Page 34 - Membranes for Industrial Wastewater Recovery and Re-Use

14 Membranes for Industrial Wastewater Recovery and Re-use

2.1 The membrane

2.1.1 Membrane and membrane process definition
There are a number of definitions of the word “membrane”, which can vary
considerably in comprehensiveness and clarity. It is a very widely used term, and
can mean anything from a cell wall to damp proofing material. Three definitions,
arbitrarily chosen from pertinent technical literature from the last 20 years are
provided below:

“An intervening phase separating two phases and/or acting as an
active or passive barrier to the transport of matter between phases” -
the European Society of Membrane Science and Technology (now the
European Membrane Society).
“An interphase separating two homogenous phases and affecting the
transport of different chemical components in a very specific way” - Prof.
Heine Strathmann, former Head of the Department of Membrane
Technology, University of Twente.
“A material through which 01ie type of substance can pass more readily
than others, thus presenting the basis of a separation process” - Prof.
George Solt, former Director of the School of Water Sciences, Cranfield.

For the purposes of this discussion of membrane technology for wastewater
treatment, Solt’s definition can be considered adequate: it is that property of the
membrane which permits the separation of components in and/or from water
that is of key interest. For many processes the membrane acts to reject the
pollutants, which may be suspended or dissolved, and allow the “purified” water
through it. In some cases, however, the membrane may act so as to extract
pollutants from the wastewater, or else transfer specific components (such as
oxygen) into it. Currently employed extractive processes include electrodialysis
(ED), dialysis, pervaporation (PV) and gas transfer (GT). In these cases the
membrane is employed to allow selective permeation of specific components
dissolved in the water. Of far more industrially importance, however, are the
filtration processes of reverse osmosis (RO), nanofiltration (NF), ultrafiltration
(UF) and microfiltration (MF). In these processes it is the bulk water that passes
through the membrane under an applied pressure, leaving the pollutants in
concentrated form on the unpermeated side of the membrane. If hemodialytic
applications are ignored, then the pressure-driven filtration processes account
for around 75% of the remaining membrane sales, almost all of these due to
aqueous separation applications in municipal and industrial water treatment.

2.12 Membrane structure
Although membrane materials vary vastly according to chemical composition
and process type, the principal objectives in membrane manufacture are always
the same. An ideal malerial will: