Page 148 - ro membanes

6.7 ADDITION OF REDUCING COMPOUNDS 131
the membrane elements over time. Therefore, the chlorine dioxide system selected for a given RO desalination project would have to be equipped with provisions to remove this residual amount of chlorine or with dechlorination system.
An undesirable site product from the generation of chlorine dioxide is the creation of chlorites (ClO2). They are carcinogenic and are not removed by the pretreatment process. Chlorites, however, are usually well removed by SWRO desalination membranes and partially removed by brackish and nanofiltration membrane elements. Therefore, the impact of chlorites generated by the chlorine dioxide system on the quality of desalinated water has to be investigated on a project-by-project basis.
6.6.3 Chloramines
Another type of oxidants, which have found wide applications for water reclamation, are chloramines. Chloramines are created by the sequential addition of chorine and ammonia to the source water and have been found to be very efficient because they are weak enough not to cause the oxidation of the RO membrane film.
Although chloramination is a very common and an efficient practice for the biofouling of RO membranes used for treating wastewater or brackish water of low bromide content (i.e., bromide concentration below 0.05 mg/L), it is not recommended for desalination applica- tions where the saline source water has a high content of bromides, such as seawater, for example, where bromide levels are usually in a range of 68e95 mg/L. When blended with ammonia, bromides in seawater create bromamines, which are several times stronger oxi- dants than chlorine and cause a permanent loss of RO membrane salt rejection within a very short period (usually less than 1 week). Therefore, chloramination is not commonly practiced for seawater desalination applications.
6.6.4 Nonoxidizing Biocides
Most nonoxidizing biocides are proprietary formulas, which have toxic effects on the aquatic species and are usually small-molecular-weight compounds that can penetrate bacte- rial cell walls and inhibit their metabolism and enzymatic system. Experience with nonoxi- dizing biocides indicates that they are efficient only when applied at large dosage over a short period of time (Voutchkov, 2014). Long-term applications at small dosages usually have limited benefit because most biofilm-forming bacteria can adapt to nonoxidizing bio- cides over time.
6.7 ADDITION OF REDUCING COMPOUNDS
Because RO membranes are damaged by exposure to oxidants, such as chlorine, when the saline source water is conditioned with chlorine or other strong oxidants, these oxidants will need to be removed before membrane separation. Typically, RO membranes would degrade irreversibly after an exposure of 200e1000 h at a free chlorine dosage of 1 mg/L. Higher chlo- rine dosages would shorten this time to only several days. Usually, membrane degradation is expedited if the pH of the water is alkaline.