Question

3. The following problems refer to the ecological impact of pig slurry leaking into a local river. a. Calculate the COD for pig slurry that has an average carbon, hydrogen, and oxygen ratio of CH,O and a concentration of 47g/L of CH2O in water. b. Assuming that the BOD/COD ratio for the pig slurry waste is 0.60, calculate the BOD for this waste. The BOD calculated here is the ultimate BOD.

Answer 1

3

a)

COD (Chemical oxygen Demand) is a measure of oxygen requirement of a sample that is susceptible to oxidation by strong chemical oxidant.It is generally determined by using K₂Cr₂O₇ as oxidant.  It expressed as mass of oxygen consumed by volume of solution : milligrams per litre (mg/L). It is used to easily quantify the amount of organics in water.

pig slurry have C,H,O ratio as : CH₂O (1:2:1)

Complete oxidation reaction of CH₂O :

CH₂O + O₂ -----> CO₂ + H₂O

One mole of CH₂O unit reacts with one mole of O₂ .

Concentration of CH₂O in water = 47 mg / L

(Molar mass of a  CH₂O unit = 30 g /L .; 1 g = 1000 mg )

No. of moles of CH₂O unit = 47*10^-3 (g / L ) / 30 g /mol = 1.57*10^-3 mol / L

Thus moles of O₂ required = 1.57*10^-3 mol / L

( Molar mass of O₂ : 32 g / mol )

Amount of  O₂ required = 1.57*10^-3 mol / L * 32 g / mol = 0.05 g /L

or COD = 50 mg/L

b) Though there is no definite relationship between COD and Biochemical Oxygen Demand (BOD) of the waste. But, we have been given a ratio of BOD and COD for pig slurry waste :

BOD / COD = 0.60

we have COD = 50 mg/L

so, BOD = 0.60*50 mg/L = 30 mg /L