The membrane performance was also observed during the
start-up period. As observed in Fig. 3, the flux across the membrane
drastically decreased (35–17 L/m2 h) from day 0 to 80, coupled
with increases in the TMP (35–90 kPa) and total resistance
(4.1 1012 m1 to 1.8 1013 m1). After the decrease in the flux,
the membrane was separated from the system for physical cleaning (flushed with deionized water) on day 80 and then retightened
in the membrane housing for another 80-day operation. A significant flux recovery of 33 L/m2 h was obtained immediately after
the cleaning process, indicating that the membrane fouling was
most likely due to sloughing or erosion of a loosely attached fouling layer on the membrane surface, which was easily removed by
physical flushing (Zhang et al., 2007). However, after another 80-
day operation, the membrane suffered a more severe fouling phenomenon than it had in the previous 80 days. The permeate flux finally decreased to 13 L/m2 h, which was lower than the flux on day
80. Moreover, the TMP (94 kPa) and total resistance
(2.3 1013 m1) both increased compared to the values on day
80, indicating that more frequent physical cleaning should be applied to prevent the occurrence of irreversible fouling (Lin et al.,
2013).