Frequently asked questions
How does industrial reverse osmosis work?
Industrial reverse osmosis use spiral wound membranes mounted in high pressure containers. The membrane stack is two; very long semi permeable 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.
Is any pretreatment required before R.O?
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 Meta Bi Sulfite. The carbon is preferred because the chemical addition can enhance bacterial growth in the system which can plug the membranes.
What about membrane plugging?
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.
How do I clean a system?
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.
How can I prevent plugging ?
Initially the incoming water if 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.
How much maintenance is involved with a system
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.
How much floor space does a reverse osmosis system occupy?
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.
How much water is rejected?
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 over sizing the pumps and system size to get the required flow rate.
What types of membranes are there?
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
Do I have to shut down for cleaning?
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.
Do I have to pH adjust before the reverse osmosis system?
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.
How automatic is automatic?
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.
How pure will the water be?
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.
What does a carbon filter do?
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 materials. 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.