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BOD_PPT__1_

Biochemical Oxygen Demand (BOD)

1. Introduction to BOD

  • Water can become "polluted" by organic matter during its flow.

  • Bacteria in water decompose organic waste, leading to increased oxygen consumption.

  • High levels of organic waste lead to increased bacterial activity and high BOD levels.

2. Definition of BOD

  • BOD is the measure of oxygen consumed by microorganisms to decompose organic matter.

  • Typically measured over 5 days at 20°C.

  • Total BOD affects food webs more than water quality.

3. Nutrients Affecting BOD Levels

  • Nitrates and Phosphates

    • Act as plant nutrients, causing rapid growth and subsequent decay.

    • This decomposition increases organic waste, raising BOD levels.

4. Differences in Oxygen Demand Measurements

  • Biochemical Oxygen Demand

    • Measures oxygen used for both organic (carbonaceous) and inorganic (e.g., sulfides, ferrous ion) degradation.

  • Biological Oxygen Demand

    • Exclusively measures oxygen used by bacteria to degrade organic substances.

5. Aerobic vs. Anaerobic Decomposition

  • Aerobic Decay Products

    • Carbon: CO2

    • Nitrogen: NO3

    • Hydrogen: H2O

  • Anaerobic Decay Products

    • Carbon: CH4

    • Nitrogen: NH3

    • Hydrogen: CH4, H2S, H2O

6. Environmental Significance of BOD

  • BOD is crucial for assessing biodegradability and the strength of waste.

  • Important for assessing pollution levels in surface and groundwater.

  • Facilitates measurement of discharged effluents on oxygen resources.

  • Data guides engineering decisions for wastewater treatment plants.

7. BOD Standards

  • Optimal BOD for discharged effluents is <30 mg/L.

  • Drinking water typically has a BOD <1 mg/L.

  • When BOD reaches 5 mg/L, water purity is questionable.

Water Quality

BOD Levels (mg/L)

Very Good

1-2

Fair

3-5

Poor

6-9

Very Poor (High Pollution)

100 or greater

8. Factors Affecting BOD

  • Temperature

    • Elevated temperature lowers dissolved oxygen (DO) and increases metabolic activity.

    • High temperatures can lead to anaerobic conditions and increased bacterial activity.

    • This can cause algal blooms, reducing oxygen levels in water.

9. Eutrophication

  • Occurs due to oversupply of nutrients leading to excessive plant and algae growth.

  • Decaying matter creates hypoxic conditions by consuming oxygen.

  • Phosphorus is the primary limiting factor in this process.

10. Effects of High BOD Levels

  • High BOD levels favor organisms that tolerate low DO (e.g., leeches).

  • Organisms requiring higher oxygen levels (e.g., caddisfly) may not survive.

11. BOD Testing - Basic Steps

  1. Sample Collection:

    • Use grab or composite samples, with composite preferred.

    • Samples should be tested within 6 hours, kept at ≤ 4°C if delayed.

  2. Dilution:

    • Dilute samples to balance oxygen demand and supply.

    • Ensure high-quality organic-free dilution water.

  3. Pretreatment:

    • Neutralize extreme pH values to 7.0 and remove chlorine before testing.

  4. Seeding:

    • Add live bacteria to samples where necessary.

12. Laboratory Procedure for BOD Testing

  • Fill BOD bottles with dilution water and add the sample appropriately.

  • Incubate at 20°C for 5 days; test DO content afterward to calculate BOD.

  • BOD Calculation:

    • BOD (mg/L) = (Initial DO - Final DO) x bottle volume

13. Conclusions of BOD Testing

  • Major interferences include substances inhibiting microbial growth (e.g., chlorines, heavy metals).

  • Algal growth increases DO levels, impacting BOD readings.

  • Variation in BOD measurements can occur due to multiple factors (temperature, day-to-day changes).