Oxidation

Question 1

What does aerobic oxidation produce?

Answer 1

New cells and relatively stable end products quickly

Question 2

What does anaerobic oxidation produce?

Answer 2

New cells and relatively unstable end products slowly

Question 3

What process is important in self purification?

Answer 3

Aerobic oxidation

Question 4

Describe aerobic oxidation process and application?

Answer 4

Takes place in natural wastewater treatment such as fixed film and activated sludge.
O2 is required to stabilise carbon and nitrogen compounds. End products are stable. Biochemically efficient and fast.

Question 5

Describe anaerobic oxidation process and application?

Answer 5

Takes place in freshwater which are rich in organic matter and also in sludge digestors. End products are unstable and smelly (can be used for biogas). Biochemically inefficient and slow.

Question 6

Describe a BOD test

Answer 6

Test to determine BOD of a sample.
Sample is tested over 5 days at a 20 degrees sealed container in the dark. BOD5 measures amount of oxygen consumed by MO in the 5 days which is representative of 65% of the total MO in sample.

Question 7

How do we try to replicate the natural oxidation processes in our sewage systems?

Answer 7

MO break down organic matter (pollution) and consume oxygen. We can use heterotrophs and autotrophs in our wastewater to break down this organic matter and clean our water.

Question 8

Why must we be careful if pipe gradients are flat? What can we do to prevent this?

Answer 8

Bacteria can release gas which will accumulate and degrade pipes. Need corrosion resistant pipes and ventilation in the sewer systems.

Question 9

What is the carbonaceous oxidation phase?

Answer 9

Phase in which the MO use up the readily assimilable OM.

Question 10

What limits the carbonaceous oxidation?

Answer 10

Lo which represents the amount of food available for the MO

Question 11

What is the nitrogenous oxidation phase?

Answer 11

Represents the O2 required for nitrification.

Question 12

When does the effects of nitrification become apparent in raw water samples?

Answer 12

Slow growth rate of nitrifying bacteria does not become important until 8-10 days have elapsed

Question 13

When does the effects of nitrification become apparent in treated effluent samples?

Answer 13

Large number of nitrifying bacteria means effects of nitrification become apparent after 1 or 2 days

Question 14

Why is nitrification important when measuring BOD? How is this reduced?

Answer 14

It exerts significant O2 demand and modifies the BOD curve. Adding ATU inhibits these effects so that only the carbonaceous demand is measured.

Question 15

What is the assumption for the basic BOD model?
What does this mean?

Answer 15

Rate at which organic matter is oxidised is proportional to the amount of oxidised material left.
Rate is initially high and gets progressively slower.

Question 16

What are the coefficients in the BOD(t) equation?

Answer 16

Lo = ultimate BOD, limit of carbonaceous phase (mg/l)
K1 = breakdown coefficient (day-1)
L(t) = concentration of organic matter (mg/l)

Question 17

Derive the BOD(t) equation

Answer 17

dL/dt = -K1L
L(t) = Loe^(-K1t)
BOD(t) = Lo-L(t)
BOD(t) = Lo(1- e^(-K1t))

Question 18

What is BOD

Answer 18

Amount of oxygen consumed by MOs whilst breaking down OM

Question 19

How can we find Lo?

Answer 19

For a given temperature can determine DO saturation level where D is the difference between saturation and now
Higher temperature = DO solubility reduces
Use D(t) equation to get Lo

Question 20

What is BOD ultimate?

Answer 20

BOD ultimate = Lo

Question 21

What does the oxygen balance require for OM consumption?

Answer 21

Maintain a balance between BOD and processes which reoxygenate the water since aerobic oxidation is important de-oxygenation factor.

Question 22

How does reoxygenation occur?

Answer 22

Turbulence, sedimentation, bacterial decay etc.

Question 23

What is the most important factor for reaeration? What does this mean for the rate of reaeration?

Answer 23

Oxygen saturation deficiency
The greater the difference between the level of dissolved oxygen in water and the saturation level, the faster the rate of reaeration.

Question 24

What is reaeration?
What does it depend on?

Answer 24

The rate of solution of oxygen in water
Depends on solubility and diffusion

Question 25

What does solubility depend on?

Answer 25

Temperature - Osat down with T up Pressure - Osat down with P down Salt Concentration - O2sat down with salt up

Question 26

What is diffusion and what does it mean for reaeration?

Answer 26

Process where rate of flow is proportional to concentration gradient. Substance flows from area of high concentrations to low concentrations. The lower the O2 in the water the faster it replenishes.

Question 27

What is the basic model for reaeration?

Answer 27

Oxygen dissolves in water at a rate proportional to the dissolved oxygen

Question 28

What are the parameters in the D(t) equation?

Answer 28

D(t) = deficit of dissolved oxygen K2 = reaeration coefficient Do = original DO deficit at t=0

Question 29

How do you calculate D?

Answer 29

D = cs - DO cs = saturation concentration of DO DO = dissolved oxygen at that time

Question 30

Derive the D(t) equation?

Answer 30

dDO/dt = K2(cs-DO) D = cs - DO dD/dt = -K2D D(t) = Doe^(-K2t)

Question 31

What is the oxygen sag curve of a river and its assumptions?

Answer 31

Assumes only processes are decomposition of OM and DO replenishment by reaeration. Combine the effects saying the total rate of change of DO deficit is equal to the rate at which material oxidises minus the rate of reaeration. Curve of DO concentration against time or distance.

Question 32

What does the oxygen sag D(t) equation describe?

Answer 32

How the oxygen level in a river changes downstream of an oxygen demanding input.

Question 33

Where does the the maximum D occur on the oxygen sag curve? What equilibrium occurs?

Answer 33

Critical turning point where DO minimum occurs and dD/dt = 0. K1L(t) = K2D(t)

Question 34

What limits the oxygen sag curve?

Answer 34

DO saturation concentration

Question 35

What does K1 and K2 vary with?

Answer 35

K1 = increases with pollution K2 = increases with speed of flow

Question 36

When can we apply the conservation of mass?

Answer 36

When we know the flows and concentrations of both river and discharge into river we can find the concentration of the mixed fluid

Question 37

What are assumptions of the oxidation models?

Answer 37

- assumes point pollution is a single point - flow doesnt vary over time - not considered turbulence which adds O2 - not considered photosynthesis which consumes and releases O2 - not considered sedimentation which has an oxygen demand

Question 38

Why might it be necessary to dilute samples when performing a BOD test?

Answer 38

Sample with high BOD need to be diluted so they don’t use up all the oxygen in the water before the 5 days is up

Question 39

How do you calculate BOD of a sample which has been diluted?

Answer 39

BOD5 = DOinitial - DOfinal Find BOD for diluted sample Find BOD for water BOD of sample = x (Fraction of sample)x + (Fraction of water)BODwater = BODdilutedsample