NAMS 2002 Workshop - ICOM 2008

ehaviour was investigated in a 10 L external MBR (microfiltration), at constant
operating conditions. Initial permeability was fixed at 265 L.h -1 .m -2 .bar -1 , with a
relative standard deviation of 8%. A mixed liquor suspended solids (MLSS)
concentration of 10 g.L -1 was chosen. When MLSS was increased from 8 g.L -1 to
16 g.L -1 , at a transmembrane pressure (TMP) of 2.5 bar and a crossflow velocity
of 3 m.s -1 , there was a 20% decrease in permeate flux. Critical flux, defined as
the minimum flux that creates an irreversible deposit on the membrane [1], is the
key parameter used to predict fouling. However, MBRs are typically operated at
fluxes above the critical flux of the system. At 4 m.s -1 crossflow velocity, critical
flux appeared between 0.7 and 0.9 bar, with a permeate flux of 70-85 L.h -1 .m -2 .
Critical flux also depends on the membrane state. At 5 m.s -1 , a new membrane
had an irreversible fouling between 0.7 and 0.9 bar whereas a previously fouled
membrane had a critical flux below 0.7 bar. One method for reducing membrane
fouling consists of increasing the crossflow velocity. At a fixed TMP of 2.5 bar,
when crossflow velocities were increased from 2 to 5 m.s -1 , the permeate flux
increased from 142%. Backpulses can also be used to reduce membrane fouling.
For one-day experiments, with 2 and 4 m.s -1 crossflow velocities and 1 bar TMP,
1 s backpulses per minute did not improve flux permeate. A four-day experiment
at 5 m.s -1 and 1 bar gave the same result. Therefore, backpulses can influence
filtration but not during short term experiments. Previous research [2] has
indicated extracellular polymeric substances (EPS) as the most significant factor
affecting fouling in MBRs. EPS effects on fouling mechanisms occurring in the
external MBR are currently being investigated. First, experiments without the
membrane will show the effect of the pump shear stress on the EPS
concentration. The same type of experiments, performed with the membrane, will
show the effect of the membrane on the EPS concentration.
[1] Espinasse B, Bacchin P and Aimar P. On an experimental method to measure critical flux in
ultrafiltration. Desalination; 2002, 146:91-96.
[2] Le-Clech P, Chen V and Fane TAG. Fouling in membrane bioreactors used in wastewater
treatment. Journal of Membrane Science; 2006, 284:17-53.