Question

7.0 E 2 of a 50:50 mixture on n-propanol (CH3CH2CH2OH) and n-butanol (CH3CH2CH2CH2OH) are discharged into a body of water containing 1.0 E 10 L of H2O. What is the BOD of this water?

Answer 1

Dear Friend please read the following and you will find the way how to calculate BOD. With the provided data

                              (range of BOD is in between 4-14)

                The biochemical oxygen demand (BOD) is an empirical test, in which standardised laboratory procedures are used to estimate the relative oxygen requirements of wastewaters, effluents and polluted waters. Micro- organisms use the atmospheric oxygen dissolved in the water for biochemical oxidation of organic matter, which is their source of carbon. The BOD is used as an approximate measure of the amount of biochemically degradable organic matter present in a sample. The 5-day incubation period has been accepted as the standard
for this test (although other incubation periods are occasionally used).

Substances oxidisable by molecular oxygen, such as ferrous iron, sulphite, sulphide and aldehyde, and alcohols (n-propanol & n-butanol) impose a load on the receiving water and must be taken into consideration. The total oxygen demand of such a substrate may be determined by using a calculated initial dissolved oxygen (DO) or by using the sum of the immediate dissolved oxygen demand (IDOD) and the 5-day BOD. Where a differentiation of the two components is desired, the IDOD should be determined. It should be understood that the IDOD does not necessarily represent the immediate oxidation by molecular dissolved oxygen but may represent an oxidation by the iodine liberated in the acidification step of the iodometric method. The depletion of dissolved oxygen in a standard water dilution of the sample in 15 minutes has been arbitrarily selected as the IDOD. To determine the IDOD, the dissolved oxygen of the sample (which in most cases is zero) and of the dilution water are determined separately. An appropriate dilution of the sample and dilution water is prepared, and the dissolved oxygen of the sample dilution minus the observed dissolved oxygen after 15 minutes is the IDOD (mg l^-1) of the sample dilution.

When BOD has been determined in a diluted sample:

If the BOD of the dilution water reaches the limit of 0.5 mg l^-1 or approaches it, the correction may be of importance, especially for samples of water having a low BOD but requiring a dilution. In such cases, correction for BOD may be used. The calculation is then:

where:
BOD = BOD of the sample
BODm = BOD of the mixture (sample + dilution water)
BODd = BOD of the dilution water (blank)
V = volume (ml) of sample in 1 litre of the mixture
Sm = DO of the mixture before incubation
St = DO of the mixture after incubation (after t days)
Dm = DO of the dilution water before incubation
Dt = DO of the dilution water after incubation for t days

Thank you, all the best.