Membrane filtration, microfiltration, ultrafiltration, reverse osmosis

The growing significance of using membranes in the water treatment process is due to various factors. Insufficient freshwater resources first led to the development of membranes for use in desalination.

Membranes for all types of water treatment

In recent years other factors have helped to extend the use of membrane techniques in all areas of water and wastewater treatment :

ever stricter and often contradictory regulations on water quality: a process that physically removes pathogens, membranes make it possible for disinfection to occur without producing any undesirable by-products.
the growing significance of water reuse: wastewater has now become a resource in cities as well as in arid areas. Membranes are an ideal tool for recycling water due to their extensive abatement range.
technical advances: membrane technology has been used in the water treatment process for less than 50 years. After a long research and development phase, its industrial application accelerated once the main patents passed into the public domain.

Basic Principles

Membranes are semi-permeable thin barrier sheets.

Synthetic membranes are used to remove different solutions and particles in the water treatment process.

There are five types of membrane processes (see table below).

	Conventional Process	Membrane Process
	Sand Filtration	MF	UF	NF	RO	ED
Gradient	Pressure					Electric Potential
Transport of the main species	Water					Ions
Removal of the smallest species	Suspended Matter	Colloids Bacteria	Large Organic Molecules, Viruses	Small Organic Molecules, Divalent Ions	All Dissolved Species
Operating Pressure (bar)	0.1 to 2	0.2 to 2	1 to 5	5 to 20	20 to 80	1 to 3
Typical Flux (l/m 2/h)	2,000 to 10,000	100 to 1,000	50 to 200	20 to 50	10 to 50
Water Treatment Applications	Clarification	Clarification	Clarification	Water softening, colour, natural organic matter, micro- pollutant removal	Desalination of seawater and brackish water	Desalination of brackish water, ion removal (NO₃)

Different Pressure Processes

Nanofiltration (NF) and reverse osmosis (RO) as well as microfiltration (MF) and ultrafiltration (UF) are often described as being pressure processes. They are in fact very different from each other :

MF is a direct continuation of conventional filtration that makes it possible to remove particles smaller than a micron such as colloids and bacteria.
UF goes even further. Thanks to this process viruses and heavy molecular organic compounds can be removed.

These two processes use a straining technique and therefore separation depends on the size of the pores and which species are chosen, while in NF and RO the pores are not visible.

The retention level for inorganic and organic solutions depends on their solubility and their diffusivity in the membrane. Retention also depends to a large extent on the net charge on the membrane surface.

Reverse osmosis (RO): high pressures

RO can only occur at higher pressures than those of the osmotic pressures of feed water.

While the osmotic pressure of brackish water is relatively low (1.4 and 3.4 bars for 4.0 g/l of calcium sulphate and sodium chloride solution), that of seawater is relatively high (27 bars for 35 g/l of sodium chloride).

Electro dialysis (ED): transportation of ions under the influence of an electric potential

ED is a fundamentally different process.

Rather than have the water pass through the membrane, the ions are transported through the membrane under the influence of an electric potential. Micro-porous positive and negative membranes are alternatively filled with electrodes in order to specify which columns will be enriched with or depleted of salts. ED's role is limited to the removal of ionic species.

There is also a modified version of ED, called "electrodialysis reversal" which consists of reversing the polarity of the electrodes at various frequencies in order to minimize scaling and clogging.

20 to 50% of treated water is converted

The conversion rate is a concept that applies to all membrane processes.

This is the fraction of feed water converted into treated water.

This level ranges from 20% to 50% for the desalination of seawater by RO (it is limited due to the high osmotic pressure).

It reaches a maximum of 80 % to 95 % when using RO, NF or ED to treat brackish water or freshwater (process limitations being due to possible scaling from the concentrate).