Membrane and Desalination Technologies - TCE Moodle Website

322 J. Paul Chen et al.
many factors such as temperature and pH. Enzymes act slowly, and consequently longer
cleaning times are required.
Inorganic fouling in the form of scale formation is generally removed by acidic cleaning
(54, 96, 97). Membranes treating water with high inorganic potential, such as a high TDS
or hardness level, often include low pH cleaning in their chemical cleaning procedures.
Table 7.7 shows a survey of chemical methods used to counter scaling on membrane.
Although chemical cleaning agents are categorized with respect to their action on different
type of foulant, their combined effects are much more complex. These cleaning agents may
interfere with the cleaning effects of each other. Some may provide efficient control over
particular foulants while adversely affecting fouling control of another foulant. For example,
some cationic polymers while effective in silica-scale inhibition reduce calcium carbonatescale
inhibition by 30–40% (98). Humic and fulvic acids act as good calcium-scale inhibitor,
but they promote biofouling in the membrane system.
8.2. Physical Cleaning Methods
Physical cleaning methods depend on mechanical forces to dislodge and remove foulants
from the membrane surface. Physical methods used include forward flushing, reverse flushing,
backwashing, vibrations, air sparge and CO2 back permeation.
MF and UF used in pre-treatment to RO are more frequently cleaned by physical cleaning
and less frequently by chemical cleaning. Cleaning frequencies reported in literature varied
widely. Physical cleaning frequency is approximately every 40 min with a chemical clean
scheduled every 6 months (98). An air backwash frequency of 15–20 min is sufficient for
Table 7.7
Chemicals used in removing scale from RO membranes
Target foulants Recipe for chemical cleaning Results References
CaSO4
CaCO3
Metal hydroxide
deposits
2% EDTA and citric acid
stabilized by ammonia
to pH 7
Floclean 103A by Pfizer
(low pH formulation)
l Remove calcium scale
effectively
l Removed stubborn metal
hydroxide deposits
l 30% improvement in permeate
flow
l 33% reduction in differential
pressure
Ca and Mg deposits 1% EDTA at pH 10 l Remove Ca and Mg deposits
Fe deposits 1–2% sodium hydrosulphite
at pH 3.2–3.8
effectively
(96)
(90)
(97)
l Removal is faster in dynamic
cleaner than in static cleaning
l Remove Fe deposits effectively (97)