Diagnosing Organic Issues Related to Larger RO Operations
Wednesday, April 15th, 2020
By Matthew Wirth
Reverse osmosis (RO) machines have unique problems and diagnosis is often an educated guess. Sometimes the problems are obvious, but sometimes the problems are multifaceted with more than a singular issue. The key to this is the term educated. Troubleshooting requires understanding how RO works; the pressure system, the membrane elements and hydraulic requirements. With an understanding of how and what ROs do, one can extrapolate from available data the causes of malfunction. In this article, we will look at a general description of an RO system, membrane crossflow and discuss how organics can quickly derail the RO machine’s functionality.
Equipment and diagnostic instrument needs
In addition to the pump and membranes, larger ROs need monitoring equipment. Without flowrate data, TDS monitoring, pressure conditions and pH, it is difficult to figure out the causes of reduced performance and physical failure. The following are things to consider:
• The pump must produce the pressure required to meet the operating specification of the membrane elements. This is found on the manufacturer’s specifications for a specific membrane element.
— Substituting membranes requiring hydraulics outside the performance curve of the pump will affect performance and cause operational issues. One example is the thread to 8”x 40” elements with 400 ft2 of membrane surface. Prior to 400 ft2, the common ft2 for an eight-inch element was 365 ft2 of membrane. An advantage to the 400s is increased permeate (more product water) or lower flux rate and pressure at the system’s given flow. This means the system does less work to create the needed permeate. This is all good if the machine has the ability to accept the change. Say the RO machine has a VFD (variable frequency drive) and the RPM can be adjusted to slow the motor. If the machine is less sophisticated, the pump may not deliver the flow and pressure required to manage greater flux and still meet the concentrate flows for necessary crossflow. Match the membrane elements with the capability of the RO machine.
• Membranes have crossflow requirements, pressure specifications and feed-water quality specifications (see Image C).
— Working outside the parameters of these operational conditions is not recommended. (Some examples: If you over-flux a membrane element, it can be damaged and produce poor water. Using a water source with chlorine/chloramine levels outside a manufacturer’s recommendations can (and likely will) result in damage to the membrane. Running recoveries too high for the element can result in telescoping the windings and compromise the element [see Image X].)
Image X. Telescoping membrane element
TDS monitors—conductivity monitor
• Knowing the feedwater TDS and the permeate TDS shows the operating performance of the system. Realize that ppm of TDS is often a calculated measurement based on the water conductivity. Many larger ROs use conductivity to monitor water quality. A rule of thumb is to divide the ppm value by 0.64 to convert to a conductivity value approximation.
• The pump inlet feed pressure and pump outlet pressures and ΔP (change in pressure) are paramount to the operation of the system.
• The RO high-pressure pump boosts the inlet feed pressure, therefore the high-pressure pump requires a minimum and maximum pressure to operate optimally. One needs to know the feed-line pressure to operate and log the performance of the machine.
— Most larger RO machines have low feed-water pressure safety shut-offs to protect the high-pressure RO pump. Be aware that some machines come back online once the pressure returns and others alarm and/or shut down until they are manually restarted.
• To operate the high-pressure system pump and set the system recovery, one must know the pressure delivered by this pump.
• The ΔP is the difference in pressure between the system pressure and the permeate pressure.
— The ΔP for a single membrane is 12 psi and 50 psi for an array (see Image C).
— Increased ΔP is one of the first indicators of organic fouling and other issues. If the pressures are out of balance, something is likely plugged or there is a compromised interior seal and/or membrane pocket.
• Hydraulic dynamic flowrates are indicators of RO performance.
— Systems without flow indicators are difficult to troubleshoot and operate. The permeate and concentrate flows show the system’s performance. In addition, they are indicators of system flow imbalance. Without knowing these values, diagnosing problems is difficult at best.
RO system design related to crossflow and flux rate
A complete water analysis is the first and most important part of an RO system design. If there are seasonal variations (which are common for surface sources) or varying sources (which are common with municipal supplies), get all the pertinent water quality data. Size to worst-case data to ensure year-round system functionality. Note: The usable source data is the current source data; test results from years past are not usable.
Part of any RO system is appropriate pretreatment. The quality of the water feeding the RO determines if it will function correctly. An RO with a clean, low TDS water source can function more aggressively than with questionable high TDS water. Design the pretreatment to produce the RO feed-water parameters and ensure that it operates uninterrupted during peak use. Check that the source pressure and the ΔP across the pretreatment do not affect the performance of the RO pump. The RO pump needs a specific flow and pressure. The pretreatment system cannot be a choke point that robs water and pressure from the RO pump. It is highly recommended that the source-water feed to the RO is not on a line that loses water pressure when other upstream systems call for water.
The RO system requires a steady pressure and flow. If the pretreatment is chemical and the source water is hard, the application may require tertiary treatment downstream. One example of tertiary treatment is a permeate polisher, which is a water softener designed to work in water that has low conductivity and low pH. Softening permeate is common on large systems fed with hard water. The water hardness and subsequent scaling issue are managed with chemical pretreatment. In many cases with large RO machines, it is not cost effective or practical to soften the water before the RO because of the scale of water use and the size of the equipment.
Carefully read the operational specification for the membrane elements and consider both the flux and crossflow requirements. Consider that a reduced rate of permeate water flow for a given area of membrane element reduces the collection of foulants at the membrane surface. Flux rates (flow per square foot of membrane over time) for surface water supplies should range from eight to 14 GFD (gallons per square foot of membrane area per day) and 14 to 18 GFD for well water.1
RO recovery rate should be conservative. Example: A normal 8”x 40” (8040) membrane element has 365 square feet of membrane rolled around the permeate tube. A flux rate of 10 gallons per day per ft2 equals 3,650 gallons in 24 hours. Many 8040 membrane elements are rated at two to three times that output, but that is a maximum. A conservative percent recovery of the feed water minimizes the concentration of foulants.
Image A. Contaminants moving properly in the crossflow (bulk flow)
In challenging water, maximize the crossflow velocity in the elements. Crossflow creates the tortuous path across the membrane surface to keep foulants up and off the membrane and allow water to pass through to the permeate side of the element. The concentrate percentage flow determines the crossflow: in larger systems, this is 50 to 30 percent of the total feed rate. In addition, a conservative design maximizes the crossflow velocity of the feed and concentrate streams. A higher crossflow velocity reduces the concentration of mineral salts, organics and foulants at the membrane surface by increasing their diffusion back into the crossflow feed stream above and along the membrane surface (see Image A). Inadequate crossflow allows contaminants to settle on top of the membrane surface and block its production (see Image B).
Image B. Contaminants lodged on the surface of the membrane
The membrane element must match the desired recovery rate for the temperature, pump feed pressure, feed-water conductivity and available upstream feed flow and pressure. An RO pump boosts the feed pressure to the desired system needs. If feed pressure and flow drop below the specifications for the pump, either the system will alarm or the pump may be damaged. Example: A low-energy membrane works at a pump feed pressure of 100 psi, but this membrane will not function well in a system with pump feed pressures of 250 psi. In waters with low temperature, consider low-temperature elements. Match the membrane element to the water and the machine.
Osmotic pressure and transmembrane pressure
The water’s conductivity and/or TDS dictate osmotic pressure and thus the transmembrane pressure requirement to make RO water. Osmotic pressure is the pressure required to prevent the flow of water across a semi-permeable membrane separating two solutions having different ionic strengths: the permeate and concentrate. For RO systems, it is this osmotic pressure that has to be overcome in order to produce permeate. The rule of thumb is for every 100 ppm of TDS difference between feed and permeate, one psi of osmotic pressure exists.2
Increasing the water temperature makes it less viscous (thinner), making it easier for the water to pass through the membrane. A 10°F-increase in feed-water temperature will decrease the feed-pump pressure requirement by 15 percent. Be aware that both the conductivity and the temperature of the feed will affect the performance of a membrane element. In reviewing the element specifications, note that there is a change in water condition stated for the performance of a membrane. Example: Average salt rejection after 24 hours operation. Individual flowrate may vary +25% / -15%.2 Testing conditions: 2,000 ppm NaCl solution at 225 psi (1,551 kPa) operating pressure, 77°F (25°C), pH 7.5 and 15% recovery.3
Image D. Biofilm dripping from an eight-inch membrane element
Delta P (ΔP)
There needs to be pressure differential between a permeate side of the membrane element and the feed-water side. It is a pressure differential that creates RO and allows the system to overcome transmembrane pressures and desired permeate flow. Without a ΔP, an RO system would not work. Problems arise when the pressure differential is too high. The published ΔP (maximum pressure drop) for an 8040 membrane (see Image C) is 12 psi and the recommended ΔP for an array is 50 psi.3
Image E. Biofilm on the end cap of an eight-inch membrane housing
Organic (bio) fouling
All RO membranes are subject to organic fouling. Even with careful sanitary care and consideration when loading new membranes, the system may become contaminated and the result is a slimy, smelly organic loading on the membranes and in the array housings. In Image D, there is actually biological material dripping from the end of the membrane element. In Image E, notice the slime buildup on the end cap of the array housing. In both of these cases, the RO machine was experiencing increased pressure differential across the membrane elements. This was the indication that the system was experiencing biological fouling. The correction for this problem is either to replace the membranes after fully sanitizing the system or conducting a clean-in-place (CIP) procedure.
Image F. Dirty CIP high pH solution
There are two parts to CIP for RO systems. One includes a low pH acid cleaning to remove calcification and other mineral deposits. To remove organics from an RO system using CIP, the system is flushed with a high pH solution to clear the organic matter. The order in which the low or high pH is run depends on the baseline issue of concern. If the system is loaded with organics, it becomes difficult for a low pH solution to cut through the biofilm and attack the mineral deposits. If this is the case, perform the high pH CIP first to remove the biofilm and expose the mineral deposits to the low pH acid CIP stage.
There is a saying concerning system cleaning: keep it hot, keep it strong and keep it moving. CIP solutions recirculate through the system at a given rate based on the CIP instructions for the chemical used. There are multiple CIP solutions and instructions for cleaning RO systems and membrane elements. The best practice is to follow the instructions to the letter. In cases of extreme fouling, it may become necessary to dump the CIP solution several times during the process (see image F). Note that some larger production RO systems are not able to receive CIP. If the site wishes to reuse the existing membranes, there are companies that clean elements offsite and this is an option if the installation can be down while the membranes are cleaned. FYI: If membranes go out for cleaning, they have a finite shelf-life. Check with the company that did the cleaning for the recommendations on membrane element storage after the cleaning procedure.
If the system has experienced increased pressure differential across the membranes and/or a housing, it is often due to organics. A quick way to diagnose if there is an organic issue due to biofouling is just safely shut the system down, release all the energy from the system, ensure that all the power is locked out and pull off one of the end caps on the array housing. If there is a distinct putrid odor and/or signs of slime buildup in the walls of the array, the end cap or membrane element, then action is required. RO systems of any kind or size will not function optimally in the presence of biofilm within the system.
Be aware that organic fouling is an issue with RO systems. While it is common, it is often overlooked. Considering the biological control is removed from the water prior to the water seeing the membranes, it is actually not surprising that organics can get a foothold within a system and grow. Presuming the system should not have an organic issue is an incorrect assumption.
Image C, courtesy of Suez Water. All other images, courtesy of Pargreen.
- Troubleshooting Your RO, Hydranautics (A Division of Nitto Denko Corp.), Informational Document, January 23, 2001.
- Reverse Osmosis Chemistry, Manchester, 2018, Retrieved March 6, 2020. http://reverseosmosischemicals.com/reverse-osmosis-guides/reverse-osmosis-glossary-terms/osmotic-pressure-reverse-osmosis-systems
- Suez AG Series Fact Sheet. Retrieved March 6, 2020. http://www.gemembranes.com/ge-osmonics-ag-ro-water-membranes.pdf
About the author
Matthew Wirth is an Industrial Water Specialist with 40 years of experience working with large process systems, including RO, IEx, process filtration and media filtration. He is the General Manager of Water at Pargreen Sales Engineering in Chicago, IL. Wirth received his engineering training at the South Dakota School of Mines and Technology in Rapid City, SD and earned a Bachelor’s Degree from Concordia University in St. Paul, MN. He can be reached via email, email@example.com or phone (630) 628-1330.
About the company
Since 1966, Pargreen, Inc has been supplying advanced filtration solutions to a variety of industries worldwide. Pargreen Process Technologies & Water Technologies Systems provide IEx, separation and filtration for a variety of manufacturing processes. Visit the website for more information: www.pargreen.com