The MBR Book: Principles and Applications of Membrane

they can be put, which then provides an indication of the pore size. The latter can be
defined either in terms of the effective equivalent pore diameter, normally in �m, or the
equivalent mass of the smallest molecule in daltons (Da) the membrane is capable of
rejecting, where 1 Da represents the mass of a hydrogen atom. For UF membranes
specifically the selectivity is thus defined by the molecular weight cut-off (MWCO) in
daltons. For the key membrane processes identified, pressure is applied to force water
through the membrane. However, there are additional membrane processes in which
the membrane is not necessarily used to retain the contaminants and allow the
water to pass through, but can instead be used either to:
(a) selectively extract constituents (extractive) or
(b) introduce a component in the molecular form (diffusive).
Fundamentals 23
The range of membrane processes available is given in Table 2.1, along with an outline
of the mechanism by which each process operates. Mature commercial membrane
applications in water and wastewater treatment are limited to the pressure-driven
processes and electrodialysis (ED), which can extract problem ions such as nitrate
and those ions associated with hardness or salinity. Membrane technologies as
applied to the municipal sector are predominantly pressure driven and, whilst the
membrane permselectivity and separation mechanism may vary from process to
another, such processes all have the common elements of a purified permeate product
and a concentrated retentate waste (Fig. 2.2).
The rejection of contaminants ultimately places a fundamental constraint on all
membrane processes. The rejected constituents in the retentate tend to accumulate
at the membrane surface, producing various phenomena which lead to a reduction
Table 2.1 Dense and porous membranes for water treatment
Pressure-driven Extractive/diffusive
Reverse osmosis (RO)
Separation achieved by virtue of differing
solubility and diffusion rates of water (solvent)
and solutes in water
Nanofiltration (NF)
Formerly called leaky RO. Separation achieved
through combination of charge rejection,
solubility–diffusion and sieving through
micropores (�2 nm)
Ultrafiltration (UF)
Separation by sieving through mesopores
(2–50 nm)*
Microfiltration (MF)
Separation of suspended solids from water by
sieving through macropores (�50 nm)*
*IUPAC (1985).
Electrodialysis (ED)
Separation achieved by virtue of differing ionic
size, charge and charge density of solute ions,
using ion-exchange membranes
Pervaporation (PV)
Same mechanism as RO but with the (volatile)
solute partially vapourised in the membrane
by partially vacuumating the permeate.
Membrane extraction (ME)
Constituent removed by virtue of a
concentration gradient between retentate and
permeate side of membrane
Gas transfer (GT)
Gas transferred under a partial pressure
gradient into or out of water in molecular form