Tertiary Wastewater Treatment and Nutrient Removal

Secondary wastewater treatment effectively removes organic matter, but it often leaves behind dissolved inorganic nutrients—primarily nitrogen and phosphorus. When discharged into lakes, rivers, or coastal waters, these nutrients act as fertilizers, triggering explosive algal blooms in a process called eutrophication. This depletes oxygen, kills aquatic life, and can produce toxins, making advanced nutrient removal not just an engineering challenge but an ecological imperative. Tertiary treatment, or advanced treatment, specifically targets these residual nutrients and fine solids to protect sensitive water bodies and meet stringent regulatory limits.

The Drivers and Goals of Nutrient Removal

The primary goal of tertiary treatment is to reduce concentrations of nitrogen and phosphorus to levels that prevent eutrophication. Nitrogen is typically regulated as Total Nitrogen (TN), which includes ammonia ($N{H}_3$/$N{H}_4^{+}$), nitrite ($N{O}_2^{-}$), nitrate ($N{O}_3^{-}$), and organic nitrogen. Phosphorus is regulated as Total Phosphorus (TP). While secondary treatment can remove some nutrients, targeted tertiary processes are required to achieve low discharge limits, often below 10 mg/L for TN and 1 mg/L or even 0.1 mg/L for TP. These processes are either biological, leveraging specialized bacteria, or chemical, involving the addition of precipitating agents.

Biological Nitrogen Removal: Nitrification and Denitrification

Biological nitrogen removal is a two-step microbial process. The first step, nitrification, converts ammonia into nitrate. This is performed in an aerobic tank by two groups of slow-growing bacteria. Nitrosomonas bacteria oxidize ammonia to nitrite, and Nitrobacter bacteria then oxidize nitrite to nitrate. The simplified reactions are: $$2N{H}_4^{+}+3O_2\to 2N{O}_2^{-}+4H^{+}+2H_{2}O$$ $$2N{O}_2^{-}+O_2\to 2N{O}_3^{-}$$

Successful nitrification requires sufficient oxygen, a long sludge age to retain the slow-growing bacteria, and adequate alkalinity, as the process consumes about 7.1 mg of alkalinity (as $CaC{O}_3$) per mg of ammonia-nitrogen oxidized.

The second step, denitrification, converts nitrate into nitrogen gas ($N_2$), which escapes harmlessly into the atmosphere. This occurs in an anoxic tank (with no free oxygen but with nitrate present). Denitrifying bacteria use nitrate as their oxygen source, consuming organic carbon in the process. The general reaction is: $$6N{O}_3^{-}+5C{H}_{3}OH\to 3N_2+5C{O}_2+7H_{2}O+6O{H}^{-}$$

This process recovers some alkalinity and requires a readily biodegradable carbon source, which can be the incoming wastewater or an added external source like methanol.

Biological and Chemical Phosphorus Removal

Phosphorus removal can be achieved biologically or chemically. Enhanced Biological Phosphorus Removal (EBPR) exploits the metabolism of phosphorus-accumulating organisms (PAOs). In an anaerobic tank at the head of the process, PAOs consume volatile fatty acids (VFAs) and store them internally, releasing orthophosphate from their cells into the water. When these organisms are then cycled to an aerobic tank, they take up far more phosphate than they released, sequestering it within their biomass. The phosphorus-rich sludge is then wasted from the system. Effective EBPR depends heavily on providing a strong anaerobic zone with ample VFAs and ensuring the sludge is not retained too long, which can lead to phosphorus release.

Chemical phosphorus precipitation is a more direct and reliable method. A metal salt—typically aluminum (e.g., alum, $A{l}_2(S{O}_4{)}_3$) or iron (e.g., ferric chloride, $FeC{l}_3$)—is added to the water. The metal ions react with soluble phosphate to form an insoluble precipitate that settles out. For example, with alum: $$A{l}^{3+}+P{O}_4^{3-}\to AlP{O}_4\downarrow$$

Chemical dosing is often controlled in real-time based on phosphate concentration. While highly effective, it increases sludge production, can be costly, and may affect pH.

Polishing and Advanced Processes

After biological and chemical nutrient removal, effluent may still contain fine suspended solids and trace organic contaminants. Filtration for suspended solids polishing is a common tertiary step. Deep-bed granular media filters (sand, anthracite) or cloth disc filters capture these fine particles, which often have nutrients adsorbed to them, providing a final "polish" to the effluent.

For the most challenging contaminants, such as pharmaceuticals or industrial chemicals, advanced oxidation processes (AOPs) may be employed. AOPs generate highly reactive hydroxyl radicals ($\cdot OH$) that non-selectively oxidize and destroy organic compounds. Common methods include combining ultraviolet (UV) light with hydrogen peroxide ($H_2{O}_2$) or using ozone ($O_3$) with peroxide. These are energy-intensive processes typically reserved for water reuse applications or where trace contaminants are a major concern.

Integrated Treatment Systems: Bardenpho and A2O

Real-world treatment plants combine these unit processes into integrated, multi-zone systems. Two prominent designs are the A2O (Anaerobic-Anoxic-Oxic) process and the modified Bardenpho (Barnard Denitrification and Phosphorus Removal) process.

The A2O process is a plug-flow configuration. Wastewater first enters an anaerobic zone for phosphorus release by PAOs, then an anoxic zone for denitrification (using nitrate recycled from the end of the system), and finally an aerobic zone for nitrification, carbon oxidation, and phosphorus uptake. It effectively removes both nitrogen and phosphorus in a single sludge system.

The modified Bardenpho process is a five-stage system designed for very low nitrogen levels. It sequences tanks as: first anoxic → first aerobic → second anoxic → second aerobic → final clarifier. The first aerobic zone performs nitrification. The second anoxic zone provides additional denitrification using endogenous carbon sources (the bacteria themselves), and the final short aerobic zone strips any nitrogen gases and prevents phosphorus release in the clarifier. This staged approach achieves exceptional Total Nitrogen removal, often below 3 mg/L.

Common Pitfalls

1. Insufficient Alkalinity for Nitrification: In soft waters, nitrification can consume all bicarbonate alkalinity, crashing the pH and halting the process. Engineers must test influent alkalinity and supplement with lime or sodium hydroxide if needed. A simple check is ensuring there is at least 7.1 times more alkalinity (mg/L as $CaC{O}_3$) than the ammonia-nitrogen to be oxidized.
2. Carbon Competition in EBPR Systems: In an A2O configuration, ordinary heterotrophic bacteria in the anaerobic zone will compete with PAOs for the precious VFAs. If these bacteria win, EBPR fails. The solution is to ensure a strong, truly anaerobic environment (zero nitrate) and to potentially provide a dedicated VFA source, like fermented primary sludge.
3. Over-reliance on Chemical Phosphorus Removal: While chemicals guarantee low phosphorus, automatic overdosing is wasteful, increases sludge handling costs, and can inhibit downstream biological processes if metal residuals carry over. Implementing phosphorus probes for real-time feedback control optimizes dosing and reduces operational expenses.
4. Ignoring the Impact of Internal Recycles: Integrated processes like A2O use large internal mixed liquor recirculation flows from the aerobic zone back to the anoxic zone. This recycle stream contains dissolved oxygen, which can "poison" the anoxic zone and impair denitrification. Proper design includes deoxygenation zones or considers the oxygen load in sizing calculations.

Summary

• Tertiary wastewater treatment is essential to prevent eutrophication by removing dissolved nitrogen and phosphorus to very low concentrations.
• Biological nitrogen removal is a two-stage process: aerobic nitrification (ammonia to nitrate) followed by anoxic denitrification (nitrate to nitrogen gas), each with distinct microbial and environmental requirements.
• Phosphorus can be removed via Enhanced Biological Phosphorus Removal (EBPR), which cycles sludge between anaerobic and aerobic conditions, or via chemical precipitation using metal salts like alum or ferric chloride.
• Final filtration polishes effluent by removing fine solids, while advanced oxidation processes (AOPs) use hydroxyl radicals to destroy trace organic contaminants for water reuse.
• Integrated systems like the A2O and modified Bardenpho processes efficiently combine anaerobic, anoxic, and aerobic zones in a single sludge system to achieve simultaneous and advanced nutrient removal.