Question

What does $BO{D_5}$ represent?
A.Biological oxygen demand in five days
B.Dissolved oxygen left after five days
C.Dissolved oxygen consumed in five days
D.Microorganisms killed by ozone in sewage treatment plants in five hours.

Answer 1

BOD is defined as biological oxygen demand which is defined as the amount of oxygen required by bacteria to break the un-necessary molecules present in a certain volume of water. The unit in which BOD is measured is ppm.

Complete step by step solution:
First of all we will read about the concentration terms.
Solute: The compound which is mixed in a given substance, is known as solute.
Solvent: The compound which is given and in which solute is mixed, is known as solvent.
Molar Concentration: It is defined as the amount of substance per unit volume of solution.
Molarity: It is defined as the number of moles in the given volume of the solution. The unit of the molarity is as: $mol/litre$.
Molality: It is defined as the number of moles of solute per kilogram of solvent. The unit of molality is: $mol/kg$.
Normality: It is defined as the gram equivalent weight per unit litre of solution. The unit of normality is: $eq/litre$.
Equivalent mass: It is defined as the mass of one equivalent.
Ppm (parts per million): It is also expressed in milligrams per litre. It is defined as mass of substance per unit volume of water.
BOD is defined as biological oxygen demand which is defined as the amount of oxygen required by bacteria to break the un-necessary molecules present in a certain volume of water. The unit in which BOD is measured is ppm. For pure water BOD value is $1{\text{ milligram/liter}}$. And for moderate water BOD value is $2 - 8{\text{ milligram/liter}}$. For mostly polluted water BOD value is $20{\text{ milligram/liter}}$.
$BO{D_5}$ represents the biological oxygen demand for five days. BOD represents oxygen which is used by microbes to eliminate contamination from waste water.

So option C is correct i.e. $BO{D_5}$ represents dissolved oxygen consumed in five days.

Note: COD: It is defined as Chemical oxygen demand which is defined as the measurement of all the chemicals (either organic i.e. containing carbon and hydrogen, or inorganic) present in the water or wastewater.