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3. Chemical Cleaning of Element
Chemical cleaning of element should be conducted when the trans-membrane pressure rises
to excess. Such a pressure increase can be caused when contaminants clog the pores of the
membrane surface. The timing of chemical cleaning is determined as follows:
(1)
Every 6 months or when the trans-membrane pressure rises by 5 kPa from its
initial operating level at the same permeated water flow rate, whichever comes
first.
(2)
If the trans-membrane pressure rises rapidly, please conduct chemical cleaning
much earlier. The earlier chemical cleaning is much effective to recover and keep
the permeability of the membrane.
(3)
In the case that the trans-membrane pressure rises by 5kPa within 6 months, it is
recommended to observe how many months it takes for the trans-membrane
pressure to rise by 5kPa and conduct chemical cleaning periodically. This
measure is effective to prolong the life of membranes.
4. Chemical Agents Available for Chemical Cleaning
It is quite important to select chemicals for the chemical cleaning in accordance with the type
of adherent contaminant. Cleaning under inappropriate cleaning conditions or using the
wrong chemicals may cause poorer filtration performance or damage the membrane. Please
select chemicals suitable for each contaminant. Table VIII-1 shows suitable chemicals and
standard cleaning conditions.
Table VIII-1 Cleaning Chemicals and Standard Cleaning Conditions by Contaminant
Contaminant
Chemical
Solution concentration
Amount used
Hold time
Organic
matter
Sodium
hypochlorite
2,000-6,000 mg/L
(Effective chlorine
concentration)
(pH is about 12)
5 L/element
1 to 3 hours
Inorganic
matter
Oxalic acid
0.5-1.0wt%
5 L/element
1 to 3 hours
Inorganic
matter
Citric acid
1-3wt%
5 L/element
1 to 3 hours
