Reverse Osmosis (RO) process consists of generating an aqueous solution with a low salt content from another aqueous solution that has a high salt content, via a water-permeable membrane.
It is the technology used to produce desalinated water from seawater. Just as in MF and UF, the driving force for achieving the separation of the salt is caused by a difference in transmembrane pressure.
Nevertheless, in RO, the separation process is due to the different solubility and diffusivity in the membrane of the aqueous solution components. The operating values for the transmembrane pressure difference and solution concentration are 7 – 70 bar and 200 – 30000 ppm respectively.
Osmosis is the process that occurs when a membrane with selective permeability for water separates two aqueous saline solutions with different concentrations that are at the same pressure and temperature. The water changes naturally from a more dilute solution to a more concentrated one via the membrane. This process stops when the increase in hydrostatic pressure on the side of the membrane with the more concentrated solution, poses sufficient resistance to prevent the water from the dilute solution from passing through. The pressure difference between the two solutions when it reaches this state of equilibrium is called transmembrane osmotic pressure difference (Dp). If the aim is to reverse the flow of water generated by the osmosis, it is necessary to apply a pressure to the concentrated solution side that causes a transmembrane pressure difference (Dp) that is higher than the osmotic pressure.
This makes the water flow from the concentrated solution to the dilute solution (RO), resulting in desalinated water from saline aqueous solutions that is of sufficient quality to be used for human consumption and other applications.
The benefits of the membranes used in RO can be summarized in the following points:
- The multiple ions retain the single ions better.
- The dissolved gases, such as ammonia, carbon dioxide, sulfur dioxide, oxygen, chlorine and hydrogen sulphide, have a good permeability.
- The rejection of acids and weak bases is greater than pH values when they are in their ionized form.
- The rejection of neutral organic molecules increases with the molecular weight, compounds with molecular weights greater than 100 D have high values of rejection coefficient. The nature of the membrane material has an important influence on the value of this parameter.
Negative values of rejection coefficient have been observed in solutes such as phenol and benzene in cellulose acetate membranes.
Specific contamination problems
Due to the high rejection values in RO processes, contamination is the most important cause of membrane malfunction. The most frequent causes of contamination are due to 1) deposits on the surface of the membrane of crusts or scales of calcium carbonate, calcium sulphate, complex silicates, barium sulphate, strontium sulphate, calcium fluoride, etc., depending on the composition of the feed and as a result of the fact that the concentrations of salt in the concentrate can exceed the solubility product of the salt; 2) particle sediments such as colloids, products of the corrosion of the iron pipes, precipitates of iron hydroxide, algae, etc.; 3) bio-contamination due to the growth of microorganisms on the surface of the membrane, as certain membrane materials such as cellulose acetate or polyamides can be a useful substrate for microorganisms and 4) contamination due to organic compounds such as oil or grease found in industrial wastewater. The type of cleaning for the membranes will depend on the characteristics of the feed water, the type of membrane and the nature of the contamination. As a general guideline, proceed with alternating periods of rinsing of the membranes, making sure that the cleaning solutions circulate at a high speed over the surface of the membranes with periods in which the membranes are submerged in the cleaning solutions.
The standard cleaning agents used are 1) hydrochloric, phosphoric or citric acids and chelating agents such as EDTA to eliminate saline precipitate crusts, and oxalic acid to eliminate iron sediments 2) alkalis combined with surfactants to eliminate microorganisms, sediments and organic compounds and 3) sterilization of the membranes with chlorine solutions to eliminate microorganisms.
Successive cleaning causes the membranes to deteriorate. Depending on the application, the lifespan guaranteed by the manufacturer is normally 1 – 2 years. With a good cleaning program, the lifespan of the membranes can be extended to 3 years, though a lifespan of 5 years is unlikely.
The objectives of installed RO plants are distributed in the following way: 50% in desalination of seawater and brackish water; 40% in the production of ultrapure water for the electronic, pharmaceutical and energy production industries; 10% as decontamination systems for urban and industrial water.
- Desalination of brackish water
The salinity of this type of water is 2000 mg/L – 10000 mg/L. In its treatment, pressures of 14 bar – 21 bar are used to achieve rejection coefficients greater than 90% and to obtain water with saline concentrations of lower than 500 mg/L, which are the values recommended by the WHO as a requirement of potability. Treatment plants use membrane modules rolled in spirals. It is estimated that the costs for this type of plant are in the region of 0.25 $US/L of treated water/day, with the operating costs being equivalent.
- Desalination of seawater
Depending on the geographical area, the salinity of this type of water is 30000 mg/L – 40000 mg/L. To meet the potability requirements, polyamide hollow fiber membranes are used which allow rejection coefficients to be achieved of greater than 99.3% with working pressures of 50 bar – 70 bar. The operating costs of this type of treatment plant are estimated to be 1 – 1.25$US/ L of treated water/day, which means that this treatment system is not competitive in comparison with other systems, such as multistage evaporation processes, if water needs exceed 40000 m3 of treated water/day.
- Production of ultrapure water
RO allows water of the quality demanded by the electronic industry to be obtained from drinking water (concentration of dissolved solids < 200 mg/L). The main problem with this type of installation is the bio-contamination of the membranes, which is why the installation of sterilization systems that use UV radiation is necessary. Table 11 compares the characteristics demanded for drinking water with those for ultrapure water.
- Wastewater treatment
This RO application is limited by the high operating costs due to the problems of contamination of the membranes.In the case of industrial wastewater, RO is used in those industries where it is possible to improve the efficiency of the process by recuperating valuable components that can be recycled in the production process: galvanoplasty industries and paint for metal structures, or where reuse of the treated water represents an important reduction in the consumption of water: textile industry.
In the case of urban water, RO is a treatment that would be indicated as a tertiary treatment, if it were possible for water to be obtained with a quality that made it appropriate for consumption, at a cost of 0.5 – 0.75 $US/m3. The main problem for the consolidation of this type of treatment is the social reaction. Nevertheless, in areas of Japan and California, where there are extreme water limitations, RO plants are being used to treat water that comes from the biological treatment of domestic water, with the water treated by RO being used to refill groundwater resources.