5. Reactor Design and Operational Parameters
Biological treatment processes are carried out in reactors, which can be modeled as one of three idealized types.
5.1. Idealized Reactor Types
- Batch Reactor: All reactants are added at once; composition changes over time.
- Plug Flow Reactor (PFR): Fluid flows in an orderly manner without overtaking, resulting in concentration gradients along the reactor. The reactor dispersion number is zero.
- Completely Mixed Reactor (CMR): Contents are perfectly stirred, resulting in uniform composition throughout. The reactor dispersion number is infinity.
Most real-world reactors are non-ideal and fall somewhere between these two extremes.
5.2. Continuous Flow Treatment Models
Lawrence and McCarty proposed three primary models for continuous flow systems:
- A completely mixed reactor without biological solids recycle.
- A completely mixed reactor with biological solids recycle.
- A plug flow reactor with biological solids recycle.
5.3. Key Design and Operation Parameters
| Parameter | Symbol | Definition |
| Biological Solids Retention Time (SRT) | θ_x | The average time a microorganism is kept in the system. (Total active mass / Total mass withdrawn daily). Also called Sludge Age or Mean Cell Retention Time. |
| Process Loading Factor | U | The mass of substrate utilized per day per unit mass of active microorganisms. Also called Food to Microorganisms (F/M) Ratio or Specific Utilization. |
| Hydraulic Retention Time (HRT) | t̄ | The average time wastewater stays in the reactor. (Reactor Volume / Volumetric Feed Rate). Also called Detention Time. |
| Volumetric Loading Rate | B_v | The mass of substrate applied per day per unit volume of the reactor. |
| Process Treatment Efficiency | E | The percentage of substrate removed from the influent. |