4 Introduction to pr is and selecti The constituents of concern found in wastewater are removed by physical, chemical, and biological methods. The individual methods usually are classified as physical unit operations, chemical unit processes, and biological unit processes. Treatment methods in which the application of physical forces predominate are known as physical unit operations. Examples of physical unit operations include transfer, filtration, and adsor hich the al or conversion of constituents is brought about reactions are known as chemical unit processes. Examples of chemical unit processes include disinfection. oxidation. and precipitation Treatment methods in which the removal of constituents is brought about by biological activity are known as biological unit processes. Biological treatment is used primarily to remove the biodegradable organic constituents and nutrients in wastewater. Examples of biological activated-sludge and trickling-filter processes. Unit operations and ombinations in treatment flow diagrams ontact basin Gravity thickeners ons and conversions Dechlorination Gri Liquid biosolids completion and eftluent pumping storage lagoons generally a function of the temperature. and the type which the reactions take place). Hence. both the effects of mperature and the ty important in the selection of treatment process physical constraints(约束 must be considered in Fig 4-1 Overview of a biological nutrient Anaerobic Dew Influent pump Septage station an removal(BNR) wastewater-treatment Harford County, MD. The capacity is 76,000 m /d) The fundamental basis for the analysis of the physical, chemical, and biological unit operations and processes used for wastewater treatment is the materials mass balance principle in which an accounting of made before and after Therefore, the purpose of this chapter is to introduce and discuss(1)the types of reactors used for wastewater treatment;(2)the preparation of mass balances to determine process performance;(3) modeling ideal flow in reactors; (4)the analysis of reactor hydraulics using tracers: (5)modeling nonideal flow in reactors;(6) reactions, reaction rates, and reaction rate coefficients;(7)modeling treatment kinetics, which involves the coupling of reactors and reaction rates;(8) treatment processes involving mass transfer; and(9)important factors involved in process analysis and selection The information in this chapter is intended to serve as an introduction to the subject of process analysis, and to provide a basis for the analysis of the unit operations and processes that will be presented in ubsequent chapters. 4-1 Reactors used for the treatment of wastewater 4-14-1 4 Introduction to Process Analysis and Selection The constituents of concern found in wastewater are removed by physical, chemical, and biological methods. The individual methods usually are classified as physical unit operations, chemical unit processes, and biological unit processes. Treatment methods in which the application of physical forces predominate are known as physical unit operations. Examples of physical unit operations include screening, mixing, sedimentation, gas transfer, filtration, and adsorption. Treatment methods in which the removal or conversion of constituents is brought about by the addition of chemicals or by other chemical reactions are known as chemical unit processes. Examples of chemical unit processes include disinfection, oxidation, and precipitation. Treatment methods in which the removal of constituents is brought about by biological activity are known as biological unit processes. Biological treatment is used primarily to remove the biodegradable organic constituents and nutrients in wastewater. Examples of biological treatment processes include the activated-sludge and trickling-filter processes. Unit operations and processes occur in a variety of combinations in treatment flow diagrams. The rate at which reactions and conversions occur, and the degree of their completion, is generally a function of the constituents involved, the temperature, and the type of reactor (i.e., container or tank in which the reactions take place). Hence, both the effects of temperature and the type of reactor employed are important in the selection of treatment processes. In addition, a variety of environmental and other physical constraints(约束) must be considered in process selection. Fig. 4-1 Overview of a biological nutrient removal(BNR) wastewater-treatment plant (Harford County, MD. The capacity is 76,000 m3 /d) The fundamental basis for the analysis of the physical, chemical, and biological unit operations and processes used for wastewater treatment is the materials mass balance principle in which an accounting of mass is made before and after reactions and conversions have taken place. Therefore, the purpose of this chapter is to introduce and discuss (1) the types of reactors used for wastewater treatment; (2) the preparation of mass balances to determine process performance; (3) modeling ideal flow in reactors; (4) the analysis of reactor hydraulics using tracers: (5) modeling nonideal flow in reactors; (6) reactions, reaction rates, and reaction rate coefficients; (7) modeling treatment kinetics, which involves the coupling of reactors and reaction rates; (8) treatment processes involving mass transfer; and (9) important factors involved in process analysis and selection. The information in this chapter is intended to serve as an introduction to the subject of process analysis, and to provide a basis for the analysis of the unit operations and processes that will be presented in subsequent chapters. 4-1 Reactors used for the Treatment of Wastewater