Membrane and Desalination Technologies - TCE Moodle Website

626 P. Kajitvichyanukul et al.
affected by the presence of turbidity, iron, manganese, hydrogen sulfide, and chlorine in
water. High levels of particulate matter in the water such as iron can foul or clog the
membrane, decreasing water flow. Because RO devices are point-of-use units, it may be
necessary for a point-of-entry device to be installed to treat these problems if they exist. Many
systems now have a built-in continuous monitor that indicates a high TDS level, a sign that
the system is not operating properly. It may also be necessary to test regularly for specific
health-related contaminants such as nitrates or lead.
Replacement of RO components in POU/POE is a routine maintenance process. Pre- or
postfilters must be replaced once a year or more often, depending on the volume of water fed
through the system and the quality of the feed water. Customers have to follow the manufacturer’s
instructions for the maintenance procedure in each part of the RO system. For RO
maintenance, some systems have built-in monitors such as conductivity meters, pressure
gauges, and other devices, which can be used to detect the RO operating problems. However,
many RO units are not provided with these. Maintenance in accordance with the manufacturer’s
recommendations is required for all parts of RO units, as described above.
3.6. Fouling and Cleaning of Reverse Osmosis
Membrane fouling is classified as the most serious problem encountered in RO systems.
It can reduce the performance of RO tremendously. Clogged RO membranes, filters, or flow
controls will decrease water flow and the system’s performance. If fouling is detected in the
early stages, the membrane can often be cleaned and regenerated. Completely clogged or torn
RO membranes must be replaced. The cleaning procedure varies depending on the type of
membrane and fouling.
Generally, fouling refers to the deposition of undesirable material on the membrane, which
leads to several negative effects including reduced membrane fluxes, decreased permeate
flow, increased differential pressure, increased pressure drops, and decreased salt rejection,
necessitating periodic cleanings and causing risks of mechanical and chemical damage. All
these effects result in the deterioration of plant performance and increased treatment costs
(67, 68).
Foulings are classified into four categories: scaling, colloidal fouling, organic fouling, and
biological fouling. Scaling occurs when the concentration of salts present in the concentrate
exceeds the concentration of the solubility product. The fouling formed by deposits from
small amounts of relatively inert suspended solids, which typically are silica, silicates, and
previously precipitated scales, is called colloidal fouling. Metallic oxides also play a critical
role in the cohesion of the scale formed or the fouling layer (69, 70). Divalent cations (e.g., Ca þ2 )
were also proved to have a dramatic effect on fouling (71).
Organic fouling occurs when surface water contains residual material from the decay of
organic matter with a high percentage of multi-functional organic acids (e.g., humic acids).
Precipitated organic acids are the most common form of organic fouling and are easily
dissolved at elevated pH. These compounds are measured as DOC (dissolved organic
carbon). Amphoteric and hydrophobic organic matters are primary organic foulants for
polyamide nanofiltration membranes. These two species displayed different interaction