Everyone is aware of the fundamental importance of water to life on Earth, whether plant, animal or human. The process of life itself is based upon the unique character of the water molecule, and the method by which living things interact with water is through membranes. Over the past 50 years, water treatment technology has increasingly adopted the natural forces of membranes, and in the next 50 years, membranes have the potential to revolutionize the water treatment industry.
A membrane is a barrier that allows the passage of specific types of particles and prevents the passage of other particles. In nature, membranes are composed of lipids and proteins manufactured within the cells of a living host. Manmade membranes are made from polymers—essentially plastics, such as polyamide or polysulfone—manufactured in pools of solvents. The pores in the membrane surface are created by the natural barrier between the water-soluble (hydrophilic) solvents and the water-insoluble (hydrophobic) membrane surface. This is similar to the separation observed between oil and water.
The particles in water can be classified into four types. Charged particles (typically dissolved solids) carry an ionic charge, which interacts with the polar water molecules. These particles can vary in charge strength and size. This category includes cations and anions. Uncharged particles (suspended solids) carry no ionic charge, and include silt, tannins and colloids. They vary in size and configuration. Important uncharged particle subsets include organic particles, which contain a carbon-hydrogen bond, and microbes, which are living organic particles.
The pore size defines the primary function of a membrane. Typical pore sizes range from 0.001 to 1 µ. (For reference, the periods on this page have a diameter of 615 µ.) For convenience, names have been established for membranes with particular pore size ranges, from microfilter to osmosis. However, the purpose of membranes is to provide a barrier that is selective to a particular type or size of particle.
Process of Life
Water interacts with solid particles across the surface of membranes. The action at the surface of membranes is different for various pore sizes. The smallest pore size, referred to as osmosis, has an effect upon dissolved charged particles. Osmosis is defined by the movement of water molecules from a position of low concentration of charged particles to a position of high concentration of charged particles. Osmosis is caused by the affinity of the polar water molecule to associate with charged particles in order to balance the polar charges.
The other important interaction of water with particles is diffusion. In this phenomenon, particles move within the water to attain uniform concentration throughout. All particles that can fit through the pore will disperse. This phenomenon can be observed in everyday life in the diffusion of tea within a cup. Diffusion is active on all inanimate particles in water, whether charged or uncharged.
All living beings on Earth employ these two membrane phenomena. Osmosis is used in our bodies to regulate electrolyte concentration and control blood pressure, primarily in the kidneys. Diffusion is employed to facilitate the transfer of nutrients into our organs, and the release of waste products for disposal.
An important application of ultrafiltration (UF) technology takes place in the human digestive system. All animal species’ digestive systems include trillions of living microbes, many of which provide the service of digesting foods into products the body can use as an energy source or as building blocks for new living cells. The surfaces of the digestive system consist of membranes, which allow the ingestion of these products for transfer to various parts of the body.
The water treatment and processing industries have applied these living membrane processes to their technologies. Osmosis was first observed in 1748, but the application of energy in the form of water pressure to reverse the natural process of osmosis, appropriately termed reverse osmosis (RO), was not applied in the industry until the 1950s. The development of plastic technology (polymer chemistry) has been the driving force behind this application. Today, the RO process has water treatment applications ranging from home faucets to municipal desalination plants.
UF accelerates the process of diffusion by applying additional energy in the form of pressure. It initially was recognized in the early 1900s and has rapidly evolved since its application in hemodialysis as an artificial kidney in 1944. Although UF technology has mirrored the development of RO—due to plastic technology—it has only recently become widespread in water treatment, especially in residential and small commercial applications. UF covers a range of pore sizes, and specific UF membranes target a specific range of particle sizes intended to filter at a specific micron rating.
Developed in the 1960s, membrane bioreactor (MBR) technology is an application of the human digestive system. Bacterial species are cultivated in a wastewater medium and encouraged to “digest” the waste into its components. UF membranes are used to harvest the selected products. The utility of MBR accelerated because of developments in polymer chemistry and microbiology. Today, microbes can be cultured for the digestion of an array of water contaminants.
Membrane efficiencies are a factor of surface area and uniformity of pore structure. Improvements in precision of membrane manufacturing is the next step in membrane improvement. The more uniform the pore structure, the more efficient the surfaces become. Advances in the precision of membrane structure already have significantly improved efficiency in RO technology, especially in desalination. The effect upon UF will be even more dramatic. Advances in precision could have the same effect on water treatment as they had on U.S. industry in the 1940s, when the country’s manufacturing capabilities outpaced the entire world.
Promise of Potential
In the distant future, technology may cultivate more intricate living membrane processes of active transport. Within living beings, transport membranes actively select and transfer specific solids from the side of lower concentration to the side of higher concentration, reversing the process of diffusion. This would be “reverse diffusion.” To do this, the membrane harnesses natural potential energy contained in the adenosine triphosphate molecule. Imagine the ability to dilute only specific contaminants from water.
Advances in media and chemical technology have provided improvements in the ability to efficiently modify the contents of water solutions. Advances in membrane technology, on the other hand, have inspired dramatic increases. The better we understand and apply living membrane technology, the more dramatic advances will become.