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 4-1
4-1 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
Wastewater treatment involving physical unit operations and chemical and biological unit processes is carried out in vessels or tanks commonly known as"reactors Types of Reactors The principal types of reactors used for the are(1) the batch reactor(2) the complete-mix reactor(also known as the continuous-flow stirred-tank reactor(CFSTR) reactor(also known as a tubular-flow reacto (4) complete-mix reactors in series. (5) the acked-bed reactor and(6)the fluidized-bed Batch Reactor. In the batch reactor. flow is flow enters. is treated. and then is discharged the reactor are mixed completely. For example the bod test is carried out in a batch reactor although it should be noted that the contents are not mixed completely during the Uflow incubation period. Batch r ften chemicals Packing matera Fig. 4-2 Definition sketch for Outfiow ofreactors used for wastewater treatment (a) batch reactor; (b)complete-mix reactor (c]plug-flow open reactor, (dplug-flow closed reactor(tubular reactor); (e)complete-mix reactor in series. o) packed-bed reactor ( g)paked-bed upflow reactor; (h)expanded-bed upflow reactor Complete-Mix Reactor. In the complete-mix reactor, it is assumed that complete mixing occurs instantaneously and uniformly throughout the reactor as fluid particles enter the reactor. Fl leave the reactor in proportion to their statistical population. Complete mixi round or square reactors if the contents of the reactor are uniformly and continuously redistributed. The actual time required to achieve completely mixed conditions will depend on the reactor geometry and the Plug-Flow Reactor Fluid particles pass through the reactor with little or no longitudinal mixing and ch they entered The particles retain their identity and remain in the reactor for a time equal to the theoretical detention time. This type of flow is approximated in long open tanks with a high length-to-width ratio in which longitudinal dispersion is minimal or absent or closed tubular reactors(e.g, pipelines Complete -Mix Reactors in Series. The series of complete- mix reactors is used to model the flow regime that exists between the ideal hydraulic flow patterns corresponding to the complete-mix and lug-flow reactors. If the series is composed of one reactor. the complete-mix regime prevails. If the series consists of an infinite number of reactors in series. the plug-flow regime prevails. Packed-Bed Reactors. The packed-bed reactor is filled with some type of packing material, such as rock, slag, ceramic, or, now more commonly, plastic. With respect to flow, the packed-bed reactor can be operated in either the downflow or upflow mode, Dosing can be continuous or intermittent(e. g. trickling filter). The packing material in packed-bed reactors can be continuous or arranged in multiple stages. with Fluidized-Bed Reactor. The fluidized-bed reactor is similar to the packed-bed reactor in many respects but the packing material is expanded by the upward movement of fluid (air or water) through the bed. The expanded porosity of the fluidized-bed packing material can be varied by controlling the flowrate of the fuid Application of re The principal applications of reactor types used for wastewater treatment are reported in Table 4-1
4-2 Wastewater treatment involving physical unit operations and chemical and biological unit processes is carried out in vessels or tanks commonly known as "reactors." Types of Reactors The principal types of reactors used for the treatment of wastewater, illustrated on Fig. 4-2, are (1) the batch reactor, (2) the complete-mix reactor (also known as the continuous-flow stirred-tank reactor (CFSTR) in the chemical engineering literature), (3) the plug-flow reactor (also known as a tubular-flow reactor), (4) complete-mix reactors in series, (5) the packed-bed reactor, and (6) the fluidized-bed reactor. Batch Reactor. In the batch reactor, flow is neither entering nor leaving the reactor (i.e, flow enters, is treated, and then is discharged, and the cycle repeats). The liquid contents of the reactor are mixed completely. For example, the BOD test is carried out in a batch reactor, although it should be noted that the contents are not mixed completely during the incubation period. Batch reactors are often used to blend chemicals or to dilute concentrated chemicals. Fig. 4-2 Definition sketch for various types of reactors used for wastewater treatment (a) batch reactor;(b)complete-mix reactor;(c)plug-flow open reactor; (d)plug-flow closed reactor(tubular reactor);(e)complete-mix reactor in series; (f)packed-bed reactor;(g)paked-bed upflow reactor;(h)expanded-bed upflow reactor Complete-Mix Reactor. In the complete-mix reactor, it is assumed that complete mixing occurs instantaneously and uniformly throughout the reactor as fluid particles enter the reactor. Fluid particles leave the reactor in proportion to their statistical population. Complete mixing can be accomplished in round or square reactors if the contents of the reactor are uniformly and continuously redistributed. The actual time required to achieve completely mixed conditions will depend on the reactor geometry and the power input. Plug-Flow Reactor. Fluid particles pass through the reactor with little or no longitudinal mixing and exit from the reactor in the same sequence in which they entered. The particles retain their identity and remain in the reactor for a time equal to the theoretical detention time. This type of flow is approximated in long open tanks with a high length-to-width ratio in which longitudinal dispersion is minimal or absent or closed tubular reactors (e.g., pipelines). Complete-Mix Reactors in Series. The series of complete-mix reactors is used to model the flow regime that exists between the ideal hydraulic flow patterns corresponding to the complete-mix and plug-flow reactors. If the series is composed of one reactor, the complete-mix regime prevails. If the series consists of an infinite number of reactors in series, the plug-flow regime prevails. Packed-Bed Reactors. The packed-bed reactor is filled with some type of packing material, such as rock, slag, ceramic, or, now more commonly, plastic. With respect to flow, the packed-bed reactor can be operated in either the downflow or upflow mode. Dosing can be continuous or intermittent (e.g., trickling filter). The packing material in packed-bed reactors can be continuous or arranged in multiple stages, with flow from one stage to another. Fluidized-Bed Reactor. The fluidized-bed reactor is similar to the packed-bed reactor in many respects, but the packing material is expanded by the upward movement of fluid (air or water) through the bed. The expanded porosity of the fluidized-bed packing material can be varied by controlling the flowrate of the fluid. Application of Reactors The principal applications of reactor types used for wastewater treatment are reported in Table 4-1
Tab. 4- Principal applications ofreactor types used for wastewater treatment ted-sludge biological treatment in a sequence batch reactor, mixing of orking solution ated lagoons, aerobic sludge digestions C Activated-sludge biological treatment g-flow reactors Packed-bed submerged and submerged trickling-filter biolog ical treatment units, depth atural treatment systems, air stripping Fluid ized-bed luidized-bed reactors for aerobic and anaerob ic biological treatment, upflow ludge blanket reactors, air stri Operational factors that must be considered in the selection of the type of reactor or reactors to be used in the treatment process include(1) the nature of the wastewater to be treated. (2)the nature of the reaction ocess.( 4) the rocess performance requirements. and(5) local environmental conditions. In practice, the construction costs and operation and maintenance costs also affect reactor selection. Because the relative importance of these factors varies with each factor should be considered separately when the type of reactor is to be selected Hydraulic Characteristics of Reactors Complete-mix and plug-flow reactors are the two reactor types used most commonly in the field of wastewater treatment. The hydraulic flow characteristics of complete-mix and plug-flow reactors can be described as varying from ideal and nonideal, depending on the relationship of the incoming flow to outgoing flow Ideal Flow in Complete-Mix and Plug-Flow Reactors. The ideal hydraulic flow characteristics of complete-mix and plug-flow reactors are illustrated on Fig. 4-3 in which dve tracer response craves are presented for pulse (slug-dose) and step inputs(continuous iniection). On Fig. 4-3, t is the actual time and t is equal to the theoretical hydraulic detention time defined as follows where t=hydraulic detention time, T V=volume of the reactor. L: Q=volumetric flowrate, LT-I If a pulse(slug) input of a conservative (i.e, nonreactive) tracer is injected and dispersed instantaneously in an ideal-flow complete-mix reactor, with a continuous inflow of clear water, the output tracer concentration would appear as shown on Fig. 4-3 (a-1)If a continuous step input of a conservative tracer at concentration Co is injected into the inlet of an ideal complete-mix reactor, initially filled with clear water, the appearance of the tracer at the outlet would occur as shown on Fig 4-3(a-2 In the case of an ideal plug-flow reactor, the reactor is initially filled with clear water before being deal,ebr subjected to a pulse or a step input of tracer. If an the appearance of the tracer in the effluent for a pulse input, distributed uniformly across the reactor cross section, would occur as shown on Fig. 4-3(b-1). If a continuous step input of a tracer were injected into such a reactor at an initial concentration Co. the tracer would appear in the effluent as shown on Fig 4-3(b-2) Fig. subject to pulse and step inputs ofatrmcer 4-3
4-3 Tab. 4-1 Principal applications of reactor types used for wastewater treatment Type of reactor Application in wastewater treatment Batch Activated-sludge biological treatment in a sequence batch reactor, mixing of concentrated solutions into working solutions Complete-mix Aerated lagoons, aerobic sludge digestions Complete-mix with recycle Activated-sludge biological treatment Plug-flow Chlorine contact basin, natural treatment systems Plug-flow with recycle Activated-sludge biological treatment, aquatic treatment systems Complete-mix reactors in series Lagoon treatment systems, used to simulate nonideal flow in plug-flow reactors Packed-bed Nonsubmerged and submerged trickling-filter biological treatment units, depth filtration, natural treatment systems, air stripping Fluidized-bed Fluidized-bed reactors for aerobic and anaerobic biological treatment, upflow sludge blanket reactors, air stripping Operational factors that must be considered in the selection of the type of reactor or reactors to be used in the treatment process include (1) the nature of the wastewater to be treated, (2) the nature of the reaction (i.e., homogeneous or heterogeneous), (3) the reaction kinetics governing the treatment process, (4) the process performance requirements, and (5) local environmental conditions. In practice, the construction costs and operation and maintenance costs also affect reactor selection. Because the relative importance of these factors varies with each factor should be considered separately when the type of reactor is to be selected. Hydraulic Characteristics of Reactors Complete-mix and plug-flow reactors are the two reactor types used most commonly in the field of wastewater treatment. The hydraulic flow characteristics of complete-mix and plug-flow reactors can be described as varying from ideal and nonideal, depending on the relationship of the incoming flow to outgoing flow. Ideal Flow in Complete-Mix and Plug-Flow Reactors. The ideal hydraulic flow characteristics of complete-mix and plug-flow reactors are illustrated on Fig. 4-3 in which dye tracer response craves are presented for pulse (slug-dose) and step inputs (continuous injection). On Fig. 4-3, t is the actual time and τ is equal to the theoretical hydraulic detention time defined as follows: τ= V/Q where τ = hydraulic detention time, T V = volume of the reactor, L3 Q = volumetric flowrate, L3T -1 If a pulse (slug) input of a conservative (i.e., nonreactive) tracer is injected and dispersed instantaneously in an ideal-flow complete-mix reactor, with a continuous inflow of clear water, the output tracer concentration would appear as shown on Fig. 4-3 (a-1) If a continuous step input of a conservative tracer at concentration Co is injected into the inlet of an ideal complete-mix reactor, initially filled with clear water, the appearance of the tracer at the outlet would occur as shown on Fig. 4-3(a-2). In the case of an ideal plug-flow reactor, the reactor is initially filled with clear water before being subjected to a pulse or a step input of tracer. If an observer were positioned at the outlet of the reactor, the appearance of the tracer in the effluent for a pulse input, distributed uniformly across the reactor cross section, would occur as shown on Fig. 4-3(b-1). If a continuous step input of a tracer were injected into such a reactor at an initial concentration Co, the tracer would appear in the effluent as shown on Fig. 4-3(b-2). Fig. 4-3 Output tracer response curves from reactors subject to pulse and step inputs of a tracer (a)complete-mix reactor; (2)plug-flow reactor
Nonideal Flow in Complete-Mix and Plug-Flow Reactors. In practice the flow in complete-mix and olug- flow reactors is seldom ideal. For example, when a reactor is designed, how is the flow to be introduced to satisfy the theoretical requirement of instantaneous and complete dispersion? In practice, there is al ways some deviation from ideal conditions, and it is the precautions taken to minimize these effects that are important Nonideal flow occurs when a portion of the flow that enters the reactor during a given time period arrives at the outlet before the bulk of the flow that entered the reactor during the same time period arrives. Nonideal flow is illustrated on Fig 4-3a and 4-3b. The important issue with nonideal flow is that a portion of the flow will not remain in the reactor as long as may be required for a biological or chemical reaction to go to completion 4-2 Mass-balance Analysis The fundamental approach used to study the hydraulic flow characteristics of reactors and to delineate the changes that take place when a reaction is occurring in a reactor(eg, a container), or in some definable portion of a body of liquid, is the mass-balance analysis Inflow Outflow Q C ystem boundary amass balance Fig 4-4 Definition sketch for the balanc jsis for nix reactor with o are mixed completely The ph pical commplefe-mix actinated shudge reactor used for the biological treatment of wastewater The Mass-Balance Principle form of the mass can be altered (e. g. liquid to a gas). The mass-balance analysis affords a convenient way of defining what occurs within treatment reactors as a function of time. To illustrate the basic concepts involved in the preparation of a mass-balance analysis, consider the reactor shown on Fig 4-4. The system boundary is drawn to identify all of the liquid and constituent flows into and out of the system. The control volume is used to identify the actual volume in which change is occurring In most cases, the system and control volume boundaries will coincide. For a given reactant. the general mass-balance analysis is given by 1. General word statement Rate of accumulation Rate of flow of of reactant within reactant out of the tant within the e system boundary ystem boundary ystem boundary 2. The corresponding simplified word statement is Accumulation= inflow- outflow t generation (4) The mass balance is made up of the four terms cited above. Depending on the flow regime or treatment process, one or more of the terms can be equal to zero. For example, in a batch reactor in which there is no inflow or outflow the second and third terms will be equal to zero. A positive sign is used for the ecause the necessary (e.g. re=-kC for a decrease in the reactant or r.=+ kc for all increase in the reactant). Preparation of Mass Balances In preparing mass balances it is helpful if the following steps are followed, especially as the techniques involved are being mastered
4-4 Nonideal Flow in Complete-Mix and Plug-Flow Reactors. In practice the flow in complete-mix and plug-flow reactors is seldom ideal. For example, when a reactor is designed, how is the flow to be introduced to satisfy the theoretical requirement of instantaneous and complete dispersion? In practice, there is always some deviation from ideal conditions, and it is the precautions taken to minimize these effects that are important. Nonideal flow occurs when a portion of the flow that enters the reactor during a given time period arrives at the outlet before the bulk of the flow that entered the reactor during the same time period arrives. Nonideal flow is illustrated on Fig.4-3a and 4-3b. The important issue with nonideal flow is that a portion of the flow will not remain in the reactor as long as may be required for a biological or chemical reaction to go to completion. 4-2 Mass-balance Analysis The fundamental approach used to study the hydraulic flow characteristics of reactors and to delineate the changes that take place when a reaction is occurring in a reactor (e.g., a container), or in some definable portion of a body of liquid, is the mass-balance analysis. Fig. 4-4 Definition sketch for the application of materials mass-balance analysis for a complete-mix reactor with inflow and outflow. The presence of a mixer is used to represent symbolically the fact the contents of the reactor are mixed completely. The photo is of a typical complete-mix activated sludge reactor used for the biological treatment of wastewater. The Mass-Balance Principle The mass-balance analysis is based on the principle that mass is neither created nor destroyed, but the form of the mass can be altered (e.g., liquid to a gas). The mass-balance analysis affords a convenient way of defining what occurs within treatment reactors as a function of time. To illustrate the basic concepts involved in the preparation of a mass-balance analysis, consider the reactor shown on Fig. 4-4. The system boundary is drawn to identify all of the liquid and constituent flows into and out of the system. The control volume is used to identify the actual volume in which change is occurring. In most cases, the system and control volume boundaries will coincide. For a given reactant, the general mass-balance analysis is given by 1. General word statement: ＝ － ＋ 2. The corresponding simplified word statement is Accumulation ＝ inflow － outflow ＋ generation (1) (2) (3) (4) The mass balance is made up of the four terms cited above. Depending on the flow regime or treatment process, one or more of the terms can be equal to zero. For example, in a batch reactor in which there is no inflow or outflow the second and third terms will be equal to zero. A positive sign is used for the rate-of-generation term because the necessary sign for the operative process is past of the rate expression (e.g., rc = -kC for a decrease in the reactant or rc = + kC for all increase in the reactant). Preparation of Mass Balances In preparing mass balances it is helpful if the following steps are followed, especially as the techniques involved are being mastered. Rate of accumulation of reactant within the system boundary (1) Rate of flow of reactant into the system boundary (2) Rate of flow of reactant out of the system boundary (3) Rate of generation of reactant within the system boundary (4)
1. Prepare a simplified schematic or flow diagram of the system or process for which the mass balance is to be prepared 2. Draw a system or control volume boundary to de its over which mass balance is to be extremely import e mass List all of the pertinent data and assumptions that will be used in the preparation of the materials balance on the schematic or flow diagram 4. List all of the rate expressions for the biological or chemical reactions that occur within the control volume 5. Select a convenient basis on which the numerical calculations will be based It is recommended that the above steps be fol slowed routinely. to avoid the errors that are often made in the preparation of mass-balance analyses Application of the Mass-Balance Analysis To illustrate the application of the mass-balance analysis, consider the complete-mix reactor shown on Fig 4-4. First, the control volume boundary must be established so that all the flows of mass into and out of the system can be identified. On Fig 4-4a, the control volume boundary is shown by the inner dashed line To apply a mass-balance analysis to the liquid contents of the reactor shown on Fig 4-4. it will be volumetric flowrate into and out of the control volume is constant 2. The liquid within the control volume is not subiect to evaporation(constant volume The liquid within the control volume is mixed completely. 4.A chemical reaction i a reactant a is occurring within the reacte The rate of change in the concentration of the reactant a that is occurring within the control volume is overned by a first-order reaction(rc=-KC). Using the above assumptions, the mass balance can be formulated as follows 1. Simplified word statement Accumulation=[nflow-5utflow+ generation 2. Symbolic representation(refer to Fig. 4-4) y=C -kC+ry Substituting-kC for r'eyields =C-4C+(-kC where dC/dt=rate of change of reactant concentration within the control volume, MLT V= volume contained within control volume L3 0=volumetric flowrate into and out of control volume L'T-l Co=concentration of mactunt entering the control volume ML-3 C=concentration of reactant leaving the control volume ML-3 rc= first-order reaction, (-kC). ML-ST k=first-order reaction rate coefficient, T-I Before attempting to solve any mass-balance expression, a unit check should always be made to assure that units of the individual quantities are consistent. If the following units are substituted into the above. dC/dt=g/' / Co, C=g/ V =QCo -QC +(-kC)V (g/n.)m'=m /s(g/m)-m'(g/m)+(-1/s)(8/m)m
4-5 1. Prepare a simplified schematic or flow diagram of the system or process for which the mass balance is to be prepared. 2. Draw a system or control volume boundary to define the limits over which mass balance is to be applied. Proper selection of the system or control volume boundary is extremely important because, in many situations, it may be possible to simplify the mass-balance computations. 3. List all of the pertinent data and assumptions that will be used in the preparation of the materials balance on the schematic or flow diagram. 4. List all of the rate expressions for the biological or chemical reactions that occur within the control volume. 5. Select a convenient basis on which the numerical calculations will be based. It is recommended that the above steps be followed routinely, to avoid the errors that are often made in the preparation of mass-balance analyses. Application of the Mass-Balance Analysis To illustrate the application of the mass-balance analysis, consider the complete-mix reactor shown on Fig. 4-4. First, the control volume boundary must be established so that all the flows of mass into and out of the system can be identified. On Fig. 4-4a, the control volume boundary is shown by the inner dashed line. To apply a mass-balance analysis to the liquid contents of the reactor shown on Fig. 4-4, it will be assumed that: 1. The volumetric flowrate into and out of the control volume is constant. 2. The liquid within the control volume is not subject to evaporation (constant volume). 3. The liquid within the control volume is mixed completely. 4. A chemical reaction involving a reactant A is occurring within the reactor. 5. The rate of change in the concentration of the reactant A that is occurring within the control volume is governed by a first-order reaction (rc = -kC ). Using the above assumptions, the mass balance can be formulated as follows: 1. Simplified word statement: Accumulation ＝ inflow － outflow ＋ generation 2. Symbolic representation (refer to Fig. 4-4): c dC V QC QC rV dt = − + 0 Substituting -kC for rc yields 0 ( ) dC V QC QC kC V dt = − + − where dC/dt = rate of change of reactant concentration within the control volume, ML-3T -1 V = volume contained within control volume, L3 Q = volumetric flowrate into and out of control volume, L3T -1 Co = concentration of mactunt entering the control volume, ML-3 C = concentration of reactant leaving the control volume. ML-3 rc = first-order reaction, (-kC). ML-3T -1 k = first-order reaction rate coefficient, T-1 Before attempting to solve any mass-balance expression, a unit check should always be made to assure that units of the individual quantities are consistent. If the following units are substituted into the above equations: V=m3 dC/dt = g/m3·s Q = m3 /s Co, C = g/m3 k = 1/s the resulting unit check yields 0 ( ) dC V QC QC kC V dt = − + − (g/m3·s)m3=m3 /s(g/m3 )-m 3 (g/m3 )+(-1/s)(g/m3 )m3
g/s=g/s-g/s-g/s(units are consistent) The analytical procedures that are adopted for the solution of mass-balance equations usually are governed by(1)the nature of the rate expression, (2)the type of reactor under consideration, (3)the mathematical form of the final materials-balance expression (i.e, ordinary or partial differential equation), and(4)the corresponding boundary conditions Steady-State Simplificati Fortunately in most applications in the field of wastewater treatment. the solution of mass-balance that the steady-statelie. long-term) concentration is of principal concern. If it is assumed that only the steady-state effluent concentration is desired, then above equation can be simplified by noting that, under steady-state conditions. the rate accumulation is zero(dC/di=0). Thus, the equatin can be written as When solved for rc, the equation yields the following expression Q The solution to the expression given by the equation will depend on the nature of the rate expression(e.g 4-3 Analysis of Nonideal Flow in Reactors Using Tracers The discussion of nonideal flow in this section will serve as an introduction to the modeling of nonideal flow considered in the following section. Attention is called to this subject because often it is neglected o not considered properly. Because of a lack of appreciation for the hydraulics of reactors. many of the. treatment plants that have been built do not perform hydraulically as designed. Factors Leading to nonideal flow in reactors As noted previously, nonideal flow is often defined as short circuiting that occurs when a portion of the flow that enters the reactor during a given time period arrives at the outlet before the bulk of the flow that entered the reactor during the same time period arrives. Factors leading to nonideal flow in reactors include ature differences. In flow reactors, nonideal flow( short rents due to temperature differenc colde or warmer than the water in the tank a portion of the water can travel to the outlet along the bottom of across the top of the reactor without mixing completely(see Fig 4-5a). 2. Wind-driven circulation patterns. In shallow reactors. wind-circulation patterns can be set ort a portion of the incoming water to the outlet in a ion of the actual detention time(see Fig 4-5 3. Inadequate mixing. Without sufficient energy input, portions of the reactor contents may not mix with the incoming water(see Fig. 4-5c). 4 Poor d the design of the inlet and outlet of the reactor relative to the reactor aspe ratio, dead zones may develop within the reactor that will not mix with the incoming water(see Fig. 4-5d 5. Axial dispersion in plug-flow reactors. In plug-flow reactors the forward movement of the tracer is due to advection and dispersion. Advection is the term used to describe the movement of dissolved or colloidal material with the current velocity. Dispersion is the term used to describe the axial and longitudinal transport of material brought about by velocity differences, turbulent eddies. and molecular diffusion. The distinction between molecular diffusion, turbulent diffusion, and dispersion is considered in the notion e subsequent discussion dealing with"Modeling Nonideal Flow In reactors. "In a tubular plug-flow reactor(.g, a pipeline), the early arrival of the tracer at the outlet can be reasoned partially by remembering that the velocity distribution in the pipeline will be parabolic 八人 Ultimately, the inefficient use of the reactor
4-6 g/s=g/s-g/s-g/s(units are consistent) The analytical procedures that are adopted for the solution of mass-balance equations usually are governed by (1) the nature of the rate expression, (2) the type of reactor under consideration, (3) the mathematical form of the final materials-balance expression (i.e., ordinary or partial differential equation), and (4) the corresponding boundary conditions. Steady-State Simplification Fortunately, in most applications in the field of wastewater treatment, the solution of mass-balance equations, such as the one given by the equations, can be simplified by noting that the steady-state(i.e., long-term) concentration is of principal concern. If it is assumed that only the steady-state effluent concentration is desired, then above equation can be simplified by noting that, under steady-state conditions, the rate accumulation is zero (dC/dt = 0). Thus, the equatin can be written as c dC V QC QC rV dt = − + 0 When solved for rc, the equation yields the following expression: 0 ( ) c Q r C C V = − The solution to the expression given by the equation will depend on the nature of the rate expression (e.g., zero-, first-, or second-order). 4-3 Analysis of Nonideal Flow in Reactors Using Tracers The discussion of nonideal flow in this section will serve as an introduction to the modeling of nonideal flow considered in the following section. Attention is called to this subject because often it is neglected or not considered properly. Because of a lack of appreciation for the hydraulics of reactors, many of the treatment plants that have been built do not perform hydraulically as designed. Factors Leading to Nonideal Flow in Reactors As noted previously, nonideal flow is often defined as short circuiting that occurs when a portion of the flow that enters the reactor during a given time period arrives at the outlet before the bulk of the flow that entered the reactor during the same time period arrives. Factors leading to nonideal flow in reactors include: 1. Temperature differences. In complete-mix and plug-flow reactors, nonideal flow (short circuiting) can be caused by density currents due to temperature differences. When the water entering the reactor is colder or warmer than the water in the tank, a portion of the water can travel to the outlet along the bottom of or across the top of the reactor without mixing completely (see Fig. 4-5a). 2. Wind-driven circulation patterns. In shallow reactors, wind-circulation patterns can be set up that will transport a portion of the incoming water to the outlet in a fraction of the actual detention time (see Fig. 4-5b). 3. Inadequate mixing. Without sufficient energy input, portions of the reactor contents may not mix with the incoming water (see Fig. 4-5c). 4. Poor design. Depending on the design of the inlet and outlet of the reactor relative to the reactor aspect ratio, dead zones may develop within the reactor that will not mix with the incoming water (see Fig. 4-5d). 5. Axial dispersion in plug-flow reactors. In plug-flow reactors the forward movement of the tracer is due to advection and dispersion. Advection is the term used to describe the movement of dissolved or colloidal material with the current velocity. Dispersion is the term used to describe the axial and longitudinal transport of material brought about by velocity differences, turbulent eddies, and molecular diffusion. The distinction between molecular diffusion, turbulent diffusion, and dispersion is considered in the subsequent discussion dealing with"Modeling Nonideal Flow In Reactors." In a tubular plug-flow reactor (e.g., a pipeline), the early arrival of the tracer at the outlet can be reasoned partially by remembering that the velocity distribution in the pipeline will be parabolic. Ultimately, the inefficient use of the reactor
volume due to short circuiting resulting from temperature differences. the presence of dead zones resulting from poor design. inadequate mixing, and dispersion(see Fig 4-6) can result in reduced treatment rformance. Morrill examined the effects of short circuiting on the performance of sedimentation tanks Fig 4-5 Definition sketch for short circuiting caused by(a)density currents caused by temperature differences;(b)wind circulation patterns;(c)inadequate mixing;( fluid advection(via)and dispersion Need for Tracer Analysis One of the more important practical considerations involved in reactor design is how to achieve the ideal conditions postulated in the analysis of their performance. The use of dves and tracers for measuring the residence time distribution curves is one of the simplest and most successful methods now used to assess ce of full-scal tors.(2)the contact time in chlorine contact basins. 3) the assessment of the hydraulic approach co lIons Ir reactors and (4)the assessment of flow patterns in constructed wetlands and other natural treatment stems.Tracer studies are also of critical importance in assessing the degree of success that has been d with corrective measures Types of Tracers Over the vears. a number of tracers have been used to evaluate the hydraulic performance of reactors. Important characteristics for a tracer include: The tracer should not affect the flow (should have essentially the same densi when v The tracer must be conservative so that a mass balance can be performed. It must be possible to inject the tracer over a short ti The tracer should be able to be analyzed conveniently v The tracer should not be absorbed on or react with the exposed reactor surfaces. v The tracer should not be absorbed on or react with the particles in wastewater. Dves and chemicals that have been used successfully in tracer studies include congo-red fluorescein fluorosilicic acid(H2SiF6), hexafluoride gas( SF6). lithium chloride( LiCD. Pontacyl Brilliant Pink B. tassium, potassium p anate rhodam T, and sodium chloride(NaCD. Pontacyl Brilliant Pink B(the acid form of rhodamine WD) is especially useful in the not readily adsorbed onto surfaces. Because fluorescein. rhodamine WT. and Pontacyl Brilliant Pink B can be detected at very low concentrations using a fluorometer, they are the dye tracers used most commonl in the evaluation of wastewater-treatment facilities. Lithium chloride is commonly used for the study of dium chloride. used commonly in the past. has a tendency to form density currents less mixed Hexafluoride gas(SF6) is used most commonly for tracing the movement of groundwater Conduct of Tracer Tests In tracer studies, typically a tracer (i.e, a dye, most commonly) is introduced into the influent end of the Tracer response reactor or basin to be studied The time of its arrival at the effluent end is determined by collecting a series of grab samples for a given Mixer to period of time or by measuring the arrival of a Uy banks ELuent weir The method used to introduce the tracer willo tracer using instrumental methods(see Fig. 4-6) control the type of response observed at the downstream end. Two types of dye input are used, the choice depending on the influent and effluent configurat Stacked plan view of Fig 4-6 Schematic of setup used to control of tracer using positive curve for continuous nput tracer studies of plug-flow reactor (a)slug of tracers added to flow; (b)continuous input of tracer added to flow. Tracer response curve is measured continuousl The first method involves the injection of a quantity of dye(sometimes referred to as a pulse or slug of dye)over a short period of time. Initial mixing is usually accomplished with a static mixer or an auxiliary mixer. With the slug injection method it is important to keep the initial mixing time short relative to the 4-7
4-7 volume due to short circuiting resulting from temperature differences, the presence of dead zones resulting from poor design, inadequate mixing, and dispersion (see Fig. 4-6) can result in reduced treatment performance. Morrill examined the effects of short circuiting on the performance of sedimentation tanks. Fig. 4-5 Definition sketch for short circuiting caused by (a)density currents caused by temperature differences; (b)wind circulation patterns; (c)inadequate mixing; (d)fluid advection(平流) and dispersion Need for Tracer Analysis One of the more important practical considerations involved in reactor design is how to achieve the ideal conditions postulated in the analysis of their performance. The use of dyes and tracers for measuring the residence time distribution curves is one of the simplest and most successful methods now used to assess the hydraulic performance of full-scale reactors. Important applications of tracer studies include (1) the assessment of short circuiting in sedimentation tanks and biological reactors, (2) the assessment of the contact time in chlorine contact basins, (3) the assessment of the hydraulic approach conditions in UV reactors, and (4) the assessment of flow patterns in constructed wetlands and other natural treatment systems. Tracer studies are also of critical importance in assessing the degree of success that has been achieved with corrective measures. Types of Tracers Over the years, a number of tracers have been used to evaluate the hydraulic performance of reactors. Important characteristics for a tracer include : ✓ The tracer should not affect the flow (should have essentially the same density as water when diluted). ✓ The tracer must be conservative so that a mass balance can be performed. ✓ It must be possible to inject the tracer over a short time period. ✓ The tracer should be able to be analyzed conveniently. ✓ The molecular diffusivity of the tracer should be low. ✓ The tracer should not be absorbed on or react with the exposed reactor surfaces. ✓ The tracer should not be absorbed on or react with the particles in wastewater. Dyes and chemicals that have been used successfully in tracer studies include congo-red, fluorescein, fluorosilicic acid (H2SiF6), hexafluoride gas (SF6), lithium chloride (LiCl), Pontacyl Brilliant Pink B, potassium, potassium permanganate, rhodamine WT, and sodium chloride (NaCl). Pontacyl Brilliant Pink B (the acid form of rhodamine WT) is especially useful in the conduct of dispersion studies because it is not readily adsorbed onto surfaces. Because fluorescein, rhodamine WT, and Pontacyl Brilliant Pink B can be detected at very low concentrations using a fluorometer , they are the dye tracers used most commonly in the evaluation of wastewater-treatment facilities. Lithium chloride is commonly used for the study of natural systems. Sodium chloride, used commonly in the past, has a tendency to form density currents unless mixed. Hexafluoride gas (SF6) is used most commonly for tracing the movement of groundwater. Conduct of Tracer Tests In tracer studies, typically a tracer (i.e., a dye, most commonly) is introduced into the influent end of the reactor or basin to be studied. The time of its arrival at the effluent end is determined by collecting a series of grab samples for a given period of time or by measuring the arrival of a tracer using instrumental methods (see Fig. 4-6). The method used to introduce the tracer will control the type of response observed at the downstream end. Two types of dye input are used, the choice depending on the influent and effluent configurations. Fig 4-6 Schematic of setup used to control tracer studies of plug-flow reactors (a)slug of tracers added to flow; (b)continuous input of tracer added to flow. Tracer response curve is measured continuously. The first method involves the injection of a quantity of dye (sometimes referred to as a pulse or slug of dye) over a short period of time. Initial mixing is usually accomplished with a static mixer or an auxiliary mixer. With the slug injection method it is important to keep the initial mixing time short relative to the
detention time of the reactor being measured. The measured output is as described on Fig. 4-3(a-I and b-D) In the second method, a continuous step input of dye is introduced until the effluent concentration matches the influent concentration. The measured response is as shown on Fig. 4-3(a-2 and b-2). Another response curve can be measured after the dye injection has ceased and the dye in the reactor is flushed out Analysis of Tracer esponse Curves 10=48.7 min types are used Tracers of var 8000m3d 4000m3d commonly to assess the hydraulic performance of reactors used for Typical examples of tracer response curves are shown on Fig 4-8 ime, min Fig. 4-8 Typical tracer response curves: two different types of circular clarifiers and open channel Uv disinfection systen Tracer response curves, measured using a short-term and continuous injection of tracer, are known as C (concentration versus time)and F(fraction of tracer remaining in the reactor versus time)curves, respectively. The fraction remaining is based on the volume of water displaced from the reactor by the step Input of tracer. 4-4 Reactions, Reaction Rates, and Reaction Rate Coefficients From the standpoint of process selection and design, the controlling stoichiometry and the rate of the reaction are of principal concern. The number of moles of a substance entering into a reaction and the number of moles of the substances produced are defined by the stoichiometry of a reaction. The stoichiometry of reaction refers to the definition of the quantities of chemical compounds involved in a reaction. The rate at which a substance disappears or is formed in any given stoichiometry reaction is defined as the rate of reaction. The rate expressions discussed in this section will be integrated with the hydraulic characteristics of the reactors. discussed previously. to define treatment kinetics. Types of Reactions The two principal types of reactions that occur in wastewater treatment are classified as homogeneous and Homogeneous Reactions. In homogeneous reactions, the reactants are distributed uniformly throughout the fluid so that the potential for reaction at any point within the fluid is the same, Homogeneous reactions are usually carried out in the batch, complete-mix, and plug-flow reactors(see Figs. 4-2a, b, c, and d) Homogeneous reactions may be either irreversible or reversible Simple reactions b. Parallel reactions A+E c Conse reactions D Examples of reversible reactions are As will be discussed subsequently, for both irreversible and reversible reactions, the rate of reaction will 4-8
4-8 detention time of the reactor being measured. The measured output is as described on Fig. 4-3(a-1 and b-l). In the second method, a continuous step input of dye is introduced until the effluent concentration matches the influent concentration. The measured response is as shown on Fig. 4-3(a-2 and b-2). Another response curve can be measured after the dye injection has ceased and the dye in the reactor is flushed out. Analysis of Tracer Response Curves Tracers of various types are used commonly to assess the hydraulic performance of reactors used for wastewater treatment. Typical examples of tracer response curves are shown on Fig. 4-8. Fig. 4-8 Typical tracer response curves: two different types of circular clarifiers and open channel UV disinfection system Tracer response curves, measured using a short-term and continuous injection of tracer, are known as C (concentration versus time) and F (fraction of tracer remaining in the reactor versus time) curves, respectively. The fraction remaining is based on the volume of water displaced from the reactor by the step input of tracer. 4-4 Reactions, Reaction Rates, and Reaction Rate Coefficients From the standpoint of process selection and design, the controlling stoichiometry and the rate of the reaction are of principal concern. The number of moles of a substance entering into a reaction and the number of moles of the substances produced are defined by the stoichiometry of a reaction. The stoichiometry of reaction refers to the definition of the quantities of chemical compounds involved in a reaction. The rate at which a substance disappears or is formed in any given stoichiometry reaction is defined as the rate of reaction. The rate expressions discussed in this section will be integrated with the hydraulic characteristics of the reactors, discussed previously, to define treatment kinetics. Types of Reactions The two principal types of reactions that occur in wastewater treatment are classified as homogeneous and heterogeneous (non-homogeneous). Homogeneous Reactions. In homogeneous reactions, the reactants are distributed uniformly throughout the fluid so that the potential for reaction at any point within the fluid is the same, Homogeneous reactions are usually carried out in the batch, complete-mix, and plug-flow reactors (see Figs. 4-2a, b, c, and d). Homogeneous reactions may be either irreversible or reversible. Examples of irreversible reactions are a. Simple reactions A——>B A + A——> C aA + bB ——> C b. Parallel reactions A + B ——> C A + B ——>D c. Consecutive reactions A + B ——> C A + C——> D Examples of reversible reactions are A B A + B C + D As will be discussed subsequently, for both irreversible and reversible reactions, the rate of reaction will
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 particle 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, in the transformat ion of i 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 either anaerobically or aerobically, depending on local conditions. The bacter ial transformation of toxic Chemical reactions mportant chemical reactions that occur in the Chemical disinfection, environment include hydrolysis, photochemical, decomposition of organic and oxidation-reduction reactions. Hydrolysis compounds, specific ion reactions occur between contaminants and water. exchange element
4-9 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
Filtration Removal of suspended and colloid al solids by TSS, colloidal particles contact), sedimentation, interception, impaction, and Flocculation Flocculation is the term used to describe the Colloidal and aggregation of smaller particles into larger particles particles that can be removed by sed imen tation and filtration. Flocculation is brought about by Brownian motion, differential velocity gradients, and differential settling in which large particles overtake smaller The process whereby a gas is taken up by a liquid is| O2, CO2, CH4, NH3, H2S absorption/desorptio known as absorption. For example, when the issolved oxygen concentration in a body of water with a free surface is below the saturation oncentration in the water, a net transfer of oxygen occurs from the atmosphere to the water. The rate of transfer(mass per unit time per unit surface area)is proportional to the amount by which the dissolved oxygen is below saturation. The addition of oxygen to water is also known as reaeration. Desorption occurs when the concentration of the gas in the liquid exceeds the saturation value, and there is a transfer from the liquid to the atmosphere Natural decay In nature, contaminants will decay for a variety of Plants, animals, algae, reasons, including mortality in the case of bacteria fungi, protozoa, eubacteria and photooxidation for certain organic constituents.(most Natural and radioactive decay usually follow archaebacteria, first-order kinetics radioactive substances Solar radiation Solar radiation is known to trigger a number of Oxidation of inorganic and chemical reactions. Radiation in the near-ultraviolet organic compounds UV and visible range is known to cause the During the day, algal cell in water bodies will Algae, produce oxygen by means of photosynthesis. submerged macrophy tes Dissolved oxygen concentrations as high as 30 toNHA, PO3,pH,etc 40 mg/L have been measured. During the evening hours algal respiration will consume oxygen. Where heavy growths of algae are present, oxygen depletion has been observer during the evening hours Distribution i The suspended solids discharged with treated Tss wastewater ultimately settle to the bottom of floccul ation and hindered by ambient turbulence In rivers and coas sufficient to distribute the suspended solids over the entire water 4-1
4-10 Filtration Removal of suspended and colloidal solids by straining (mechanical and chance contact), sedimentation, interception, impaction, and adsorption. TSS, colloidal particles Flocculation Flocculation is the term used to describe the aggregation of smaller particles into larger particles that can be removed by sedimentation and filtration. Flocculation is brought about by Brownian motion, differential velocity gradients, and differential settling in which large particles overtake smaller particles and form larger particles. Colloidal and small particles Gas absorption/desorptio n The process whereby a gas is taken up by a liquid is known as absorption. For example, when the dissolved oxygen concentration in a body of water with a free surface is below the saturation concentration in the water, a net transfer of oxygen occurs from the atmosphere to the water. The rate of transfer (mass per unit time per unit surface area) is proportional to the amount by which the dissolved oxygen is below saturation. The addition of oxygen to water is also known as reaeration. Desorption occurs when the concentration of the gas in the liquid exceeds the saturation value, and there is a transfer from the liquid to the atmosphere. O2,CO2,CH4,NH3,H2S Natural decay In nature, contaminants will decay for a variety of reasons, including mortality in the case of bacteria and photooxidation for certain organic constituents. Natural and radioactive decay usually follow first-order kinetics Plants, animals, algae, fungi, protozoa, eubacteria (most bacteria), archaebacteria, viruses, radioactive substances, plant mass Solar radiation Solar radiation is known to trigger a number of chemical reactions. Radiation in the near-ultraviolet (UV) and visible range is known to cause the breakdown of a variety of organic compounds. Oxidation of inorganic and organic compounds Photosynthesis During the day, algal cell in water bodies will produce oxygen by means of photosynthesis. Dissolved oxygen concentrations as high as 30 to 40 mg/L have been measured. During the evening hours algal respiration will consume oxygen. Where heavy growths of algae are present, oxygen depletion has been observer during the evening hours. Algae, duckweed, submerged macrophytes, NH4 + , PO3 3- , pH, etc Distribution The suspended solids discharged with treated wastewater ultimately settle to the bottom of flocculation and hindered by ambient turbulence. In rivers and coastal areas, turbulence is often sufficient to distribute the suspended solids over the entire water depth TSS