Anaerobic Treatment

Question 1

Which is harder to remove with biological treatment: dissolved organics or new bacteria?

Answer 1

Dissolved organics are harder to remove. New bacteria is easy to remove via sedimentation and filtration.

Anaerobic treatment creates new bacteria, while aerobic treatment does not.

Question 2

What are the three biological growth requirements?

Answer 2

1. Energy - for growth, maintenance, movements
2. Carbon - for growing new cells
3. Macronutrients and trace metals

Question 2

What is ATP?

Answer 2

ATP (adenosine triphosphate) is the cell’s “transport truck” for energy. It contains large amounts of chemical energy stored in high-energy phosphate bonds.

Energy is released when ATP is broken down into ADP (adenosine diphosphate).

Question 3

What do LEO and GER represent?

Answer 3

LEO (e− donor): Lose e− = oxidized

GER (e− acceptor): Gain e− = reduced

Question 4

List some examples of macronutrients for all bacteria.

Answer 4

S, Na, K, Ca, Mg, Fe

Extra note: Anaerobes have particular needs: Ni, Co, Zn, Mo

Question 5

What type of bacteria are specialists, and what type of bacteria are generalists?

Answer 5

Anaerobic bacteria are specialists (has a limited and narrow diet), while aerobic bacteria are generalists (can feed on a wide variety of things)

Question 6

Why should we care about acid concentration in reactors?

Answer 6

Acid concentration is an indicator of the reactor’s “health”.

Question 7

What happens when acid production > acid consumption?

Answer 7

• Acids ↑, pH ↓, methanogens grow slower
• Acids ↑, more, pH ↓, methanogens die
• Process is “stuck” (fermentation keeps producing acid)

Question 8

What can be done to ensure pH is maintained at an appropriate level in anaerobic processes?

Answer 8

• use a long SRT: grow lots of methanogens
• control feeding rate
• add alkalinity (pH buffer)

Question 9

Describe how anaerobic processes affect the kinetic constants (k, ke, K, and Y).

Answer 9

1. k, ke, K are low, because anaerobic processes are slower (bad)
2. Y is low, because anaerobic processes produce less sludge (good)

Question 10

How are anaerobic processes used for sludge digestion?

Answer 10

• concentrates the sludge to reduce disposal costs (due to low growth yield, Y)
• reduces sludge volume
• reduces sludge mass

Question 11

What is hydraulic retention time (HRT)?

What is solids retention time (SRT)?

Answer 11

Hydraulic retention time (HRT):

• average time a parcel of liquid stays in the system
• determines capital cost (size, construction cost)

Solids retention time (SRT)

• average time a particle stays in the system
• determines efficiency of biological treatment

(refer to pages 646-652 of textbook)

Question 12

How would you decrease cost while improving treatment?

Answer 12

SRT > HRT

Question 13

What is the most common special high-rate reactor used to retain anaerobic cells?

Answer 13

Upflow anaerobic sludge blanket (UASB) is the most common.

For extra context:
Special high-rate reactors have been developed to retain anaerobic cells:
SRT = 20 - 60 d&raquo_space; HRT = 6 - 12 hr

Others include: expanded granular sludge bed (EGSB), anaerobic filter (AF), anaerobic fluidized bed (FB).

Question 14

Why is anaerobic treatment common at WWTPs?

Answer 14

To reduce volume of waste activated sludge (WAS) to be disposed.

Question 15

What purpose does anaerobic sludge digestion serve?

Answer 15

Particulate substrate, needs lots of hydrolysis.
SRT (15 - 20 d) = HRT (15 - 20 d)

(this doesn’t make sense but it’s what the slides say)

Question 16

What are the results/effects of by-product formation from anaerobic sludge digestion?

Answer 16

By-product formation generates desirable off-gasses (methane, hydrogen) as well as undesirable (H2S) off-gasses.

Partial energy recovery is also possible for heat and electricity.

Question 17

How do we design sludge digesters?

Answer 17

• based on volatile solids (VS) reduction (VSR)

- empirical SRT and loading rates