Reverse Osmosis is currently the most advanced technique being used for purifying water of all unwanted minerals contained in it. The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use screens of various sizes to separate particles from the water streams.
In order to understand what reverse osmosis is, it is necessary to know what osmosis is first. Osmosis – the natural phenomenon- is the flow of water from a region of low salt concentration to a region of high salt concentration through a semi permeable membrane.
In other words if two containers of water, one at high salt concentration and one at low salt concentration were connected by a semi permeable membrane then the water molecules would flow from the low concentration to the high concentration until both solutions were balanced out.
Reverse Osmosis System acts as a molecular filter to removes the entire spectrum of contaminants of Sodium, Salt, Nitrates, Fluoride, Lead, Heavy metals, Cysts, Cryptosporidium, Bacteria, Tastes, Odors, sugars, proteins, particles, dyes THMs, Chlorine, Radium, Dissolved Solid, Pharmaceuticals, Sewage Treatment Plant Waste and much more. The BEST in water purification great tastingDissolved ions / > 93%
Organics / 99%
Particles / > 99%
Bacteria / > 99%
Pyrogens / > 99%
Most reverse osmosis technology uses a process known as crossflow to allow the membrane to continually clean itself. The process of reverse osmosis requires a driving force to push the fluid through the membrane, and the most common force is pressure from a pump. With a high-pressure pump, pressurised saline feed water is continuously pumped to the module system. Within the module, consisting of a pressure vessel (housing) and a membrane element of 0.0001 micron, the feed water will be split in a low saline product called permeate across through the membrane to the core and a high saline brine of dissolved minerals called concentrate or reject remains on the feed side. A flow regulating valve called concentrate valve, controls the percentage of feed water that is going to the concentrate stream and the permeate which will be obtained from the feed as a safe to drink.
The rate of production is relatively slow. Normally a storage tank is used hold the finished water until it is needed. When the tank is full, the system will automatically stop making R.O. water.
Not all of the water flows through the membrane. Only 15-50% of the water becomes permeate. The remainder stays on the feed side of the membrane and flushes away the dissolved minerals. this water is called the "concentrate". The concentrate plays an important role in the operation of the membrane. As the permeate flows through the membrane, the concentrate retains almost all of the minerals that were in the original feed water. The TDS (total dissolved solids) of the concentrate rises. As it flows past the membrane, it carries away the minerals, in effect washing the surface of the membrane. It is eventually piped out to the drain. If the concentrate did not carry away the minerals, the membrane would foul or plug-up, acting like a filter which fills with particles and must be changed. Proper concentrate flow across the membrane will generally mean many years of high quality water from the membrane.
Other methods of water treatment, such as water softening and deionization, are also important and it is a combination of these along with R.O. that will normally produce the highest quality water It should be noted that an R.O.
The rejection of the chemicals by the membranes is achieved not only by molecular size exclusion but also by factors such as solubility and ionic charge and rejections of close to 99% are generally achieved for both salt (as sodium chloride) and total organic carbon (TOC). The process is also often used in seawater desalination applications.
Membranes The working principle of Reverse Osmosis system is detaching concentrated solution and diluted solution by a semi-permeable membrane.
A membrane is a selective barrier that permits the separation of certain species in a fluid by a combination of sieving and sorption diffusion mechanisms. In terms of energy, membrane separations have an important advantage in that, unlike evaporation and distillation, no change of phase is involved in the process, thus avoiding latent heat requirements. No heat is required with membranes, thus it is possible to produce products with functional properties superior in some respects to those produced by conventional processes. Membrane technology also enables to simultaneously concentrate, fractionate, and purify the products.
The membranes come in a variety of pore sizes and characteristics.
Membranes are available in several different configurations such as tubular, hollow fiber, plate-and-frame, and spiral-wound. Some of these designs may work better than others for a particular application, depending on such factors as viscosity, concentration of suspended solids, particle size, and temperature.
- Spiral Membranes, are produced by winding consecutive thin layers of feed spacer, membrane, permeate collection channel, and membrane around a perforated center tube for permeate collection. Spirals offer the advantages of a wide range of membranes and lower energy costs due to reduced pumping requirements and higher packing density. The features of spiral construction also allow the module to be operated at elevated pressure and temperature, resulting in greater output.
- Hollow Fiber membranes are anisotropic (asymmetric). The self-supporting membrane can be back-flushed to maximize the efficiency of the cross-flow filtration process. This operating feature, combined with chemical stability, ensures reliability and long life expectancy. The hollow fiber geometry allows a high membrane surface area to be contained in compact modules providing high capacity while utilizing minimal space.
- Tube Membranes consist of a membrane cast on the inside of a support tube, 1/2" to 1" in diameter. Typically, the 1" tube is used singly, and the 1/2" tubes are bundled into modules. The tubular design provides superior performance at high concentrations of solids. Plugging is minimized and high product recovery is achievable.
This is an outline of the components of a spiral wound reverse osmosis membrane and their functions:
This material prevents salt passage while allowing water to permeate. The membrane is typically made from polyamide with a polysulphone support layer.
Permeate Carrier (Tricot)
Sandwiched between layers of membrane, this material carries permeate water to the permeate tube.
A perforated tube which collects permeate and, upon which, the membrane leaves are attached. The ends of the permeate connector are profiled to hold interconnectors. These allow permeate to travel from element to element, and finally to the take-off point on the pressure vessel.
Anti-telescoping Device (ATD)
This device is fitted over the feed and concentrate end of the membrane. It is designed to prevent the membrane from elongating ("telescoping") due to pressure differential from feed to concentrate. It also holds the brine seal.
Feed Spacer (Vexar)
Fitted between membrane leaves, the vexar forms a flow channel for the feed water to pass over. It is designed to generate turbulence, breaking down boundary layers close to the membrane and helping to reduce scaling and fouling potential.
The Two types of membranes commonly used:
- CTA membrane (Cellulose Triacetate): It is specially useful in the removal of chlorine from water. Hence, it is used widely in commercial water filters to purify tap water.
- TFC membrane (Thin Film Composite): It is highly resistant to bacteria and has to be used in combination with carbon filters, if the water to be purified is chlorinated.
The Thin Film Composite membranes used in our systems due to its resistance to bacterial attack and degradation due to alkaline or acidic conditions, and for its higher rejection rate (95-98%) and longer life (durability) than the CTA membrane. it also serve at high pH and high TDS range.
Thin film composite. TFC membranes have high salt rejection rates, usually operate at a lower pressure then CA or HF and have exhibited good performance under wide ranging pH and temperature conditions. They are not degradable by microorganisms and hold their flux rates over long periods of time. They have low chlorine tolerance that is because chlorine oxidizes its surface which damage its ability to reject salts, so chlorine removal is needed as pretreatment step. TFC membranes are produced in spiral wound module configuration.
Most TFC (thin film composite) membranes produce water with a 90-98% plus rejection of the salts depending on the temperature and pressure and the particular salt.
TFC membranes composed of three layers of materials - a thin (0.25 um) barrier coating on the surface of a micro porous layer of polysulfone. both supported by a polyester web. The barrier coating can be made of polymers such as polyamines or polyethers.
Thin Film membrane elements can be very effectively cleaned due to their tolerance for wide range of pH and temperature. However, if cleaning is delayed, it becomes increasingly difficult to remove foulants or scales from the membrane surface. Cleaning will be more effective if it is tailored to the specific fouling problem.
Comparison of Operating and Cleaning Parameters for Thin-Film Composite Membrane and a CA Membrane
Typical TFC Membrane Rejections Rates
Contaminate / %Rejection / Contaminate / %Rejection
Aluminum / 96-98% / Ammonium / 80-90%
Arsenic / 50-90% / Bacteria / 95-99%
Borate / 30-50% / Boron / 50-70%
Bromide / 90-95% / Cadmium / 93-97%
Calcium / 93-99% / Chloride / 92-98%
Chromate / 85-95% / Copper / 96-99%
Cyanide / 90-97% / Fluoride / 92-95%
Hardness (as CaCO3) / 93-97% / Iron / 96-98%
Lead / 95-98% / Magnesium / 93-98%
Manganese / 96-98% / Mercury / 94-97%
Nickel / 96-99% / Nitrate / 90-95%
Orthophosphate / 96-98% / Phosphate / 95-98%
Polyphosphate / 96-98% / Potassium / 92-96%
Radioactivity / 93-97% / Silica / 90-98%
Silicate / 92-95% / Silver / 93-96%
Sodium / 92-98% / Sulfate / 96-99%
Thiosulfate / 96-98% / Zinc / 96-98%
Scaling of the membrane surface occurs due to the precipitation of sparingly soluble salts, such as calcium carbonate, barium sulfate and iron compounds.
As water passes through the membrane, dissolved minerals from the feed-water become more concentrated in the reject. If concentrations exceed solubility limits, crystals will precipitate on the surface of membrane. Scaling occurs first in the last elements of the system because feed-water is more concentrated and there may be more polarization.
fouling increases with increased flux rates and decreased feed flow. In addition, fouling tends to occur at the feed end where flux is the highest.
Fouling of membranes is due to the suspended, organic, Bacteria and other microorganisms or emulsified materials that may be present in the feed water to the reverse osmosis system. Examples of such materials are: silica, oil, clay, iron, sulfur and humic acids. These substances can be present in a very fine or colloidal form. Even the typical 5 micron cartridge filters used upstream from a reverse osmosis system may not completely remove these foulants. This foulants decreases the capacity and/or an increase of the pressure and, as a result, of the energy use. fouling increases with increased flux rates and decreased feed flow. In addition, fouling tends to occur at the feed end where flux is the highest.
Some foulants/scales are nearly impossible to clean off: e.g. aluminum, oil, grease, calcium, barium, or strontium sulfate scale, calcium phosphate. So if an element is fouled or scaled with these, it will need to be replaced.
For non-residential systems, another option to avoid calcium carbonate and calcium sulfate scaling is by the use of antiscalants. These are injected directly into the feed water upstream from the cartridge filter. Dosage of antiscalant depends on the feed water analysis but usually is between 2 to 5 ppm. In simplified terms, the antiscalants delay the scale formation process. This delay is sufficient to avoid precipitation of calcium carbonate and calcium sulfate on the membrane surface. As this delay is for a finite period, scaling can take place in systems on shut down. For this reason, it is a good practice to flush the membranes with permeate or feed water at shut down. By this flush, the concentrated solution in the membrane is displaced by the permeate or feed water.
Adjusting the pH of the feed water is another way to control calcium carbonate scaling. The net effect of lowering the feed pH with acid injection is to convert bicarbonate alkalinity to carbon dioxide and thereby prevent the formation of calcium carbonate.
A Membrane life is a function of feed water source as ph, TDS, temperature, microorganisms, suspended solids, chlorine content, pretreatment, frequency of cleaning, system design, and operating conditions as temperature, pressure. For economic analysis, a 5 year life is normally used.
Effect of Feed water Pressure on Flux and Salt Rejection
The higher the net pressure on a membrane element, the higher the permeate rate, while , the permeate TDS will decreases.
Effect of Feed water Temperature on Flux and Salt Rejection
The water temperature is one of the key factors in the performance of the reverse osmosis membrane element. The higher the temperature, the more the product flow and vice versa. All reverse osmosis membrane elements and systems are rated at 77º Fahrenheit (25ºCelsius).
Effect of Increasing Salt Concentration on Flux and Salt Rejection
Effect of Increased Recovery on Flux and Salt Rejection
The recovery is the ratio of permeate flow to feed flow. In the case of increasing recovery, the permeate flux will decrease and stop if the salt concentration reaches a value where the osmotic pressure of the concentrate is as high as the applied feed pressure. The salt rejection will drop with increasing recovery.
Effect of Feed water pH on Water Flux and Salt Rejection
Effect of chlorine:
The residual free chlorine present in most municipal water supplies will damage the thin film composite structure of the membranes used in this unit. Carbon filtration or sodium bisulfite injection should be used to completely remove the free chlorine residual.
The Water Quality Association (WQA) classifies water into various categories based on the Total Dissolved Solids (TDS) present in the water. The classification is as follows-
- Fresh Water < 1000 mg/lit TDS.
- Brackish Water 1000- 10,000 mg/lit.
- Highly Brackish Water 10,000-15,000 mg/lit.
- Saline water- 15,000-30,000 mg/lit.
- Sea water- 30,000- 40,000 mg/lit.
- Brine- 40,000 TDS and up.
POTABLE WATER / DOMESTIC / SEMI RESIDENTIAL:
Potable Water Systems are used at Home & Villas, Apartments / condominiums, Schools, Healthy water stations, Construction sites, Military sites and etc.
Commercial Systems are widely used in Hospitals / Medical Facilities, Kidney dialysis, Food/Beverage processing, Ice plants, Laboratories, Horticulture's / Greenhouses, Film processing, Printing, Vehicle washing and etc.
Industrial Systems are used for Deionizer pre-treatment, Cooling tower make-up, Boiler feed water, Humidifiers, Electronics manufacturing, Chemical processing and recovery, Pharmaceuticals, Metal working lubricants, Plating make-up and etc.
These plants are specially used in kidney dialysis center of hospitals, medical purposes and treatment of critical patients suffering from kidney failure.
Features of Reverse Osmosis
1. Removal of dissolved salts
Reverse osmosis can stably and effectively remove dissolved salts, dissolved organic substances (trihalomethane, its precursors, agricultural chemicals, etc.), and micro fine particles (living germs, dead germs, and many other micro fine particles) from water. Thus it is ideal for a wide area of applications ranging from production of ultra pure water to desalination of seawater.
2. Energy-saving separation technique
Reverse osmosis keeps water from evaporating, making it energy-saving separation technique that requires less energy consumption.
3.Utilizable as a concentration and recovery method
Reverse osmosis does not need heating, it can concentrate and recover valuable process materials dissolved in a solution without any degradation which might otherwise occur in such materials.
4. Compact equipment
Modules can be arranged in a three dimensional configuration to provide excellent space efficiency, Small size and has a lot of flexibility in where to install the unit.
5. Simple operation and control
Reverse osmosis is a simple process, its operation and control are uncomplicated, while maintenance is easy and free from trouble.
6. Clean technology.
No Regeneration with chemicals, Reverse osmosis systems do not use of harmful chemicals, so it used in many power plants. Hence it considered environmental friendly
7. The cost is low compared to other water filtration systems
8. Long lifetime of membranes
The RO system uses an automated technique called cross-flow to clean its clogged membrane.
Reverse Osmosis pre-treatment and post treatment options include:
pretreatment is essential to avoid membrane fouling by sediments, hardness, organic matter, bacteria, silica, metal oxides or even chlorine.
Also, RO permeate water is often more acidic than the feed water due to dissolved carbon dioxide. Common post-treatment are pH neutralization and re-mineralization, sometimes it is need to preserve the water free from microorganisms so it is proper to add disinfection unit.
- Multi-media filters
- Carbon filters
- Water softeners
- Chemical dosing to raise pH, remove silica, feed antiscalent
- Mixed-bed polishers
- Raw water break tanks
- UV Treatment
- Ozone generator
- Permeate storage and distribution
Reverse Osmosis System