Reverse osmosis (RO) plays a vital role in most powerplant water treatment systems today, particularly those serving gas-turbine-based generating facilities (Fig 1). It is used in the production of water for power enhancement (evaporative coolers, fogging) and emissions reduction, makeup for heat-recovery steam generators, treatment of plant wastewater, etc.
Proper membrane care is critical to producing the quantity of permeate required at the quality necessary over the expected lifetime. The editors approached Rob Goodlett of Avista Technologies Inc, San Marcos, Calif, shortly after his presentation at the 2009 Southwest Chemistry Workshop to learn what RO-related tasks GT owner/operators should be factoring into their outage plans.
But before getting into details, short backgrounders on both organizations are useful:
- Avista Technologies’ core business is the supply of specialty chemi¬cals and technical support services for membrane-based filtration and RO systems.
- Southwest Chemistry Workshop is an annual meeting serving the generation sector of the electric-power industry. It focuses primarily on water treatment issues challenging owner/operators of plants powered by fossil and nuclear fuels. The 2010 conference, hosted by Capital Power USA, will be held June 8-10 in San Diego. Your contact is Frank Spencer (fspencer@capitalpowerusa).
Goodlett began by saying that the only things any owner/operator should have to do for the RO system during an outage are membrane replacement or offsite cleaning; everything else should have been done beforehand.
It’s important to monitor RO system performance daily, he continued, and to clean membranes and conduct other maintenance as operating data indicate the need. Failure to address issues in timely fashion can have a permanent adverse effect on system performance and/or reduce membrane life.
All pressure-driven membrane separation systems foul over time (Fig 2). The symptoms of fouling include a decrease in normalized permeate flow, an increase in salt passage, and an increase in differential pressure across the elements. RO element manufacturers recommend cleaning when permeate flow drops by 10% to 15% or when there is a 10% to 15% increase in differential pressure across the elements. Performance monitoring lets operators know when these limits have been reached.
Service companies like Avista help plant O&M personnel measure performance with proprietary normalization software. For example, Avista’s normRO, provided free to customers, considers changes in water temperature, pressure, and conductivity to differentiate between membrane fouling and changes in feedwater conditions. Data, recorded by operators on daily rounds, are entered into the program, thereby enabling performance trending so maintenance can be scheduled in advance.
Outage preparations should include an analytical review of current operating conditions. Goodlett also suggested an autopsy or cleaning study to determine membrane health and the degree of fouling, and to select site-specific membrane cleaners (Fig 3).
Success in the lab equates to success in the field, he added. Confidence in the cleaning formulation is particularly important because of the limited time available during an outage to restore membranes to as close to original condition as is possible. The cleaning process must meet expectations on first use.
Fig 4 illustrates the importance of effective cleaning. The red line shows that proper cleaning of a healthy membrane effectively restores production to the original baseline value each time. By contrast, ineffective cleanings, represented by the green line, restore less and less of the original flow each time. Note the similarity between these curves and ones you’ve seen previously to illustrate the benefits of compressor cleaning.
Additionally, when cleanings are not effective, you must clean more frequently. This increases downtime and operating costs, and shortens membrane life. Note that ineffective cleaning can be caused by poor procedures, the wrong formulation, and cleaning intervals that are too long.
Ensuring that your RO system is at full capacity and running well following an outage is critical. The last thing you want to deal with when bringing units back into service is interruptions in process water supply. If you don’t have personnel with sufficient RO experience onsite, consider hiring a qualified service organization to review daily processes and data collected, and to recommend a suitable maintenance program.
Goodlett told the editors there were five types of foulants to address when preparing a cleaning formulation: metals, scale, silt, organics, and chemical (Fig 5). He provided the following descriptive material:
Metal foulants most frequently include iron, manganese, and aluminum; sometimes zinc, copper, and nickel are found. Iron and manganese commonly occur in groundwaters as soluble divalent ions (Fe++, Mn++). Should air or chlorine be introduced into these waters, the iron and manganese may be oxidized and precipitate on membrane surfaces as hydrous oxides. These compounds include oxides (such as MnO) and hydroxides (such as Mn(OH)2).
Other sources of iron fouling: (1) Corrosion processes that occur within well headers or feed piping and (2) carryover from flocculator/clarifiers or multimedia filters when iron salts are used as coagulants. Fouling by aluminum oxide is common. The usual source is carryover of flocculated solids from filtration processes using aluminum salts as coagulants.
Scale. Supersaturation of sparingly soluble salts (those characterized by low solubilities) can cause scaling of membrane surfaces and feed spacers. The most common scale-formers are calcium carbonate and sulfates of calcium, barium, and strontium (Fig 6); less common scales are silica and calcium fluoride. Silica scale, when it does occur, is very troublesome because of the difficulty in removing it from membrane elements without damaging them. Sidebar 1 offers more detail.
Goodlett pointed out that scale often is found on the downstream end of the last membrane in the system, or on the inside of the last pressure vessel. Higher product-water conductivity may be your first indicator of scaling. He stressed that scale is important to eliminate as quickly as possible because it is abrasive and may harm the membrane surface.
Colloids and silt are the most common foulants. Examples include clays, colloidal silica, rust particles, and bacteria, which are found in virtually all surface water supplies (Fig 7). Colloids are particles less than about 1 micron in diameter and do not settle from a standing solution. Silt particles have larger diameters and will settle from solution. Indicators of colloidal fouling, Goodlett said, include high delta P across the first array and reduced permeate production.
Organics. Humic and fulvic acids frequently are found in surface waters, formed during the decay of leaves and other vegetation. The degree of fouling these acids cause depends on their exact nature and feedwater ionic composition. Today’s polyamide membranes are more susceptible to severe fouling than older cellulose acetate membranes.
Two more facts to remember: (1) Acids with high molecular weights are more serious foulants than those of lower weight. (2) Calcium and magnesium ions may contribute to humic and fulvic acid fouling by binding together anionic groups of membranes and acids.
Biologically derived organic foulants include the slime exuded from bacteria and filamentous fungi, as well as the microorganisms themselves. Slime may originate in the feedwater and be carried downstream to your membrane separation system. But more frequently it forms insitu through the growth of microorganisms on membrane surfaces (Fig 8).
Goodlett said biological slime is easy to identify in RO vessels and inlet vessels by sight and smell. Another telltale clue is high differential pressure across the first array. Biocides for mitigating slime formation are covered in Sidebar 2.
Chemical fouling generally is caused by feeding two or more incompatible chemicals ahead of the RO system. The most common example is the precipitation of polymeric antiscalants by organic coagulants which typically plug the front ends of first-array membranes. High differential pressure across the primary array is an indicator.
Plant owner/operators can clean their membrane systems with generic chemicals or special commercial formulations. The former, which include citric acid and trisodium phosphate, have a cost advantage. However, Goodlett points out, a selection based solely on cost may actually turn out more expensive for one or more of these reasons:
- Commercial cleaning formulations typically are more effective than generic chemicals, so you need less of them.
- It takes time to mix generic cleaners.
- Generics can damage membrane element components if a high or low pH excursion occurs (Fig 9).
Good commercial formulations are more effective than generic chemicals, he explained, because they contain a “team” of ingredients that work together to remove foulants. The general categories of ingredients are bulleted below to help you understand what functions they perform.
- Buffers maintain the cleaning solution’s pH in the range necessary for the formulation’s constituents to work at maximum efficiency, as well as to protect membranes and other RO system components.
- Surfactants “wet” surfaces and enable the cleaning formulation’s other components to penetrate layers of foulant.
- Builders are complex divalent ions—such as calcium and magnesium—that may render surfactants inactive.
- Chelants sequester heavy metals—such as iron and manganese—as well as the constituents of many scales.
- Dispersants disperse colloids and silt components that cannot be dissolved by the cleaning formulation. They work in concert with surfactants.
- Hydrotopes are organic substances added to some formulations to dissolve targeted organics—such as bacterial slime.
- Redox controllers raise or lower the oxidization potential of the cleaning solution. To illustrate:
Reducing agents help dissolve iron and manganese; mild oxidizing agents help remove organic foulants.
Goodlett told the editors that when users find it difficult, or virtually impossible, to get their membranes clean with a given formulation—possibly one that has been effective in the past—a cleaner evaluation study should be conducted. Foulants tend to occur in combinations based upon the feed source and the physical and chemical nature of the water supply often changes over time. It’s not unusual to find gremlins in your makeup water.
Typically, one membrane element from the affected system is removed and samples with representative fouling are removed. They are wet-tested and cleaned with several candidate formulations. At Avista, this is done in the company’s RO Vision™ test cell apparatus.
Refer back to Fig 3 for pictures of test cells, which have transparent tops that allow technicians to observe the cleaning and collect membrane performance data. A foulant analysis also may be conducted as part of the study. Knowing the chemical nature of the foulants helps chemists optimize the pretreatment process.
For example, when fouling is heavy and/or tenacious, it sometimes is necessary to soak the system overnight in the appropriate cleaners. During the soaking period it often is helpful to occasionally “bump” the cleaning pump to agitate the solution within the membrane elements.
A suitable cleaning system for membrane separation systems is relatively simple and one that most plants might build in-house (Fig 10). Size the cleaning tank, which should be made of reinforced plastic or other non-corrosive material, to hold a volume equal to that of the water contained in the first-stage pressure vessels, cleaning-system hoses, and cartridge filter holder.
Some help here for your volumetric estimate: Pressure vessels for 4-in.-diam × 40-in.-long membrane elements hold 2 gallons per element; 6-in. elements, 3.5 gal; 8-in. elements, 6 gal. The volume of liquid retained per foot of 2-in.-diam hose is 0.16 gal; 3 in., 0.37 gal; 4 in., 0.65 gal.
Equip the tank with a cover, heater, and temperature and level controls. Include an exhaust fan in your design if the tank will not be located in a well-ventilated area. Specify a centrifugal cleaning-solution pump made of Type-316 stainless steel to deliver 10 gpm to pressure vessels retaining 4-in. membrane elements (23 gpm for 6-in. elements, 40 for 8 in.).
Be sure the hoses that connect the cleaning skid to the RO system have an adequate pressure rating for the task, Goodlett cautioned, and that the hose is not kinked before operating. Also, locate the return line below the liquid line in the tank to reduce splashing and foaming.
Other best practices he recommended implementing:
- Install a 5-micron cartridge filter after the cleaning pump to remove undissolved powder cleaners.
- Locate a safety strainer upstream of the cleaning pump.
- Provide appropriate valves to control system pressure and permit drainage and flushing of tank and lines.
- Install a flowmeter to establish the proper rates through the pressure vessels.
- Provide a bypass line around the cleaning loop and back to the tank to help control flow to the pressure vessels and to mix or dissolve cleaning formulations.
Good procedures are essential for effective cleaning. Here are key points to remember:
Flow and pressure. Clean one stage at a time, circulating the cleaning solution through each stage for at least one hour. Flow should be at the maximum rate recommended by the membrane element manufacturer. If unknown, use the flow rates suggested for system design presented in the preceding section.
Clean at the minimum pressure needed to achieve the desired flow. Low pressure minimizes permeation and reduces the fluid force that holds foulants to the membrane surface. In general, do not exceed 60 psig.
Dilution water used in the preparation of cleaning solutions should be RO permeate or demineralized water.
Flush the system with RO permeate or demineralized water prior to and after cleaning. If two formulations are used sequentially, flush the system between cleanings.
Temperature of the cleaning solution should be at the maximum permitted by the element manufacturer. For thin-film polyamide elements, Goodlett considered 120F both safe and effective for solutions having a pH value of less than 10.5; for cellulose acetate he said not to exceed 95F.
Cleaner quantity. Be sure to calculate the total volume of water in the cleaning loop to determine how much of the cleaning formulation is required, he continued. This includes the amount of water in the cleaning tank, pressure vessels, hoses, and cartridge-filter housing. The numbers presented in the previous section will help here.
To give you a feel for the amount of water you’re dealing with, Goodlett ran through a quick calculation for a system with a 4:2 array having six 8-in.-diam × 40-in.-long elements per vessel. His sample system had 200 ft of 3-in. hose to connect the cleaning skid with the RO system, the cartridge filter housing held 15 gal, and the cleaning tank volume equaled the volume of the hoses, pressure vessels, and filter housing.
Total volume required to clean the first stage: 466 gal. The second stage needed a minimum volume of 322 gal. With this information, together with the cleaner concentration recommended by laboratory tests, you can determine the quantity of the cleaning formulation required.
Goodlett noted that you can repurpose the cleaning solution used in the first stage for the second stage provided the pH is still in the recommended range and the solution is not turbid. ccj