Demystifying Membranes - Part II
This overview helps separate the facts from the fallacies related to membrane technologies used in wastewater treatment
This is the second article in a two-part series on membrane elements and treatment systems. "Demystifying Membranes - Part I" was published in Environmental Protection's July-August 2003 issue. The first article compares the advantages and disadvantages of four types of membrane separation technologies. Part II clears up some common misunderstandings about the properties of membrane technologies.
As explained in the first article, the four pressure-driven membrane separation technologies consist of reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). Applications for using membranes to separate solids from liquids can generally be broken into three categories:
- Water purification -- removing contaminants from a water supply to produce purified water for some downstream activity. Examples include drinking water, boiler feed, pharmaceutical manufacturing and semiconductor rinsing.
- Wastewater -- treating wastewater from a manufacturing operation or municipal effluent source for water reuse and/or solute or particulate recovery. Examples include metal finishing rinse water treatment, aqueous cleaning bath and rinse water treatment, oily wastewater treatment and chemical mechanical polishing (CMP) effluent treatment.
- Processing -- separating and/or recovering components within a manufacturing process. Examples include milk dewatering, lactalbumin protein recovery from cheese whey and pharmaceutical protein fractionation.
Using membrane technologies for water purification is straightforward, and the results are generally readily predictable; most of the confusion and negative opinions surrounding these technologies are based on attempts to apply membrane technologies to the categories of wastewater treatment and processing applications. A reason for this is related to "system recovery." Recovery is defined as that percentage of the feedwater flow that passes through the membrane and becomes permeate. The system manufacturer designs the recovery into the system, and as it is increased, the concentration of rejected materials left behind in the concentrate stream increases dramatically. For water purification applications, system recovery rarely exceeds 80 percent, whereas, for wastewater and processing applications, recovery is usually 90 percent or higher.
Another reason for the relative simplicity of water purification applications is that the nature of the contaminants found in natural water supplies, as well as their behavior resulting from the concentration effects of membrane processing, is predictable. For example, virtually all of the spiral wound membrane element manufacturers provide computer programs that will design an RO system for water purification applications. On the other hand, wastewater and processing applications are generally neither predictable nor straightforward, as explained below.
Since the birth of focused environmental awareness in the early 1970s, protection of the environment has steadily been moving up on the priority list of concerned citizens worldwide. Issues such as the ozone hole, the greenhouse effect, overflowing landfills, acid rain, destruction of the rain forests and overpopulation have created an attitude of conservation and environmental responsibility throughout the world.
Historically, industrial pollution prevention has been regulatory driven, and these regulations, coupled with active enforcement, are the direct result of consumer influences. Today, "pollution prevention" is emphasized over "pollution control." In other words, wastes must be controlled, reduced or recycled at the source as opposed to traditional "end-of-pipe" treatment, treating the wastes as they leave the manufacturing facility. Many companies are discovering that by proactively pursuing these activities, they realize economic benefits, as well as the reputation of environmental stewardship.
On the "raw water" side, many industries are discovering that the quality of their available water supplies has a negative impact on the quality of their products. In other words, manufacturing processes are becoming so sophisticated as to be sensitive to the quality of the incoming water.
Wastewater and processing applications present particular challenges because they are:
- Usually unique and often one-of-a-kind applications.
- Operated at very high recoveries, because a goal is to make the concentrate volume as small as possible.
The outcome of this is that it is virtually impossible to predict the behavior of membrane technologies in these applications, and therefore testing is mandatory.
Facts and Fallacies
With my almost 30 years of experience in the membrane industry, I have had the opportunity to hear numerous comments from system designers as well as end users regarding the applicability and appropriateness of membrane technologies in all sorts of applications. The following is my list of fallacies and the actual facts associated with each.
Membranes are very sensitive to pH and temperature extremes.
Since the introduction of thin-film composite reverse osmosis and nanofiltration membrane polymers in the early 1980s, these parameters are no longer an issue. They can withstand a pH range of 3 to 11 in normal operation, and even a wider range for cleaning purposes. These polymers can also tolerate temperatures down to freezing and as high as 130 degrees Fahrenheit, even higher with specially manufactured modules. It is true that lower water temperatures significantly reduce the permeate output of all membranes; however, this is related to the viscosity of water rather than characteristics of the polymer. The polymers utilized in most ultrafiltration and microfiltration membranes are even more chemical and temperature resistant than those used in nanofiltration and reverse osmosis.
Bacteria will "eat" the membranes.
Whereas this is a problem with cellulose acetate RO membranes, the thin-film composite polymers are inert to the effects of bacteria; however, bacteria will attach themselves to the membrane surface and form biofilms.
Chlorine will attack RO and NF membranes.
This is not a fallacy; virtually all oxidizing agents will attack thin-film composite membrane polymers and cause irreversible damage. In general, chlorine will not affect MF and UF membrane polymers.
Membranes don't work because they are so easily plugged or fouled.
Although membrane fouling is the single greatest performance problem facing membrane systems, today there are much improved pretreatment technologies, as well as a vastly superior understanding of causes of such fouling. In many large applications, extensive pilot testing is conducted leading to the design of an optimum pretreatment component for the membrane processing system. As discussed in the first article in this series, device configurations such as tubular, plate and frame and capillary fiber are less susceptible to fouling when compared to spiral wound membrane elements.
There have been so many failures in the application of membrane technologies to wastewater treatment that they just must not work.
Virtually every failure can be traced to a single cause -- insufficient testing (if any at all). Because, as stated earlier, most wastewater applications are unique, there's no history or experience behind the treatment of this particular stream. As a result, the reaction of a given membrane polymer to the particular chemistry and high-concentration environment of this stream is unknown. Each potential membrane polymer must be tested, as well as one or more of the four membrane device configurations previously detailed. Where one polymer and/or device configuration may fail miserably in a given application, another one might work perfectly.
Membrane technologies are too expensive.
The increasing demand for higher quality water and incentive to reclaim water are strong incentives to utilize the unique characteristics of these technologies. Ten years ago, nobody would have dreamed of using membrane technologies to treat surface water sources to obtain municipal drinking water. Yet within a few years, we may see capillary fiber UF or MF in virtually every application.
In those instances where membrane technologies have been successfully installed in wastewater treatment and processing applications, many of the successes have not been adequately publicized for any one of a number of reasons. These include:
- The end user (ultimate purchaser of the system), for competitive reasons, chooses to keep the application confidential. This is particularly evident in competitive industries such as pharmaceutical manufacturing.
- The original equipment manufacturer (OEM) hasn't bothered to publicize the application. Generally, it takes a fair amount of initiative to write an article or case history and submit it to the appropriate trade journal for publication. Very often, the membrane element manufacturer is a large company with the infrastructure to write articles to publicize its products, but is not aware of the application because the company is usually not in direct contact with the end user.
- The end user has hired a consulting firm to select the appropriate membrane polymer and device configurations, perform the testing and help them go out for bid. In this scenario, the OEM may not even be aware of the application -- the manufacturer has simply supplied a system based on the bid specifications. Although the end user may not consider the application proprietary, they may not be aware of its uniqueness, and consider any publicity surrounding this application unimportant.
The point is, although there is a paucity of membrane technology applications operating in these non-water purification applications when compared to the vast potential, there are a lot more than we actually hear about.
Borrowing from the old Virginia Slims advertisement claiming "You've come a long way, baby," membrane technologies have matured immeasurably since they first appeared on the commercial scene. Most of this maturation is the result of improvements in membrane polymer chemistry and device configurations. Unfortunately, most of the membrane element manufacturers are lacking in application expertise -- particularly the ability to identify and investigate new potential applications, and the majority of system integrators (OEMs) lack a sufficient knowledge of both the membrane polymers and device configurations currently available on the market.
The good news is that there are a handful of consulting engineering firms, academic institutions and other organizations that, because they neither sell systems nor components, can claim complete objectivity with regard to sources and properties of membrane elements and treatment systems. They have the knowledge of the many membrane elements and have experience in working with them. Some have test equipment available and the application expertise to perform the testing required to design the optimum membrane processing system, and also have the capability to prepare technical specifications and bid documents to allow the client to go out for bid to qualified systems suppliers. Ideally, the firm should have the capabilities of representing the client during the construction, installation and start-up phase, as well as to supervise operator training.
A significant issue today is not the availability of membrane polymers and device configurations to address the myriad of solids-liquid separation applications, but the end user's lack of awareness of the membrane technologies that may work and the difficulties in identifying the experts capable of performing the testing and design of the total treatment system.
As we see more and more of these fallacies corrected, we will witness membrane separation technologies taking their rightful places as true chemical engineering unit operations.
This article originally appeared in the 05/01/2004 issue of Environmental Protection.