Spiral-wound membrane elements (sepralators) were used mainly for basic water purification until the mid to late 1970s, when broader uses began to receive attention. New applications often required special consideration such as; sanitary construction for the dairy industry, high temperature for food processes and waste streams, and high pressure capability for desalination. In addition, aggressive chemicals either in the feed stream or in necessary cleaners had to be dealt with. As the sophistication of the membrane industry grew, so did increased use of specialty polymers, improved adhesives, more engineering thermoplastics, and even stainless steel in sepralators and their ancillary parts. This has allowed the expanded scope of application, which continues to grow today.
Original designs used a plastic tape outer cover with a concentrate (or 'brine') seal to direct flow through the feed channels. The permeate tube protruded from the material package on each end; requiring this thermoplastic part to bear the entire axial compressive load for the series of sepralators (up to six) placed in one housing. An FRP outer cover was next developed for both handling protection and improved hydraulic load-bearing. A 'close-coupled' design followed where the sheet materials are trimmed flush with the permeate tube end. This maximizes the available space in the housing and efficiently transfers the DP load between sepralators. Most commercial sepralators today employ this design.
The Full-Fit™ design evolved next and represents a major improvement. The outer cover and brine seal are eliminated, allowing higher feed flows (at the limiting pressure differential) which has been found to help reduce fouling in some applications. It also eliminated the 'dead-flow' area behind the brine seal where bacteria proliferated and from where cleaning/storage chemicals were not readily flushed. This design has become standard in pharmaceutical and food/beverage applications, where cleanliness and ability to sanitize are essential. Also, many high-fouling applications, from biotech to waste treatment, have been made feasible by the Full-Fit membrane element . The attributes of this design were further enhanced by tapering it to fit in a filter cartridge housing, combining cleanliness, convenience and economy.