Reverse osmosis (RO) is a common method of water filtration, and in particular, desalination. Of commercial desalination using filter membranes, RO makes up 80%. It operates by applying pressure to brackish water on one side of a membrane that is only permeable to water, thus creating purified water on the other side of the membrane. In practice, however, the intake water is chlorinated in order to remove organic matter such as algae that can cause problems with the system. This in turn results in chlorine by-products, some of which can cause chronic problems or even be lethal at sufficient concentrations. In addition, desalinated water is often blended with available freshwater, which may also contain the same organic compounds, thereby making total concentration levels unacceptable. The concern addressed in this study is whether these by-products are released in harmful concentrations either for human consumption or the aquatic environment (Agus and Sedlak 2010). The study focused on a pilot plant in Carlsbad, CA and though the study found that some of the organic compounds produced were prevalent enough to be tasted, none were harmful.––Erin Partlan
Agus, E., Sedlak, D., 2010. Formation and fate of chlorination by-products in reverse osmosis desalination systems. Water Research 44, 1616—1626.
Eva Agus and David Sedlak looked at the Carlsbad desalination plant and also obtained water samples from the coastal regions of California, Florida, and Singapore. These samples were used as a comparison to the Carlsbad data and were chosen as they are likely locations for the placement of water-processing plants. In the Carlsbad tests, samples were taken at two different times (summer and winter) and with two different chlorination dosages. To model blending of desalinated water with freshwater, samples were taken from the Colorado River, NV and the San Pablo Reservoir, CA, which are rich and poor in bromide respectively, a compound that can pass through RO filters relatively well. For the samples from the various coastal regions, after controlling for pH, one chlorination dosage was used to match the Carlsbad tests. For all samples, measurements were taken at various times over a period of three days.
The Carlsbad plant samples had less organic compounds than expected, and is thus attributed to the fact that it was designed for higher water purity and also, as a pilot-scale plant, gives less opportunity for the formation of by-products due to a shorter holding time. The organic compounds that were detected were found to vary both seasonally and with chlorine concentration. As a result, the authors recommend that tests be run during different times of the year in order to ensure that none of the water produced is harmful. The study also found that even after blending the desalinated water with the chosen freshwater samples, while the bromide concentrations increased greatly, they remained safe for consumption. It is also noted that the results of this test were conservative since no pre-treatment of the freshwater was performed, a process that would normally be done in a commercial plant.
For the samples from the coastal locations, none were found to exceed standards for organic compounds after chlorination. However, it was found that the types and proportions of compounds varied by locations, even though the method and dosage of chlorine were held constant. It is likely that the initial variation in organic compounds dictates the type of reactions that occur after chlorination. Therefore, the authors recommended that tests be performed at the site of a desalination plant to ensure that the compounds present in its outflows are not harmful. In particular, they note that some compounds would be tasted at the levels produced and may be a factor in the consumer desirability of the desalinated water.