6.1 Beyond Carbonaceous BOD Removal

While conventional secondary treatment is highly effective at removing carbonaceous BOD, it is often insufficient for removing the nutrients nitrogen and phosphorus. The discharge of these nutrients into surface waters can lead to eutrophication—an over-enrichment that causes excessive growth of algae and other aquatic plants, leading to oxygen depletion and severe ecological damage. Consequently, regulations often require advanced wastewater treatment specifically engineered for biological nutrient removal (BNR).

6.2 Biological Nitrogen Removal

The biological removal of nitrogen from wastewater is a sophisticated process that relies on two distinct microbial groups operating under different environmental conditions. In colder climates, where reaction rates are slower, a three-stage process is often considered necessary for effective removal, as depicted in Figure 22.

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Figure 22: Typical Three-Stage Treatment Process for Nutrient Removal

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The Three-Stage Process

1. Stage 1 (Carbonaceous BOD Removal): The first step is a conventional aerobic process designed to reduce the primary organic load (BOD) to a low level (e.g., below 50 mg/L). This is a classic case of microbial competition. The heterotrophic bacteria responsible for BOD removal have a much higher maximum specific growth rate (μ_max) than the slow-growing autotrophic nitrifiers. If we don’t remove their primary food source first, the nitrifiers will be out-competed and washed out of the system before they can perform their function.
2. Stage 2 (Nitrification): This is a strictly aerobic process where ammonia (NH₄⁺) is oxidized to nitrate (NO₃⁻). This is accomplished in two steps by two distinct groups of autotrophic bacteria:
   • Nitrosomonas bacteria oxidize ammonia to nitrite (NO₂⁻): NH₄⁺ + 1.5 O₂ → NO₂⁻ + H₂O + 2H⁺
   • Nitrobacter bacteria then oxidize nitrite to nitrate (NO₃⁻): NO₂⁻ + 0.5 O₂ → NO₃⁻ Nitrification is a sensitive process. The nitrifying bacteria are slow-growing, sensitive to low temperatures, and can be inhibited by substances such as free ammonia and free nitrous acid at elevated concentrations.
3. Stage 3 (Denitrification): This is an anoxic process (oxygen is absent, but nitrate is present) where the nitrate produced during nitrification is converted to harmless nitrogen gas (N₂), which then escapes to the atmosphere. This process is carried out by a wide range of facultative heterotrophic bacteria. For the reaction to proceed, these bacteria require a source of carbon (an electron donor). Since the carbonaceous BOD was largely removed in Stage 1, a supplemental carbon source, such as methanol (CH₃OH), must often be added to drive the denitrification reaction: 6NO₃⁻ + 5CH₃OH → 3N₂ + 5CO₂ + 7H₂O + 6OH⁻

Alternative configurations exist, such as combining carbon oxidation and nitrification in a single, long-SRT aerobic stage, or using separate fixed-film reactors for nitrification and denitrification.

6.3 Biological Phosphorus Removal

Conventional Removal

Conventional biological treatment processes remove only a modest amount of phosphorus (typically 20-40%). This removal occurs primarily through assimilation, as phosphorus is an essential nutrient required for the synthesis of new microbial cells.

Enhanced Biological Phosphorus Removal (EBPR)

To achieve higher levels of removal, specialized processes have been developed to promote a phenomenon known as “luxury phosphorus uptake.” In these systems, a specific group of bacteria, known as phosphorus-accumulating organisms (PAOs), are encouraged to take up and store phosphorus in amounts far exceeding their normal metabolic needs. The key factors required to promote this behavior include:

• A reactor configuration that includes an anaerobic zone followed by an aerobic zone (plug-flow is beneficial).
• A slightly alkaline pH.
• Adequate dissolved oxygen in the aerobic zone.
• Low carbon dioxide concentration.

Chemical Augmentation

While EBPR can be effective, the most reliable and common method for achieving very low effluent phosphorus concentrations involves augmenting the biological process with the addition of chemical salts. Chemicals such as alum (aluminum sulfate) or ferric salts (ferric chloride) are added to the wastewater, causing the soluble phosphate to precipitate out as an insoluble metal-phosphate solid, which is then removed with the sludge.

6.4 Concluding the Lecture

Biological wastewater treatment is a sophisticated and highly adaptable field of environmental engineering. It is a discipline that integrates principles from microbiology, biochemistry, and chemical engineering to solve critical environmental problems. From understanding the fundamental metabolic reactions of a single bacterium, to designing complex multi-stage reactor systems for advanced nutrient removal, the overarching goal remains the same: to harness and optimize natural biological processes for the protection of public health and the preservation of our aquatic environments.