Process Monitoring and Control

Question 1

Process monitoring (classic)

Answer 1

On-line
- ph
- dissolved O2
- temperature
- conductivity
- redox
- flow rates (influent, sludge recycle, nitrate recycle, waste sludge)
Off-line
- COD, BOD, SS, N and P components
At line
- nitrate, ammonium, phosphate
- performs in an automatic way that is normally done during the lab analysis

Question 2

Constant pH

Answer 2

• around neutral pH value
• if the incoming flow is too acidic or alkaline, then one exploits most often a buffer tank instead of adjusting the pH in the biodegradation tank itself
• if only nitrification is occurring (and no denitrification) then one will observe a pH drop

Question 3

Constant oxygen concentration in the biodegradation tank

Answer 3

• DO is 1 to 2 mg/L
• the dissolved oxygen concentrations monitored with sensors and the blowers are switched on and off
• one should maintain a minimum O2 level to ensure biodegradation at minimal cost

Question 4

Constant sludge concentration in the biodegradation tank

Answer 4

• MLSS = 4g/L
• the ML(V)SS is monitored and the waste sludge flow rate (fraction w) is adjusted to maintain the ML(V)SS at the desired level
• due to changing influent (quantity or quality), the sludge loading and sludge retention time will inevitably change

Question 5

Constant sludge loading rate (F/M ratio)

Answer 5

• 0.15 gBOD/gMLSS/day
• the ML(V)SS is monitored, together with incoming flow rate and the incoming (b)COD concentration
• the waste flow rate fraction w is adjusted to maintain the F/M value constant at the desired level
• the sludge retention time can still change

Question 6

Constant Sludge Retention Time

Answer 6

• SRT = 20 days
• the (V)SS concentration in the biodegradation tank and in the recycle stream, together with the incoming flow rate are monitored
• the waste flow rate fraction w is adjusted to maintain the SRT constant at the desired level
• this is the best way to sustain a microbial population with slow growers

Question 7

Process monitoring (advanced)

Answer 7

Filamentous bulking monitoring
- SVI = sludge volume index
- FI = filament index
- Eikelboom’s classification (microscopy)
- process is subjective and time consuming

Question 8

Activated sludge characteristics

Answer 8

• Global characteristics:
  > number
  > total surface
  > total length
• individual characteristics
  > size (perimeter, length and width)
  > shape (elongation, roughness, convexity)

Question 9

Respirometry

Answer 9

• Monitoring the Oxygen Uptake Rate (OUR)
• OUR = -dO/dt
• measure for the specific substrate degradation rate
• detect toxicity
  > if the oxygen concentration remains constant (no uptake), there is no biodegradation which indicates a toxic event for the micro-organisms
  > if the OUR is measured regularly and the values suddenly decrease to levels lower than normally measured, chances are high that there are toxic components in the influent
• assess the adaptation of an activated sludge population to a specific influent (component)
  > when administering each day (a surplus of ) influent to the activated sludge (i.e. enough to ensure that the activated sludge can be maximally active)
  > the maximal specific OUR will increase the more the activated sludge ‘get used’ to the influent components
• to assess the fraction of readily and slowly biodegradable organic matter
  > from surface beneath the OUR curve
• to determine growth parameters
  > minimise the difference between computer generated values and measured values

Question 10

Maximal Specific Oxygen Uptake Rate

Answer 10

• sOUrmax
• administering a surplus of C-source
• dividing by the ML(V)SS
• activated sludge was used to glucose but is fed with starch and then again with glucose
• assessing the functional stability of the population