Pre-coagulation in the UF system is generally categorized into
standard coagulation and in-line coagulation in accordance to the
presence and absence of the sedimentation.
Standard coagulation can remove destabilized pollutants or their
aggregates. Coagulant hydrolytes/precipitates can also partly be
eliminated during the sedimentation process. Liang et al. [37]
compared coagulation, coagulation–sedimentation, and coagulation–
sedimentation–filtration prior to ultrafiltration applied in drinking
water production from algae-rich reservoir water, and found that
coagulation–sedimentation was the most effective; however, filter
presented an adverse effect. Dong et al. [38] presented an opposing
view that in-line coagulation could form an initial floc cake layer on the
membrane surface for adsorption of the contribution parts of NOM.
The phenomenon suggests that there is probably a kind of aggregate
benefiting the following ultrafiltration, which can be settled or filtered
during the subsequent conventional process. And Kimura et al. [39]
studied the efficacy of pre-coagulation/sedimentation on the control of
irreversible fouling in a polysulfone ultrafiltration membrane by a pilot
scale experiment in a drinking water production plant, and found that
pre-coagulation/sedimentation could not remove fractions of organic
substances (such as polysaccharides and protein), which accounted for
the irreversible fouling in UF in the experiment.
The properties and specific components of the raw water may
account for the different results, but the controversial results bring a
new insight into the integrated process. It is that during the whole
coagulation process, there must be a proper time, from initiating
coagulation to ending sedimentation, to form a kind of special flocs to
minimize UF fouling. Additionally, the conventional, though classic,
coagulation should be improved, and new coagulation way can be
established to reduce membrane fouling, Park et al. [40] used a precoating
method in both standard coagulation and in-line coagulation,
demonstrated that removal of DOM could make the membrane more
filterable, and found that 13.0 mg/L FeCl3 presented a more enhanced
filterability than 4.1 mg/L poly-aluminumchloride due to the formation
of a liable permeable cake layer.