Membrane and Desalination Technologies - TCE Moodle Website

628 P. Kajitvichyanukul et al.
Adjustment of Sulfate Scaling Salts (CaSO 4, SrSO 4, and BaSO 4) for Reverse Osmosis. In the
specified method, the ion products, IPc, of the considered salt in the concentrate stream and
the solubility product, K sp, of that salt under conditions in the concentrate stream have to be
compared. Generally, scale-control measures are not needed if IPc < Ksp.
The cleaning chemicals used to prevent fouling can be divided into acid and alkaline
cleaners. Acid cleaners are used to remove inorganic precipitates. Examples of acid cleaners
are HCl and phosphoric acid (H3PO4). Sulfuric acid is not recommended for cleaning purpose
due to the risk of calcium sulfate precipitation. Alkaline cleaners are used to remove organic
fouling including biological matter. The major alkaline cleaner is sodium hydroxide (NaOH).
RO permeate can be used for preparing cleaning solutions. However, prefiltered raw water
may be used.
For biofouling, controlling the microbiological activity using surface area or bacteriacontaminated
water as a feed source in the system design process as follows can prevent the
accumulation of microorganisms. As the sunlight is essential for the microorganism growth,
the components of the pre-treatment system such as pipes, filters, and retention tanks should
be covered to avoid the sunlight exposure. It is also suggested that the stand-by devices with
large surfaces such as sand or cartridge filters should be avoided. If they are not avoidable,
drains should be installed to allow discharge of the sanitization chemicals. The most common
way to eliminate microorganisms from an RO system is the application of antimicrobial
chemicals. This practice is practical when mechanical cleaning is impracticable and costly
due to equipment downtime. The most widely used biocides are chlorine and aldehyde.
However, these biocides often fail to disinfect surfaces of water systems successfully. It is
well known that microorganisms living in biofilms are more resistant and thus require much
higher concentrations of biocides to be disinfected (78). Prevention of biofouling is frequently
achieved by continuous dosage of biocides. However, in many cases, permanent
dosage of biocides is undesirable for environmental reasons. Biocides and their toxic byproducts
contaminate sewage water, where they still act as unwanted antimicrobial chemicals.
Furthermore, biocides require special attention in terms of storage and handling in order
to meet legal regulations (72).
Chlorine is the important biocide used for the different types of RO membranes. Continuous
chlorination can be used with cellulose membranes to protect the system from biofouling
and eliminate the particle trapping slime. However, chlorine and other oxidizing disinfectants
are harmful to some type of RO membranes such as TFC. If the TFC membrane is used in RO
units, an activated carbon must be in place to remove the oxidizing chemicals before the
chlorinated water reaches the TFC membrane (30).
Other techniques to prevent biofouling are periodic testing and monitoring and using
activated carbon filter. Microorganisms, dead or alive, can clog RO membranes. If coliform
bacteria or other special contaminants are a known or suspected problem, periodic testing is
recommended. For the application of activated carbon (AC) prefilters, it is recommended that
this pretreatment process should be used with chlorinated water. Application with nonchlorinated
water supplies is not recommended because they provide a place for microorganisms to
multiply and lead to increased biofouling of the RO membrane surface (30). It is important to