Questions Engineering

How is BOD measured?

Airtract

Nitika Kale

Always deliver more than expected

Biochemical Oxygen Demand (BOD), is defined as a chemical procedure for determining the quantity of oxygen needed by organisms to break down the organic material in wastewater. Organic matter such as dead plants, leaves, food waste present in water, the bacteria start breaking down these waste materials.

Methods to measure BOD

Dilution Method

This is considered as the standard method for measuring BOD. It is recognized by Environmental Protection Agency (EPA) U.S. This analysis is carried out by taking 300ml incubation bottles containing buffered dilution water which is heavily dosed with seed microorganisms and kept in the darkroom at 20° Celsius for pentad (5-days). The 5-days time is due to avoid DO (Diluted Oxygen) production exerted by photosynthesis. To measure carbonaceous BOD (cBOD), a nitrification inhibitor should be added after adding dilution water to the sample, and as a result, nitrogenous BOD (nBOD) is exhibited. 

Calculation 

Unseeded- BOD5 = [(D0-D5)]/P

Seeded- BOD5 = [(D0-D5)-(B0-B5)f]/P

Where:

D5- DO after 5-days incubation.

B0-diluted seed sample after preparation.

B5- Diluted seed sample after 5 days incubation.

F-  Seed volume in dilution solution to seed volume in BOD Test.

D0- Is the dissolved oxygen.

P-  P represent the Decimal dilution factor.

Manometric method

In this method, keep the sample in a tightly packed container which is fitted with a pressure sensor. Generally, adding lithium hydroxide to the container above the sample level. Compared to the above method ( method for dilution) this is simple and doesn't need dilution of the sample and no need of seeding, and also this method allows to read the values of BOD directly.

Some of the alternative methods include Biosensor, software sensor, a fluorescent Redox indicator, and titration and so on.

Measurement problems

There are many problems which lead to some disturbances in the measurement; they are either due to dirty BOD bottles and DI water or due to bad seed and bubbles and so on. Some measures that should be taken into account while performing the measurements are provided below:

  • Calibration to the instrument should be made once in a day before taking the reading. 
  • Sensors should have fresh electrolyte and membranes along with maintained anode and cathode for better performance.
  • A warm-up time of 5-15 minutes should be given to the probes before calibrating and conducting tests.

Safety and hygiene

  •  Wear gloves while performing tests as some chemicals may be dangerous.
  •  Clean all the spills and drops of buffers or wastewater as soon as possible.
  • Wear an apron while dealing with chemicals.
  •  Always try to use a pipette bulb instead of pipetting with the mouth. 
  • One more essential thing is that add acids /bases into the water and never add water into acids/bases. 

Conclusion

Manometric is an accurate, handy method for the determination of BOD. The results obtained after conducting the technique show that there will be some deviation between BOD7measured by manometric and titration method.


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