Modelli matematici applicati ai processi di filtrazione a membrana

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Modelli matematici applicati ai processi di
filtrazione a membrana Mathematical modelling
of MBR system
Biomath, Ghent University, Belgium 06-06-2006 Tao
Jiang
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Overview of the presentation

• Modelling the biological performance of MBR
• Modelling of MBR fouling

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Biological difference of MBR and TAS

• Complete retention of solids and partial
  retention of colloidal/macromolecular fraction
• Operational parameters
• Long SRT
• Short HRT

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Colloidal fraction in MBR

• Colloidal 0.001 µm - 1µm
• MBR membrane pore size 0.03-0.4 µm
• non-settable flocs in TAS lt 5-10 µm
• Additional removal of solids by MBR
• Small flocs (0.45-10 µm)
• Partial retention of colloids
  (pore size - 0.45µm)

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Colloidal concentration in MBR sludge

• TAS Effluent 30-60 mg/L
• MBR sludge (lt0.45µm) 50-200 mg/L
• MBR effluent (ltpore size) 5-20 mg/L
• Membrane retention 70-95

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Colloidal fraction is S or X?

• By size
• Colloidal fraction lt 0.45 µm ? S
• By retention
• 70-90 retention ? X
• By biological degradation
• Slow biodegradable ? X

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Colloidal fraction is X

• Colloidal fraction is X, although smaller than
  0.45 µm
• No significant error in TSS measurement, if the
  colloidal fraction is missing (CODColltltCODTSS)

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Influence of long SRT and short HRT

• High MLSS concentration
• MLSSSRT/HRT..
• Increased sensitivity of X (advantage of
  calibration)
• Inert particulate COD build up in MBR
• XI SRT/HRTXI,in
• Careful wastewater characterization
• Low active biomass fraction

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Membrane model

• Simple option (BNR study)
• Point settler and include the colloidal fraction
  into X
• Complete option (membrane fouling study)
• Define new variable S_SMP (X)
• Define retention of S_SMP by membrane

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Modelling of settler vs. membrane

• TAS (settler)
• Difficulty in calibrating settling model
• Possible biological processes in settlers
• MBR (Membrane)
• Point separation (no volume)
• No biological processes
• Complete retention of X
• Partial retention of colloidal fraction

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Modelling of a lab-scale MBR
Parameter /variable Reference values
Influent rate 108 L/day
Aerobic 17 min
Anoxic mixing 11 min
Anoxic recirculation 12 min
SRT 17 days
HRT 6.4 hr
MLSS 7 g/L
Filtration flux 31.8 L/(m2h)
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WEST Configuration
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WEST Experimentation
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Simulation results - Particulate
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Simulation results - effluent
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Simulation results - user defined
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Objective of modelling MBR fouling

• Prediction of membrane fouling (TMP vs t)
• Facilitate integrated design, upgrading,
  operation
• Cost reduction

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Influence of biology on fouling

• Feed to membrane is activated sludge
• The composition of activated sludge is
  determined by the influent and operation of
  biological process
• How biology influence fouling
• What is the main foulant?
• Influence of MLSS, SRT, HRT, DO?

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Foulant in MBRs

• The main foulant in MBRs is up to the influent
  composition, design and operation
• Particulate and colloidal can be the main
  foulant
• Colloidal fouling is getting more attention
  (soluble microbial products)

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Steps in the modelling of fouling

• Identify the main foulant
• Quantify the amount of foulant and their fouling
  potential
• Estimate the deposit rate of foulant on/in the
  membrane
• Predict additional resistance due to the foulant
• Estimate the reversibility of foulant by
  backwashing and chemical cleaning

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conclusion
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• Modelling the biolgical performance of MBR is
  simpler than TAS
• Modelling of MBR fouling, especially fouling
  prediction is extremely difficult and pre-mature