RO technology has come a long way.
The concepts of osmosis were studied as early as 1748 among scientists. But it was only in early 1960‘s when asymmetric cellulose acetate membranes with relatively high water fluxes and separations were made available that RO separation processes became possible and practical for industrial purposes. Since then, the continual development and improvement of RO membranes have resulted in many RO applications. Apart from seawater and brackish water purification processes, RO membranes have also been used in wastewater treatment, production of ultrapure water, water softening and many others.
Osmosis. And the reverse of it.
Osmosis is a natural phenomenon in which a solvent (in this case, we refer to water as an example) passes through a semipermeable media (e.g. filtration membrane) from the content of lower solute concentration to the content of higher solute concentration, due to osmotic pressure differential between both solutions. The flow of water continues until the chemical potential equilibrium is achieved between both solute contents. At this point, the pressure difference between the solute contents is equal to the osmotic pressure of the solution. To reverse the flow of water, an external pressure greater than the osmotic pressure may be applied on the solute of higher concentration. With the filtration media as a barrier to most of the contaminants but not water molecules, separation of water from the solution occurs as pure water flows to the solute content of lower concentration. This “reverse” process is termed “reverse osmosis”.
RO membranes. Common types.
Two commonly used RO membranes today are thin-film composite (TFC) and cellulose acetate (CA) membranes. TFC membranes are generally made of layers of dissimilar materials formed together as a single membrane. This allows the use of material combinations to optimise the performance and durability of the membranes. CA membranes (also referred to as “asymmetric membranes”), the oldest form of commercial RO membranes, are made of the same material for different layers, though with dissimilar structures. TFC RO membranes have gained wider use today as they offer better salt and organics rejection, hence more highly purified water. They generally purify water at lower feed pressures, are more durable and easier to operate and maintain.
RO membrane modules.
There are generally 4 main types of RO membrane module configurations: plate-and-frame, tubular, hollow-fibre (capillary structure) and spiral wound modules. The most popular module in today’s industry for RO applications (and nanofiltration process) is the spiral wound module. The reasons could be of its most economical form of packaging, high membrane packing area per element, and the relatively low cost of the materials that are used to construct the membrane element/module.
Spiral-wound RO membrane modules.
A spiral wound RO membrane element available in the market today typically consists of membrane flat sheets, feed channel spacers, permeate collection layers, permeate collection tube and sealed carriers (or called Anti-telescoping device – ATD). ATDs are designed and fitted over the feed and concentrate ends of a membrane element. They are to prevent the membrane leaves from elongating (“telescoping”) due to the pressure differential across an element. They are also used to hold the brine seals.
Typical filtration spectrum of RO membranes.
A typical RO membrane has a pore size range of 0.0001 – 0.001 microns.
Among all the separation materials in the industry, it is by far the finest in terms of pore sizes available. In addition to removing all organic molecules and viruses, RO membranes also remove most minerals that are present in the water. They also remove monovalent ions, which means they are able to filter out salt (desalination) and metallic ions from water.
Maturing technology –
not without technical challenges.
While a maruting technology, the applications of RO membranes continue to face three main technical challenges – scaling, fouling and degradation problems, among others. These problems tend to reduce a RO system’s productivity in permeate’s output, and correspondingly increase the operating costs (especially from energy consumption) and membranes’ replacement costs significantly.
Scaling. This occurs on membrane’s surface when the concentration of scale-forming elements such as calcium carbonate, calcium sulfate, barium sulfate, reactive silica etc exceeds saturation and become additional solids in feed water. These low-soluble elements are difficult to be removed from the membrane’s surface. They reduce the membrane’s filtration efficiency and cause more frequent cleaning.
Fouling. A number of elements may cause membrane’s fouling when they are deposited on the membrane’s surface. These may include suspended solids, microbes and organic and colloidal materials. Soluble heavy metals, such as iron, can be oxidised within membrane modules and cause fouling too. Costly replacements of membrane are often required to achieve the recovery originally intended.
Degradation. Membrane degradation occurs when membranes are subject to conditions which destroy the polymer materials used to make the membranes. Hydrolysis at high and low pH, exposure to oxidisers such as chlorine are some examples that RO membranes may degrade fast over time.
Effective pretreatment – an essential process.
Effective pre-treatment processes to condition the feed water as preventive measures against membrane’s scaling, fouling and/or degradation would be essential towards optimising the performance of a RO system. The presence of accurate feed water’s parameters and careful and thorough analyses of feed water data would be some of the first due diligence steps towards designing and making high-performing RO systems. Subject to feed water’s qualities, some common pre-treatment options include multimedia filters (MMF), activated carbon filters (ACF), softeners, micro- or ultra-filtration systems, among others. Pre-treatment options >
Energy consumption – getting economical.
RO process requires relatively high pressure on the high concentration side of the membrane to overcome the natural osmotic pressure on the other side. Subject to the feed water’s qualities, typically, a RO system requires 2-17 bar (30-250 psi) of pressure to filter fresh and brackish water, and 40-82 bar (600-1200 psi) to desalinate seawater (which has around 27 bars or 390 psi of natural osmotic pressure). Running pumps to generate such range of flow pressure consumes substantial power over time.
Managing the energy consumption of RO applications.
While RO membranes are widely applied for industrial and municipal purposes today, high energy consumption remains a key concern. This is particularly so for seawater desalination. However, the advancement of RO membranes in achieving greater flux per unit of membrane’s surface area, and the availability of high-efficiency pumps these days help mitigate energy consumption costs of RO applications. But most significantly, the availability of various innovative Energy Recovery Devices (ERD) has made seawater desalination an economically viable option for producing potable and industrial water today.
From brackish water to seawater.
Wide spectrum of applications.
RO technology can be used to remove up to 95-99% of Total Dissolved Solids (TDS – More >), and most other contaminants, present in water of different sources. It is capable of purifying water of many sources, to qualities required by a wide spectrum of applications. With the high efficiency of most established membranes and the cost savings from availability of energy recover devices, RO technology has become a very widely adopted way worldwide to achieve high quality water needed for domestic consumption and a wide spectrum of industrial uses. It has also been used popularly for wastewater reclamation, and for industrial production processes such as concentration of fruit juice, maple syrup production etc.