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CHEE 342 > Module 4 (Quiz 2) > Flashcards

Module 4 (Quiz 2) Flashcards

1
Q

List major problems of landfills as the source of CH4 release

A
  • Loss of energy + other resources
  • Expensive and long-term monitoring
  • Soil + groundwater pollution
  • Release of GHGs
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2
Q

World in 2100 will need ___ times that of the current amount of energy

A

3

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3
Q

List benefits of biogas plants/anaerobic digesters

A
  • Reduces reliance on landfills
  • Reduces greenhouse gas emissions
  • Renewable source of E
  • Supports rural areas
  • Recovers nutrients
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4
Q

How can organic waste be recovered using anaerobic digesters?

A

Converted to:
- Fertilizer (non-converted)
- Biogas > electricity + heat
- Biogas > biomethane > gas grid, fuel

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5
Q

T/F: Another application for anaerobic fermentation is a pretreatment step for aerobic bio processes (ie. activated sludge)

A

True

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6
Q

Name a category of anaerobic digestion

A
  1. Operating temp (meso = 30-40ºC; thermo (50-65ºC)
  2. Flow regime = batch, semi-batch, continuous
  3. Rector configuration (single stage, two stage)
  4. Substrate (anaerobic digestion, anaerobic co-digestion)
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7
Q

List ADVANTAGES of anaerobic ferment.

A
  • Lower biomass yields compared to aerobic
  • Potential for E prod in form of methane of H2
  • Lower operating cost (no aeration req)
  • Lower env footprint (less GHG contribution)
  • Capacity to handle high-strength org waste streams
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8
Q

List DISADVANTAGES of anaerobic ferment.

A
  • Slow rep processes (long retention time) > produces less of itself
  • Large V of tank
  • Odor (sulfur-containing compounds)
  • Safety (due to CH4 + H2 gas gen)
  • Corrosion (H2S prod as by-product)
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9
Q

List stages of anaerobic digestion (AD)

A
  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis
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10
Q

T/F: If the stage of AD (methanogenesis) is inhibited, the process is called aerobic fermentation.

A

FALSE: anaerobic fermentation (or acidification)

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11
Q

Hydrolysis: The ___ stage of AD, through which particulate materials (_______ organics) are converted to _________ compounds. If there is a high concentration of these organics already, acidogenesis may be the first stage of AD.

A

1st
insoluble
soluble

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12
Q

Acidogenesis: The ___ stage of AD, which is carried out by acidogenic bacteria. Results in prod of ________ fatty acids, CO2, and H2. A larger fraction of the long-chain fatty acid ____ is converted to H2 than that for _______ and ________.

A

2nd
volatile
COD
sugars
amino acids

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13
Q

Acetogenesis: Refers to further fermentation to convert _________ products of acidogenesis to produce ________, CO2, and H2, which will be used for _____ in the final step. This conversion requires that ___ by at low concentrations in the system (<10^-4).

A

intermediate
acetate
CH4
H2

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14
Q

Methanogenesis: Carried out by methanogens. Two groups are involved (1. ________ methanogens and 2. ________ methanogens). The first splits _____ into methane and CO2. The second uses ___ as the electron donor and ____ as the electron acceptor to produce methane.

A

acetoclastic
hydrogenotrophic
acetate
H2
CO2

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15
Q

T/F: On average, 55-65% of methane produced during AD is from acetate by acetoclastic methanogens

A

FALSE: 70-75%

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16
Q

T/F: Composition of biogas produced typically contains 70-75% methane and 25-30% CO2.

A

FALSE: 60-65% methane and 20-30% CO2.

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17
Q

T/F: A higher lipid fraction in the waste results in a higher CH4 fraction in the biogas

A

True

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18
Q

__________ methanogens obtain their E from the oxidation of hydrogen and use CO2 as their C source, which results in a low biomass yield.

A

Hydrogenotrophic

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19
Q

What are the two major reactions for producing methane?

A

4 H2 + CO2 > CH4 + 2 H2O

CH3COOH > CH4 + CO2

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20
Q

What is one of the challenges with the breakdown of methylamine to produce methane?

A

Presence of ammonium in the digester effluent; therefore need a way to treat the resulting ammonia that is produced/leaves the digester (ie. nearby treatment plant)

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21
Q

T/F: Sulfate-reducing bacteria are nuisance organisms and can be toxic to methanogens at high concentrations.

A

True

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22
Q

T/F: To reduce the sulfur in wastewater samples, we can add cobalt to form sulfur-cobalt precipitates.

A

FALSE: Iron, not cobalt

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23
Q

T/F: In the eqn CH4 + 2O2 > CO2 + 2H2O, 64g O2/mole CH4 is the ratio

A

True

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24
Q

What is the volume of methane per mole methane at STP (0ºC 1 atm)? Volume CH4 per gram COD?

A

22.414L CH4/mol CH4

0.35 L CH4/g COD

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25
Q

What are the two RLS of anaerobic processes?

A
  1. Hydrolysis conversion
  2. Soluble utilization rate for ferm + methanogenesis
26
Q

What is the range of retention time (days) for hydrolysis?

A

20-40 days

27
Q

List pre-treatment methods to overcome RLS of hydrolysis

A
  1. Thermal (using surplus E; recovering thermal E)
  2. Mechanical
  3. Biological
  4. Chemical
28
Q

What three things happen after pre-treatment?

(of anaerobic process)

A
  • Reduced amount of suspended solids (into volatiles)
  • Increased soluble fraction of org matter (soluble COD)
  • Reduced viscosity (flow easier in digester)
29
Q

What are BMP tests?

A

BMP tests are analogous to BOD tests: they preserve samples in bottles and monitor the methane production during an anaerobic process. (Batch operation mode, with digester effluent as the inoculum)

30
Q

What happens when temperature during a BMP test is increase? (hint: think about first order rxn kinetics rate constants)

A
  • Increase T = increase k
  • Increase k = decreases t (retention time)
  • Rxn proceeds quicker, more methane produced in shorter amount of time
31
Q

T/F: Production kinetics for VFAs are slower than corresponding utilization and methane production kinetics for methanogens.

A

False: faster

32
Q

T/F: Unstable digester operation can develop under transient loading conditions in which VFA production rate exceeds methanogenic VFA utilization rate.

A

True

33
Q

As VFA concentration increases, what happens?

A

pH drops (ie. acetate and propionate increases)

34
Q

what is external alkilinity? Name examples

A

Substance (ie. sodium/K bicarbonate) added to digester to ensure enough buffering capacity and prevent pH drop

35
Q

What happens during VFA accumulation?

A

pH drops > reduced methanogenic activity > VFAs accumulate more > pH drops further > REPEAT

36
Q

Methanogenic inhibition can occur when acetate concentration exceeds _____ g/m3.

A

3000

37
Q

What are some causes of unstable conditions in an anaerobic digester?

A
  • Transient temp change
  • Sudden increase in organic load
  • High [ ] inhibiting substances
38
Q

Define SRT/SRT_min

A

SRT = Sludge Retention Time (SRTs are selected based on kinetics of treatment goals. t 20, 25, 35ºC, the SRT_min (minimum time required for growth of the organisms in the reactor before they’re washed out from the system) is 8, 6, and 3 days.

39
Q

Anaerobic processes are sensitive to:

A

pH, nutrient limits, inhibitory substances

40
Q

T/F: a pH value near neutral is preferred for methanogenic activity (above 6.8, it is inhibited)

A

FALSE: below 6.8 inhibits activity

41
Q

T/F: if the waste stream contains enough concentration of fatty acids, sufficient alkalinity can be produced by the breakdown of fatty acid chains to produce CH4

A

FALSE: alkalinity is produced by the breakdown of protein and amino acids to produce NH3 (combines with CO2 and H2O to form NH4(HCO3)

42
Q

T/F: BMP tests are done to determine V_max of a system, which is the max acetate utilization capacity

A

True

43
Q

What is the acetate capacity number (ACN)?

A

Ratio of V_max (g acetate COD/m3d) to the avg estimated acetate utilization rate V_avg (g acetate COD/m3d)

44
Q

T/F: ~50% of the COD removed/used in the system is assumed to be converted to acetate before CH4 production

A

FALSE: 70%

45
Q

What is the Higher Heating Value (HHV)?

A

Upper lim of the available thermal energy produced by a complete combustion of fuel

46
Q

T/F: When calculating HHV, we use the percentages of each element in the fuel compound on a wet basis

A

FALSE: dry basis

47
Q

T/F: the E value of biogas is proportional to its methane, carbon dioxide and ammonia content.

A

FALSE: just methane content

48
Q

Why will the mole fraction of H2S in biogas be low?

A

Because of metal precipitation (to reduce the odor-causing compounds)

49
Q

What is the net energy production?

A

E produced (in the form of methane) - E used (to heat the feed strea to optimal temp)

50
Q

What are some advantages of anaerobic treatment processes vs. aerobic processes?

A
  1. energy considerations - may be the net E producers instead of E users compared to aerobic processes
  2. lower biomass yields - sludge processing + disposal costs reduced
  3. less nutrients required - less biomass production, imp for deadline with industrial organic waste streams
51
Q

What are some disadvantages of anaerobic treatment processes vs. aerobic processes?

A
  1. longer startip time to develop biomass inventory
  2. may require alkalinity addition
  3. may require further treatment with an aerobic treatment process
  4. bio nitrogen and phosphorus removal not possible
  5. more sensitive to -ve effect of lower temps on rxn rates
  6. may be more susceptible to upsets due to toxic substances or wide feeding changes
  7. potential for odor production and corrosiveness of gas
52
Q

What is “co-digestion” in the context of the AD process?

A

Simultaneous digestion of two (or more) feedstock

53
Q

What are two forms of digest that can be sent through an AD bioreactor?

A
  1. Biomass (crop residue): high C, low alkalinity, low nutrient (high C/N ratio)
  2. Manure: high ammonia, high alkalinity, rich in nutrients (low C/N ratio)
54
Q

Define a “synergistic effect”.

A

Result of two or more processes interacting tg to produce an effect greater or smaller than the cumulative effect that those processes produce when used individually.

+ve synergy: F(A+B) > F(A) + F(B)
-ve synergy: F(A+B) < F(A) + F(B)

55
Q

What is synergy wrt anaerobic co-digestion?

A

Methane production yield (per g COD added) obtained from the co-digestion of two or more feedstocks will be higher (or lower) than the summation of methane yield values from digesting each feedstock separately.

56
Q

List the steps to determine synergistic effects during anaerobic co-digestion (using BMP assays)

A
  1. Determine digester operating temp + the theoretical methane yield (mL/g COD) at that T
  2. Measure the COD content for each feedstock (g/L)
  3. Determine Food to Microorganism ratio (F/M; g COD feed added/g VSS of inoculum)
  4. Measure VSS in inoculum (g/L)
  5. Determine inoculum V in each BMP bottle (mL)
  6. Calc mass VSS needed to be added to each bottle
  7. Calc mass of total COD that should be added to each bottle (g COD)
  8. Make exp design for various co-digestion scenarios by selecting diff mixing COD ratios of feedstocks. Also include sole-feedstock scenarios.
  9. Calc V of each feedstock needed to be used in each [co-]digestion scenario
  10. Run BMP experiment (monitor CH4 over 30-60 days)
  11. Calc synergestic effect
57
Q

Out of the 4 stages of the AD process, which one determines the process kinetics, and why?

A

Hydrolysis: involves breakdown of insoluble materials to soluble materials

58
Q

T/F: Hydrolysis is considered as a 2nd-order rxn kinetics process.

A

FALSE: first-order

59
Q

What is the first-order reaction model used to asses biogas/biomethane production rate?

A

Vt = Vu(1-e^(-kt))

Vt = cumulative specific CH4 yield (mL/g COD added) at t
Vu = ultimate CH4 yield (mL/g COD added) obtained @ end of test
k = first-order specific hydrolysis constant (1/d)
t = digestion time (d)

60
Q

T/F: By rearranging the first-order kinetics expression for methane production to solve for -kt, we obtain an “ln” expression which gives a negative curve graph.

A

FALSE: the rearrangement of the eqn yields a linearized model with a negative slope (which is the k constant)

61
Q

List 3 characteristics of a BMP test used to determine process kinetics.

A
  • 100-10000mL in replicates
  • inoculum = digester effluent from an operating digester
  • substrates are included
  • operation mode = batch
  • operates in mesophilic or thermophilic conditions (35-60ºC)
62
Q

What is the eqn for anaerobic biodegradability?

A

(COD equiv of Vu (g) / Total COD added to digester (g)) x 100%

CHEE 342 (4 decks)

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