How Microorganisms Remove COD in Wastewater: A Complete Guide

Microorganisms In Water

COD in wastewater, short for Chemical Oxygen Demand, is one of the most important indicators of organic pollution in both domestic and industrial effluents. But how is this pollution removed efficiently and sustainably? The answer lies in nature’s tiniest engineers: microorganisms.

In this article, we’ll explore how microorganisms break down COD, the biochemical steps involved, and why microbial treatment is essential for modern wastewater treatment systems.

What Is COD in Wastewater?

Chemical Oxygen Demand (COD) is a measure of the amount of organic substances in water that can be chemically oxidized. It indicates the potential pollution load in the water, essentially a “menu” of carbon-rich materials like:

• Carbohydrates

• Proteins

• Fats

• Cellulose

• Synthetic organic compounds

The higher the COD level, the more polluted the water, and the harder it is to treat. However, this also means more "food" for microbial communities that digest these pollutants.

Microbial Wastewater Treatment: Nature’s Cleanup Crew

At the heart of every biological wastewater treatment plant is a diverse ecosystem of microorganisms, each playing a specialized role in removing COD:

1. Bacteria – The Primary Degraders

Bacteria are the main workhorses in wastewater treatment. They secrete enzymes that break down large molecules into smaller ones, absorb them, and convert them into energy and cellular material.

2. Protozoa – Quality Control and Clarification

Protozoa feed on free-floating bacteria and suspended particles, improving water clarity and maintaining microbial balance.

3. Metazoan (e.g., rotifers, nematodes) – Sludge Densifiers

These higher organisms appear in stable treatment systems and help improve sludge settleability by consuming smaller microbes and debris.

How Microorganisms Remove COD: The 4-Step Biochemical Breakdown

The microbial degradation of COD in wastewater follows a precise, four-phase biochemical process:

1. Hydrolysis – Breaking Down Complex Molecules

Large organic molecules like fats, proteins, and carbohydrates are too big for microbial cells to absorb. Hydrolytic bacteria release enzymes (e.g., proteases, amylases, lipases) that cut these into smaller components:

• Proteins → Amino acids

• Starches → Glucose

• Fats → Fatty acids + Glycerol

  
  

2. Acidogenesis – Initial Fermentation

Smaller molecules are fermented by acidogenic bacteria into volatile fatty acids (VFAs) like acetic, propionic, and butyric acids, along with hydrogen and carbon dioxide. This stage significantly reduces COD.

3. Acetogenesis – Preparing Methanogenic Substrates

Specialized bacteria convert the VFAs and alcohols into acetic acid, which is the primary substrate for methane-producing microbes.

4. Methanogenesis – Final Digestion and Energy Recovery

Methanogenic archaea use acetic acid and hydrogen to produce methane (CH₄) and carbon dioxide. This step completes the COD removal and creates biogas, a renewable energy source.

Aerobic vs Anaerobic COD Removal: Which Is Better?

Aerobic Treatment – Fast and Effective

In systems like activated sludge tanks, air is pumped into the water to supply oxygen. Aerobic bacteria rapidly oxidize organic matter into CO₂ and water, forming activated sludge flocs. This method is widely used in municipal sewage treatment for its high efficiency and speed.

Anaerobic Treatment – Slower but Energy-Producing

Conducted in oxygen-free reactors such as UASB, IC, or EGSB, this process is ideal for high-COD industrial wastewater (e.g., food, brewing). Though slower, it consumes less energy (no aeration needed) and produces biogas for energy recovery.

Sludge Management: The Final Step

After COD is removed, the microbial biomass (sludge) must be separated from the treated water:

• Sedimentation tanks allow sludge to settle.

• Clear supernatant is discharged or reused.

• Recycled sludge maintains microbial activity.

• Excess sludge is thickened, digested, dewatered, and safely disposed of or reused.

  
  

Benefits of Microbial COD Removal in Wastewater Treatment

• Efficient Organic Matter Degradation

• Low Energy Demand (especially in anaerobic systems)

• Biogas Generation

• Reduced Environmental Impact

• Adaptability to Different Wastewater Types

Conclusion: The Invisible Heroes of Water Treatment

Microorganisms in wastewater treatment play an indispensable role in removing COD, ensuring that polluted water can be safely discharged or reused. Through their complex metabolic pathways, they convert harmful organic pollutants into:

• Inert gases (CO₂, CH₄)

• Water

• Renewable energy (methane)

• New microbial biomass

  
  

This biological approach not only mimics nature’s water-cleaning process—it does so with speed, efficiency, and the bonus of resource recovery. Behind every drop of treated water is a microscopic but mighty biological system hard at work.

FAQs

1. What is COD in wastewater treatment?

   COD (Chemical Oxygen Demand) measures the amount of organic matter in wastewater that can be oxidized. It indicates how polluted the water is.

2. How do microorganisms remove COD?

   Microorganisms digest organic matter through processes like hydrolysis, fermentation, acetogenesis, and methanogenesis—converting it into harmless gases and biomass.

3. Which method is better for COD removal: aerobic or anaerobic?

   Aerobic methods are faster and widely used in cities. Anaerobic methods are slower but energy-efficient and suitable for high-strength industrial wastewater.

4. Can COD be completely removed from wastewater?

   Yes, modern treatment systems can reduce COD to levels that meet discharge standards, and in some cases, even allow water reuse.

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