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 c 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 engineering literature).(3) the plug-flow eactor(also known as a tubular-fl (4) complete-mix reactors in series. (5)the packed-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 read ften chemicals Packing sing concentrated chemicals Fig. 4-2 Definition sketch for various types Outfiow ofreactors used for wastewater treatment (a) batch reactor; (b)complete-mix reactor; c)plug-flow open reactor, (aplug-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. Fluid pa 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 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 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 4-14-2 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