5.1 From Principles to Practice

Having established the foundational principles of microbiology, kinetics, and reactor design, we now turn our attention to the major categories of biological treatment systems used in practice. While all of these systems leverage the same underlying metabolic processes, their physical configurations are specifically engineered to suit different types of wastewater, operational constraints, and treatment objectives. As shown in the typical plant layout in Figure 12, these biological processes typically form the core of secondary treatment, following primary sedimentation and preceding any advanced or tertiary treatment steps.

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Figure 12: Typical Wastewater Treatment Sequence

• Explanation of Figure 12: This schematic illustrates the conventional flow of wastewater through a treatment plant. The biological systems we will discuss—such as activated sludge, trickling filters, and rotating biological contactors—are categorized under Secondary Treatment. Their primary goal is to remove the soluble and colloidal biodegradable organic matter (BOD) that remains after primary settling has removed the heavier, settleable solids. The excess biological solids (sludge) generated during this stage are typically combined with primary sludge for further treatment, often via anaerobic digestion.

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5.2 Aerobic Fixed-Film (Stationary-Contact) Systems

Core Principle

In fixed-film systems, the active biomass is not held in suspension within the liquid. Instead, it is intentionally grown as a slimy layer, or biofilm, on the surface of a stationary solid media. The wastewater is then passed over or through this media, bringing the substrate and oxygen into contact with the immobilized microbial population. These systems are noted for their stability and resilience to shock loads, partly due to the large mass of attached biomass and the mass transfer limitations that can shield the inner layers of the biofilm from sudden changes in influent concentration.

Focus on Trickling Filters

The trickling filter is one of the oldest and most established types of fixed-film biological treatment.

• Mechanism of Action: In a trickling filter, wastewater is distributed over the top of a packed bed of media, which can consist of stones, slag, or, more commonly in modern designs, engineered plastic materials. As the wastewater trickles downward through the bed, a microbial biofilm develops on the media surfaces. The process of purification is illustrated in Figure 13. Organic matter from the wastewater diffuses into the biofilm and is metabolized by the microorganisms. Oxygen from the air, circulating through the voids in the media, diffuses into the biofilm from the outside. As the biofilm grows thicker, the inner layer closest to the media surface can become anoxic or anaerobic because oxygen cannot penetrate that deep. Eventually, the biofilm becomes too thick and the inner anaerobic activity causes it to lose its grip on the media. It then detaches in a process called sloughing. These sloughed solids are carried out with the effluent and removed in a downstream secondary clarifier.

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Figure 13: Process of BOD Removal in Trickling Filters

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• Comparison of Trickling Filter Types: Trickling filters are broadly classified as either low-rate or high-rate, based primarily on their hydraulic and organic loading rates. The key differences are summarized in the table below, based on data from Table 1.

• Evaluation of Recirculation: High-rate filters almost always employ recirculation, where a portion of the treated effluent is pumped back and blended with the raw influent. This practice has several significant advantages, including:
  • Continuous Seeding: The recirculated flow contains active microorganisms, which continuously seeds the filter and stimulates biochemical activity.
  • Uniform Loading: It dilutes the strong influent and helps distribute the organic load more evenly over the day.
  • Reduced Clogging: The increased hydraulic flow helps to continuously scour the media and prevent clogging or ponding. However, recirculation also has disadvantages, most notably the increased capital and operating costs associated with pumping, and the potential for cooling the wastewater in cold climates, which can reduce biochemical reaction rates.
• Design Considerations: The performance of a trickling filter is modeled using equations that relate BOD removal to factors like filter depth (H), hydraulic flow rate per unit area (Qₐ), and media characteristics. These models, such as Equations (21-25), are often empirical or semi-empirical, based on first-order kinetics. They generally show that treatment efficiency increases with greater filter depth and decreases with higher hydraulic loading rates.

Focus on Rotating Biological Contactors (RBCs)

An RBC is another prominent fixed-film technology that shares principles with trickling filters but has a distinct mechanical design.

• Mechanism of Action: An RBC unit consists of a series of large, lightweight plastic disks mounted on a horizontal shaft. The shaft is placed in a contour-bottomed tank, and the disks are rotated slowly (1-2 rpm) so that they are approximately half-submerged in the wastewater. As the disks rotate, a biofilm develops on their surfaces. This rotation serves a dual purpose: it brings the biomass into contact with the wastewater to absorb substrate, and then lifts it out of the water and into the air to absorb oxygen. This creates an efficient system for mass transfer of both food and oxygen to the biofilm.
• Applications and Efficacy: RBCs have been successfully applied to treat a wide variety of high-strength municipal and industrial wastewaters, including those from pulp and paper mills and refineries. They are known for their operational stability, ease of use, and effectiveness in toxicity reduction. For instance, studies have shown that RBCs can effectively treat toxic paper mill wastewater and refinery waste containing phenols.

5.3 Aerobic Suspended-Contact Systems

Core Principle

In contrast to fixed-film systems, suspended-contact (or suspended-growth) systems maintain the active biomass in suspension within the liquid wastewater in a reactor. Both the microorganisms and the substrate are kept in constant motion, typically through mechanical mixing or diffused aeration.

Focus on the Activated Sludge Process

The activated sludge process is the most widely used suspended-growth system for biological wastewater treatment.

• General Process Description: The process involves mixing wastewater with a concentrated, biologically active mass of microorganisms known as activated sludge. This mixture, called the mixed liquor, is held in an aeration basin where oxygen is supplied to maintain aerobic conditions. After a specified aeration time, the mixed liquor flows to a secondary clarifier where the activated sludge solids are separated from the treated water by gravity settling. A significant portion of these settled solids is then recirculated back to the inlet of the aeration basin to maintain a high concentration of active microorganisms. The remaining portion, which represents the net growth of biomass in the system, is removed as excess sludge for disposal.
• Conventional Activated Sludge: The original design for the activated sludge process operates as a long, rectangular plug-flow reactor, as depicted in Figure 14. The settled wastewater and return activated sludge are introduced at the inlet end of the tank. This creates a high organic load and thus a high oxygen demand at the beginning of the tank. As the mixed liquor flows along the tank’s length, the BOD is consumed, and the oxygen demand diminishes. To match this demand profile, modern conventional plants often use tapered aeration, supplying more air at the inlet and progressively less air toward the outlet.

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Figure 14: Conventional Activated Sludge

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Analysis of Process Modifications

Over the decades, numerous modifications to the conventional process have been developed to address specific challenges or improve performance. The typical operating parameters for these variations are summarized in Table 2.

1. Step Aeration (Figure 15): To equalize the high initial oxygen demand seen in conventional plants, the step aeration process introduces the primary effluent at several points along the length of the aeration tank. The return sludge is introduced at the head of the tank. This distributes the organic load more evenly, resulting in a more uniform oxygen demand and a more biologically active state throughout the reactor.
2. Short-Term / High-Rate Aeration: This process operates on the far-left side of the curve in Figure 7 (high F/M), deliberately targeting the log-growth phase to maximize throughput. This high-loading process results in a smaller required reactor volume (footprint) but provides only an intermediate level of treatment (60-85% BOD removal), with the accepted trade-off of producing a large quantity of less-stabilized sludge.
3. Contact Stabilization / Biosorption (Figure 16): This process uses a two-tank system and is ideal for wastewaters with a high proportion of suspended or colloidal BOD. The wastewater is first mixed with highly activated return sludge in a small “contact tank” for a short period (0.5-1.5 hours), where BOD is rapidly removed by adsorption. The mixed liquor is then settled. The settled sludge is sent to a separate, larger “activation tank” where it is aerated for a longer period to metabolize the adsorbed organics and regenerate its activity before being returned to the contact tank.
4. Completely Mixed (Figure 17): In this configuration, the aeration tank is designed to function as a completely mixed reactor (CMR). Influent wastewater is rapidly dispersed throughout the entire basin. The primary advantage of this design is its inherent biological stability and its ability to dampen the effects of shock loads (sudden spikes in organic or toxic concentrations), as any slug of influent is immediately diluted by the entire reactor volume.
5. Pure Oxygen Systems (Figure 18): Instead of using air (which is ~21% oxygen), these systems use high-purity oxygen. The primary advantage is the ability to achieve a much higher dissolved oxygen concentration in the mixed liquor. This allows the system to support a significantly higher concentration of biomass (MLSS values of 3,000–10,000 mg/L) than conventional air systems. This, in turn, permits much higher volumetric loading rates, leading to smaller reactor volumes and potentially better sludge settling characteristics.
6. Extended Aeration (Figure 19): This system is engineered to live on the far-right side of Figure 7 (low F/M), maintaining a very low food-to-microorganism ratio. It is a low-load process characterized by a very long aeration time (20-30 hours) and a long SRT. The goal is to force the biomass deep into endogenous respiration, thereby minimizing the net sludge yield at the cost of a larger reactor volume. Oxidation ditches are a common type of extended aeration plant.

• Aerated Lagoons: An aerated lagoon can be thought of as an activated sludge process without sludge recycle. It is a large, earthen basin where wastewater is aerated for a very long period (70-120 hours). Because there is no sludge recycle, the concentration of microbial solids is very low, which necessitates the long detention times to achieve adequate treatment.

5.4 Anaerobic Treatment Systems

Core Principle and Advantages

In anaerobic treatment, specialized bacteria stabilize organic matter in the complete absence of free oxygen. This process has long been used for treating high-solids sludges, but is now also applied to liquid wastes. The primary advantages over aerobic processes are:

1. Low Biological Cell Production: A much lower percentage of the organic matter is converted into new biomass (sludge).
2. Methane Production: A high percentage of the organic matter is converted into biogas (primarily methane and carbon dioxide), which can be captured and used as a renewable energy source.

The Two-Stage Mechanism

Anaerobic digestion is a complex sequential process involving multiple groups of bacteria, which can be simplified into two main stages, as shown in Figure 20.

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Figure 20: Sequential Mechanism of Anaerobic Waste Treatment

• Explanation of Figure 20:
  1. Stage 1 (Acid Formation): A group of bacteria known as acid formers (or acidogens) break down complex organic materials like proteins, fats, and carbohydrates into simpler organic compounds, primarily volatile fatty acids (e.g., acetic acid, propionic acid). This stage accomplishes very little in terms of COD or BOD reduction.
  2. Stage 2 (Methane Formation): A second, distinct group of microorganisms called methane formers (or methanogens) consumes the organic acids produced in the first stage and converts them into methane (CH₄) and carbon dioxide (CO₂). This is the critical stabilization step where the bulk of the COD is removed.

Anaerobic Process Designs

As identified by McCarty and shown in Figure 21, there are three basic designs for anaerobic treatment:

1. Conventional Process: This is a simple flow-through reactor, often a mixed digester, with no solids recycle. The Hydraulic Retention Time (HRT) is therefore equal to the Solids Retention Time (SRT). This design is suitable for concentrated wastes like sludges, where the required long SRT can be achieved with a reasonable HRT.
2. Anaerobic Activated Sludge: This process incorporates solids separation and recycle, similar to its aerobic counterpart. This decouples the HRT from the SRT, allowing the system to maintain the long SRT required by the slow-growing methanogens while treating more dilute wastes at a shorter, more economical HRT.
3. Anaerobic Filter: This is a fixed-film anaerobic process where wastewater is passed through a bed of media on which a film of anaerobic bacteria is grown. This design provides a very long SRT by retaining the biomass on the media.

Operational Sensitivity and Inhibitors

Anaerobic processes, particularly the methane-forming bacteria, are notoriously sensitive to environmental conditions and inhibitory substances. For example, while stimulatory at low concentrations, key cations become strongly inhibitory at higher levels (Table 3): Sodium at >8000 mg/L, Potassium at >12000 mg/L, Calcium at >8000 mg/L, and Magnesium at >3000 mg/L. Beyond cations, other common inhibitors pose a significant threat. Soluble sulfides are highly toxic, with concentrations above 200 mg/L causing strong inhibition of the methanogenic population. Similarly, free ammonia (NH₃), which is more prevalent at higher pH, is also very toxic, with concentrations above 150 mg/L leading to severe process upset.

5.5 Concluding the Survey

This survey has covered the major process configurations used to remove conventional pollutants like BOD and suspended solids. However, modern wastewater treatment often requires the removal of specific nutrients to protect receiving waters from environmental damage. This advanced treatment objective is the final topic of our lecture.