Water samples with BOD values of 4 ppm and 18 ppm, respectively, are :

Q. Water samples with $BOD$ values of $4ppm$ and $18ppm$, respectively, are:

Q. The COD values of three water samples A, B and C are 60 ppm, 990 ppm and 120 ppm respectively. The most polluted water sample is:

Q. A water sample contains 81 ppm of $Ca(HC{O}_3{)}_2$, 73 ppm of $Mg(HC{O}_3{)}_2$, 68 ppm of $CaS{O}_4$ and 60 ppm of $MgS{O}_4$. What is the hardness of the Water sample? (Atomic weights of calcium, magnesium and sulphur are respectively 40, 24 and 32).

Q. Biochemical oxygen demand (BOD) is a measure of organic material present in water. BOD value less than $5ppm$ indicates a water sample to be

Q. Biochemical Oxygen Demand, (BOD) is a measure of organic material present in water. BOD value less than 5 ppm indicates a water sample to be________.