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BOD Determination

For determination of BOD the sample is diluted with distilled water such that the BOD of the diluted sample is less than about 6.0 mg/ L. The diluted sample is aerated to increase the DO level and seeding done to provide microorganisms. The initial DO concentration in the sample before the incubation and final DO in the sample after 5 days of incubation are measured. The difference between the initial and the final DOs is the DO consumed and is the BOD of the diluted sample. From this value the BOD of the sample is determined multiplying with the dilution factor and expressed as BOD5, in mg/L.

While conducting BOD test, domestic wastewater is added as seeding material, a source of microbial population, to decompose the organic waste. The microbial seeds being used in different laboratories at different time are not of uniform nature. Moreover, the microbial community varies from season to season at any place. There are variations in microbial diversity in terms of species and number, which lead to variations in the results of BOD estimation. To overcome these problems associated with use of conventional wastewater seed, a standard mixture of microbial cultures named as BODSEED, has been developed in collaboration with Centre for Biochemical Technology, CSIR, Delhi. BODSEED is inexpensive, convenient and safe to handle as compared to conventional wastewater seed.

Ultimate BOD

Often it is desirable to translate BOD5 to total BOD or ultimate BOD (L) or the BOD (Y,) at some other time, t. The translation of BOD value to any other time period is done, by using the following equation:

= La (1 – 10KI) Eq. 1


Y, = BOD at any time t, mg/ L

Lo = the total or ultimate BOD, mg/L K = reaction rate constant, d-1 (base 10)

The reaction rate constant is also expressed to (base- e) as k.

The reaction rate constant is also expressed to (base- e) as k. The value of k for a particular wastewater must be determined by experiment. The value of k varies between 0.05 to 0.2 cll. The value of reaction rate constant can also be expressed to base 10 as K. The K value varies with temperature of wastewater, and the relationship with temperature is approximately given by the following equation:

K(T) = K(20°) (1.047) T– 2°° Eq. 2


K(T°) = reaction rate constant (base 10) at T°C, and K (20°) is the reaction rate constant at 20°C.

For Indian conditions the Central Pollution Control Board (CPCB) has stipulated the 3-day BOD at 27° C as the standard. The Bureau of Indian Standards (BIS) has also approved the methodology for adoption of test at 27°C for 3 days instead of 20°C for 5 days incubation for India and published as Indian Standard vide IS: 3025 (Part 44): 1993.

BOD is the most important and commonly used parameter in wastewater quality monitoring and designing of wastewater treatment plant. The test takes considerable time say 3 days at 27° C or 5 days at 20° C, as well as consumes lot of energy. To overcome these constraints, attempts have been made by the CPCB in collaboration with Centre for Biochemical Technology (CBT), CSIR, Delhi to carry out the test in short time through the technique using BIO-SENSOR probes. This technique involves selection of suitable microbial composition for using Bio-sensor so as to degrade the wide range of wastes.