Natural Organic Matter (NOM), which can be broadly divided into two fractions of hydrophobic
and hydrophilic, is abundant in natural water resources and in many ways may affect the unit operations in
water treatment. It plays an important role in membrane fouling in the water treatment process. Fouling of
membranes decreases the applicability of the membrane process and thus a deeper understanding of
membrane fouling is needed. The aims of this study were to investigate rejection NaCl and humic acid as
hydrophobic fraction and flux decline behavior with a spiral wound RO membrane at a pilot scale and
effect of water pH and conductivity (EC) on fouling and retention of RO membrane. The results showed
that flux reduction increased with increasing ionic strength and humic acid concentration and lower pH.the
rejection efficiency of organic and salt increased with an increase in pH and a decrease of ionic strength.
Generally, rejection of humic acid and salt ranged 98-99.3% and 95.8-97.13% respectively. Permeate salt
concentration was also significantly reduced immediately upon exposing membranes to humic acid. The
different behavior is most probably caused by the differences in the fouling layers and foulant
characteristics as well as by the electro neutrality effect. The improved salt rejection was likely due to
Donnan exclusion by humic material close to membrane surfaces.AN