(290s) Analysis of Fouled Water Treatment Membranes and Determination of Foulant Irreversibility

An ever-present problem
associated with nanofiltration membranes for water treatment is fouling. Fouling
of membrane elements often causes a significant increase in hydraulic resistance
and applied pressure drop, which increases operating cost and decreases life of
the membrane. This project focused on the (1) analysis of fouled sulfonated
polysulfone membranes and (2) determining which foulants could not be removed by
cleaning, and thus form irreversible fouling layers.

In Task 1 distilled water
containing 10 mM Na⁺ was filtered through sulfonated polysulfone
membranes at 70 psi for 2 hours for initial pore precompaction and for flux
stabilization. The membranes were fouled with a solution of 2 mg/L of bovine
serum albumin (protein), and 1 mM calcium in distilled water. Experiments were
run for 1 minute (instantaneous fouling), 5 minutes, 15 minutes, 30 minutes, 1
hour, 2 hours, 4 hours, 6 hours, 8 hours (permeate flux is equal to 50% of the
initial permeate flux (i.e. J/Jo = 0.5). Time intervals were tested to model the
evolution of hydraulic and cake resistances as a function of time. After each
experiment, membrane autopsies characterized the fouling layer (i.e. the
involuntary/adverse layer) with respect to chemical structure and morphology by
the use of attenuated total reflectance fourier transform infrared spectroscopy
(ATR-FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM),
total organic carbon analysis (TOC), and conductivity equipment.

The protein solution (2 mg/L of
bovine serum albumin, and 1 mM calcium in distilled water) showed a lower flux
than the pore precompaction solution (distilled water containing 10 mM Na⁺)
initially (Figure 1). Furthermore, the protein solution's flux slowly declined
for 6 hours when it started to stabilize. The AFM analysis showed an increase in
surface roughness across the membrane as fouling time increased (Figures 2 and 3
show the instantaneous fouling and the fouling on the membrane after 8 hours).
The ATR-FTIR analysis showed changes in the bond structure of the membranes,
furthermore, peaks around 1200, 1400 and 1700cm^-1 were observed to
have changed (Figure 4). The conductivity analysis shows that over time (e.g. as
fouling time increases) the solution conductivity decreases and that the
rejection percentage increases (Figure 5).

The information in Task 1 only
provides half of the picture. The other half is related to the irreversibility
of the fouling layer. Task 2 aims at relating foulant characteristics and cake
resistance to the reversibility of the cake layer. In Task 2, the experimental
runs in Task 1 will be repeated and following each run, the membranes will be
hydraulically cleaned. Membrane resilience will be determined by backflushing
the membranes with distilled water for 30 minutes at 70 psi. After backflushing
the membranes an instantaneous flux will be measured (recovered clean water
flux). Relative to Task 1, experiments will be performed as a function of
different time intervals in order to relate fouling layer chemical structure and
morphology to the effectiveness of the physical (e.g. backwashing) method. By
performing identical experimental runs as in Task 1 the amount of foulant
detachment will be determined. Task 2 will be completed by August 2005.