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Environmental Issues > Aerobic Digestion > Flashcards

Aerobic Digestion Flashcards

1
Q

What is sludge?

A

Sludge is microorganisms or biomass

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2
Q

What is BOD to the biomass and what are the consequences?

A

The biomass (microbes) sees the BOD as food (the BOD is acting as a substrate or nutrient). Air will be supplied to the biomass if there is aerobic digestion. After the biomass has fed on the BOD there will be decreased BOD, there will be byproducts including CO2 and CH4 and more biomass i.e. growth especially if aerobic.

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3
Q

What are the four main elements of life for the microogranisms that are in sludge?

A
  • Carbon
  • Nitrogen
  • Phosphorus
  • Energy source
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4
Q

Describe the requirement of the microorganisms for Carbon, Nitrogen, Phosphorus, Energy?

A
  • Carbon = growth
  • Nitrogen = proteins and enzymes
  • Phosphorus = DNA and handling energy in living cells
  • Energy source = respiration
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5
Q

For optimal bacterial growth, what is the ratio of C:N:P?

A

100:20:5

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6
Q

What is the kinetics for the growth of microbial sludge?

A

apparent growth rate = (specific growth rate of organisms - extinction rate) x concentration of active organisms

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7
Q

What is the formula for the specific growth rate?

A

Specific growth rate = actual growth rate of biomass/ concentration of active organisms

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8
Q

Remember what is the assumption being made about sludge at the moment?

A

That sludge is primarily bacteria, with some protozoa and yeast

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9
Q

What is the Monod Model?

A

Specific growth rate = (maximum specific growth rate x concentration of limiting substrate)/ (Ks + concentration of limiting substrate)

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10
Q

What does Ks represent?

A

The order of magnitude for the concentration of nutrient that becomes limiting.

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11
Q

What happens when S is a lot less that Ks?

A

The specific growth rate varies linearly with the concentration of the limiting substrate.

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12
Q

What happens when S is a lot greater than Ks?

A

Then the specific growth rate is about equal to the maximum specific growth rate.

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13
Q

What is the definition of the maximum specific growth rate?

A

It is the maximum growth rate that the microbes can manage in the best of conditions when no nutrients are limiting. No nutrients are limiting when the substrate concentration is very far from Ks.

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14
Q

How does the specific growth rate relate to the maximum growth rate when S = Ks?

A

Then specific growth rate is half of the maximum specific growth rate.

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15
Q

How is Ks determined?

A

Ks is specific to any limiting reagent.

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16
Q

What is the definition of the yield?

A

It is the rate of biomass production (without discount deaths) divided by the rate of substrate consumption. It is the biomass that can be produced per unit of substrate that can be consumed in a given period of time.

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17
Q

What is the formula for the yield?

A

Yield = specific growth rate x concentration of active organisms/ - rate of substrate consumption

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18
Q

What is the difference between the yield and the observed yield?

A

The observed yield accounts for the extinction of biomass. It is the net growth of the microbial mass.

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19
Q

What is the formula for the observable yield?

A

Observable yield = (specific growth rate - extinction rate) * concentration of live cells/ - r s

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20
Q

What is rs?

A

It is the rate at which the BOD is consumed by the bacteria.

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21
Q

What is the aim of aerobic digestion?

A

We convert dirty water with BOD to clean water with less BOD and some cells.

22
Q

How may the cells be washed out?

A

By overloading the digester or wiped out by discharge of chemicals.

23
Q

What two classes are municipal treatment units divided into?

A
  • Biological Filters

- Active Sludge Units

24
Q

For what size of population are biological filters used?

A

For small scale and capacity, near small centres of populations

25
Q

Which microbial populations do biological filters use?

A

Slime-exuding microbial populations

26
Q

Describe the operation of biological filters.

A

The filter is packed with coarse particles such as gravels. Microbes exude a layer of slime which covers the surface. Water trickles down over the surface. O2 and nutrients transfer from water to slime layer. CO2 transfers back through water film to atmosphere. Biomass builds up in slime and must be removed periodically.

27
Q

What happens in the aeration tank for the activated sludge process?

A

The sludge is suspended in aeration tanks where it is fed with waste water and air. This is where it digests BOD.

28
Q

What happens in the clarifier for the activated sludge process?

A

It concentrates and separates the sludge in the clarifier. Most of the sludge is recycled.

29
Q

What is the significance of the recycle?

A

Recycling the live (activated) sludge back to the digester boosts the productivity by increasing the concentration of cells in the digester. This results in a greater conversion of BOD (i.e. removal) and smaller digester.

30
Q

What are the consequences of increasing the recycle ratio at constant BOD load?

A
  • Remove more BOD
  • Achieve same BOD removal with smaller V (less capital cost)
  • But raise pumping costs
31
Q

What sort of process is activated sludge?

A

It is an intensified process, increasing productivity by forcing both aeration and sludge concentration.

32
Q

How do we estimate the minimum compressed air rate?

A

21 % of air = (BOD out - BOD in) x wastewater flowrate

33
Q

Why is excess air required?

A

The mass transfer process may not be inefficient

34
Q

What does the performance of the aerobic digestion depend on?

A

The BOD load from the feed (s0) and the recycle ratio (alpha)

35
Q

What are digesters called in aerobic digestion?

A

Aeration tanks

36
Q

Why is it important that there are multiple tanks that can operate in series?

A

You can control oxygen supply in one tank to reduce nitrates to nitrogen.

37
Q

What is the main cost associated with aerobic digestion?

A

The provision of compressed air.

38
Q

What compounds can act as inhibitors to sludge bacteria?

A

Chlorinated compounds, phenols, heavy metals

39
Q

What happens when sludge bacteria is inhibited?

A

Readily biodegradable BOD may be too slow to degrade.

40
Q

What happens if there are inhibitors present?

A

There should be pre-treatment such as adsorption on resin or precipitation of heavy metals

41
Q

What nitrogen elements may be present in the feed?

A

Organic N2 and ammonia NH3

42
Q

To what is nitrogen and ammonia degraded?

A

Ammonia, Nitrite, Nitrate and organic nitrogen

43
Q

How do we deal with ammonia product?

A

Aerobic conditions sludge can easily consume ammonia provided that the conditions are not acidic nor toxic and it will produce organic nitrogen.

44
Q

What is required for the removal of ammonia by biomass?

A

Nitrification.

45
Q

How do we remove biological nitrate?

A

By using nitrate as an oxidising agent. NO3 - to NO2 - to N2 gas. This is denitrification.

46
Q

What is the problem with nitrates in high concentrations?

A

Eutrophication agents and harmful.

47
Q

What are the advantages of placing denit before the main stage of aerobic digestion?

A

Uses the initial, plenty BOD content of effluent as carbon source for denit bacteria.

48
Q

What are the disadvantages of placing denit before the main stage of aerobic digestion?

A

Organic nitrogen further down digester where oxygen is present will degrade to new nitrate content, potentially slipping through the unchecked clarifier.

49
Q

What are the advantages of placing denit after the main stage of aerobic digestion?

A

Screens off the nitrate just before discharge of treated effluent to clarifier. It should catch it all.

50
Q

What are the disadvantages of placing denit after the main stage of aerobic digestion?

A

Where will BOD come from for feeding denit bacteria? It may rely on decaying aerobic sludge, hence BOD contamination of treated effluent.

51
Q

How de we make denit a hybrid?

A

Before main stage of aerobic digestion plus recycle of treated water.

52
Q

What are the advantages of the hybrid?

A
  • Uses the initial, plenty BOD content of effluent as carbon source for denit bacteria
  • Recycling of water addresses the slip through issue to a large extent and greatly improves conversion rate of nitrates to nitrogen and even that of BOD.

Environmental Issues (15 decks)

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