4.0 Suspended-Contact Reactors: Where Biomass is in Motion
4.1 The Activated Sludge Process: The Workhorse of Wastewater Treatment
The Activated Sludge process is the most common type of suspended-contact system. In this process, a concentrated population of microorganisms, known as “activated sludge,” is continuously mixed with wastewater in an aeration basin. The fundamental process loop can be summarized in three steps:
- Aeration In a large tank, incoming wastewater is mixed with the activated sludge. Oxygen is supplied by mechanical aerators or by bubbling compressed air through diffusers at the bottom of the tank. This oxygen-rich mixture of water and biomass is called the “mixed liquor.”
- Sedimentation The mixed liquor flows from the aeration tank into a quiet tank called a clarifier. Here, the heavier activated sludge solids settle to the bottom, separating from the clear, treated water, which flows out from the top.
- Recirculation A portion of the settled sludge is continuously pumped back to the beginning of the aeration basin. This recirculation is crucial because it ensures a high concentration of active microorganisms is always available to treat the incoming wastewater. The excess sludge that grows in the system is removed and sent for further treatment.
4.2 Important Variations on the Activated Sludge Theme
The basic activated sludge process is incredibly versatile and has been modified in several ways to meet different treatment objectives. Below is a comparison of three common variations.
| Process Type | Key Operational Feature | Primary Benefit or Purpose |
| Conventional | Wastewater and return sludge are mixed at the inlet of a long, narrow tank, creating a plug-flow pattern. | Creates a high initial oxygen demand that gradually decreases as the water flows through the tank. |
| Completely Mixed | Wastewater is rapidly dispersed throughout the entire aeration tank, creating a uniform mixture. | Distributes the organic load and oxygen demand evenly, making the system more resilient to shock loads (e.g., for an industrial facility with variable discharge). |
| Extended Aeration | Characterized by a very long aeration time, which forces the biomass into the endogenous respiration phase. | Minimizes the net production of excess sludge (ideal for smaller, remote facilities where sludge handling and disposal are difficult or expensive). |
Having explored systems that rely on oxygen, we now turn our attention to reactors that are designed to function in its complete absence.