Abstract:This review aims to understand Reverse Osmosisas a method of water purification and holistically analyze and contrast it withthe other popularly used methods. It will also suggest the attempts made tomake the process more effective and throw light on some ways of reducing theinvestment involved in the setup and better the environmental impacts causedduring running of RO units. List of Abbreviations: RO-Reverse osmosisPpm-parts per millionTFC- thinfilm compositesTDS-Total Dissolved SolidsCTA-Cellulose TriacetateCA-Cellulose AcetateUF-UltrafiltrationUV-Ultraviolet Contents1) Introduction 2) Process of RO3) Membrane Types4) Importance and Efficiency of process· Domesticscale· Industrialscale5) Cons of the process6) Improving RO systems7) Conclusions8) References IntroductionReverse Osmosis (RO) is a widely followed, yetexpensive, water purification technology. It is a technology that evolved frombasic science of liquid flow across membranes.
Osmosis is a process involvingthe flow of liquid from lower concentration of salt to higher concentrationwhen separated by a semipermeable membrane. This is the process which plantsuse to absorb soil nutrients through the roots in the cellular level. RO is theopposite process.
It involves pressurizing a solvent from a region of highsolute concentration through a semipermeable membrane to a region of low soluteconcentration by applying a pressure in excess to osmotic pressure (thatpressure which is required to just stop osmosis when applied in the region ofhigher solute concentration). RO is very important in the process ofdesalination of sea water. Pure water is thus separated from sea (brackish)water. Brackish water is pressurized against one side of the semipermeablemembrane resulting in movement of salt less water across the membrane and producingdrinking water from the other side.
ProcessDuring the process molecules are pressurizedthrough a semi-permeable membrane to a higher solute concentration. Themembrane here behaves as a type of filter being extensively porous with tinypores that remove microscopic debris from the water hence making it pure andpotable. The semi-permeable membrane (0.0001 microns size) is called so becauseonly lets water molecules through while other contaminants are collected andflushed away. Membranes used for RO have dense barrier layer in the polymerbase. Usually this process requires a pressure of 30 to 250 psi to be appliedon the other side of the membrane for normal tap water and 600-1000 psi forbrackish sea water.
Membrane TypesCommon membrane materials include polyamidethin film composites (TFC), cellulose triacetate (CTA) and cellulose acetate(CA) with the membrane material being spiral wound around a tube, or hollowfibres held together as bundles. Hollow fibre membranes have a greater surfacearea and hence capacity but are more easily blocked than spiral wound membranesRO membranes are rated for their ability to reject compounds from contaminatedwater. A rejection rate can be computed calculated for every specific ion typeor contaminant element as well as for the general reduction of total dissolvedsolids (TDS).Comparatively TFC membranes are stronger anddurable as well as possess higher rejection rates than CTA or CA membranes.They are also more resistant against high pH values, microbe attack and highTDS. CA/CTA’s have a better ability to tolerate chlorine. There is another membrane type: Sulphonated polysulphonemembranes (SPS) which is chlorine tolerant and can tolerate higher pH’s. Theseare best used where the water is soft and is of high pH or where high nitrates concentrationis present.
Effectiveness and ImportanceRO can effectively remove suspended anddissolved impurities: pathogens, dust and mineral matter apart from that isused in various industrial processes. TDS is a measure of all soluble chemicals:minerals (cations and anions) and microorganisms and biomolecules in water. TDSis a very important parameter in the design of RO systems. It is often measuredand reported in units of ppm.
The standard values are around 300 ppm for tapwater and 2,000 ppm for ground water. Contribution to TDS is mainly by sodiumchloride salt. RO can ensure a purity of 0 to 50 ppm of TDS values which isconsidered safe to drink for a normal person. TDS values lesser than these arenot recommended to drink as they are short of dissolved minerals that isnecessary for humans to intake through water. 1. Domesticuse: RO units are responsible for reducing barium,chromium, radium, and other ions whose presence in drinking water can causecancer, kidney damage, and birth defects.
Figure 1:Household RO treatment unit (Source: Google images) 2. Industrialuse: In industry, boiler water is demineralizedusing RO before using it in the power plant. Further with few cycles ofdistillation the water becomes as pure as possible and doesn’t leave mineraldeposits in machinery or corrode the turbines. Any such corrosion leads to poorsteam production as boiler efficiency is brought down and hence poorer powerproduction in the turbine. Thus anatural conclusion would be to understand that their profitability is largelybased on the cost effectiveness and efficiency of water purification system.
In food industry, RO is extensively used forconcentrating fruit juices. RO is found to be more economical than theconventional heating method to concentrate fruit juice. Moreover, heatsensitive drinks such as enzyme or protein based drinks can be concentratedthrough RO. A common example is milk and concentrated whey protein given tochildren and body builders.RO also finds its application in hydrogenproduction plant as treated water prevents scale formation on the electrodesurface. Cons of using ROThe household RO system has one maindisadvantage.
Due to low back pressure (pressure from tap water) it generallywastes two to three gallons of water for every gallon of pure water produced.This water is used in cleaning the membrane first as the membrane is givenlateral flow of water. The wasted water is highly salty and diverting largevolumes of this into the sewage would cause acting against the smooth movementsewer contents. The rejected water will not be tolerated by normal gardenplants due to very high salt concentration. It will also lead to corrosion asit would form deposits on floor and sanitary ware.Secondly, natural trace minerals present inwater can be removed by reverse osmosis.
These minerals not only provide a goodtaste to water, but they also serve a vital function in the body’s system andcan hence be unhealthy for the body if removed from water. Persons drinkingsuch water would have to take minerals necessary through supplementary tablets.High power consumption is a major disadvantageof RO especially in larger scale apart from leading to environmental imbalancespecifically in the application in desalination of brackish water.
Mostindustrial RO systems use large amounts of energy thus affecting carbonfootprint. Average annual RO based desalination consumes power equivalent topower consumed by a refrigerator running in the house. Annual power required tosupply RO based seawater desalinated water in each household could be comparedto 10% increase in power consumption of that house.
Desalination of water can be harmful to the aquaticecosystem as its effects include sucking in fish eggs with intake water in thesystem; using harsh chemicals to clean membranes which in-turn can get retainedin water and enter bodies of consuming organisms; and release of large volumesof highly salty liquid brine back into the water body resulting change in environmentfor water organisms. Comparison between RO, UV and UF systemsThe table below compares RO, UV and UF modesof water purification which is with special reference to household/municipalwater usage. Improving the RO systemsRO systems are mainly inefficient due to dischargeof 1 part of water in every 4 parts purified (in most cases). In recent years,one can find the development of novel RO filter systems called as”zero-discharge”. This technology involves discharging the backwash water intothe water supply pipes. It further refers to the fact that the backwash wateris not discharged into the sewer system.
This may require additional powerintake but the net effect is making it water resource friendly as the issues ofwasting water and simultaneously releasing high contaminant concentration waterinto domestic sewers or in dump-lands is taken care of. A recent research paper published that acombination of RO and evaporation can suggest a very energy efficient andsimpler way to maximize recovery of water that is given as input to the RO system. The paper proposed a very innovativemethod of using high pressure nano-filtration and crystallizer/filtration. Thepaper further suggested a water recovery efficiency greater than 95% fromdumpsite leachate. ConclusionsRO is a very powerful way of waterpurification. Although it is inspired from nature it is energy intensive. ROmust be resorted to only in cases where the salt content of water to be usedfor drinking is extremely high. Even here reduction of the salt content to thelevel of 10 or 20 ppm may not be very favorable.
If the salt content of thewater is considerably high even for non-potable purposes, rainwater harvestingcan be resorted to.In cases where the water contains coliformbacteria, the source must be traced to and the contamination must beeliminated. Although RO may be advised, elimination of the cause is the saferand preferred method and ultimately the more cost effective option too.Resorting to RO for water with high salt contentmust be preceded by assessment of the volume likely to be subjected per day andone must ask the supplier how long will the filters work effectively with thatvolume, moreover the cost of replacement of RO membranes must also be consideredapart from the monthly running costs in addition to the capital cost.Nevertheless for heavy purposes such asdesalination and massive water purification for food products industry, RO isthe best and most suitable method compared to most other methods such as UV andUF especially in cases where virus free water and water free from any kind ofdissolved salt ions is desired.
Resources1. www.wikipedia.com2. https://www.diffen.