Basic Wastewater Terminology: What Is Reverse Osmosis?
Reverse osmosis (RO), also called inverse osmosis, is a membrane separation operation that uses a pressure difference as the driving force to separate the solvent from a solution. When pressure is applied to the feed liquid on one side of the membrane and exceeds its osmotic pressure, the solvent flows in the reverse direction to natural osmosis (i.e. back through the membrane). Thus the permeated solvent - the permeate - is obtained on the low-pressure side of the membrane, while the concentrated solution - the concentrate - is obtained on the high-pressure side. If RO is applied to seawater, fresh water is obtained on the low-pressure side and brine on the high-pressure side.
During reverse osmosis, the solvent permeation rate, i.e. the solution flow energy N, is:
N=Kh(Δp-Δπ)
where Kh is the hydraulic permeation coefficient, which increases slightly with rising temperature; Delta p is the static pressure difference across the membrane; and Delta pi is the osmotic pressure difference of the solutions on the two sides of the membrane. The osmotic pressure pi of a dilute solution is:
where i is the number of ions produced by dissociation of the solute molecule; C is the molar concentration of the solute; R is the molar gas constant; and T is the absolute temperature.
Reverse osmosis typically uses asymmetric membranes and composite membranes. The equipment employed for RO mainly consists of hollow-fibre or spiral-wound membrane separation devices.
RO membranes reject various inorganic ions, colloidal matter and macromolecular solutes in water, thereby yielding purified water. They can also be used for the pre-concentration of macromolecular organic solutions. Because the RO process is simple and energy-efficient, it has developed rapidly over the past 20 years. It is now applied at large scale in the desalination of seawater and brackish water (see brine), boiler feed-water softening and wastewater treatment, and is combined with ion exchange to produce high-purity water; its scope of application is expanding and it has begun to be used in the concentration of dairy and fruit-juice products and in the separation and concentration of biochemical and biological preparations.
One uses carbon nanotubes to form the tiny pores of the membrane; the other
Place equal volumes of a dilute solution (e.g. fresh water) and a concentrated liquid (e.g. seawater or brine) on opposite sides of a container, separated by a semi-permeable membrane. The solvent in the dilute solution will naturally pass through the semi-permeable membrane and flow toward the concentrated side; the liquid level on the concentrated side will rise above that on the dilute side by a certain height, creating a pressure difference - this is the state of osmotic equilibrium, and that pressure difference is the osmotic pressure. The magnitude of the osmotic pressure depends on the type, concentration and temperature of the concentrated liquid, and is independent of the nature of the semi-permeable membrane. If a pressure greater than the osmotic pressure is applied to the concentrated side, the solvent in the concentrated solution will flow toward the dilute solution; this flow direction is opposite to that of the original osmosis, and the process is called reverse osmosis.
Lonsdale et al. proposed the solution-diffusion model to explain the reverse osmosis phenomenon. They regarded the active skin layer of RO as a dense, pore-free membrane, and assumed that both solute and solvent dissolve into the homogeneous non-porous surface layer, each diffusing through the membrane driven by the chemical potential caused by concentration or pressure. The difference in solubility and the difference in diffusivity of solute and solvent in the membrane phase affect the energy with which they pass through the membrane. The specific process is divided into three steps: first, the solute and solvent are adsorbed and dissolved on the surface of the feed side of the membrane; second, with no interaction between solute and solvent, they pass through the active layer of the RO membrane by molecular diffusion driven by their respective chemical potential differences; third, the solute and solvent desorb on the permeate-side surface of the membrane.
In the above process of solute and solvent passing through the membrane, it is generally assumed that the first and third steps proceed very rapidly, so that the permeation rate is determined by the second step - the molecular diffusion of solute and solvent through the membrane driven by their chemical potential differences. Because of the membrane's selectivity, gas or liquid mixtures can be separated. And the permeation capacity of a substance depends not only on its diffusion coefficient but also on its solubility in the membrane.
Preferential adsorption-capillary flow theory
When a liquid dissolves different kinds of substances, its surface tension changes in different ways. For example, when alcohols, acids, aldehydes, esters and other organic substances are dissolved in water, the surface tension decreases, but dissolving certain inorganic salts instead slightly increases it. This is because the distribution of the solute is non-uniform - the concentration of the solute in the surface layer of the solution differs from that inside the solution; this is the surface adsorption phenomenon of solutions. When an aqueous solution contacts a polymeric porous membrane, if the chemical nature of the membrane causes it to negatively adsorb the solute and preferentially positively adsorb water, a pure-water layer of a certain thickness adsorbed by the membrane forms at the membrane-solution interface. Under external pressure this layer passes through the capillary pores on the membrane surface, thus yielding pure water.
In cellulose acetate, due to hydrogen bonding and van der Waals forces, the membrane contains two parts: crystalline and amorphous regions. Regions where the macromolecules are firmly bound and arranged in parallel are crystalline, while regions where the macromolecules are completely disordered are amorphous; water and solute cannot enter the crystalline regions. Near the cellulose acetate molecules, water forms hydrogen bonds with the oxygen atoms on the carbonyl groups of cellulose acetate, forming so-called bound water. Once cellulose acetate adsorbs the first layer of water molecules, the entropy of the water molecules drops sharply, forming an ice-like structure. In the larger pore spaces of the amorphous regions, the occupancy of bound water is low; in the centre of the pores exists normally structured water, and ions or molecules that cannot form hydrogen bonds with the cellulose acetate membrane enter the bound water and migrate by ordered diffusion, passing through the membrane by continuously changing their hydrogen-bonding positions with the cellulose acetate.
Under pressure, water molecules in the solution form hydrogen bonds with the activation sites of cellulose acetate - the oxygen atoms on the carbonyl groups - while the original hydrogen bonds of the water molecules are broken; the water molecules detach and move on to the next activation site, forming new hydrogen bonds. Through this continuous formation and breaking of hydrogen bonds, water molecules leave the dense active layer at the membrane surface and enter the porous layer of the membrane. Because the porous layer contains abundant capillary water, the water molecules can flow freely out of the membrane.
Single-stage RO is suitable for water with conductivity below 500 uS/cm;
effluent conductivity 1-10 uS/cm
Raw water is collected in a raw-water tank and pressurised by a booster pump. The raw water is delivered by the booster pump to a quartz-sand filter, an activated-carbon filter and a cation softener for preliminary treatment. After pre-treatment, the water passes through a precision filter (also called a security filter) and enters the RO main unit for reverse osmosis treatment. The RO main unit is the principal purified-water treatment system; the treated water is conveyed through a water-air mixer, and after purification is filled by dedicated filling equipment into large-barrel or small-bottle purified water.
A unified 'dry-closed, wet-open' RO mechanism model - there are several classic models
1. Preferential-adsorption capillary-pore model: weakness - under the electron microscope, no pores are found in the dry-state membrane; the wet-state membrane specimen is not an electron-microscope sample.
2. Solution-diffusion model: does not assume pores exist.
3. Dry-closed, wet-open model: proposed by Deng Yu et al. in the 1980s-1990s, it is the most appropriate modern unified model that can explain models 1 and 2 - the 'dry-closed, wet-open' RO model, which unifies the two most classic RO mechanism models, the pore model and the solution-diffusion model. That is:
when the membrane is dry, its pores shrink and densify, the pores close, and the dry film prepared for microscopy cannot be seen under the electron microscope;
when the membrane is wet, the membrane material swells, the pores are swollen open by the solvent, and the pores open. Combined, this is the 'dry-closed, wet-open' desalination model.
Non-pressurised osmotic adsorption method (1990s)
The non-pressurised adsorption-osmosis seawater desalination method, also called the 'forward osmosis method', lets water permeate forward through a porous membrane into a super-absorbent adsorbent or a solution or solid whose salt concentration even exceeds that of seawater, requiring no external pressure; however, the special salt 'extract' in the solution is easily evaporated and requires little heating (how does the heating energy compare with the pressurisation energy of RO?). It is divided into solid-salt and liquid-salt directions. Solid-salt desorption consumes less energy.
Seawater desalination technology: non-pressurised adsorption-osmosis seawater desalination method: an invention by Deng Yu in the 1990s, included in Chemical Abstracts.
The other two methods have also achieved innovation and improvement in membrane structure
Reverse osmosis technology is typically used for the desalination of seawater and brackish water; water softening; wastewater treatment; and purification, concentration and separation in the food, pharmaceutical and chemical industries. In addition, RO applied to pre-desalination achieves good results, reducing the load on ion-exchange resins by over 90% and cutting resin regenerant consumption by 90% as well. Thus it saves cost and also benefits environmental protection. RO can also remove particles, organic matter and colloids from water, and plays a good role in reducing contamination of ion-exchange resins and extending their service life.
The pores of the membrane are formed by proteins that guide water molecules through the cell membranes of living cells.
Preparation of space water, purified water, distilled water, etc.; water for liquor manufacture and proof reduction; pre-treatment water for the pharmaceutical and electronics industries; concentration, separation, purification and prepared-water production in chemical processes; boiler make-up water desalination and softening; seawater and brackish-water desalination; water and wastewater treatment for the paper, electroplating, dyeing, printing and food industries; advanced treatment of municipal sewage.
Membrane separation technology, represented by polymeric separation membranes, as a novel, high-efficiency fluid-separation unit operation, has achieved remarkable rapid development over the past 30 years and is now widely applied across all sectors of the national economy.
Among various membrane separation technologies, reverse osmosis is the one most successfully applied, fastest developing and widest spread in China in recent years. It is estimated that since 1995, RO membrane usage has grown at an average annual rate of 20%; conservative statistics show that in 1999 the industrial RO membrane element market supplied 6,000 eight-inch membranes and 26,000 four-inch membranes. The market in 2000 and 2001 was even stronger, with membrane consumption rising substantially year on year. It is estimated that RO technology applications have created annual output value of over 1 billion RMB in the water-treatment industry.
The largest field of domestic RO membrane industrial application remains large-scale boiler make-up water and various kinds of industrial pure water, followed by the market scale of drinking water; applications in electronics, semiconductors, pharmaceuticals, medical care, food, beverages, liquor, chemicals and environmental protection have also formed a certain scale.
Ultra-low-pressure membranes: thanks to advantages such as power savings and lower material costs from reduced pressure ratings of related mechanical components, the share of ultra-low-pressure membranes has been growing since 1999, most prominently in small systems that mainly use 4-inch membranes; the use of ultra-low-pressure membranes in large systems is also on the rise, and the largest unit currently using them has an output of 650 t/h.
Low-fouling membranes: membrane fouling is the greatest hazard in RO applications. Several low-fouling membranes with strong anti-fouling performance, long service life, low and easy cleaning frequency have now been introduced.
Negatively charged: membrane manufacturers have now developed low-pressure composite membranes with a positively charged surface, mainly used in high-purity-water systems requiring high resistivity. The positively charged membrane ES10C produced by Japan's Nitto Denko has achieved 10-15 M Ohm resistivity high-purity water in three-stage RO systems in the semiconductor industry; the three-stage RO systems of three Hyundai Electronics plants in Korea, with a combined final output of 800 t/h, produce water at 8-9 M Ohm resistivity; a 170 t/h three-stage RO system at a Shanghai semiconductor plant also meets these figures. In addition, two-stage RO systems at several domestic pharmaceutical plants, at 5-20 t/h scale, have achieved RO permeate resistivity of 1.7-3 M Ohm.
High-temperature-resistant, food-grade and sanitary-grade RO membranes: ordinary water-treatment RO membranes operate at 0-45 deg C, but for special occasions requiring sterilisation at 90 deg C, high-temperature- and chemical-resistant RO membranes can be used. In addition, various food-grade or sanitary-grade RO membranes with special element structures have also begun to be applied domestically.
Abroad, RO seawater desalination plants with a daily output of 100,000 tonnes of water already exist, and large spiral-wound membrane seawater desalination units currently in operation have a single-unit capacity of 6,000 tonnes/day. Domestic RO seawater desalination plants built or under construction produce 350-1,000 tonnes/day; the maximum single-pass RO water-utilisation rate abroad reaches 45%, while domestically it is mostly 35%; in addition, RO desalination membranes installed on domestic fishing boats mostly use small 2.5-inch membrane elements. At present fewer than 10 companies domestically mass-produce seawater desalination plants; the 18,000-tonne/day 'sub-seawater' desalination plant under construction in Hebei is the largest domestic RO plant using seawater desalination membranes. In the future the application of seawater desalination membranes in China will enter a new era, and before long China will also build 10,000-tonne/day-class desalination plants. Furthermore, domestic commercial production of seawater-desalination RO membrane elements has already begun.
The main difficulty lies in developing RO membranes that are cheap, stable and undamaged under long-term pressure. China began to master independent RO membrane production technology in the early 21st century; with strong state support, the programme was included in the State Planning Commission's key high-tech industrialisation development programme and undertaken and successfully developed by Hangzhou Beidouxing Membrane Products Co., Ltd., a subsidiary of the Hangzhou Water Treatment Development Centre under the State Oceanic Administration. At present 95% of the RO membrane market consists of imported membranes, while domestic membranes hold only about 5% of the market - China's RO technology still has a long way to go.
