Tab. 4- Principal applications ofreactor types used for wastewater treatment ted-sludge biological treatment in a sequence batch reactor, mixing of orking solution Complete-mix ated lagoons, aerobic sludge digestions C reactors in series plug-flow reactor 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) step input of a co 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 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 aunts from reactors subject to pulse and step inputs ofatmcer 4-34-3 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