Page 135 - ro membanes

118 6. CONDITIONING OF SALINE WATER
A relatively simple method to diagnose the spontaneous generation of oxidant in the membrane vessels is to measure the oxidationereduction potential (ORP) of the saline source water fed to the individual RO trains and that of the concentrate from the same RO trains. If the spontaneous generation of a significant amount of oxidant occurs in the membrane ves- sels of a given RO train, the ORP of the RO train concentrate is measurably (typically over 10%) higher than the ORP of the feed water to this train.
If such a condition occurs and sodium bisulfite is being fed to the RO system at the same time, it is recommended to discontinue the use of this chemical and check the ORP of the RO train feed water and concentrate within 20 min of the discontinuation of the bisulfite feed to verify whether the problem is solved. If this problem persists, it is desirable to reduce or dis- continue the addition of ferric chloride and consider the supply of new batch of this product from another supplier or to switch to the use of ferric sulfate.
Due to the uncertainty of the quality of ferric chloride available to the desalination plant operators and the potential for changes of content of manganese and other impurities from one batch of chemical to another, the most suitable solution to the challenges described earlier is to use ferric sulfate instead of ferric chloride for coagulation. Ferric sulfate is generated from the chemical reaction of pure iron and sulfuric acid and therefore, it usually does not contain impurities and does not cause the problems associated with the use of low-quality ferric chloride. Therefore, although of higher unit chemical cost and higher application dosage for the same amount of solids, the use of ferric sulfate is preferable and strongly recommended to troubleshoot the negative impact of the use of low-purity ferric chloride on plant cartridge filters and RO membranes.
Source water temperature is an important factor affecting the coagulation process, espe- cially for source waters with solid particles that have low electric charge. Because in this case, the driving force for the coagulation process is mainly random particle movement and collision, lower temperatures tend to hinder the available kinetic energy for particle movement and therefore cause lower coagulation efficiency. Such negative impacts are typi- cally observed for source water temperatures below 20C (68F).
It is important to note that in many cases, plant operators try to compensate for such temperature impact by increasing coagulant dosage. This approach, however, is not recom- mended because the overdosing of chemical will result in an elevated fouling rate of the downstream cartridge filters and RO membranes.
Instead, it is preferable to increase the flocculation time to 15 min or more by reducing the volume of the pretreated saline source water.
Two other approaches suitable to troubleshoot this operation challenge are pH adjustment and increase of the motor speed of the flash mixers in the coagulation tank. These corrective actions are applicable if the coagulation system is equipped with coagulation contact tank and the flash mixer motors have two-speed or variable frequency drives. Usually, lower tem- perature results in higher optimum pHdfor source water temperature in a range of 10e20C (50e68F) the optimum pH is usually between 8.2 and 7.8. Therefore, in this case, if the acid is added for optimizing of the coagulation process, it is recommended that the acid feed is dis- continued when the source water temperature drops below 20C (68F). The pH adjustment- based troubleshooting approach usually works if the source water particles are negatively charged because the pH adjustment chemical increases the charge of the coagulant and its electrostatic attraction force. If source water particles do not have significant electric charge