Membrane separation process: A sustainable technology for water purification, industrial wastewater recycling and reuse

“The current population of slightly more than six billion consumes the resources (water) of one planet Earth. By about 2050, when the population is expected to reach about nine billion, and if standards of living continue to rise, the amount consumed will be the resources of about three planet Earths. Obviously, this scenario is not sustainable.” (Daigger, 2008)

In highly populated urban areas and where surface water is used for domestic water supply such as in Metro Manila, a more efficient wastewater treatment must be employed. An environmentally sustainable technology for water treatment should be energy-efficient with minimal or no chemical consumption, and capable of water recycling and reuse that minimizes the direct disposal of wastewater to the aquatic environment. Membrane separation technology has the promise to dramatically improve the sustainability of our water resources.

Membrane separation processes are not new or even recent technology. The use of membrane separations started in 1960. In 1980, large membrane filtration plants were already installed worldwide, and microfiltration, ultrafiltration, reverse osmosis and electrodialysis membrane processes were established. 

What is a membrane? A membrane is a highly engineered thin barrier that has the ability to reject various mineral salts, heavy metals, organic molecules, bacteria, parasites, and even viruses, while allowing the permeation or passage of water. Separation is based on the molecular size, shape or character of the species. Membranes may be as thin as a fraction of a micrometer or several millimeters thick. Most people may think that a membrane resembles that of a filter, like the filter paper we use for brewing our coffee. However, a membrane is much more complex in both structure and function. The ability of a membrane to reject dissolved particles depends on the multitude of pores, of incredibly small size, that penetrate its surface. The membrane pores can reject particles as small as 0.0005 micrometer or 0.5 nanometer (nm) and allow water permeation with size equivalent to 0.298 nm. [A micrometer (mm) is a metric unit of length equal to a millionth of a meter while a nanometer is a billionth of a meter. Human hair is approximately 75 mm in diameter. The naked eye can only see particles as small as 40 mm. The smallest bacterium is about 0.22 mm while the size of a virus is even smaller at 0.01 mm.] Membranes or synthetic membranes can be produced from organic materials (http://en.wikipedia.org/wiki/Organic) such as polymers and liquids, as well as inorganic materials (http://en.wikipedia.org/wiki/Inorganic). Most of commercially utilized synthetic membranes in separation industry are made of polymeric structures (http://en.wikipedia.org/wiki/Polymeric).

The growing interest in membrane technology for water and wastewater treatment is based on the following advantages:

Unlike conventional technology, membrane technology has better removal efficiencies. Membrane separation processes can separate a wide range of contaminants ranging from suspended solids to microorganisms. Membrane technology has the capability to address more stringent drinking water regulations, since it prevents the passage of Cryptosporidium, Giardia, bacteria, and virus. Membrane technology, therefore, avoids the risk of microbial outbreaks without any chemical pretreatment. In wastewater treatment, membranes produce a very high effluent quality that meets strict discharge regulations. Thus, effluents can be reused for industrial applications, irrigation, and even as a source of potable drinking water.

Membrane systems have flexibility to handle changing feed water conditions and capacity increases. The operation is simple and automated which ensures that system integrity is met. The separation process can be batchwise or continuous.

The technology is suitable for small and distributed communities. A membrane filtration system requires a smaller footprint than conventional technologies. Membrane technology needs only half of the footprint of a conventional wastewater treatment plant, thus saving space and money. The capacity of the existing plant can be increased without additional footprint whether for plant upgrade, expansion or for a new plant, thereby providing great capital savings. The membrane has a modular design which makes it possible for easy scale-up. 

Low energy consumption. Membrane separation processes do not involve any phase change, which makes the process energy-efficient.

Membrane separation provides the lowest cost of treated water, as compared to most conventional technologies over the life of the plant. With the increase in number of membrane manufacturers, there is a steady reduction in membrane equipment costs which makes water and wastewater treatment cost much lower. Recycling wastewater from sewage is much cheaper than purifying seawater. Recycling will take care of wastewater disposal problems and water pollution. In Singapore, all over America, Italy, and other parts of the world, where there is water scarcity and water stress, recycling is done unobtrusively. In the recycling plants, the treated water is recharged into the ground and allowed to mix with fresh groundwater before use.

Membranes can process very bad water, for example, as bad as floodwater and sewage wastewater with very high concentrations of suspended particles and organic compounds. The use of membrane technology will increase the use of lower quality water, which can be an alternative source of potable and non-potable water supply.

These benefits must be reviewed by the government to recognize that membrane technology is an ideal separation process for water and wastewater treatment, and also for environmental applications.

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Dr. Michelle C. Almendrala is in the faculty of the School of Chemical Engineering and Chemistry, Mapua Institute of Technology; an associate member of PAASE (Philippine-American Academy of Science and Engineering); a member of the Philippine Institute of Chemical Engineers; and a member of the Water Environment Association of the Philippines Inc. (a member-association of the Water Environment Federation, USA). She was selected as a principal candidate for a Fulbright Scholar Advanced Research award in the United States during the academic year 2009-2010 for her research on “Recycling of Biobutanol Fermentation Broth by Membrane Ultrafiltration” at the Department of Chemical and Biomolecular Engineering of the Ohio State University in Columbus, Ohio. Her research interests are membrane separation applications in wastewater treatment and recycling; rice bran oil extraction, fruit juice clarification and concentration by osmotic distillation using hollow fiber membrane. E-mail her at [email protected].