Membrane Materials

The efficiency of membrane filtration processes in wastewater treatment is decisively influenced by the selection of the membrane material for the wastewater to be treated with regard to the particles and dissolved compounds it contains. Therefore, the membrane material has to be chosen carefully by the supplier in cooperation with the plant engineers. It is usually necessary to perform investigations in laboratory or pilot scale and to test different membrane materials and modules. Table 12.3 presents an overview of common membrane materials.

Depending on the composition and properties of the wastewater to be treated as well as on the membrane's mechanical stability, it is possible to use either organic or inorganic solid materials. Once commonly used, cellulose membranes are now used less often in recent times compared to polymer membranes, like polysulfone PS, polyacrylonitrile PAN and polyethersulfone PES. Because of their resistance to high temperatures and chemical stress, the use of inorganic materials like ceramics, aluminum, refined steel and glass is becoming more important (MUNLV 2003).

Table 12.3 Common membrane materials and membrane structures used in membrane processes (according to ATV-DVWK 2002a).

Membrane

Membrane

Active layer

structure

material

material

Microfiltration

Symmetric, porous

Polymer and ceramic

Polypropylene PP Polyvinylidenfluoride PVDF Polysulphone PSU Aluminium oxide Refined steel Titanium dioxide Zirconium dioxide

Ultrafiltration

Asymmetric, porous

Polymer phase inversion, composite membrane and ceramic

Polysulphone PSU Reg. cellulose Polyacrylnitrile PAN Polyethersulphone PES Titanium dioxide Zirconium dioxide Polyvinylidenfluoride PVDF

Nanofiltration

Asymmetric, dense

Polymer phase inversion, composite membrane

Polyamide PA (Zirconium dioxide) Polyethersulphone PES Cellulose acetate CA

Reverse osmosis

Asymmetric, dense

Polymer phase inversion, composite membrane

Polyamide PA 95% Cellulose acetate AC 5%

Membranes are either symmetric or asymmetric in structure. That means they exhibit a homogenous or inhomogeneous dispersion of material. An asymmetric membrane is constructed by two layers of one material with different porosities (a phase inversion membrane), or by two layers of different materials (a composite membrane).

The very thin and dense active layer of about 1 pm on the feed side is responsible for the membrane's performance and permeability. The porous second layer is the supporting structure. Higher fluxes are attainable, especially for solution diffusion membrane processes, with asymmetric membranes. Both composite and phase inversion membranes can be constructed as porous or non-porous membranes (Rautenbach 1997).

0 0

Post a comment