By Diego Bonta and Denise Haukkala
Reverse osmosis (RO) elements are used in a variety of applications, from residential tap water, power generation and industrial processing to water re-use. Regardless of the applications, most RO element life is limited by the same kinds of performance issues. Large plants and industries that use RO systems have sophisticated tools that monitor and help troubleshoot element performance. Large plants have advanced pretreatment arrays and cleaning regimes to protect and extend RO element life.
Residential RO systems, on the other hand, do not come equipped with all the advanced instrumentation, resources and testing facilities to troubleshoot element performance problems. By learning to identify the primary causes of element performance degradation, however, corrective actions can be taken to extend the life of RO systems. Hopefully these measures can be implemented as part of a new installation, before a unexpected field failure occurs.
There are three common types of foulants that impact the performance of RO elements in residential and small commercial systems:
- Scaling. Mineral deposits such as calcium or iron that precipitate out of the feed water onto the element. Deposits tend to first form on the element near the discharge, where minerals are the most concentrated and unstable.
- Biofouling. A general term for all microbial growth that can propagate and form a barrier film on the membrane
- Particles. A general term for any suspended solids in the feed water like rust, sand or carbon fines that may plug the membrane.
Perhaps there are suspicions that a field system is affected by one or more of the three foulants listed above. There are a few straightforward tools that can be utilized to identify which problem is occurring. The good news is that in many cases, using one of the five human senses can act as a guide to detect the root cause of failure. The first sign that there is an issue with a POU RO system is typically taste; the product water has a more salty or mineral taste than normal. Many POU systems include an electronic conductivity sensor that measures the level of dissolved solids in the water. These electronic sensors will either measure conductivity directly or trigger an alarm, such as a light color change on the RO unit when a preset conductivity setpoint has been exceeded. In addition to a salt level change, the rate of water flow coming out of the POU tap may deteriorate significantly, or the storage tank may go empty too quickly when there is a fouling problem.
At this stage, quality diagnostic procedures should be performed to confirm the nature of the problem before the membrane is condemned. These include but are not limited to:
1) determine the percent rejection of the RO water; 2) verify the correct flow of reject water going to drain using a graduated cylinder; 3) measure the product water flow using the graduated cylinder; 4) measure the (empty) storage tank air charge and 5) verify that various sealing surfaces are robust. After diagnostic procedures are performed, most technicians are convinced that the element’s performance has been compromised and the element can be removed from the system to be further inspected. While this inspection process may destroy the integrity and function of the element, it is typically past the point of recovering and should be changed out. The point of the inspection at this stage is to diagnose what the problem was and prevent the same issue in the future.
When the element is first removed, the first step is to smell the element. If it smells noxious, this is a good indication of microbes contaminating the membrane. Next, try squeezing the element. If it sounds crunchy, the element is probably scaled with deposits of calcium carbonate. After removing the element, peel off the outer tape and unroll the membrane so that the membrane surface and mesh spacer can be examined. The following membrane appearance or textures would indicate a fouled membrane (see Figure 1):
B) Scaling: White, chalky deposit often containing calcium carbonate is more pronounced at the tail end of elements (municipalities are not required to remove hardness and calcium carbonate, which is one of the most abundant minerals found in potable water)
C) Particulates: Discoloration or large particles on the membrane surface could mean there is something wrong with POU pretreatment.
Another useful diagnostic tool is a small amount of acid or base to drop onto the membrane surface. It may not be practical to carry hydrochloric acid or caustic to do this test, so consider using a drain cleaner for base or a lime cleaner for an acid as alternatives, both of which can be found in the plumbing section of most hardware stores. Check the active ingredients to confirm that the correct type of chemical is in the cleaner. Use caution and follow the instructions of the product before using. Place a few drops of acid on the membrane surface and look for bubbling to indicate scaling (CaCO3). If the acid turns the membrane yellow, iron fouling is the likely cause of membrane failure. Figure 2 shows:
B) forming a yellow solution from reacting with iron. A few drops of a base chemical, which clean the membrane surface, indicate biofouling or natural organic matter as the likely cause of failure.
To avoid these problems for a POU RO system, there are several tips to keep in mind.
Know the feedwater
Most homeowners are unaware of their feed- water quality. One option is to obtain a water analysis to identify common parameters such as:
- Actual pH
- Temperature range (summer and winter)
- Specific regional ions of concern
Other considerations include the source of the water supply. If it is municipal water, what are the primary treatment methods used on the water supply? If it is well water, microbes or iron fouling can be common issues. This information may be useful when diagnosing system performance problems for both commercial and domestic drinking water systems.
The Langelier Saturation Index (LSI) predicts calcium carbonate stability in the water and is derived from water temperature, pH, alkalinity, hardness and total dissolved solids. ASTM-D 3739 is a good method to determine the LSI of a water source. Other tools can be found online to help calculate the LSI for a given water source.1
Example of an LSI formula:
LSI = pH -pHs
pHs = (9.3 + A + B) – (C + D)
where: A = (Log10 [TDS] – 1) / 10
B = -13.12 x Log 10 (°C + 273) + 34.55
C = Log 10 [Ca+2 as CaCO3] – 0.4
D = Log 10 [alkalinity as CaCO3 ]
If the LSI calculated is:
- Negative: calcium carbonate is stable in water and will not precipitate out onto the membrane.
- Positive: scaling may form.
- Close to zero (borderline scale potential): water quality and temperature changes could change the index and swing it one way or the other.
The stronger the positive or negative number, the stronger the result.
Pretreatments for scaling
The following are recommended corrective actions for positive LSI values:
- Use softened water, which reduces mineral deposits onsinks, showers and toilets.
- Use an RO permeate rinse to flush the membrane when it is idle (this is not available on all home drinking water systems).
- Use an antiscalant prefilter, such as poly- phosphate, combined with a microfilter to prevent mineral crystals from readily forming and growing. Not all antiscalant prefilters are equal, and some may cause their own problems, such as gelling or seeding crystals of their own.
Pretreatment choices for iron
Ever notice that water with iron dissolved in it, if left alone, eventually forms a layer of red rust on the bottom of the container? The same will happen with membranes if water with iron and air reaches the membrane. The iron will oxidize and precipitate onto the membrane surface. A great pretreatment for iron and its various species is an oxidizing filter (a type of filter used to change the valence state of dissolved molecules), making them insoluble and, therefore, filterable. For example, use a filter that oxidizes ferrous iron, manganous manganese and/ or anionic sulfur by use of a catalytic media such as manganic oxides, and then filters the oxidized precipitates out of the water. An alternative pretreatment to consider is water softening for removing lower concentrations of ferrous iron. Water softening functioning and robustness can be vulnerable to water pH and ferric ion levels. Care should be taken when using greensand filters, as they do not always remove all of the iron, and overdosing can attack the RO membrane, particularly with potassium permanganate.
Pretreatment of chlorine and microbes
Cars have recommended maintenance schedules for things like oil changes and brake inspections. The harsher the driving conditions, the more frequent the recommended maintenance for car care, in order to prevent a catastrophic breakdown. Similarly for RO units, pretreatments such as microfilters, carbon filters and other options should be changed on a regular basis. Following the manufacturers recommended service schedule is the best prevention for chlorine attack, biofouling and particulate fouling that will shorten the service life of prefilters. The more challenging the feedwater, the more frequent the prefilter changes will be.
Control recovery and rinsing
Recovery is the fraction of the treated water produced by the membrane element, divided by the amount of feedwater entering the system. Home drinking-water systems have a reject flow restrictor that controls the waste volume to drain, based on the total surface area of the membrane. The higher the recovery, the more easily the membrane will foul due to scaling or microbes. By lowering the recovery, the crossflow increases along an element to help flush and refresh the membrane surface. A typical domestic RO flow restrictor will flow three to five times the product flowrate for a product that is fundamentally correct in design.
By following these tips, many catastrophic element failures may be avoided and RO element life can be extended. Should a field failure occur, there are tools readily available to properly diagnose the problem(s). Based based on the diagnosis, corrective steps can be effectively applied to ensure long-term performance and reliability of the RO system.
About the authors
Diego Bonta is an Applications Development Specialist for Dow Water & Process Solutions and has held a variety of roles in the organization’s research and development. He is responsible for ensuring residential and commercial products perform to meet customer needs while maximizing their value. In addition, Bonta is responsible for developing new applications. The author earned a Bachelor’s Degree in mechanical engineering from the University of Maryland and a Master’s Degree in mechanical engineering from the University of Minnesota. Bonta is currently pursuing a Doctorate in applied economics.
Denise Haukkala is in Technical Service and Development for Dow Water & Process Solutions. She has 11 years of experience with the company and has held a number of research positions, focused in analytical and material sciences, polymer application development, and technical service and development for FILMTEC™ membrane products. Haukkala holds a Bachelor of Science Degree in chemistry from the Honor’s College at Texas Tech University.
About the company
Dow Water & Process Solutions has a 50-year legacy of providing in- novative water and process solutions to both communities and industries alike. A differentiated business unit of The Dow Chemical Company and its consolidated subsidiaries, Dow Water & Process Solutions offers a broad portfolio of ion exchange resins, reverse osmosis membranes, ultrafiltration membranes and electrodeionization products, with strong positions in a number of major application areas, including industrial and municipal water, chemical processes, pharmaceuticals, power, residential and wastewater and water reuse. More information about the unit can be found at www.dowwaterandprocess.com