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 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 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. 4-1 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 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 General word statement: ＝ － ＋ 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. 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):  Substituting -kC for rc yields  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  (g/m3·s)m3=m3/s(g/m3)-m3(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 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  When solved for rc, the equation yields the following expression:  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 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-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: 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+, PO43-  Algal synthesis The synthesis of algal cell tissue using the nutrients found in wastewater NH4+,NO3-,PO43-,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 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/desorption 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+, PO33-, 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  Sedimentation The residual solids discharged with treated wastewater will, in time, settle to the bottom of streams and rivers. Because the particles are partly organic, they can be decomposed anaerobically as well as aerobically, depending on conditions. Algae which settle to the bottom will also be decomposed, but much more slowly. The oxygen consumed in the aerobic decomposition of material in the sediment represents another dissolved oxygen demand in the water body. O2, particulate BOD  Volatilization Volatilization is the process whereby liquids and solids vaporize and escape to the atmosphere. Organic compounds that readily volatilize are known as VOCs (volatile organic compounds). The physics of this phenomenon are very similar to gas absorption, except that the net flux is out of the water surface. VOCs,NH3,CH4,H2S, other gases  Tab. 4-3 Typical rate expressions for selected processes given in Tab. 4-2 Process Rate expression Comments  Bacterial conversion rc=-kC rc=rate of conversion,M/L3T k=first-order reaction rate coefficient,1/T C=concentration of organic material remaining,M/L3  Chemical reactions rc=±kCn rc= rate of conversion,M/L3T k= reaction rate coefficient,(M/L3)n-1/T C=concentration of constituent, (M/L3)n n=reaction order (e.g.,for second-order n=2)  Gas absorption/desorption rab=kabA(Cs-C)/V rde=kdeA(C-Cs)/V rab=rate of absorption, M/L3T rde= rate of desorption, M/L3T kab= coefficient of absorption,L/T kde= coefficient of desorption,L/T A=area,L2 V=volume,L3 Cs=saturation concentration of constituent in liquid, M/L3 C= concentration of constituent in liquid, M/L3  Natural decay rd=-kdN rd=rate of decay,no./T kd=first-order reaction rate coefficient,1/T N=amount of organisms remaining,no.  Sedimentation rs=Vs(SS)/H rs=rate of sedimentation, 1/T Vs=settle velocity,L/T H=depth,L SS=settleable solids,L3/L3  Volatilization rv=-kv(C-CS) rv=rate of volatilization per unit time per unit volume, M/L3T kv=volatilization constant,1/T C= concentration of constituent in liquid, M/L3 Cs=saturation concentration of constituent in liquid, M/L3   4-5 Treatment Processes Involving Mass Transfer In this section, a group of separation operations based on the transfer of material from one homogeneous phase to another is considered. The operations are designed to reduce the concentration of a given component in one stream and to increase the concentration in another stream. Unlike mechanical separations, the driving force for the transfer of material is a pressure or concentration gradient). The separation operations are sometimes identified as equilibrium phase separations or equilibrium contact separations, because the transfer of a component will cease when equilibrium conditions prevail. Common operations and processes in wastewater treatment involving mass transfer are identified in Table 4-4. The most important mass transfer operations in wastewater treatment involve (1) the transfer of material across gas-liquid interfaces as in aeration and in the removal of unwanted gaseous constituents found in wastewater by air stripping, and (2) the removal of unwanted constituents from wastewater by adsorption onto solid surfaces such as activated carbon and ion exchange. To introduce the concepts involved in mass transfer, the basic principle of mass transfer is reviewed followed by a consideration of gas-liquid and liquid-solid mass transfer operations. Tab. 4-4 Principal applications of mass transfer operations and processes in wastewater treatment Type of reactor Phase equilibria Application  Adsorption Gas→liquid Addition of gases to water(e.g., O2,O3,CO2,Cl2,SO2), NH3 srcubbing in acid  Adsorption Gas→solid Removal of organics with activated carbon  Desorption liquid→solid solid→liquid solid→gas Removal of organics with activated carbon, dechlorination Sediment scrubbing Reactivation of spent activated carbon  Drying(evaporation) liquid→gas Drying of sludges  Gas stripping(also known as desorption) liquid→gas Removal of gases(e.g., O2,CO2,H2S,NH3, volatile organic compounds, NH3 from digester supernatant)  Ion exchange liquid→solid Selective removal of chemical constituents, demineralization  Gas-Liquid Mass Transfer Over the past 50 years a number of mass transfer theories have been proposed to explain the mechanism of gas transfer across gas-liquid interfaces. The simplest and most commonly used is the two-film theory proposed by Lewis and Whitman (1924). The penetration model proposed by Higbie (1935) and the surface-renewal model proposed by Danckwerts (1951 ) are more theoretical and take into account more of the physical phenomena involved. The two-film theory remains popular because, in more than 95 percent of the situations encountered, the results obtained are essentially the same as those obtained with the more complex theories. Even in the 5 percent where there is some disagreement between the two-film theory and other theories, it is not clear which approach is more correct. The Two-Film Theory. The two-film theory is based on a physical model in which two films exist at the gas-liquid interface, as shown on Fig. 4-9. Two conditions are shown on Fig. 4-9: (a) "absorption," in which a gas is transferred from the gas phase to the liquid phase, and (b) "desorption," in which a gas is transferred out of the liquid phase into the gas phase. The two films, one liquid and one gas, provide the resistance to the passage of gas molecules between the bulk-liquid and the bulk-gaseous phases. It is very important to note that in the application of the two-film theory it is assumed that the concentration and partial pressure in both the bulk liquid and bulk-gas phase are uniform (i.e., mixed completely). Under steady-state conditions, the rate of mass transfer of a gas through the gas film must be equal to the rate transfer through the liquid film. Using Fick's first law, the mass flux for each phase for absorption (gas addition) is written as follows: r= kG(PG-Pi)=kL(Ci-CL) where r=rate of mass transferred per unit area per unit time kG =gas film mass transfer coefficient PG =partial pressure of constituent A in the bulk of the gas phase Pi =partial pressure of constituent A at the interface in equilibrium with concentration Ci of constituent A in liquid Fig. 4-9 Definition sketch for the two-film theory of gas transfer: (a)absorption;(b)desorption kL =liquid film mass transfer coefficient Ci =concentration of constituent A at the interface in equilibrium with partial pressure Pi of constituent A in the gas CL =concentration of constituent A in the bulk liquid phase It should be noted that the gas and liquid film mass transfer coefficients depend on the conditions at the interface. The terms (PG - Pi) and (Ci-CL) represent the driving force causing transfer in the gas and liquid phase, respectively. If the terms (PG-Pi) and (Ci – CL) are divided by their respective film thickness values (δG and δL), the driving force can be expressed in terms of unit thickness. Thus, the degree of mass transport can be enhanced by reducing the thickness of the film, depending on which is the controlling film. However, because it is difficult to measure the values of kG and kL at the interface it is common to use overall coefficients KGand KL, depending on whether the resistance to mass transfer is on the gas or liquid side. If it is assumed that all of the resistance to mass transfer is caused by the liquid film, then the rate of mass transfer can be defined as follows in terms of the overall liquid mass transfer coefficient: r= KL(CS-CL) where r = rate of mass transferred per unit area per unit time KL = overall liquid mass transfer coefficient Cs = concentration of constituent A at the interface in equilibrium with the partial pressure of constituent A in the bulk gas phase CL = concentration of constituent A in the bulk liquid phase If the two expressions given above are equated, the following relationship can be derived between the overall liquid mass transfer coefficient and the gas and liquid film coefficients: r= KL(CS-CL)=kG(PG-Pi)=kL(Ci-CL) Because it was assumed that all of the resistance to mass transfer is caused by the liquid film, the following relationships, based on Henry's law, must apply at the interface: PG = HCs and Pi = HCi It will now be noted that the overall driving force (Cs – CL) in the above equation can be written as (Cs - CL) = (Cs - Ci) + (Ci - CL) Absorption of Gases. The application of the gas-liquid mass transfer relation ship developed above will be illustrated by considering the absorption of a gas in a liquid (see Fig. 4-10a). Consider, for example, a storage basin open to the atmosphere with surface area A and depth h. If the concentration of dissolved oxygen in the basin is initially under-saturation, how long would it take for the oxygen concentration to increase by a given mount? The approach to this mass transfer problem can be outlined as follows. First, a mass balance is written for the open basin as follows: General word statement ＝ － ＋ 2. Simplified word statement Accumulation = Inflow - Outflow + Increase due to absorption 3. Symbolic representation at equilibrium dC/dt(V) = 0-0+rVV where dC/dt = change in concentration with time. ML-3T-1, (g/m3.s) V = volume in which constituent concentration is increasing, L3, (m3) rv = mass of constituent transferred per unit volume per unit time, ML-3T-1, (g/m3.s) Fig. 4-10 Definition sketch for the absorption of a gas: (1)under turbulent conditions of gas in the gaseous and liquid phases is uniform; (b)under quiescent conditions. Liquid-Solid Mass Transfer In the discussion of gas-liquid mass transfer, it was found that mass could be transferred from either phase to the other phase. In liquid-solid mass transfer operations, constituents from the liquid phase are transferred (adsorbed) to a solid phase. Adsorption and ion exchange mass transfer processes are introduced in the following discussion. Adsorption. The process of accumulating substances that are in solution on a suitable interface is termed adsorption. The adsorbate is the substance that is being removed from liquid phase at the interface. The adsorbent is the solid, liquid, or gas phase onto which the adsorbate accumulates. The adsorption process takes place in three steps: macrotransport, microtransport, and sorption. Macrotransport involves the movement of the adsorbate (e.g., organic matter) through the water to the liquid/solid interface by advection and diffusion. Microtransport involves the diffusion of the organic material through the macropore system of the solid adsorbent, such as activated carbon, to the adsorption sites in the micropores and the solid adsorbent (see Fig. 4-11). Although adsorption also occurs on the surface of the solid adsorbent and in the macropores and mesopores, the surface area of these parts of most solid adsorbents is so extremely small compared with the surface area of the micropores that the amount of material adsorbed there is usually considered negligible. Fig. 4-11 Macropore, mesopore, micropore, and submicropore adsorption sites on activated carbon Two important characteristics of the solid adsorbent are (1) its extremely large surface area to volume ratio and (2) its preferential affinity for certain constituents in the liquid phase. As noted above, adsorption is the process of accumulating substances that are in solution on a suitable interface. Granular or powdered activated carbon (GAC or PAC) is used most commonly for the removal of selected constituents from wastewater. The accumulation of material is described by what is known as an adsorption isotherm which is used to define the mass of material adsorbed per unit mass of adsorbing material. A common adsorption isotherm is the Freundlich isotherm. Derived from empirical considerations, the Freundlich isotherm is defined as follows:  4-6 Introduction to Process Selection Process selection involves the detailed evaluation of the various factors that must be considered when evaluating unit operations and processes and other treatment methods to meet current and future treatment objectives. The purpose of process analysis is to select the most suitable unit operations and processes and the optimum operational criteria. The purpose of this section is to introduce the important factors that must be considered in process selection and to consider the basis for process design. Fig. 4-12 Typical plot of a Freundlich adsorption isotherm Important Factors in Process Selection The most important factors that must be evaluated in process analysis and selection are identified in Table 4-5. Each factor is important in its own right, but some factors require additional attention and explanation. The first factor, "process applicability,” stands out above all others and reflects directly upon the skill and experience of the design engineer. Many resources are available to the designer to determine applicability, including past experience in similar types of projects. Available resources include performance data from operating installations, published information in technical journals, manuals of practice published by the Water Environment Federation, process design manuals published by EPA, and the results of pilot-plant studies. Where the applicability of a process to a given situation is unknown or uncertain, pilot-plant studies must be conducted to determine performance capabilities and to obtain design data upon which a full-scale design can be based. Some of the factors (items 2 through 6) listed in Table 4-5 have been discussed previously in Chaps. 2 and 3. The following discussion will deal briefly with process design based on reaction kinetics, mass transfer, and the use of loading criteria, the subjects considered in this chapter. As part of the discussion, the conduct of bench and pilot plant studies is considered along with process variability. The other factors in Table 4-5 will be discussed throughout the remainder of the book. They are identified here to indicate the diverse nature of the information that must be available to make a proper evaluation of unit operations and processes used for the treatment of wastewater. Tab. 4-5 Important factors that must be considered when evaluating and selecting unit operations and processes Factor Comment  1.Process applicability The applicability of a process is evaluated on the basis of past experience, data from full-scale plants, published data, and from pilot-plant studies. If new or unusual conditions are encountered, pilot-plant studies are essential.  2.Applicable flow range The process should be matched to the expected range of flowrate. For example, stabilization ponds are not suitable for extremely large flowrates in highly populated areas.  3.Applicable flow variation Most unit operations and processes have to be designed to operate over a wide range of flowrates. Most processes work best at a relatively constant flowrate. If the flow variation is too great, flow equalization may be necessary.  4.Influent wastewater characteristics The characteristics of the influent wastewater affect the types of processes to be used (e.g.,chemical or biological) and the requirements for their proper operation.  5.Inhibiting and unaffected constituents What constituents are present and may be inhibitory to the treatment processes? What constituents are not affected during treatment?  6.Climatic constraints Temperature affects the rate of reaction of most chemical and biological processes. Temperature may also affect the physical operation of the facilities. Temperatures may accelerate odor generation and also limit atmospheric dispersion.  7.Process sizing based on reaction linetics or process loading criteria Reactor sizing is based on the governing reaction kinetics and kinetic coefficients. If kinetic expressions are not available, process loading criteria are used. Data for kinetic expressions and process loading criteria usually are derived from experience, published literature, and the results of pilot-plant studies  8.Process sizing based on mass transfer rates or process loading criteria Reactor sizing is based on mass transfer coefficients, If mass transfer rates are not available, process loading criteria are used. Data for mass transfer coefficients and process loading criteria usually are derived from experience, published literature, and the results of pilot-plant studies.  9.Performance Performance is usually measured in terms of effluent quality and its variability, which must be consistent with the effluent discharge requirements  10.Treatment residuals The types and amounts of solid, liquid, and gaseous residuals produced must be known or estimated. Often, pilot-plant studies are used to identify and quantify residuals.  11.Sludge processing Are there any constraints that would make sludge processing and disposal infeasible or expensive? How might recycle loads from sludge processing affect the liquid unit operation or processes? The selection of the sludge processing system should go hand in hand with the selection of the liquid treatment system  12.Environmental constraints Environment factors, such as prevailing wind directions and proximity to residential areas, may restrict or affect the use of certain processes, especially where odors may be produced. Noise and traffic may affect selection of a plant site. Receiving waters may have special limitations, requiring the removal of specific constituents such as nutrients.  13.Chemical requirements What resources and what amounts must be committed for a long period of time for the successful operation of the unit operation or process? What effects might the addition of chemicals have on the characteristics of the treatment residuals and the cost of treatment?  14.Energy requirements The energy requirements, as well as probable future energy cost, must be known if cost-effective treatment systems are to be designed.   Process Selection Based on Reaction Kinetics In process selection and sizing based on reaction kinetics, particular emphasis is placed on deriding the nature of the reactions occurring within the process, the appropriate values of the kinetic coefficients, and the selection of the reactor type. Nature of the Kinetic Reactions. The nature of the reactions occurring within a process must be known to apply the reaction kinetics approach to design. Selection of reaction rate expressions for the process that is to be designed is typically based on (1) information obtained from the literature, (2) experience with the design and operation of similar systems, or (3) data derived from pilot-plant studies. For example, it is of critical importance to know if the reaction is zero-, first-, retarded first-, or second-order, or if the reaction is a saturation type. Clearly, as demonstrated in this chapter, the order of the reaction will have a significant effect on the type and size of the reactor. Selection of Appropriate Kinetic Rate Coefficients. Selection of appropriate kinetic rate coefficients for the process that is to be designed is also based on (1)information obtained from the literature, (2) experience with the design and operation of similar systems, or (3) data derived from pilot-plant studies. In cases where significantly different wastewater characteristics occur or new applications of existing technology or new processes are being considered, pilot-plant testing is recommended. Selection of Reactor Types. Operational factors that must be considered in 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 kinetics governing the treatment process, (3) special process requirements, and (4) local environmental conditions. As noted previously, for biological treatment with the activated sludge process there is no difference in the size of the reactor required (i,e. Vcomplete-mix = Vplug-flow). For example, a complete-mix reactor might be selected over a plug-flow reactor, because of its dilution capacity, if the influent wastewater is known to contain toxic constituents that cannot be removed by pretreatment. Alternatively, a plug-flow or multistage reactor might be selected over a complete-mix reactor to control the growth of filamentous microorganisms. In practice, the construction costs and operation and maintenance costs also affect reactor selection. Process Selection Based on Mass Transfer In addition to process selection based on reaction kinetics and loading criteria, a number of treatment processes will be based on mass transfer considerations, as introduced in this chapter. The principal operations in wastewater treatment involving mass transfer are aeration, especially the addition of oxygen to water; the drying of biosolids and sludge; the removal of volatile organics from wastewater; the stripping of dissolved constituents such as ammonia from digested supernatant; and the exchange of dissolved constituents as in ion exchange. Fortunately, there is a considerable body of literature on these subjects as well as a vast amount of practical experience. Process Design Based on Loading Criteria If appropriate reaction rate expressions and/or mass transfer coefficients cannot be developed, generalized loading criteria are frequently used. Early design loading criteria for activated sludge biological treatment systems were based on aeration tank capacity [e.g., kg of BOD/m3 (lb BOD/103 ft3)]. For example, if a process that is loaded at 10 kg/m3 produces an acceptable effluent and one loaded at 20 kg/103 m3 does not, the successful experience tends to be repeated. Unfortunately, records often are not well maintained, and the limits of such loading criteria are seldom defined. Examples of loading criteria are presented in the design chapters for unit operations and processes. It should be noted that with the new activated-sludge biological treatment process variations and new aeration equipment, the use of loading factors should be avoided. Bench Tests and Pilot-Plant Studies Where the applicability of a process for a given situation is unknown, but the potential benefits of using the process are significant, bench-scale or pilot-scale tests must be conducted. The purpose of conducting pilot-plant studies is to establish the suitability of the process in the treatment of a specific wastewater under specific environmental conditions and to obtain the necessary data on which to base a full-scale design. Factors that should be considered in planning pilot-plant studies for wastewater treatment are presented in Table 4-6. The relative importance of the factors presented in Table 4-6 will depend on the specific application and the reasons for conducting the testing program. Tab. 4-6 Considerations in setting up pilot-plant testing programs Reasons for conducting pilot testing Consideration  Reasons for conducting pilot testing Consideration  Pilot-plant size Test new process Simulate of another process Predict process performance Document process performance Optimize system design Satisfy regulatory agency requirement Bench-or lab-scale model Pilot-scale tests Full-scale tests  Physical design factors Scale-up factors Size of prototype Flow of variations expected Facilities and equipment required and setup Materials of construction  Non-physical design factors Available time, money, and labor Degree of innovation and motivation Quality of water or wastewater Location of facilities Complexity of process Similar testing experience Dependent and independent variables  Design of pilot testing program Dependent variables including ranges Independent variables including ranges Time required Test facilities Test protocols Statistical design of data acquisition program  For example, testing of UV disinfection systems is typically done: (1) to verify manufacturers' performance claims, (2) to quantify effects of effluent water quality constituents on UV performance, (3) to assess the effect(s) of system and reactor hydraulics on UV performance, (4) to assess the effect(s) of effluent filtration on UV performance, and (5) to investigate photo reactivation and impacts. Bench-scale tests are conducted in the laboratory with small quantities of the wastewater in question. Pilot-scale tests axe typically conducted with flows that are 5 to 10 percent of the design flows . In some instances full-scale tests have been conducted, especially where scale-up issues are complex and computational methods are too complex. Reliability Considerations in Process Selection Important factors in process selection and design are treatment plant performance and reliability in meeting permit requirements. In most permits, effluent constituent require merits based on 7-day and 30-day average concentrations are specified. Because waste water treatment effluent quality is variable for a number of reasons (varying organic loads, changing environmental conditions, etc.), it is necessary to ensure that the treatment system is designed to produce effluent concentrations equal to or less than the permit limits. Two approaches in process selection and design are (1) the use of arbitrary safety factors, and (2) statistical analysis of treatment plant performance to determine a functional relationship between effluent quality and the probable frequency of occurrence. The latter approach, termed the "reliability concept," is preferred because it can be used to provide a consistent basis for analysis of uncertainty and a rational basis for the analysis of performance and reliability. Treatment plant reliability can be defined as the probability that a system can meet established performance criteria consistently over extended periods of time. Thus, reliability is comprised of two components: inherent reliability and mechanical reliability.