The scheme involves mere physical movement of materials, no chemical or biological changes occur. In addition, the function of the various operations in the scheme, such as screening, may be applied not only to the primary treatment of sewage as the figure indicates but to other plant operations as well. For example, bagasse may be screened from sugar cane juice in the expression of sugar in a sugar mill, or the larger particles resulting from the cleaning of pineapples in a pineapple factory may be screened from the rest of the wastewater. To master the function of screening, it is not necessary that this be studied in a wastewater treatment plant, in
FIGURE 1 A primary treatment system.
FIGURE 1 A primary treatment system.
a sugar mill, or in a pineapple factory. It can be studied in any setting where screens are used.
Furthermore, the functions of the operation of the primary clarifier may be applied not only to the treatment of sewage as indicated in the schematic but also to the clarification in a water treatment plant, as well as in the clarification of tailings in a mining operation. Similar statements may be made about the operation of the grit channel. In other words, to master the function of clarification and grit removal, it is not necessary that these be studied in a sewage treatment plant or in a water treatment plant. It can be studied in any setting where clarifiers and grit removal are used.
The foregoing operations are physical; they are therefore physical treatments. These physical treatments are called unit operations and as gleaned from the previous discussion they may be defined as physical treatments that are identified only according to their functions without particular reference to the location of the units utilizing the functions. For example, screening may be studied without particular reference to any sugar mills or pineapple factories. The unit operations of clarification may be studied without particular reference to any wastewater or water treatment plants or mining operations. The unit operations of grit removal may also be studied without particular reference to any sewage plant but only to any setting where grit removal is involved.
The unit operations discussed previously are all physical operations. In the biological or chemical scene where materials are changed, unit operations have counterparts called unit processes. Examples of unit processes are coagulation and biological oxidation. In coagulation, a chemical called coagulant undergoes a chemical reaction. This chemical reaction may occur in any plant or factory or any location at all where the function of the chemical reaction in coagulation is utilized. For example, coagulation is employed in water treatment to enhance the settling of the turbidity of raw waters. Coagulation may also be used in the clarification of sugar cane juice to remove the fibers that the juice may contain.
The other unit process, biological oxidation, is used in sewage treatment; it may also be used in biofiltration applied in water treatment. The biological reaction that occurs in either sewage treatment or biofiltration are the same. In other words, the function of the biological reaction is the same whether the reaction occurs in sewage treatment or in biofiltration.
Coagulation and biological oxidation are identified on the basis of the function of their characteristic chemical or biological reactions irrespective of the plant, factory, or any other location that uses the reactions. The function of a coagulation reaction is coagulation whether the reaction occurs in a water treatment plant or in a sugar plant; and the function of a biological oxidation reaction is biological oxidation whether the reaction occurs in a sewage treatment plant or in a water treatment plant. The setting is immaterial; what is of concern is the function of the chemical reaction. Unit processes may therefore be defined as chemical (or biological) treatments that are identified only according to the functions of the chemical (or biological) reactions irrespective of where the units utilizing the reactions are occurring.
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