Aerobic systems operate in the presence of free oxygen and are divided into two main categories based on how the biomass is maintained.

6.1. Fixed-Film Systems

In these systems, biomass grows on a stationary support medium.

• Trickling Filters: Wastewater is sprayed over a bed of stones or plastic media, where it trickles down through a microbial slime layer. Oxygen is supplied by air circulating through the media. High-rate filters utilize recirculation of the treated effluent to improve efficiency, while low-rate filters do not.
• Comparison of Low-Rate and High-Rate Trickling Filters | Parameter | Low-Rate Filters | High-Rate Filters | | :— | :— | :— | | Hydraulic Loading (m³/day/m²) | 1.0 to 4.1 | 8.1 to 40.7 | | Organic Loading (g BOD/day/m³) | 80 to 400 | 400 to 4800 | | Recirculation | Generally absent | Always provided (R = 0.5 to 3) | | Effluent Quality | Highly nitrified, lower BOD | Not fully nitrified, higher BOD |
• Rotating Biological Contactors (RBCs): Large plastic disks are partially submerged in wastewater and rotated slowly. Biomass grows on the disks, absorbing substrate when submerged and oxygen when exposed to the air. RBCs have proven effective for treating toxic industrial wastes, such as those from pulp and paper mills and refineries.

6.2. Suspended-Contact Systems: The Activated Sludge Process

In this process, biomass (activated sludge) is continuously mixed with wastewater in an aerated basin. The mixture (mixed liquor) is then sent to a clarifier where the sludge settles and a portion is recycled back to the aeration basin.

Operating Parameters for Activated Sludge Process Variations | Parameter | Conventional | Step Aeration | Extended Aeration | Completely Mixed | | :— | :— | :— | :— | :— | | Organic Loading Rate (g BOD₅/day/m³) | 480–640 | 800–2400 | 160–320 | 2000–2880 | | Process Loading Factor (U) (kg BOD₅/day/kg MLVSS) | 0.2–0.5 | 0.2–0.5 | 0.05–0.2 | 0.6–1.0 | | Sludge Age (θ_x) (days) | 3–4 | 3–4 | 20–30 | 3–5 | | Aeration Time (t̄) (hours) | 6–7.5 | 6–7.5 | 20–30 | 3–5 | | BOD₅ Removal (E) (%) | 90–95 | 90–95 | 85–90 | 85–90 |

Modifications of the Activated Sludge Process:

• Conventional: A plug-flow design where settled waste and return sludge are mixed at the inlet.
• Step Aeration: Wastewater is introduced at multiple points along the aeration tank to distribute the organic load more evenly.
• Contact Stabilization (Biosorption): Suited for wastes with high suspended/colloidal BOD. Involves a short contact time for rapid BOD absorption, followed by a longer reaeration period in a separate tank to stabilize the sludge.
• Completely Mixed: The aeration tank is designed to be completely homogeneous, which buffers against shock loads.
• Pure Oxygen: Uses pure oxygen instead of air, allowing for higher biomass concentrations and higher volumetric loading rates.
• Extended Aeration: Operates with a very long aeration time and high sludge age, placing the system in the endogenous phase to minimize excess sludge production.
• Aerated Lagoons: A simpler system without sludge recirculation, requiring long detention times.