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Environmental Issues > Water Pollution > Flashcards

Water Pollution Flashcards

1
Q

What percentage of fish kills are from natural sources and which from pollution?

A
  • 45 % from natural sources

- 9 % from pollution

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2
Q

What is the most significant cause for fish kills?

A

Low dissolved oxygen

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3
Q

What is the definition of hypoxia?

A

When dissolvd oxygen concentration is low

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4
Q

What is the definition of anoxia?

A

When there is no dissolved oxygen concentration at all

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5
Q

What are three natural causes for oxygen depletion?

A
  • excess population growth causing an increase in fish numbers
  • rotting vegetation
  • barriers to transfer oxygen from atmosphere e.g. total icing of water surface
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6
Q

What are two human causes for oxygen depletion?

A
  1. Effluents rich in nutrients (typically organic compounds that are sufficiently biodegradable)
  2. Substances that stimulate plant growth
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7
Q

How are the concentrations of nutrients that deplete oxygen expressed?

A

The concentration is expressed as a decrease in oxygen concentration that would result from their oxidation. This is the Biological Oxygen Demand (BOD).

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8
Q

What is the name for oxygen depletion due to plant growth?

A

Eutrophication and in the extreme, hypertrophication

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9
Q

Why do organic compounds typically cause depletion of dissolved oxygen?

A

They are easily reacted with oxygen. However this reaction is spontaneous or fast.

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10
Q

What is the agent for oxidising organic hydrocarbons and why does this then lead to a depletion of oxygen?

A

Microorganisms. Microorganisms in the water (typically bacteria) will see the oxidisable pollutant as food. To use this food for their energy needs, they also consume dissolved oxygen.

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11
Q

How is dissolved oxygen concentration measured?

A

Appropriate selective electrodes

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12
Q

What is the saturated concentration of oxygen in freshwater?

A

At 0 oC concentration is 14.2 mg/L

At 25 oC concentration is 9.2 mg/L

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13
Q

What does these low saturation concentrations means for organic compounds?

A

These low values explain the ease with which organic and oxidisable compounds can so easily induce hypoxia and hence environmental damage in water.

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14
Q

In which two ways can the level of pollution with oxidisable materials be expressed?

A
  • Biochemical oxygen demand (BOD)

- Chemical oxygen demand (COD)

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15
Q

What other compounds contribute to BOD and COD?

A
  • Ammonia can contribute significantly to both BOD and COD

- Some oxidisable mineral ions can also contribute COD but depresses BOD

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16
Q

How is the Biochemical Oxygen Demand (BOD) measured?

A

BOD is measured in mg/L. It is measured by incubating the water at 20 oC for 5 days. Sometimes however it is measured over 7 days. Microbes already in the water multiply using the oxidisable pollutant as a source of carbon and energy.

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17
Q

How is the BOD then determined from the measured values of oxygen in the experiment?

A

BOD = initial oxygen concentration - final oxygen concentration

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18
Q

What is the problem with the BOD?

A

It avoids identifying the oxidizable compounds themselves on their concentrations. Instead it measures their collective depleting effect on the oxygen concentration in the water.

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19
Q

Give examples of pollutant that contribute to BOD.

rank in decreasing BOD in mg O2/ mg pollutant

A

Sodium acetate, glucose, glutamic acid, sorbitol.

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20
Q

What is the chemical oxygen demand?

A

It is the oxygen demand of the organic material that is oxidised over 2 hours using acid potassium dichromate i.e. using more aggressive oxidising conditions

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21
Q

What is greater - the COD or the BOD?

A

COD is greater than BOD. The ratio of BOD: COD is typically greater than 0.3.

22
Q

What is the implication of BOD < 0.3 COD?

A
  • inhibition of bacterial growth by some pollutants

- poorly degradable organic pollutants

23
Q

What is the contribution of NH3 and NO3?

A

They make a direct contribution to oxygen depletion on account of significant COD value. They are an indirect cause of further depletion of dissolved oxygen since they are eutrophication agents i.e. they promote the growth of algae.

24
Q

What are some sourves of BOD?

A

Human sewage, slurry from cattle raising, agricultural runoff, dairy and food industry, fish farming in marine environment

25
Q

What model is used for the effects of discharge of high BOD wastes?

A

Streeter-Phelps Oxygen Sag Model

26
Q

What is the overall process in the model?

A

BOD –> O2 deficit –> Reoxygenated water

27
Q

What is the oxygen deficit?

A

The oxygen deficit is the difference between the concentration of oxygen at the baseline to the concentration of oxygen at that point.

28
Q

What are the two reactions in the Streeter-Phelps model?

A
  1. The reaction of organic waste with oxygen to biomass, CO2 and H2O
  2. The process of reoxygenation into water
29
Q

What are the assumptions of the reactions in the Streeter-Phelps model?

A

That both reactions are of the first order. The first with respect to the BOD. The second with respect to the oxygen deficit.

30
Q

For which reactions are k1 and k2 each associated?

A

k1 is associated with conversion of BOD to D and k2 is associated with D to oxygenated water.

31
Q

In what way is water reoxygenated?

A

By the equilibrium of air with water.

32
Q

How would you find the maximum deficit from the Streeter-Phelps Sag model?

A

Differentiating respect with time and setting to zero. We are then able to find the time at which the maximum occurs and the downstream distance with which this is associated (distance = flow velocity x time)

33
Q

On what does k1 depend?

A

The temperature and water quality (bacterial content). It depends on the type of BOD. High for glucose and other metabolites related to it, low for more complex, less easily broken down materials.

34
Q

On what does k2 depend?

A

The hydrodynamic conditions. Higher for shallow, faster-flowing rivers and lower for stagnant deep pools of water.

35
Q

Give some sources of error in the Streeter-Phelps model.

A
  • Rivers are seldom plug flow reactors so the conversion of distance = speed x time is only an approximation
  • Stagnant pools can become anoxic even if flowing rivers are not
  • Weirs or other turbulent generators can reoxygenate the water much more rapidly than the rest of the river
  • Any tributary or confluence will dilute the BOD and delay hypoxia
  • Time for the maximum deficit may be several days - by which time most water will discharge to sea.
36
Q

What are some of the results of oxygen depletion?

A
  • If oxygen concentration drops below 2 mg/L even for a short time then aerobic organisms in the river begin to die off
  • “Wave” of low oxygen water from a spillage can travel downstream and wipe out life over whole waterway
  • dead organisms biomass becomes more BOD for the river to deal with, amplifying the effect
37
Q

What are the effects of the release of fertilising substances in eutrophication?

A

The promotion of uncontrolled plant growth. This will then:
- block light from reach the surface below (killing the plants there)
- consume all the oxygen at the night time (anoxia) thereby killing off most plants and animals
In the extreme this is then referred to as hypertrophication.

38
Q

BOD is the major incidence of pollution. Give some examples of other pollutants.

A
  • pH
  • Water Hardness
  • Temperature
  • Heavy Metals
39
Q

At what pH should the effluent be discharged?

A

pH 7 with an uncertainty of 1

40
Q

Are organisms more tolerant to acids or alkalis?

A

Acids

41
Q

How should acidic discharges be treated?

A

Neutralisation using calcium hydroxide and calcium carbonate.

42
Q

How should alkalis be treated?

A

Collecting discharges from different plants. Alkaline discharges could neutralise acidic discharges.

43
Q

For which metal ions is water hardness associated?

A

Ca (2+) and Mg (2+)

44
Q

Dissolved salts are not too bad in small concentrations. But what are effects for high concentrations?

A

Osmotic pressure problems for freshwater organisms, suck water out from cells. It is more difficult to absorb water. May be associated with stomach upsets and persistent stomach problems.

45
Q

What are the polluting effects of suspended solids in water?

A
  • aesthetic
  • lowers light penetration and prevents photosynthesis
  • sediment on the bottom can lead to oxygen starvation and reduced food availability. Organic sediment can be treated as BOD.
46
Q

Why is oxygen depletion amplified at greater temperatures?

A
  • The solubility of oxygen in water decreases

- The demand of oxygen increases because the metabolic rate increases

47
Q

What happens if the temperature is above 40 oC?

A

Thermal mortality sets in. Typically acquatic life are at the temperature of the water it is in.

48
Q

How is the temperature of effluents thus controlled?

A

Heat is labelled as a “prescribed” substance under the IPPC regime.

49
Q

Give some examples of heavy metals.

A

Pb, As, Hg, Cd, Cr.

50
Q

What is the greatest problem related to heavy metals?

A

Their bioaccumulation in the food chain. This results in poisons of humans and other organisms. There is often no route for excretion.

51
Q

What is the use of pesticides in industry?

A

It was developed as toxic to pests and harmless to humans.

52
Q

What is the main problem related to pesticides?

A

We need them but we don’t want them to stick around. Otherwise, the pesticides will become PoP and can lead to bioaccumulation. All those which persist above 6 weeks are being banned.

Environmental Issues (15 decks)

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