The requirements for a working reverse osmosis system (RO system) are few; it needs a pump, membrane, vessels and plumbing. But operating the system in a way that minimizes membrane fouling, maximizes membrane life, and avoids hydraulic catastrophes can be challenging. Following is an assortment of issues that have led to RO system performance problems and ways those issues can be resolved.
The most common cause of a complete failure of an RO application is inadequate pretreatment of the RO feedwater. Compromises in pretreatment methods, monitoring instrumentation, or equipment quality will usually result in operational problems in the downstream RO unit.
Filter backwash vs. rinse flows: For example, a common compromise is to use the same flow-control orifice on a multimedia filter discharge line to control both the filter backwash flow rate and the flow rate of the rinse performed after backwashing. This results in roughly the same flow rate being used for both steps.
But where a backwash flow rate based on 12 gallons per minute per square foot (gpm/ft2) of cross-sectional area is appropriate for obtaining 40 percent expansion of the media granules (at 54 degrees F), this same flow rate in the rinse cycle will compact the media granules under a pressure drop exceeding 10 psid. This will tend to push any suspended particles still in the upper section of the media filters deeply into the media bed.
Unless the rinse flow rate has been reduced to the normal service flow rate, much of the solids shed by the filters will end up in the RO cartridge prefilters and in the RO membrane elements.
Balancing filter flows: Another common mistake with media filters is not installing individual flow meters on each of multiple filters in parallel. Without these flow readings, there is no way to know if flow rates are balanced among the filters. If any particular filter starts to plug up with solids, more flow will divert to the other filters.
Keep in mind that pressure gauges on each side of the filters will not indicate if one filter is plugging more than another if they are on common inlet and outlet lines.
Watching the SDI: If the media filter is not capable of providing water that does not exceed a silt density index (SDI) of 5 — a requirement on some membrane element specification sheets — a fatal mistake is to inject a polymeric filtration aid directly before the media filters.
This mistake is particularly misleading, because it appears to dramatically improve the effluent quality of the filters. What does not show up in the effluent turbidity or SDI analysis is the residual polymer breaking through the filter. Because of the polymer’s molecular charge characteristics, it will permanently bond with the RO membrane.
Any suspended solids will now attach to the polymer rather than migrating along the membrane surface. The rate of RO fouling will increase, and cleanings will no longer restore original performance. The membrane elements will need to be replaced.
Improving prefilters: If media filters are not providing water of sufficient quality, there are ways to improve their performance. A misconception about pressurized filters is that they provide the best filtration at a flow rate of 5 gpm/ft2. Actually, filter performance will keep improving as the flow velocity is reduced until it reaches the limits of the ability of the distribution laterals to prevent channeling.
It may be necessary to coagulate fine colloids upstream using a coagulant. If so, an inorganic coagulant, such as an alum product or ferric chloride, should be used. If these materials break through the media filter, they will also foul the downstream RO, but they can be cleaned off. To avoid membrane fouling, they should be used in a reaction tank of sufficient size to allow enough reaction time for the suspended solids to bind with the coagulant before getting to the media filters.
The polyamide thin-film membrane commonly used in most RO systems cannot handle chlorine.
Some membrane manufacturers have said that their membrane could tolerate free chlorine equivalent to an exposure of 1 ppm (part per million) over a period of 1,000 hours before a doubling of salt breakthrough would occur. This guideline has often been misinterpreted to mean that it is acceptable to allow chlorine to occasionally contact the RO membrane as a means of reducing biological fouling.
But membrane damage will soon occur if it is exposed to any amount of chlorine, and it will be cumulative. The damage will be worse if iron or other transition metals have fouled out on the membrane.
Using sodium bisulfite: Sodium bisulfite is often used to reduce the chlorine concentration going into the RO. But sodium bisulfite will also react with dissolved oxygen in the water.
Any excess bisulfite will tend to reduce the oxygen concentration, which increases the potential for increased growth of anaerobic biological organisms — the species responsible for heavy slime formations that can rapidly foul the systems. A symptom of this is the sulfur dioxide, rotten-egg smell noted when membrane vessels are opened.
The optimum concentration of sodium bisulfite may be difficult to maintain. Sodium bisulfite present in the injection day tank or in chemical totes will degrade over time as it reacts with oxygen from the atmosphere. If sodium hypochlorite (bleach) is injected upstream, its concentration will also change depending on its age. Thus, getting the correct bisulfite concentration injected relative to the chlorine concentration can be challenging.
Oxidation-reduction potential (ORP) is a relatively inexpensive method of monitoring bisulfite dosage, but this method may not directly reflect the residual chlorine concentration. Other variables can also impact its reading, especially pH.
Bisulfite and permeate return: When ORP is used to control bisulfite dosage on a continuously operating system, the results may be disastrous if the RO permeate returns back to an upstream feed tank when process water is not being demanded. During times of minimal usage, the increased concentration of RO permeate in the blended feed means that little alkalinity will be present. Added bisulfite will have an increased impact on the water pH and cause it to drop.
The declining pH will cause the ORP reading to increase even if no chlorine is present. The control system will respond by adding even more bisulfite. The bisulfite injection pump will eventually max out on its dosage. All the excess bisulfite will deplete the oxygen in the water, and an anaerobic bacterial outbreak will result.
The injection of a chemical scale inhibitor is typically the least expensive way to prevent scale formation in an RO system. These chemicals work by binding with the growing scale crystals, which reduces their particle growth rate. The smaller scale particles are more likely to remain suspended and exit from the RO system in the concentrate stream.
Small-particle scale rinse: A means of rinsing super-saturated salts from the RO system prior to shutdown is essential to the success of this mechanism. The best method is to tee in pressurized permeate water with an automatic valve downstream of the inlet isolation valve to displace the water in the RO at shutdown.
This also reduces the potential for anaerobic bacterial growth during shutdowns by reducing the concentration of anions in the RO, which are required by the anaerobic bacteria. It also improves the quality of permeate during startup, so a permeate diversion system may not be needed at startups.
Blend inhibitors: Homogeneous polyacrylic acid polymers are notorious for coming out of solution due to over-injection or due to a reaction with iron or aluminum. Sometimes they will even come out of solution with hardness if the injected chemical does not mix quickly at the point of injection. Blend inhibitors of two or more chemical components tend to perform better and are less likely to cause these problems.
A variable frequency drive (VFD) used to control the output of the high-pressure pump with an RO unit will enable the unit to operate with an optimum permeate flow rate without wasting energy by having to throttle the pump outlet. But if the pump is over-sized, a substantial reduction in the motor frequency may be needed to sufficiently bring down the output pressure. This shifts the pump curve well to the left, which means that too much flow will be going through the pump and it may be damaged. The potential for this problem is magnified when two pumps in series are used but only one is controlled by a VFD.
Alarms: RO systems that will not be well-attended by trained operators should have enough alarms and controls to prevent catastrophic failures. The coordination of alarm conditions with system shutdowns is often performed with a human-machine interface (HMI).
If an HMI is employed, it is critical either that program modifications can be made on-site, or that the HMI can be bypassed if something goes wrong. An unforeseen problem should not prevent an RO system from being operated until the HMI programmer can get to the site.
Positioning flow devices: Accurate flow rate and pressure readings are critical to monitoring the performance of the RO membrane elements. Flow transducers must be installed with a sufficient length of straight pipe upstream and downstream of the transducer to meet the manufacturer’s recommendations. Otherwise, the meter may not perform under reduced flow conditions.
Meter redundancy: Flow meters should be calibrated based on an accurate measurement of the flow rate. This may be difficult to accomplish with larger systems, but some method must be devised. This may be as simple as timing the rate at which a downstream storage tank fills. Use of redundant flow meters will help note when a transducer is not reading accurately.
Valves affect pressure: Pressure transducers should not be located directly downstream of throttled valves. The high localized water velocity created by the valve will cause an aspiration effect (partial vacuum) that will result in the transducer reading less than the downstream pressure. Valved tees at the transducer location will make it possible to check all readings using the same calibrated gauge.
RO system installation
The RO concentrate stream often is plumbed to a discharge drain located beneath the highest point of the membrane pressure vessels. Unless an automatic isolation valve or a vacuum-breaking valve is installed on the concentrate line, a siphon effect will pull on the RO while it is shut down. Water will continue to flow through the line after shutdown and will pull a vacuum on the RO system.
This vacuum will cause water to partially drain from the RO pressure vessels. Victaulic-style couplings enable this draining because their standard gaskets allow air to be pulled into the system to displace the vacating water. Specialty gaskets can be purchased that maintain a better seal under vacuum conditions.
When an RO system drains, the incoming air will carry bacteria and fungi spores into the membrane elements, which may contribute to membrane fouling. When the RO restarts, water hammer may occur, which can break the fiberglass wrap and plastic anti-telescoping devices (ATDs) on the ends of the elements.
A check valve that uses a lightly weighted spring (1-2 psi) may be teed into the top of the concentrate discharge line to allow air to be sucked into the line under vacuum conditions. It should be plumbed in such a way that, whenever the RO starts up, it does not spit water at someone nearby.
To sum up: If any of the issues discussed in this article are a concern for a particular RO system, they should be resolved. While this may not eliminate all potential for problems, it can only help.
References: Byrne, W., A Practical Guide for Industrial Users, 2nd Edition. Tall Oaks Publishing, Inc., 2005. Illustrations courtesy of CEA, Encyclopedia of Water Treatment, 1998.
With 30 years of experience in the application of reverse osmosis technology, Wes Byrne provides training and consulting services through his company, CEA, and as an adjunct professor at St. Cloud Technical College, St. Cloud, MN. He wrote this article as a consultant for US Water Services Inc., an independently owned water treatment service company based in Plymouth, MN. More information about the company is available at: uswaterservices.com.