Dead-end ultrafiltration as alternative pre-treatment to reverse osmosis in seawater desalination: a case study

In recent years new developments in membrane filtration have led to a number of new applications for this technology. One of these relatively new developments is the use of dead-end backwashable, Hollow Fibre Ultrafiltration. This has been widely applied in the area of potable water production as a barrier to pathogens and viruses, and for pre-treatment of Reverse Osmosis (RO) from surface water and treated municipal effluent. Of course, the technology is also very suitable as a pre-treatment to seawater reverse osmosis (SWRO) plants. To ensure that SWRO systems operate as designed, adequate pre-treatment is essential. The need for appropriate pre-treatment to ensure optimum performance of RO systems is well understood. RO membrane suppliers generally recommend that the 15-min Silt Density Index (SDI15) should be less than 3 to minimise problems caused by suspended solids blocking the brine spacers of the RO, membrane module and to reduce fouling of the RO membrane itself. This can be difficult to achieve for conventional pre-treatment schemes treating seawater from an open intake. Hollow fibre UF provides a very high quality, low SDI feed water, regardless of the incoming seawater quality. This enables the SWRO plant to perform at or above its design capacity. The backwashable hollow fibre configuration allows the UF system to be operated in a dead end mode of filtration resulting in energy efficiency. In combination with its small footprint and competitive operational costs, hollow fibre UF is a viable alternative to conventional pre-treatment schemes. To validate this technology, the authors' companies have set up a pilot plant in Trinidad. The plant was commissioned in September 2000 and has been running since. The pilot consists of one membrane housing, containing a single 35 rn X-Flow UF membrane element, It has been producing water with SDI15 values as low as 0.4 and showed stable operation. This paper will discuss the pilot results in detail, with operational data, water analyses and estimated operating costs, showing that the pre-treatment of SWRO plants will certainly be an interesting application of UF in the years to come.