why BOD is high in raw sewage

BOD (Biochemical Oxygen Demand) is high in raw sewage because it contains a large amount of organic matter, which serves as food for microorganisms. These microorganisms consume oxygen as they break down the organic substances, leading to high oxygen demand.

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Reasons for High BOD in Raw Sewage

1. Presence of Organic Waste:

   • Human Waste: Feces and urine are rich in organic compounds like proteins, carbohydrates, and urea.
   • Food Waste: Kitchen waste, oils, and grease from households.
   • Detergents and Soaps: Organic surfactants contribute to the load.
   • Plant Material: Leaves, grass, or other biodegradable debris in runoff.

2. High Concentration of Suspended and Dissolved Solids:

   • Raw sewage has both dissolved organic compounds and suspended solids that decompose over time, requiring oxygen.

3. Pathogenic Microorganisms:

   • Bacteria and other microbes in sewage metabolize the organic content, consuming oxygen in the process.

4. Lack of Treatment:

   • In raw sewage, no processes have yet removed or stabilized organic matter, resulting in a high concentration of biodegradable material.

5. Nutrient Availability:

   • Nutrients like nitrogen and phosphorus present in sewage promote microbial activity, increasing oxygen demand.

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BOD Levels in Raw Sewage

• Typical BOD levels in raw sewage range between 200–600 mg/L, depending on the source and composition of the wastewater.

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Impact of High BOD

1. Depletes Oxygen in Water Bodies:
   • If discharged untreated, high-BOD sewage can deplete dissolved oxygen in rivers or lakes, harming aquatic life.
2. Eutrophication:
   • Nutrients and organic matter in sewage promote excessive algae growth, further reducing oxygen levels.

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How STPs Address High BOD

1. Primary Treatment:
   • Removes large solids and sediment, reducing BOD by 20–30%.
2. Secondary Treatment:
   • Biological processes (e.g., Activated Sludge Process, MBBR) degrade organic matter, significantly lowering BOD by up to 90%.
3. Tertiary Treatment:
   • Advanced methods like filtration or disinfection ensure further reduction to meet discharge standards.

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Conclusion

High BOD in raw sewage is a result of the large amounts of biodegradable organic material and microbial activity. Effective sewage treatment reduces BOD levels, protecting water bodies and enabling the safe reuse of treated water.