Page 29 - Membranes for Industrial Wastewater Recovery and Re-Use

lntroduction 9

Table 1.6 Membrane processes
Process Usual objective
Microfiltration (MF) Removal of suspended solids, including microorganisms
Ultrafiltration (UP) Removal of both large, dissolved solute molecules and
suspended colloidal particles
Nanofiltration (NF) (Selective) removal of multivalent ions and certain charged or
polar molecules
Reverse osmosis (RO) Removal of inorganic ions
Electrodialysis (ED) and dialysis (Selective) extraction of ions from water and/or
concentration
of these ions in the waste stream
Pervaporation (PV) (Selective) extraction of molecular gas and/or volatile solutes
Gas transfer (GT) Transfer of molecular gas into or out of water

1. Separation is achieved without requiring a phase change, and is therefore
more energetically efficient than distillation.
2. Little or no accumulation takes place in the process which therefore operates
continuously under steady-state condition without necessitating regeneration
cycles, unlike adsorptive separation processes.
3. Little or no chemical addition is required, unlike conventional clarification
which generally relies on the addition of chemical coagulants and flocculants.

Membrane technology development as a whole began with the first high-
performance reverse osmosis membrane produced by in the early 1960s (Loeb
and Sourirajan, 19631, which led to the installation of large seawater
desalination plant in arid regions of the world. Since that time, growth in the
total quantity of membranes sold with reference to the water treatment capacity
provided has increased exponentially with time, with growth in microfiltration
and ultrafiltration technologies over the last decade of the twentieth century
being particularly pronounced (Fig. 1.1). There has been a corresponding
exponential decrease in membrane costs with installed membrane plant
capacity, as reflected in data from an established hollow fibre microfiltration
membrane (Fig. 1.2). The increasing stringency of water quality guidelines
and standards being introduced for municipal water treatment, for example
the European Union Urban Waste Water Treatment Directive (UWWTD) and the
position taken by the UK Drinking Water Inspectorate regarding cryptosporidia,
portend continued growth in this sector for membrane technology. Indeed, the
EU wastewater treatment membrane market is predicted to increase by an
average annual growth rate of 5.9% to $2 50 million in 2006 (RCC, 2002).
Application of membrane processes within the industrial sector is also
widespread and well established in many instances. Reverse osmosis and
ultrafiltration, for example, are both widely used within the pharmaceutical
industry to remove pyrogens in the provision of water for injection (WFI). These
processes are also essential in providing ultrapure water in semiconductor
fabrication plant, where ultrafiltration is used for removing colloidal material
and reverse osmosis both for primary deionisation (followed by polishing using