Water. Desalination + reuse
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technology MANUFACTURER'S CASE STUDY Biofouling agent finds success in seawater RO plant _________ Katsumi Matsumoto, general manager, RO Chemical Technology Dept, Kurita Water Systems, Japan ___ Since Kurita Water Systems introduced a new slime control agent for membranes at the International Desalination Association congress in Dubai in 2009, the company has been using the chemical in many different commercial reverse-osmosis plants throughout the world, where it has proved to be quite effective. This article discusses the agent's advantages for RO systems and uses an SWRO desalination plant in China as a case study. BECAUSE reverse-osmosis (RO) feed water has a very high potential for slime growth and biofouling, plant operators face a series of problems, such as increased differential pressure across RO membranes, decreased flux and salt rejection. They frequently repeat RO cleaning in order to resolve the problems. Kurita Water Systems started to use the new slime control agent, which has a characteristic ability of peeling off biofouling, to RO plants. As a result, the problems described above were all greatly diminished: differential pressure was improved; the decrease of flux was inhibited; and salt rejection was improved and maintained. In addition, the interval of RO cleaning was stretched. Since the quality of RO permeate was consistently improved, the amount of deionized water obtained at the ion-exchange system after RO units was increased. Furthermore, RO membrane autopsies verified a significant decrease of biofouling on the RO membrane with the use of the new slime control agent. Biofouling on Ro MeMBRanes Generally, when using polyamide RO membrane, biocides such as organic biocide (Cl-MIT, DBNPA) or sodium hypochlorite or chloramine are used to fight biofouling problems. These biocides are quite effective in some cases, but not always perfect. Even in the use of the above biocides, biofouling is likely to gradually accumulate in the RO membranes, and the accumulated layer cannot be removed until membrane cleaning is carried out, for which the RO system operation has to be stopped. If this accumulated biofouling could be removed without shut-down of RO operation, this would be very beneficial for the prolongation of RO operation periods and RO membrane life. sliMe-contRol agent To combat biofouling problems, Kurita has developed a new and unique combinedchlorine type slime-control agent, Kuriverter IK-110, which can remove the biofouling layer without shutdown. IK-110 has three features: 1. It has the ability to peel organic fouling off gradually from the RO membrane surface 2. It can inhibit slime problems through the whole RO system; this could reduce the risk of polyamide membrane deterioration, which would be caused by dosing with sodium hypochlorite and reducing agents. 3. It does not damage polyamide membranes. The effect of IK-110 has been verified on many actual RO systems. Its application was found to be quite beneficial. PeRMeaBility tests On the application of IK-110, its RO membrane permeability has to be understood. So an RO membrane permeability test was conducted for the agent. test Method and test unit A single RO membrane element of 4 in diameter (low-pressure polyamide membrane) was used. IK-110 and 12% sodium hypochlorite were dissolved respectively into pure water in the 100 L tank. It was pumped through a cartridge filter of 20 μm pore size to a high-pressure pump. It was then pressurized to 0.75 MPa. Combined chlorine and free chlorine in feed and concentrate were measured by a DPD instrument. Result Test data obtained is shown in Table 1 (page 38) from which we derived the following results. 1. Where sodium hypochlorite remains in the RO Feed, the same concentration of chlorine moves to the RO permeate side. 2. When IK-110 is fed to the RO feed, both free chlorine and the combined chlorine are not detected at less than the detected level on the RO permeate side. This indicates that IK-110 added in RO feed goes to the concentration not the permeate. case study on sWRo Plant in china To solve the problems caused by slime, IK-110 was actually tested in a seawater desalination RO plant located on the coast of Bo Hai Bay in China. August-September 2012 | Desalination & Water Reuse | 37 |