NAMS 2002 Workshop - ICOM 2008

Membrane Fouling - UF & Water Treatment – 3
Tuesday July 15, 3:30 PM-4:00 PM, Honolulu/Kahuku
Effect of Crossflow on the Fouling Rate of Spiral Wound Elements
P. Eriksson (Speaker), GE W&PT, Vista, California, USA - peter.eriksson@ge.com
Four spiral wound ultrafiltration elements (0.3 m (12”) long) operated on an
oil/water emulsion for 140 h at 207 kPa (30 psi) feed gage pressure, each at a
different feed flow rate, which corresponded to 0.1-0.4 m/s superficial velocities
and 11-138 kPa/m (1.7-20 psi/m) pressure drops. The feed channel spacer was
diamond shaped with a thickness of 0.86 mm (0.034”). During the first 3 hours of
operation, the permeate flux vs. feed flow rate followed the normal curve for
applications where the permeate flux at low crossflow rates is limited by the
boundary layer resistance. The permeate flux increased from 28 lmh (17 gfd) at
the lowest flow rate to 134 lmh (79 gfd) at the highest flow rate, with the slope of
the flux vs. flow curve steepest at the lowest flow rate to almost level out at the
highest flow rates. After 15 hours of operation the permeate flux had decreased
30-50 percent for the two middle flow rates and much less for the lowest and
highest flow rates. This trend continued during the rest of the test, so at the end,
the permeate flux was 9.3, 12, 21 and 77 lmh (5.5, 7.0, 12, and 46 gfd) for the
respective element listed in order from the lowest to the highest feed flow rate.
Between 55 to 79 hours operating time, the flow rate for the element with the
lowest flow rate was temporarily increased to give a pressure drop of 115 kPa/m,
which was between those for the two elements with the highest flow rates. This
increased the permeate flux of the element to slightly above that of the initially
next highest feed flow rate element, but the flux was still less than half of that of
the element with the highest feed flow rate. These results imply that the
permeate flux was affected both by the boundary layer resistance, which was
reversible, and a membrane fouling part that was not reversible. The irreversible
membrane fouling rate was not much affected by the feed flow rates at the lowest
three levels, but was greatly decreased at the highest flow rate level, which
indicates that for the used feed water solution, there was a threshold feed flow
rate, above which membrane fouling was greatly reduced.
A two-stage RO unit with 8” diameter spiral wound elements operating on city tap
water experienced after one week of operation a steadily increasing feed side
pressure drop with time. Cleanings were required every 4-8 weeks to keep the
pressure drop not to exceed the maximum allowed. The main problem was
biofouling. All six elements in one of the housings in the first stage, and the first
and last element in a housing in the second stage were taken out and tested
individually. The feed side pressure drop at a constant feed flow rate was about
4.5 times the nominal one for the first three elements in the upstream housing,
and then decreased for each element in the downstream direction to be less than