What is Biological Oxygen Demand (BOD)? The Simple Explanation

Biochemical Oxygen Demand (BOD) is defined as the concentration of dissolved oxygen required by aquatic microorganisms to biochemically decompose organic matter under aerobic conditions.

What does a high BOD indicate?

High BOD levels indicate excessive organic pollutants in water. As a key parameter in environmental monitoring, BOD quantifies organic contamination through microbial degradation processes. During this aerobic decomposition, oxygen depletion occurs when dissolved oxygen levels cannot satisfy microbial demand, ultimately leading to water body deterioration.

What are common sources of high BOD?

1. Municipal Sources (Sewage)

This is the most well-known source of high BOD.

• Domestic Wastewater:

• Food scrapsfrom kitchen sinks and garbage disposals.

• Human wastefrom toilets.

• Soaps and detergentsfrom bathing and laundry.

• Commercial and Institutional Wastewater:Effluent from restaurants, hotels, hospitals, and offices, which often contains high levels of organic waste from food preparation, cleaning, and other activities.

  
  

2. Industrial Sources

Many industries use organic materials in their processes, and their wastewater can have extremely high BOD levels, often requiring pre-treatment before being sent to a municipal plant.

• Food and Beverage Processing: This is a major contributor. Waste from slaughterhouses, dairy plants, breweries, wineries, sugar refineries, and vegetable canning facilitiesis rich in fats, proteins, and carbohydrates.

• Pulp and Paper Mills: The process of turning wood into pulp and paper releases large volumes of organic waste, including lignin and cellulose.

• Textile and Tanneries: The process of tanning leather and finishing fabrics uses organic compounds that can significantly increase BOD.

• Pharmaceuticals and Chemical Plants:While often associated with toxic chemicals, these facilities can also produce wastewater with high organic content.

3. Agricultural Runoff

Farms are a significant non-point source (diffuse pollution) of high BOD.

• Animal Waste: Runoff from feedlots, dairy farms, and poultry farmscarries manure, which is highly rich in organic nitrogen and other biodegradable materials.

• Agricultural Waste: Water used for washing vegetables or from silage (fermented fodder) can have very high BOD.

• Fertilizers: While a problem for nutrients like nitrogen and phosphorus, fertilizers also contribute organic matter.

4. Natural Sources

Even without human intervention, some environments naturally have high BOD.

• Leaf Litter and Organic Debris: When large amounts of leaves, grass clippings, or dead algae enter a water body, their decomposition will consume oxygen.

• Algal Blooms: When algae die en masse, the subsequent decomposition by bacteria can lead to a massive oxygen crash.

Why is BOD Testing So Important in Water Treatment?

Hazards of Elevated BOD Levels

Ecological Impact: Excessive BOD triggers oxygen depletion through accelerated microbial decomposition of organic matter. Dissolved oxygen (DO) concentrations below critical thresholds (<2-3 mg/L) induce aquatic hypoxia, causing mass mortality of fish and benthic organisms. Eutrophic water bodies exemplify this through algal bloom-induced BOD surges, where post-bloom degradation creates anoxic dead zones.

• Wastewater Treatment Challenges: High BOD loading overwhelms biological treatment systems, resulting in:

• Compromised organic removal efficiency (effluent non-compliance)

• Impaired flocculation and settling of activated sludge

• Elevated operational costs from energy-intensive aeration

Risks of Insufficient BOD Levels

Microbial Dysfunction: Suboptimal BOD (<100 mg/L) starves heterotrophic microorganisms, leading to:

• Biomass decay through endogenous respiration

• Poor floc formation and sludge disintegration

• Reduced nutrient removal capacity (nitrification/denitrification imbalance)

Economic Implications: Low-carbon wastewater necessitates:

• Supplemental carbon dosing (methanol/acetate injection)

• Increased chemical expenditure (∼15-30% operational cost escalation)

• Process instability requiring real-time monitoring interventions

Factors Affecting BOD

How to Measure the BOD?

BOD can be measured by Yosemitech COD sensor and BOD sensor. 

Summary

Water treatment is vital for applications from bottled water to industrial processes. A key factor in this process is biological oxygen demand (BOD)—the amount of oxygen microorganisms consume while breaking down organic matter in water. Yosemitech is a global leader in the design and manufacture of quality sensors for water quality. We offer more than 30 kinds of sensors for pH, ORP, conductivity, dissolved oxygen, COD, BOD, and other specialty measurements, as well as a full line of sensor accessories and transmitters. If you have any questions or inquiry, feel free to contact us!