Sumitomo Electric Industries Ltd of Japan has developed a membrane to trap transparent exopolymer particles (TEP) in pretreatment prior to reverse-osmosis (RO) desalination.
The TEP Trap (TT) membrane comes with an apparatus that incorporates a washing mechanism to blow TEP away from the TT membrane. It is expected to be on the market in 2012.
TEP, a polysaccharide gel substance with weight concentration of approximately 0.1 ppm, present in high salinity seawater, can cause fouling and deterioration of the water penetration performance of RO membranes. In seawater, TEP swell so that the volumetric concentration becomes 100 or more times higher than the weight concentration. They cannot be removed by a sand filter.
Once sticky TEP adhere to an RO membrane, they are difficult to remove. In addition, TEP serve as a culture medium for bacteria and form a biofilm which fouls the RO membrane.
TEP can be trapped by ultrafiltration (UF) or microfiltration (MF) membranes. However, accumulated TEP clog the membrane causing the filtration flow rate to reduce substantially. It is therefore necessary to use a membrane with a large surface area, which increases the cost of desalination.
Sumitomo’s TT membrane can trap TEP while maintaining a high filtration flow rate, and the required membrane surface area is far smaller than that of UF or MF membranes. The TT membrane can therefore be expected to reduce the pretreatment cost to nearly half that of pretreatment with a UF or MF membrane alone.
The advantages of the TT membrane and apparatus, according to Sumitomo, are:
Efficient TEP removal and high flow-rate filtration
The TT membrane was developed based on Sumitomo’s expanded polytetrafluoroethylene (PTFE) membrane technology Poreflon®. With its special fibril structure, the TT membrane traps TEP by entangling them. Since the TT membrane is hardly ever clogged, it is capable of filtration at a flow rate of 10 m/d, approximately ten times higher than with an MF membrane. The TEP removal performance has been verified by quantitative analysis conducted before and after pretreatment.
Substantial reduction in the required membrane surface area
Seawater filtration through the TT membrane can generate clean water with a silt density index (SDI) of 2.0 to 3.0, which is sufficiently low as pretreated water to be fed to an RO membrane. If TT-membrane-filtered water is further passed through a UF or MF membrane, higher quality pretreated water with an SDI of 1.0 to 2.0 can be obtained. In that case as well, since the filtration flow rate through a UF or MF membrane is nearly doubled due to the prior TT membrane filtration, the total required membrane surface area can be reduced by 40% compared with that for pretreatment by UF/MF membrane alone.
Washing mechanism to efficiently blow away TEP
TEP accumulated on the TT membrane can be removed efficiently using a unique backwashing mechanism combined with physical cleaning, enabling the TT membrane to maintain a high filtration flow rate and high TEP removal performance. The conventional backwashing method requires injection of chemicals for each cycle of pretreatment. A combination of backwashing and physical cleaning has made it possible to reduce chemical consumption to half that of the conventional method.
The TT membrane and TT apparatus have been developed by Sumitomo Electric in cooperation with the Tokyo University of Marine Science & Technology, using the University’s Shimizu Sub-station facility. Valuable advice about the TEP removal mechanism and theory of TT membranes was obtained from Professor Hideto Matsuyama of the Department of Chemical Science & Engineering, Graduate School of Engineering, Kobe University.
Toshiba Corporation helped with demonstration experiments at a seawater processing test plant. Sumitomo will continue demonstration experiments for the TT apparatus, with the expectation of marketing the apparatus in fiscal 2012.
The result of this study was presented at the International Congress on Membranes & Membrane Processes (ICOM) in Amsterdam, Netherlands from 24-29 July 2011.