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CIV342 Final Exam Prep > Activated Sludge (Biomass) > Flashcards

Activated Sludge (Biomass) Flashcards

1
Q

What does the activated sludge system involve?

A

It involves the production of an “activated” mass of microorganisms capable of stabilizing a waste aerobically.

Micro-organisms actively convert dissolved organics into new cell mass.

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2
Q

What assumptions do we make for the reactor design?

A
  • all biological activity occurs only in the reactor
  • bacteria do not lose their activity in the clarifier or while being recycled

Note: “activity” means the bacteria don’t start dying/stopping what they are doing.

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3
Q

What are the three main things we want to determine using activated sludge?

A
  1. The proper reactor design to give a desired Se (BOD in effluent) and MLVSS
  2. Rate of sludge (biomass) production
  3. Oxygen Requirements
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4
Q

What is MLVSS?

A

MLVSS = mixed liquor volatile suspended solids

It represents the mass of bacteria (biomass) present in the system

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5
Q

What are the four kinetic coefficients?

A 
k = growth
k_e = decay
Y = growth yield
K = substrate concentration
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6
Q

What are the 5 steps of the general overall design procedure.

A
  1. Find 4 kinetic coefficients (adjust for temperature if necessary)
  2. Define influent parameters
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7
Q

What are the influent parameters?

A

Q, S_o, and X_vo

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8
Q

What are the 5 steps of the general overall design procedure?

A
  1. Find 4 kinetic coefficients (adjust for temperature if necessary)
  2. Define influent parameters
  3. Select an appropriate SRT (θ_x)
  4. Solve for S_e, X_v
  5. Choose optimum HRT (θ_d), and size tank knowing
    V = θ_d * Q
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9
Q

What effect does increasing the SRT (θ_x) have on the process?

A

Increasing SRT makes the process more efficient (lowers S_e), but increases biomass (X_v).

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9
Q

What effect does increasing the SRT (θ_x) have on the process?

A

Increasing SRT makes the process more efficient (lowers S_e), but increases biomass (X_v).

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10
Q

What effect does increasing HRT (θ_d) have on the process?

A

Increasing HRT does not affect the efficiency (same S_e), but decreases biomass (X_v).

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11
Q

What are the implications of increasing the tank volume?

A

The tank volume does not affect S_e, but results in more HRT (good).

However, it could result in lowered biomass concentration (Xv; bad) if SRT remains constant.

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12
Q

What is the main control parameter for process effectiveness?

A

SRT (θ_x)

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13
Q

What is S_e?

A

S_e is soluble BOD in the effluent of the clarifier (food that escapes treatment).

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14
Q

For calculating the rate of sludge production:

Assuming the system is in steady state, what rate do we consider (in CIV342) to calculate the rate of solids production?

A

Rate of new cell material growth as a result of substrate utilization.

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15
Q

What is F/M?

A

F/M is the Food:Microorganism Ratio

F/M = (total applied substrate rate)/(total microbial biomass)

16
Q

What magnitude F/M ratio provides the best performance?

A

A low to medium F/M ratio typically provides the best performance (keep the bacteria a little bit hungry).

It helps improve settling characteristics in clarifier.

17
Q

What is the optimal SRT?

A

SRT often works best around 5-7 days.

Extra notes:
High rate systems may use 3-5 days.
Extended aeration systems may use up to 20 days; provide very thorough treatment

18
Q

What is the optimal MLVSS?

A

MLVSS is observed to be optimal between 1,500 - 5,000 mg/L

19
Q

What does the sludge volume index (SVI) measure?

A

SVI measures the settle-ability of the sludge (biomass).

In order for activated sludge to work optimally, you need to be able to efficiently settle the bacteria and remove them in the clarifier.

20
Q

What should good process performance SVI yield?

A

Good process performance should yield
SVI = 50 - 150

If the SVI is too high (poor settling), you may need to adjust the process.

21
Q

What is sludge bulking?

A

Sludge bulking is slow settling due to growth of poorly-settling organisms, and may be due in part to filamentous bacteria.

It may be indicated by a high SVI.

22
Q

What are is common under challenging operating conditions?

A
  • Low F/M (starvation)
  • Low oxygen and nutrients
  • Low pH
  • Nature of the organics in the wastewater
23
Q

How much oxygen should be maintained in a completely mixed reactor?

A

Typically, try to maintain 1 - 2 mg/L O2

24
Q

What are the principle nutrients’ (nitrogen and phosphorus) requirements?

A

Nitrogen requirement: 12.4% by weight
Phosphorus requirement: 1/5 of nitrogen (N) required

Note: normally not a problem for municipal wastewater.

25
Q

What are six variations in activated sludge?

A
  1. Completely mixed reactor (no recycle)
  2. Plug flow reactor (with or without recycle)
  3. Plug flow reactor (with tapered aeration)
  4. Plug flow reactor (with step aeration)
  5. Extended aeration
  6. Contact stabilization
26
Q

What are some traits of complete mixed reactors (no recycle)?

A
  • very simple design
  • not very efficient
    • no control over SRT
27
Q

What are some traits of plug flow reactors (with or without recycle)?

A
  • in theory, more efficient

- endogenous decay near reactor effluent yield good settling sludge

28
Q

What are some traits of plug flow reactors (with tapered aeration)?

A
  • tapered O2 is applied to reactor: more at front, less at rear
  • more efficient use of O2
  • more biological activity where S is greatest
29
Q

What are some traits of plug flow reactors (with step aeration)?

A
  • apply wastewater at stages in treatment to maintain a desirable F/M

Note: no oxygen is applied even though it includes “aeration” in the name.

30
Q

What are some traits of extended aeration?

A
  • activated sludge with high hydraulic retention time (>24hrs)
  • low flow rates - often don’t need to continuously remove sludge
  • high SRT -> often means poorer settling sludge
  • long HRT (referring to size of system) and high SRT allow good performance for variable flows (small systems typically)
31
Q

What are some traits of contact stabilization?

A
  • allows smaller tankage
  • less efficient, more prone to process upset (getting messed up)
  • hard to optimize
  • long HRT in stabilization tank results in starved biomass
32
Q

What are SBRs?

A

SBRs are Sequencing Batch Reactors

They operate in “batch mode” rather than continuous.

CIV342 Final Exam Prep (8 decks)

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