4.0 Analysis of Aerobic Biological Treatment Systems
Aerobic biological treatment systems, which use oxygen as the final electron acceptor, can be broadly classified into two major categories: fixed-film systems, where the biomass grows on a stationary surface, and suspended-growth systems, where the biomass is kept in suspension within the wastewater. This section evaluates the operational principles, advantages, and typical applications of each category, providing a basis for system selection and optimisation.
4.1 Fixed-Film (Stationary-Contact) Systems
The fundamental principle of fixed-film systems is the cultivation of a dense layer of microorganisms, or biofilm, on a stationary support media. Wastewater flows past this media, allowing for the transfer of substrate and nutrients into the biofilm and the diffusion of metabolic byproducts out.
Trickling Filters
In a trickling filter, wastewater is distributed over a bed of media (such as stones or plastic). A microbial slime layer, or biomass, develops on the media surface. As wastewater trickles through the bed, organic matter is absorbed and degraded by the biomass. As the biofilm thickens, the inner layer can become anaerobic, eventually causing the film to slough off the media. These sloughed solids are then removed in a secondary settling tank. To enhance performance, treated effluent is often recirculated back to the filter. Filters are generally classified as low-rate or high-rate based on their loading.
| 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)
An RBC system consists of large, lightweight plastic disks mounted on a horizontal shaft, which are slowly rotated while being partially submerged in a tank of wastewater. The rotation facilitates alternating contact with the wastewater for substrate absorption and the air for oxygen absorption. This design combines the principles of fixed-film growth with an efficient method of aeration.
Process Stability
Fixed-film systems are generally considered more stable and less sensitive to toxic and shock loads than suspended growth systems. This enhanced stability is attributed to two primary factors:
- High Biomass Concentration: The fixed biofilm allows for a much higher concentration of microorganisms within the reactor volume.
- Mass Transfer Resistance: The biofilm structure creates a diffusion barrier, effectively shielding the microorganisms from sudden spikes in the concentration of toxins or inhibitors in the bulk liquid.
4.2 Suspended-Growth (Suspended-Contact) Systems
The activated sludge process is the primary example of a suspended-contact system. In this process, the biomass (activated sludge) is kept in continuous suspension with the substrate in an aerated basin, creating what is known as the “mixed liquor.” Oxygen is supplied mechanically or through diffusers, and the biomass is later separated from the treated water via sedimentation. A portion of this settled sludge is then recycled back to the aeration basin to maintain the desired microbial population.
Numerous modifications of the activated sludge process have been developed to address specific treatment objectives and wastewater characteristics. The key operational parameters for the most common variations are summarized below.
| Process Type | Organic Loading Rate (g BOD5/day/m³) | Sludge Age (days) | Aeration Time (hours) | BOD5 Removal (%) |
| Conventional | 480–640 | 3–4 | 6–7.5 | 90–95 |
| Step Aeration | 800–2400 | 3–4 | 6–7.5 | 90–95 |
| Short Term (High Rate) | 1600–6400 | 0.2–0.5 | 2–4 | 60–85 |
| Biosorption (Contact Stabilization) | 480–1120 | 3–4 | 0.5–1.5 (aeration) | 85–90 |
| Complete Mixing | 2400–3200 | 0.8–2.3 | 3–5 | 88–95 |
| Pure Oxygen | 2000–2880 | — | 1–3 | 85–90 |
| Extended Aeration | 160–320 | >15 | 20–30 | 85–90 |
| Aerated Lagoons | 80 | 3–5 | 70–120 | 85–90 |
Key Activated Sludge Modifications
- Conventional: This process operates in a plug-flow configuration, where wastewater and return sludge are introduced at the inlet of a long, narrow aeration basin. This creates a high organic load and a corresponding high oxygen demand at the front end of the tank, which gradually decreases as the mixed liquor flows toward the outlet.
- Step Aeration: To address the high initial oxygen demand of the conventional process, step aeration introduces the influent wastewater at multiple points along the length of the aeration tank. This distributes the organic load more uniformly, evening out the oxygen demand and maintaining a more biologically active state throughout the basin.
- Contact Stabilization (Biosorption): This two-tank process is ideal for wastewaters with a high proportion of suspended or colloidal BOD. In a short-contact aeration tank, the activated sludge rapidly absorbs the organic matter. The sludge is then separated and moved to a second “activation” tank, where it is reaerated over a longer period to metabolize the adsorbed material before being returned to the contact tank.
- Completely Mixed: In this design, the aeration basin is configured to ensure that the incoming wastewater is rapidly and completely dispersed throughout the entire volume. This creates a homogenous state, meaning the organic load on the microorganisms is uniform everywhere in the tank. This configuration is highly effective at dampening the impact of shock loads and enhances overall biological stability.
- Extended Aeration: This process is characterized by a very long aeration time and a long sludge age, forcing the system to operate deep in the endogenous growth phase. The primary goal is to minimize the net production of excess sludge by allowing the microorganisms to auto-digest a significant portion of the biomass.
Having reviewed the primary aerobic treatment configurations, the focus now shifts to the distinct principles and applications of anaerobic treatment processes.