As noted previously, the physical, chemical, and biological unit processes are carried out in vessels or tanks commonly known as “reactors.” The types of reactors that are available and their applications are introduced in this section.
Types of Reactors
The principal types of reactors used for the treatment of wastewater, illustrated on Fig. 1–4, 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 [see Fig. 1–4(a)], flow is neither entering nor
leaving the reactor (i.e., flow enters, is treated, and then discharged and the cycle repeats).
The liquid contents of the reactor are mixed completely. For example, the BOD test dis- cussed in Chap. 2 is carried out in a batch reactor (i.e., BOD bottle as shown on Fig. 2–21 in Chap. 2), 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 con- centrated chemicals.
Complete-Mix Reactor (CMR). In the complete-mix reactor [see Fig. 1–4(b)], 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 reac- tors 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.
1–8 Reactors Used in Wastewater Treatment 23
Plug-Flow Reactor (PFR). Fluid particles pass through the reactor with little or no
longitudinal mixing and exist 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 theo- retical 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 [see Fig. 1–4(c)]
or closed tubular reactors [e.g., pipelines, see Fig. 1–4(d)].
Complete-Mix Reactors in Series. The series of complete-mix reactors [see
Fig. 1–4(e)] 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 com- posed 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 Reactor. The packed-bed reactor is filled with a type of packing
medium, 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.
w o l f n I w
o l f n I
Outflow Outflow
(c) (d)
Inflow Outflow
Q, Co Q, C1 Q, C2 Q, Cn
Mixer Mixer Mixer
(e)
Inflow
Inflow Inflow
Outflow
Outflow Outflow
Off gas Off gas Off gas
Packing material
Packing material
Fluidized packing material
Air (optional)
Air (optional) Air
(optional)
(f) (g) (h)
Inflow Outflow
r e x i M r
e x i M
(a) (b)
Q, Co Q, C
V1 V2 Vn
Figure 1–4
Definition sketch for the different types of reactors used for wastewater treatment: (a) batch reactor; (b) complete-mix reactor;
(c) plug-flow open reactor; (d) plug-flow closed reactor, also known as a tubular reactor; (e) complete-mix reactors in series;
(f) packed bed downflow reactor;
(g) packed bed upflow reactor;
and (h) expanded bed upflow reactor.
Dosing can be continuous or intermittent (e.g., trickling filter). The packing medium in packed bed reactors can be continuous [see Fig. 1–4(f )] or arranged in multiple stages, with flow from one stage to another. A packed-bed upflow anaerobic (without oxygen) reactor is shown on Fig. 1–4(g).
Fluidized-Bed Reactor. The fluidized-bed reactor is similar to the packed-bed reac-
tor in many respects, but the packing medium is expanded by the upward movement of fluid (air or water) through the bed [see Fig. 1–4(h)]. The expanded porosity of the fluidized-bed packing medium can be varied by controlling the flowrate of the fluid.
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 and nonideal flow, as well as application of reactors in wastewater treatment, are described in the following discussion.
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. 1–5 in which dye tracer response curves are presented for pulse (slug injection) and step inputs (continuous injection). On Fig. 1–5, t is the actual time and t is equal to the theoretical hydraulic detention time defined as follows:
t5 V
Q (1–7)
where t5 the hydraulic detention time, T
V 5 volume of the reactor, L3
Q 5 volumetric flowrate, L3T21
If a pulse 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. 1–5(a-1). If a con- tinuous 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. 1–5(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. 1–5(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. 1–5(b-2).
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 impor- tant. 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 Figs. 1–5(a-2) and 1–5(b-2).
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.
Application of Reactors
The principal applications of reactor types used for wastewater treatment are reported in Table 1–9. Operational factors that must be considered in the selection of the type of reac- tor 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),
Concentration
Co
(a-1)
Concentration
Co
Co (b-1)
Concentration
t Time t
Time
t Time
(b-2)
Inflow Outflow
Mixer Slug input
of tracer
Inflow Outflow
Mixer
Co
Step input of tracer
Inflow
Outflow
Outflow Slug input
of tracer
Inflow
Co
Step input of tracer
Ideal
Nonideal Ideal Nonideal
Ideal, e–t /t
Nonideal
Concentration
Co
Time (a-2)
Ideal,1 e–t /t Nonideal
t
Figure 1–5
Output tracer response curves from reactors subject to slug (pulse) and step inputs of a tracer: (a-1) and (a-2) complete- mix reactor and (b-1) and (b-2) plug-flow reactor.
1–8 Reactors Used in Wastewater Treatment 25
(3) the reaction kinetics governing the treatment process, (4) the process performance requirements, and (5) local environmental conditions. Homogeneous and heterogeneous reactions and reaction kinetics are discussed in Sec. 1–9. In practice, the construction costs and operation and maintenance costs also affect reactor selection. Because the relative importance of these factors varies with each application, each factor should be considered separately when the type of reactor is to be selected.