Novel Properties and Applications for Sulfonated Polyether Sulfone NF Membranes

Novel nanofiltration (NF) membranes based on the chemistry of sulfonated polyether sulfone (SPES) polymers are gaining growing commercial use. The unique properties of these materials have opened a broad range of applications not possible with typical polyamide NF membranes. One form of SPES membranes employs a thin film, 0.3 microns thick, of the SPES material coated on top of a polysulfone ultrafiltration membrane support. The resulting composite membranes have adjustable molecular weight cut-off in the range of 500 to 3000 Daltons, salt rejections in the range of 10-30% for most natural waters, pH tolerance from 1 to 14, temperature stable to 85ºC, and are chlorine tolerant. Depending on the degree of sulfonation, the membranes have varying charge and can be tailored to specific performance. Their key benefit is the rejection of higher molecular weight organics that cause color in aqueous solutions, while passing most all of the dissolved salt ions. This makes them ideally suited for decoloring applications. The membrane can then be rolled into traditional spiral wound elements and applied to small and large scale separations. In some instances, such as high pH applications, the elements need to be made of unique materials to withstand very severe conditions. Details about the membrane and the element will be discussed.

These novel SPES membranes have been applied to municipal and industrial applications. One of the original uses was the removal of color from soy sauce. This is commercially important for users who offer soy sauce as a color-less additive in cooking. The SPES membranes have been proven to remove the dark color of soy sauce, without significantly removing the flavor components of the soy sauce.

More recently, these membranes have been used in potable water treatment processes. There are a number of applications in which the water has acceptable salinity, but high color. One example is the Irvine Ranch Water District, which has been running these membranes since 2002 for the treatment of 7.5 million gallons per day of colored groundwater. The SPES membranes are ideal as they can remove the color at high recovery, while not significantly affecting the mineral content of the permeate. At IRWD, the plant is able to run at >92% recovery. This helps ensure stability of the water without having to add post-treatment chemicals. In particular, surface water treatment plants benefit from the chlorine tolerance of SPES membranes. This allows improved control of biological and organic fouling, which is often prevalent in natural waters.

Finally, applications in the reclamation of dyebath wastewater and ion exchange blow-down from sugar wastewater treatment will be discussed. These applications have high concentrations of dissolved organics (color) and salinities as high as 10 to 20%. Additionally, they are often at high pH and high temperature, making them untreatable by normal polyamide NF membranes. Effective treatment of these difficult wastewaters is possible due to the unique properties of this unique class of membrane materials. Various case studies illustrating this will be provided.