Reverse osmosis (RO) membrane manufacturers often are asked how long membranes will last in service. The responses to that question may vary a little, but essentially the answer is, “It depends.” An equally good response is, “How long do you want them to last?” The fact is, RO membranes can last a long time. Some membrane installations have run for 10 to 15 years on the original set of elements, while other installations operate their membranes for only six months before they need a new set. The differences between these two types of operations fall into two categories: differences in pretreatment and differences in operating discipline.
RO membranes are very fine filters designed to remove dissolved minerals from water. Other materials such as natural or organic compounds, colloidal suspensions and particulates are best removed upstream of an RO system.
Choosing the right pretreatment mostly depends on the feed water source. Well water is a feed water source with a low fouling potential. It often does not require more than acidification and/or antiscalant dosing and a 5 micrometer cartridge filter. Surface water, however, tends to be highly variable and affected by seasonal changes. The potential for fouling is high. In addition to the steps taken for well water, surface water treatment can require chlorination, coagulation/flocculation, clarification, multimedia filtration, dechlorination, acidification and/or antiscalant dosing. Industrial and municipal wastewaters require the most extensive pretreatment, yet the number of steps involved is less. Technology such as ultrafiltration effectively removes a great deal of matter and requires fewer process steps than other types of pretreatment. Proper feed water analysis is essential to understanding what type of pretreatment is necessary.
Operating discipline is a catch-all phrase which describes all the daily activities by which one runs an RO system. The best RO operators keep records of their system’s performance and track the normalized permeate flow. They use this information to monitor membrane health and set schedules for membrane cleaning. Many operators use software to help them normalize performance data. Dow Water Solution offers a program called FTNORM that assists operators in knowing when to clean their membranes.
If done properly, both membrane pretreatment and operating discipline will keep the membrane surface clean and protected from damage. A look at the construction of an RO membrane such as the FILMTEC™ FT30 shows why this is important (Figure 1). The membrane is sometimes referred to as a “thin-film composite” membrane. The term “composite” refers to the fact that it has three layers: a polyester reinforcing fabric, a microporous polysulfone support layer and, on the outside, the all-important polyamide barrier layer. The polyamide layer is the thin film, or active surface, that rejects dissolved salts while allowing water to pass. This barrier layer is about 0.2 microns thick. For comparison, consider that the average human hair is 100 microns thick. It is evident that this layer is very thin. In fact, it is the thinness of the membrane that dictates a lot of the pretreatment requirements of RO membranes.
Avoid scratching the membrane
Given the thinness of the membrane, it needs to be protected from anything that would cause the membrane layer to be scratched. Much of the pretreatment upstream of an RO system is designed to remove fine particles which can scratch the membrane if allowed to enter the membrane elements. But there are other ways to scratch a membrane. One way is through hydraulic shocks. For example, if an RO system is designed without a soft-start sequence for the primary pump, the RO elements will receive a large pressure pulse when the pump is started. This pressure pulse will push the membrane scroll slightly and cause the feed channel spacers to scratch across the membrane surface. The scratches will get worse as the membranes are put through many of these severe stop-start cycles. The solution, of course, is to always use a soft start sequence when starting an RO system.
During the cleaning cycle the membrane is vulnerable to scratching. Over the years, many cleaning recommendations have been made with varying degrees of success. The worst recommendations assume that you need to increase the flow turbulence in order to clean the membranes. One popular recommendation advocates adding air during the cleaning to help “bump” the foulants off of the membrane. Nothing could be worse. The bubbles will slam into the feed screens which separate the membrane layers and cause the screen to move and scratch the membrane surface. Another similar belief is that high-flow cleaning is needed to scour biofouling off of the surface. In fact, the high-flow cleaning is counterproductive. Since a dirty element has narrower-than-normal flow spaces, the high flow will push on the brine spacer and cause it to scratch the surface as well. The best cleaning techniques let the cleaning chemicals do the work and use low flow.
Bearing in mind how thin the membrane barrier is, it is also clear why it needs to be protected from damage by oxidants such as chlorine, hypochlorite, ozone and permanganate. These chemicals burn away parts of the membrane barrier and allow salts and water to pass through more readily. Most RO system designers are aware of this and try to protect the RO from oxidation as part of their pretreatment scheme.
The market is full of cleaning chemical formulations and often the RO operator has no idea what is actually in the solution. It is very important that the RO cleaning solution be free of any oxidants, whiteners or brighteners. These compounds may appear to make the membranes work better after one cleaning, but in the end will only shorten the life of the membranes. Be sure to question your cleaning chemical supplier about the ingredients in their products.
Knowing when to clean
It’s also important not to wait too long in between membrane cleanings. Fouling and scaling will inhibit membrane performance if allowed to continue and the membranes will not go back to their original effectiveness if the cleaning is delayed. In general, RO elements should be cleaned when one or more of these events are occurring:
• The normalized permeate flow drops 10 percent
• The normalized salt passage increases 5 percent to 10 percent
• The normalized pressure drop increases 10 percent to 15 percent
For the specific problem of biofouling, Dow Water Solutions recommends a simple cleaning solution of NaOH (sodium hydroxide) or NaOH and NaDDS (sodium salt of dodecyclsulfate). Several cleaning cycles may be required to bring the system back to optimum performance. Often it helps to clean the elements several times in succession with a fresh cleaning solution rather than relying on a longer overnight soak. This will remove the last layer of biofilm so that it does not attract more dirt and matter. In the case of severe biofouling, slug dosing of a biocide may also be necessary.
While loading hefty 32-pound RO elements into pressure vessels, it is sometimes hard to keep in mind that the critical barrier layer is actually quite thin and delicate. This barrier layer needs to be protected and kept clean in order for the membrane elements to do their job. The better protected the membranes are, the longer they will last.
Cliff Gilbert is a senior account manager at Dow Water Solutions. He can be reached at 856.231.8392, or email@example.com.