1. CE 548 Introduction to Process Analysis and Selection

2. Reactors used for the treatment
Wastewater treatment involving physical, chemical or biological activities are carried out in vessels or tanks commonly known as “reactors”.
Types of reactors:
Batch reactor: The flow enters, is treated and then is discharged and the cycle repeats. Once the processing commences flow does not enter or leave the vessel. Used for small operations.

3. Reactors used for the treatment
Types of reactors:
Complete mix reactor: Complete mixing occurs instantaneously and uniformly throughout the reactor as fluid enters the reactor. Usually squar tanks, L~W. Used with most newer systems.

4. Reactors used for the treatment
Types of reactors:
Plug-flow reactor: Fluid passes through the reactor with little or no longitudinal mixing and exit the reactor in the same sequence as they entered. Long rectangular tanks L>>W. Used in most older systems.

5. Reactors used for the treatment
Types of reactors:
Complete-mix reactors in series (e)
Packed-Bed reactors (f), (g)
Fluidized-Bed reactors (h)

6. Reactors used for the treatment
Applications of the reactors:
The principle applications of reactors types used for the treatment of wastewater are reported in Table 4-1
Operational factors that are considered in the selection of the type of reactor to be used:
The nature of the wastewater to be treated
The nature of the reaction
The reaction kinetics
The process performance requirements

7. Table 4-1

8. Reactors used for the treatment
Hydraulic characteristics of reactors:
Ideal flow in complete-mix and plug-flow reactors for pulse (slug-dose)
 = V/Q
Where;
 = hydraulic detention time, T
V = volume of reactor, L3
Q = flowrate, L3

9. Reactors used for the treatment
Hydraulic characteristics of reactors:
Ideal flow in complete-mix and plug-flow reactors for step inputs (continuous injection)
 = V/Q
Where;
 = hydraulic detention time, T
V = volume of reactor, L3
Q = flowrate, L3

10. Mass-Balance Analysis
Principle: conservation of mass; mass neither created nor destroyed.
Rate of accumulation of reactant within the system boundary =
Rate of flow of reactant into the system boundary -
Rate of flow of reactant out of the system boundary +
Rate of generation of reactant within the system boundary
(1)
(2)
(3)
(4)

11. Mass-Balance Analysis
Preparation of Mass Balances
Schematic
Control volume
List all data and assumptions
List all rate expressions
Select a basis for calculation
Application of mass-balance analysis
Assumptions:
Constant flowrate into and out of control volume
No evaporation (constant volume)
Complete mixing
Reaction occurs within reactor
Rate of reaction is first-order (rc = -kC)

12. Mass-Balance Analysis
Formulation of mass balance:
Accumulation = Inflow – outflow + generation
if the reaction is steady state, there is no accumulation (dC/dt = 0), Thus equation (4-6) can be written as:

13. Modeling Ideal Flow in Reactors
Modeling of the hydraulic characteristics of reactors is important because the results can be used to determine the actual amount of time a given volume of water will remain in the reactor.
Complete Mix Reactor:
Accumulation = Inflow – outflow + generation
Using tracer, nothing is being generated; generation = 0
For pulse (slug) input:
Integrating with limits C=Co to C=C, and t=0 to t= t yields:

14. Modeling Ideal Flow in Reactors
Complete Mix Reactor:
Accumulation = Inflow – outflow + generation
Using tracer, nothing is being generated; generation = 0
For step input:
Integrating with limits C=C to C=Co, and t=0 to t= t yields:

15. Modeling Ideal Flow in Reactors
Plug-flow Reactor:
Accumulation = Inflow – outflow + generation (generation = 0)

16. Nonideal Flow in Reactors
Factors leading to nonideal flow in reactors: (Fig 4-6)

17. Nonideal Flow in Reactors
Need for tracer analysis:
Tracer analysis is used to assess the hydraulic performance of the reactor by measuring the residence time.
Application of tracer studies include:
Assessment of short circuiting in sedimentation tanks and biological reactors
Assessment of contact time in chlorine contact basins
Assessment of the hydraulic approach conditions in UV reactors
Assessment of flow patterns in constructed wetlands and other natural treatment systems
Example 4-1 Page 236

18. Reactions, Rates, and Coefficients
Types of reactions:
Homogenous: the reactants are distributed uniformly throughout the fluid. (batch, complete-mix, plug-flow)
Rate of reaction;
Heterogeneous: reaction occur at a specific site. (packed and fluidized bed reactors)

19. Reactions, Rates, and Coefficients
Types of rate expressions:
Typical rate expressions for selected processes: Table 4-6
Integration and differential methods used to determine reaction rate coefficients: Table 4-7
Example 4-5.

22. Modeling treatment process kinetics
Batch reactor:

23. Modeling treatment process kinetics
Complete mix-reactor:
Graphical solution: Example 4-6

24. Modeling treatment process kinetics
Plug-flow:

25. Modeling treatment process kinetics
Plug-flow with axial dispersion: