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Desalination of Seawater by Reverse Osmosis 587
steel. The pH of the chemical cleaning solution must fall within the tolerable range of
1.0–13.0 for polysulfone membranes and 3.0–8.0 for CA membranes (60). Generally, chemicals
used for cleaning membranes should ideally posses the following desirable properties:
(a) Loosen and dissolve foulants from membrane surface.
(b) Keep foulants in dispersed and soluble form.
(c) Avoid fresh fouling.
(d) Do not cause damage to membrane material.
(e) Easily rinsed off after cleaning.
(f) Chemically stable before, during, and after use.
(g) Cost effective.
Table 13.3 summarizes common fouling factors and cleaning and control methods. The
cleaning procedures may vary depending on the situation, but the basic steps of chemical
cleaning are as follows:
(a) Make up the cleaning solution according to the manufacturer’s instructions.
(b) Perform a low-pressure flush with permeate water or cleaning solution to displace the solution
in the vessels, the process water can be dumped to drain until the cleaning solution has filled
the vessels.
(c) Recycle the solution through the elements and back to the tank. During operation, part of the
cleaning solution is dumped to drain before returning back to the RO cleaning tank. Readjust the
pH to that of the target when it changes more than 0.5 pH units.
(d) An optional soak and recirculation sequence can be used. The soak time can be from 1 h to
overnight depending on the manufacturer’s recommendations. Additional recirculation of cleaning
solutions at a high flow will help to displace the foulants from the membrane.
(e) A low pressure cleaning rinse with permeate water is required to remove all traces of chemical
from the cleaning skid and the RO skid.
Once all the stages of a strain are cleaned, the RO membrane can be placed back into
service. The RO permeate quality usually becomes stable after a few hours to a few days,
especially after the cleanings in high pH solutions.
Frequent and appropriate cleaning procedures are vital in order to maintain the performance
of RO membranes. Although the procedures may be similar, in many cases, the
cleaning solution concentrations, volumes of solution, and stages of cleaning will differ.
The frequency of cleaning will be determined by the rate of fouling. The main factors that
influence the effectiveness of membrane cleaning are as follows (33):
l Type of cleaning chemicals used.
l Cleaning solution volume (it must be appropriate for the size of the plant).
l Contact time (soaking of membranes in solution can aid in cleaning).
l Temperature (cleaning chemicals are most effective at warm temperatures).
l Design of the cleaning circuit and operating parameters.
Although chemical cleaning agents are categorized with respect to their action on different
types of foulants, 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 foulants. For example,