COD Removal: Biological Degradation of Organic Matter

COD is one of the most important indicators used to measure the level of organic pollution in wastewater. High COD concentrations are typically associated with black water, odor problems, and high organic loading.

Common sources include:

• Domestic sewage
• Food processing wastewater
• Slaughterhouse wastewater
• Textile and dyeing wastewater
• Pharmaceutical effluent    

  The principle behind COD removal is straightforward:

  Microorganisms consume and break down organic pollutants.

  In biological treatment systems, microorganisms use organic matter as a food source. Through metabolic activity, complex organic compounds are gradually decomposed and ultimately converted into harmless substances such as carbon dioxide and water.

  Under aerobic conditions, the reaction can be simplified as:

  $$Organic Matter + O_2 → CO_2 + H_2O + Biomass$$

  Efficient COD removal is essential because excessive organic loading can negatively affect downstream nitrogen and phosphorus removal processes.

Ammonia Nitrogen Removal: Nitrification Process

Ammonia nitrogen is a toxic pollutant commonly found in municipal sewage, livestock wastewater, and industrial effluent. Elevated ammonia levels can deplete dissolved oxygen in water bodies, damage aquatic ecosystems, and create serious odor issues.

Unlike suspended solids, ammonia nitrogen cannot be removed through simple sedimentation. Instead, it must be biologically converted through nitrification.

Nitrification occurs under aerobic conditions and involves two biological steps:

Step 1: Ammonia Oxidation

$$NH_4^+ → NO_2^-$$

Step 2: Nitrite Oxidation

$$NO_2^- → NO_3^-$$

These reactions are carried out by nitrifying bacteria under controlled conditions.

Stable nitrification typically requires:

Total Nitrogen Removal: Biological Denitrification

Many people assume that once ammonia nitrogen is removed, total nitrogen compliance is automatically achieved. In reality, ammonia nitrogen is only one component of total nitrogen.

Total nitrogen includes:

• Ammonia nitrogen
• Nitrate nitrogen
• Nitrite nitrogen
• Organic nitrogen

After nitrification, nitrogen still remains in the water in the form of nitrate. To fully remove nitrogen, a denitrification process is required.

Under anoxic conditions, denitrifying bacteria use organic carbon sources to convert nitrate into nitrogen gas:

$$NO_3^- → N_2↑$$

The nitrogen gas is then released into the atmosphere, permanently removing nitrogen from the wastewater.

This process is widely used in advanced municipal and industrial wastewater treatment systems designed to meet strict discharge standards.

One important factor in denitrification is the availability of carbon sources. When the COD/TN ratio is too low, the system may experience insufficient carbon for effective nitrogen removal. In such cases, external carbon sources such as sodium acetate, glucose, or methanol are often added to improve denitrification performance.

Total Phosphorus Removal: Biological and Chemical Phosphorus Removal

Total phosphorus is one of the primary causes of eutrophication, algae blooms, and water quality deterioration in lakes and rivers.

Compared with nitrogen compounds, phosphorus is relatively stable in water, which means a single treatment method is often insufficient for consistent removal. For this reason, most treatment plants combine biological phosphorus removal with chemical precipitation.

Biological phosphorus removal relies on phosphorus-accumulating organisms (PAOs). These microorganisms release phosphorus under anaerobic conditions and absorb excess phosphorus under aerobic conditions, storing it within their cells.

The phosphorus is ultimately removed from the system through sludge wasting.

For high-phosphorus wastewater or advanced discharge requirements, chemical phosphorus removal is commonly applied as a supplementary process. Iron salts, aluminum salts, or other coagulants react with dissolved phosphorus to form insoluble phosphate precipitates, which can then be separated from the water.

This combined approach provides stable and efficient phosphorus removal performance for stringent environmental standards.

The Fundamental Logic Behind Modern Wastewater Treatment

Regardless of whether the process is:

A/O
A²/O
MBR
MBBR
SBR

the underlying treatment principles remain largely the same:

• COD → Biological degradation
• Ammonia Nitrogen → Nitrification
• Total Nitrogen → Denitrification
• Total Phosphorus → Biological uptake + Chemical precipitation

The difference between treatment technologies mainly lies in process configuration, aeration control, sludge management, recycle systems, and treatment efficiency optimization.

Conclusion

Modern wastewater treatment is fundamentally a controlled process of biological transformation and chemical separation.

By creating suitable anaerobic, anoxic, and aerobic environments, treatment systems allow microorganisms and chemical reactions to work together to remove pollutants efficiently and reliably.

As environmental regulations continue to become more stringent worldwide, wastewater treatment is moving toward:

Higher efficiency
Lower energy consumption
Intelligent operation
Sustainable resource recovery

Understanding the removal mechanisms of COD, ammonia nitrogen, total nitrogen, and total phosphorus is essential for optimizing treatment performance, reducing operational costs, and achieving long-term environmental compliance.