Polymeric membranes account for the biggest proportion of membranes currently in use. Several different polymers are used to suit the molecular weight cut off required or to achieve the desired resistance to fouling or performance when in contact with specific process fluids. Common polymers include polysulphone and polyethersulphone, which are used for the full range of UF membranes. Although cellulose acetate, the first widely used polymer, is still used in certain applications where it exhibits superior fouling characteristics, its use is limited due to its tendency to hydrolyse in alkaline conditions. Membranes can be configured in tubular, spiral, flat sheet or hollow fibre arrangements.
Tubular membranes have several advantages.
They can handle viscous liquids with high levels of suspended solids and can be chemically or mechanically cleaned. The tubular polymeric membranes tend to be housed in modules of stainless steel or plastic. Spiral membranes, as the name suggests, consist of tightly packed filter material sandwiched between mesh spacers and wrapped in a small tube. The high packing density significantly increases their surface area compared with tubular membranes. Spiral membranes require careful pre-filtration to avoid blocking if suspended solids are present.