8. Water

Question 1

What is Biochemical Oxygen Demand (BOD)

Answer 1

The oxygen required by microorganisms/bacteria to break down (oxidise) all biodegradable organic matter in a water sample

Question 2

What is chemical oxygen demand (COD)

Answer 2

The amount of oxygen used to react an oxidising agent when put in a sample

Question 3

What is Theoretical Oxygen Demand (TOD)

Answer 3

The maximum possible oxygen required if oxidation is carried out via complete combustion: calculated using molecular weights

Question 4

Draw L (remaining oxygen equivalent) and BOD exerted on a graph of BOD against time

Answer 4

Line for L is L=L0 e^-kt crossing the y axis at Lo
Line for BOD exerted is L0 (1-e(-kt)) so a mirror image of the L line. The BOD exerted has an asymptote at L0, known as the ultimate BOD.

Question 5

How would you calculate the fraction of maximum oxygen consumed in a time T after an effluent has been discharged?

Answer 5

Given a known BOD rate constant e.g. k20 = 0.1, can calculate kT =k20 * θ^{T-20} where theta is temperature coefficient.
BOD_t/BOD_0 = [L_0 (1 - e^-kT t)]/L_0

Question 6

Draw a typical dissolved oxygen sag curve ( Dissolved oxygen against time).
Mark critical time.

Answer 6

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Question 7

Why is river management strategy especially important in summer?

Answer 7

In summer, Temp is high and river flow rate is low. Low flow rate reduces dilution of effluents and high temp lowers the saturation of dissolved oxygen and increased the deoxygenation constant kd more than the reaeration constand kr

Question 8

What natural processes lead to O2 being removed from water

Answer 8

Organic decay, heat, salts

Question 9

What can be done to increase the dissolved oxygen in water source

Answer 9

Adding O2 to effluent
Reduce the ultimate BOD
Increase the re-aeration rate

Question 10

What is BOD_5

Answer 10

Indicates waste strength - it is the biological oxygen demand after 5 days

Question 11

What does the re-aeration rate constant, k, depend on

Answer 11

Physical activity of the water - turbulent mixing and areas exposed relative to volume of water

Question 12

Why is summer worse than winter for a river

Answer 12

Low flow rates reduce dilation, and give higher L_a and D_a values.
Low flow rates also reduce the re-aeration
High temperature increase deoxygenation rate