Nutrient removal is often a key regulatory driver for advanced wastewater treatment. Discharges of nitrogen and phosphorus can lead to eutrophication in receiving water bodies, and stringent effluent standards for these nutrients are increasingly common. This section details the biological processes required to remove nitrogen and phosphorus to meet these standards.

Biological Nitrogen Removal

Biological nitrogen removal is a multi-stage process that leverages different groups of bacteria under varying environmental conditions to convert nitrogen compounds into harmless nitrogen gas.

1. First Stage (Carbonaceous BOD Removal): The process typically begins with a conventional aerobic stage to reduce the primary organic load (BOD). This is necessary to ensure the subsequent nitrification stage is not overloaded.
2. Second Stage (Nitrification): This is a strictly aerobic process carried out in two steps by specialized bacteria. First, Nitrosomonas bacteria oxidize ammonia (NH₄⁺) to nitrite (NO₂⁻). Then, Nitrobacter bacteria oxidize the nitrite to nitrate (NO₃⁻). The overall reactions are:
3. Third Stage (Denitrification): This is an anaerobic process where bacteria use the oxygen from nitrate and nitrite molecules for respiration, converting them into nitrogen gas (N₂), which is then released into the atmosphere. This process requires a source of organic carbon (BOD) to proceed. If the carbon from the raw wastewater has already been consumed, a supplemental source, such as methanol, must be added.

It is critical to note that the nitrifying bacteria in the second stage are sensitive to inhibition. Free ammonia concentrations of 0.1 to 1.0 mg/L and free nitrous acid concentrations of 0.22 to 2.8 mg/L can begin to inhibit their activity.

Biological Phosphorus Removal

Conventional biological treatment typically removes only 20-40% of the influent phosphorus, primarily by incorporating it into new cell mass. Achieving higher removal rates requires specific process modifications to encourage “luxury phosphorus uptake,” a phenomenon where certain bacteria accumulate phosphorus in amounts far exceeding their normal metabolic needs. The key operational factors required to achieve this include:

• A plug-flow reactor configuration
• Slightly alkaline pH
• Adequate dissolved oxygen in the aerobic zone
• Low carbon dioxide concentration

However, the most consistently effective method of phosphate removal is often the addition of chemicals, such as alum or ferric salts, which precipitate the phosphorus so it can be removed with the sludge.

Conclusion

Successful optimisation of biological wastewater treatment is a multifaceted discipline. It demands an integrated approach that combines a deep, theoretical understanding of microbial kinetics with the diligent, practical control of key operational parameters like sludge age and F/M ratio. The ultimate goal is the strategic selection and fine-tuning of appropriate process configurations—whether aerobic or anaerobic, fixed-film or suspended-growth—to meet specific treatment objectives. From fundamental BOD removal to the complex challenges of toxicity management and stringent nutrient compliance, mastery of these principles empowers engineers and operators to achieve stable, efficient, and reliable performance from these vital environmental systems.