Q. What is Reverse Osmosis (RO)?
A. Reverse Osmosis is a process where water is demineralized using a semipermeable membrane at high pressure. Reverse osmosis is osmosis in reverse. So, what is osmosis? Osmosis is most commonly observed in plants. If you don’t water your plants they wilt. A plant cell is a semipermeable (water flows through the membrane but salts don’t) membrane with the living stuff on the inside in a salt solution. Water is drawn into the cell from the outside because pure water will move across a semipermeable membrane to dilute the higher concentration of salt on the inside. This is how water is drawn in from the ground when you water your plants. If you salt your plants (over fertilize or spill some salt on the grass), the plant will wilt because the salt concentration on the outside of the cell is higher than the inside and water then moves across the membrane from the inside to the outside.
To reverse this process, you must overcome the osmotic pressure equilibrium across the membrane because the flow is naturally from dilute to concentrate. We want more pure water so we must increase the salt content in the cell (concentrate side of the membrane). To do this we increase the pressure on the salty side of the membrane and force the water across. The amount of pressure is determined by the salt concentration. As we force water out, the salt concentration increases requiring even greater pressure to get more pure water.
Q. How does industrial reverse osmosis work?
A. Industrial reverse osmosis use spiral wound membranes mounted in high pressure containers. The membrane stack is two, very long semipermeable membranes with a spacer mesh between them that is sealed along the two long sides. This is then wound up in a spiral tube with another spacer to separate the outside of the stack. The spiral winding provides a very high surface area for transfer. Between each membrane layer is a mesh separator that allows the permeate (pure) water to flow. Water is force in one end of the spiral cylinder and out the out other end. Backpressure forces the water through the membrane where it is collected in the space between the membranes. Permeate then flows around the spiral where it is collected in the center of the tube.
Q. Is any pretreatment required?
A. There are various pretreatment configurations that will work on the front of an reverse osmosis water system. Part of the selection is based on the capabilities and experience of you maintenance staff. The better preventative maintenance you have, the easier it will be to maintain a chemical addition system. Chemical metering systems require more daily maintenance and calibration to insure consistent operation. Fixed bed systems such as softeners and carbon beds require little daily maintenance.
Water must have a very low silt (solids) content to keep the membranes from plugging up. This can be accomplished by removing the solids or keeping them in suspension while passing through the system. Chemicals can be added to the incoming water to keep the solids in suspension or efficient filtration can be used. We prefer to remove all solids before the system, which results in the lowest rate of membrane plugging.
As the water passes through the reverse osmosis system, the ionic content of the reject stream increases as water permeates the membranes. This increase in TDS can results in calcium and magnesium (the hardness ions) precipitating out in the system and plugging the membranes. Again, either the Calcium and Magnesium can be removed or a chemical can be added to keep them in solution. We prefer using a water softener to remove the hardness ions and replace them with sodium.
Chlorine must be removed for thin film membranes and should be minimal for CTA membranes. Either it can be removed by carbon treatment or reduced with a chemical addition of sodium metabisulfite. The carbon is preferred because the chemical addition can enhance bacterial growth in the system which can plug the membranes.
Q. What is required to install and use a reverse osmosis system?
A. The reverse osmosis system itself is fairly simple, consisting of a series of tube containing the membranes with a high pressure pump to force the water through the system. Pretreatment is required for all systems which is designed to eliminate slit (suspended solids), water hardness and chlorine and other oxidizers. The schematic shows a simplified front end reverse osmosis system where the city water is filtered, softened to remove hardness, the carbon is used to remove the city chlorination (membranes are sensitive to oxidizers). An alternative would be to dose the system with chemicals to remove the chlorine and hold the hardness ions in solution.
After the carbon filter, the water is passed through the membranes where the concentrate is recycled back to the front of the system for another pass and a bleed is taken off this line to drain. This recycling allows very high system efficiencies.
The permeate line will have a TDS (total dissolved solids) level of about 4% or less of the incoming water (membrane dependent). A sidestream off this line feeds a DI bottle service for DI water. The TDS is only 4% of the incoming water so the DI bottles will last 25 times longer!
Q. How much pressure is required to purify water?
A. The pressure required is dependent on the concentration of the salt solution on the reject (concentrate) side of the membrane. Running as system at 1100 PPM on the concentrate side requires over 200 PSI. Sea water systems at 33,000+ PPM run at 800+ PSI. Under sink systems at home run at 50-70 PSI.
Q. How pure will the water be?
A. Purity is determined by two things, first the “reject ratio of the membrane (92-99.5%) and secondly, the type of salts in solution. Membranes are very good at rejection high molecular weight compounds and multivalent ions. Monovalent ions such as Na+ and Cl- (Sodium and Chloride) are not rejected as well and are the leakage ions. The amount of leakage is determined by the reject ration. A 95% reject ration means that 5% of the salt concentration leaks through so a 200 PPM input stream would result in a 10 PPM output stream. A membrane rated at 99% would result in a 2 ppm output stream. The reject ratio changes over the life of the membrane and leakage increases. Each time you clean a membrane it slightly changes its properties so after many years the ratio may drop to 90% or less.
Q. What about membrane plugging?
A. As you concentrate salts on one side of the membrane, you can reach a point where salts of the hardness ions (or other ions) precipitate out. When they do, this will plug the very small pores of the membrane. Organic compounds can also plug the pores. Once plugged, the flow decreases and the membrane must be cleaned. Hardness can be eliminated by softening or continuously dosing a chemical chelating agent.
Q. How can I prevent plugging?
A. Initially the incoming water is filtered to remove particulates and colloidal substances. After this there are two ways to reduce the chance of plugging. A chemical can be added to the feed stream that keeps the hardness from precipitating out. This is simply metered directly into the pipe feeding the reverse osmosis pump. The second way is to remove the hardness with a water softener. This will reduce the chance of plugging and also acts as another filter in front of the system.
Q. How do I clean a system?
A. Cleaning is fairly simple. A volume of water is recirculated on the high pressure side of the system with a cleaning agent (for hardness or organic plugging) for an hour or so then the membrane is flushed to drain and returned to service.
Q. How much maintenance is involved with a system?
A. If properly setup with effective pretreatment, a system usually has a 1 hour cleaning cycle once per month or even less often when softening is used as a pretreatment. A softener needs a daily check of salt level. Prefilters need a weekly check. Usually filters are alarmed through flow rate so absolute monitoring is not necessary.
Q. How much floor space does a reverse osmosis system occupy?
A. Integrated systems up to 50 gpm can occupy a space of 6 feet by 15 feet by 6 feet high without storage tanks. A 30 minute holding tank after the system is usual and if recycling is used, a 10 minute storage tank to feed the reverse osmosis pump is used.
Q. How much does it cost to run a reverse osmosis system?
A. The cost to operate is a total of three variables. These are, power, chemistry (pretreatment and pH adjust) and labor.
The power requirement is about 10 hp for 30 gpm up to 15 hp for 60 gpm. This is constant while the system is running. A 10 hp, 3-phase motor costs about 15 cents per hour to operate (or less).
The chemistry costs for pretreatment involve either salt for a water softener or a polymer or sequestering agent to keep the hardness ions from precipitating out. A pH adjustment is also usual with citric acid. Total costs for a 30 gpm system runs about 10-15 dollars a day.
Labor costs for the system is usually very low due to the automated nature of the systems. If the proper pretreatment is used, little or no maintenance is required between cleanings except for chemical maintenance for the pH adjust system and softener or polymer systems. Calibration of pH probes is a weekly project. Cleaning is a simple chemical recirculation procedure taking about 1 hour.
Q. How much water is rejected?
A. This will vary with the configuration of the system. Up to 6 membranes can be connected in series and the theoretical capture rate is about 84% (rejecting 16%). We have use oversized systems and redirected the reject to the front of the system for a multiple pass system and have gotten recovery’s of about 92% (half or the reject to drain, half to the system feed tank). This does require oversizing the pumps and system size to get the required flow rate.
Q. How do I dispose of reject water?
A. Reject water is discharged directly to drain. Usually the TDS is less than 1500 PPM and there are no contaminants. If a system is used to recycle some water after a plating application, monitoring of the reject may be necessary.
Q. What types of membranes are there?
A. There are two types of membrane materials in widespread use. These are thin film (TF) and Cellulose Triacetate (CTA) membranes. The thin film membrane is chlorine sensitive and requires carbon pretreatment to remove the chlorine. The CTA membranes don’t. TF membranes have a little higher reject ratio and operate at a wider pH range than the CTA.
Q. Do I have to shut down for cleaning?
A. Small systems will have to shut down but in larger (>10 gpm) systems, the individual banks of membranes can be isolated and cleaned one at a time and only part of the flow will be lost.
Q. Do I have to pH adjust before the reverse osmosis system?
A. Complete systems have a pH adjust module to reduce the pH to between 5.5 and 6.5. This helps to prevent plugging of the membranes and aids in cleaning the system. If the system is to be used in water recycling, pH adjust is mandatory.
Q. What about bugs (bacteria) growing in the water?
A. RO systems are sterilized periodically during cleaning. The water storage from a reverse osmosis system is optionally passed through an UV sterilization system, which kills any bacteria in the system. All tanks should be black or opaque to prevent algae growth.
Q. How automatic is automatic?
A. Our standard systems have PLC controls with alarms and full sensors compliments. Full automatic controls are available including data monitoring, storage and analysis as are network interfaces. A typical system will have a holding tank with level controls feeding the reverse osmosis pump and a reverse osmosis water storage tank with level controls and duplex pumps for shop water pressurization. All this is monitored and controlled by the PLC. Gages and instrumentation include high pressure gages on the reverse osmosis pump output and concentrate output, pressure switches on the reverse osmosis feed and output (monitored by the PLC), and flow monitors (sight gages on smaller systems, electronic on larger ones) on the concentrate, permeate and recycle stream. Even the cleaning cycle can be automated on larger systems with automatic valves to isolate selected banks so down time is minimized.
Q. How long will my reverse osmosis membranes last?
A. Reverse osmosis membranes usually last many years. They rarely fail all at once. Usually they slowly start leaking more ions until some unacceptable level is reached. There are membrane systems that have been in continuous use for 20 years. It is much more cost effective in the long run to buy over capacity initially so you can get several years more useful life out of the membranes.
RECYCLING WITH REVERSE OSMOSIS
Q. Can I use an reverse osmosis to recycle water?
A. Maybe. Reverse osmosis systems are very sensitive to plugging so the recycled stream must be carefully evaluated as to ionic and particulate content. Effluent water from a MRIX system can be recycled (see drawing). Water directly from a scrubbing system probably shouldn’t be recycled without proper pretreatment.
Q. Should I evaporate to concentrate the reject? The Zero Discharge option!
A. Evaporation is an expensive way to concentrate solutions unless there is a lot of waste heat available. Evaporating 3+ gallons per minute is very expensive, both to purchase the equipment and to operate the system. Evaporation is the only practical way to get to zero discharge, but the cost is high.
Q. What does a carbon filter do?
A. Carbon is derived from either wood or coal (or coconut shell!). The starting material is heated in an inert atmosphere to convert it to pure carbon. The way it is processed results in a very high surface area and an ability to adsorb various material. The capacity for both organic and inorganic materials is high but carbon is usually used for adsorbing organics. Each organic and each type of carbon have a different adsorption capacity.
Carbon is used to adsorb organics, chromates, sulfides and chlorine. It is used to prevent strong oxidizing materials, such as peroxides, nitric acid, and chlorine, from attacking membranes and it is used as a general adsorbent in recycling systems.
Q. How would an reverse osmosis be used to recycle water?
A. The following schematic shows a recycling schematic as would be used at a plating shop. Some of the details are left off to increase the clarity of the drawing. There are actually two different systems integrated into one to obtain the best cost/benefit ratio. Inside the dashed line is a complete Metal Recover Ion Exchange (MRIX) system to remove target metals from the waste water. Instead of the MRIX system, a microfiltration unit or a clarifier with very good final filtration could be used. After the MRIX the water has been filter and is carbon treated to remove any residual oxidizers or organics.
The second system is a complete front end reverse osmosis system with reverse osmosis feed water storage and pH adjust and full city water pretreatment. The MRIX effluent water is injected into the reverse osmosis feed tank where pH is adjusted. This water can be diverted to drain by the PLC during regenerations when the TDS is very high. An shortfall in water is made up with city water. A low level at the reverse osmosis feed tank fills it with softened/carbon treated city water.
The percentage recycled by such a system depends on how much of the reverse osmosis systems output is sent to rinses returned to the MRIX system. If you are feeding rinses that bypass the MRIX, the percent of recycled water will be low. If all goes to the MRIX, the ratio can be 80-90 percent.