be an important consideration in the design of the treatment facilities in which these reactions will be carried out. Special attention must be given to the design of mixing facilities, especially for reactions that are rapid Heterogeneous Reactions. Heterogeneous reactions occur between one or more constituents that can be identified with specific sites. such as those on an ion exchange resin in which one or more ions is replaced by another ion Reactions that require the presence of a solid-phase catalyst are also classified as heterogeneous Heterogeneous reactions are usually carried out in packed and fluidized-bed reactors(see Fig. 4-2f, g, and h). These reactions are more difficult to study because a number of interrelated steps may be involved. The typical sequence of these steps is as follows: Transport of reactants from the bulk fluid to the fluid-solid interface(external surface of catalyst 2. Intraparticle transport of reactants into the catalyst particle(if it is porous) 4. Chemical reaction of adsorbed reactants to adsorbed products( surface reaction 5. Desorption of adsorbed products 6. Transport of products from the interior sites to the outer surface of the catalyst particle Rate Expressions Used in Environmental Modeling The physical, chemical, and biological processes that control the fate of the constituents dispersed to the environment are numerous and varied. Important constituent transformations and removal processes(i.e fate processes) operative in the environment, along with the constituents affected, are reported in Table 4-2. Because all of the processes summarized in Table 4-2 are rata-dependent, representative rate expressions used to model these processes are presented in Table 4-3. The important thing to note about Table 4-2 is the variety of different rate expressions that have been used to model constituent transformation and removal processes Tab. 4-2 Constituent transformation and removal processes(i.e, fate processes) in the environment Comments Constituents affected Many chemical constituents tend to attach or sorb Metal, trace organics, onto solids. The implication for wastewater NH4, PO4 discharges is that a substantial fraction of some toxic chemicals is associated with the suspended solids in the effluent. Adsorption comb ined with solids settling results in the remov al from the water column of constituents that might not otherwise Algal synthesis The synthesis of algal cell tissue using the nutrients*, NO;, PO, ,pH found in wastewater Bacterial conversion Bacter ial conversion(both aerobic and anaerobic)isBODs, nitrification, the transformation of denitrific sulfate constituents released to the environment. The reduction, an aerobic exertion of BOD and NOD is the most common fermentation (in bottom example of bacter ial conversion encountered in sediments), conversion of waterquality management. The depletion of oxygen priority in the aerob ic conversion of organic wastes is also pollutants, etc. n as deoxygenation. Solids discharged with treated wastewater are partly organic. Upon settling anaerobically or aerobically, depending on local conditions. The bacter ial transformation of toxic Chemical reactions Important chemical reactions that occur in the Chemical dis infection, environment include hydrolysis, photochemical,I decomposition of organic and oxidation-reduction reactions. Hydrolysis compounds, specific io reactions occur between contaminants and water. exchange element4-9 be an important consideration in the design of the treatment facilities in which these reactions will be carried out. Special attention must be given to the design of mixing facilities, especially for reactions that are rapid. Heterogeneous Reactions. Heterogeneous reactions occur between one or more constituents that can be identified with specific sites, such as those on an ion exchange resin in which one or more ions is replaced by another ion. Reactions that require the presence of a solid-phase catalyst are also classified as heterogeneous. Heterogeneous reactions are usually carried out in packed and fluidized-bed reactors (see Fig. 4-2f, g, and h). These reactions are more difficult to study because a number of interrelated steps may be involved, The typical sequence of these steps is as follows: 1. Transport of reactants from the bulk fluid to the fluid-solid interface (external surface of catalyst particle) 2. Intraparticle transport of reactants into the catalyst particle (if it is porous) 3. Adsorption of reactants at interior sites of the catalyst particle 4. Chemical reaction of adsorbed reactants to adsorbed products (surface reaction) 5. Desorption of adsorbed products 6. Transport of products from the interior sites to the outer surface of the catalyst particle Rate Expressions Used in Environmental Modeling The physical, chemical, and biological processes that control the fate of the constituents dispersed to the environment are numerous and varied. Important constituent transformations and removal processes (i.e., fate processes) operative in the environment, along with the constituents affected, are reported in Table 4-2. Because all of the processes summarized in Table 4-2 are rata-dependent, representative rate expressions used to model these processes are presented in Table 4-3. The important thing to note about Table 4-2 is the variety of different rate expressions that have been used to model constituent transformation and removal processes. Tab. 4-2 Constituent transformation and removal processes(i.e., fate processes) in the environment Process Comments Constituents affected Adsorption/ desorption Many chemical constituents tend to attach or sorb onto solids. The implication for wastewater discharges is that a substantial fraction of some toxic chemicals is associated with the suspended solids in the effluent. Adsorption combined with solids settling results in the removal from the water column of constituents that might not otherwise decay Metal, trace organics, NH4 + , PO4 3- Algal synthesis The synthesis of algal cell tissue using the nutrients found in wastewater NH4 + ,NO3 - ,PO4 3- ,pH, etc Bacterial conversion Bacterial conversion (both aerobic and anaerobic) is the most important process in the transformation of constituents released to the environment. The exertion of BOD and NOD is the most common example of bacterial conversion encountered in waterquality management. The depletion of oxygen in the aerobic conversion of organic wastes is also known as deoxygenation. Solids discharged with treated wastewater are partly organic. Upon settling to the bottom, they decompose bacterially either anaerobically or aerobically, depending on local conditions. The bacterial transformation of toxic organic compounds is also of great significance. BOD5,nitrification, denitrification, sulfate reduction, anaerobic fermentation (in bottom sediments), conversion of priority organic pollutants,etc. Chemical reactions Important chemical reactions that occur in the environment include hydrolysis, photochemical, and oxidation-reduction reactions. Hydrolysis reactions occur between contaminants and water. Chemical disinfection, decomposition of organic compounds, specific ion exchange, element substitution