The membrane processing technologies of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are widely used to separate suspended and dissolved materials from water solutions in numerous industrial, medical and drinking water applications.
Generally, MF involves the removal of particulate material ranging in size from 0.1 to 1.0 microns (1,000 to 10,000 angstroms); UF is used to separate materials in the 0.001 to 0.1 micron range (10 to 1,000 angstroms); and NF and RO are used for separations involving materials less than 0.001 micron (10 angstroms) in size. MF is primarily used for removal of suspended or colloidal materials; while UF, NF and RO are used for the separation of dissolved materials (solute).
The global demand for purified water is increasing rapidly with no signs of abatement. From increasingly demanding industrial requirements to a burgeoning requirement for improved drinking water quality, there are very few water uses anymore that can accommodate the available water supplies without some form of treatment.
Waterborne contaminants can be classified into the following categories.
- Suspended solids
- Dissolved organic contaminants
- Dissolved ionic contaminants
- Dissolved gases
As analytical equipment has become more sophisticated and the ability to measure contaminants in extremely small concentrations such as the part per trillion range has become possible, it is virtually impossible to produce water that is completely free from any contaminant. As a result, treated water quality requirements have become application specific, meaning that the water users have established water quality criteria based on the requirements of their specific application. The quality standards usually are written in terms of the above classes and address the maximum concentration of one or more of the classes.
Although no single technology is the optimum for removal of all of the classes, the membrane separation technologies of MF, UF, NF and RO are the most versatile and effective in reducing all of the classes of contaminants to at least some degree.
Conventional filtration technology involves pumping the entire liquid stream through the filter medium. This is known as "dead-end" filtration. A recent development known as "crossflow" or "tangential flow" filtration allows for continuous processing of liquid streams. In this process, the bulk solution flows over and parallel to the filter surface, and because this system is pressurized (from the pump), water is forced through the filter. The turbulent flow of the bulk solution over the surface minimizes the accumulation of particulate matter on the filter and facilitates continuous operation of the system. Figure 1 illustrates this technology as compared to conventional filtration.
The most significant factor affecting the performance of membrane processing systems is membrane fouling. It is the result of insoluble materials coating the membrane surface and causing a reduction in product water quality and/or flow.
Causes of Fouling
Because all of the membrane processes involve the separation of contaminants from a solution by the action of continuously pumping water through the membrane, the resulting concentration of the contaminants increases the chances of them coating the membrane surface. Although the principle of crossflow filtration is based on the feed stream moving over the membrane surfaces at sufficiently high velocities to keep insoluble materials from settling out, in the real world, fouling can and frequently does occur. As the contaminants coat the membrane surface, they tend to plug the pores, thereby reducing the flow of product water through the membrane. Another phenomenon known as "concentration polarization" also takes place. As the fouling layer builds up, dissolved materials become trapped in the layer and cannot readily disperse back into the feed stream. As their concentration increases, they may become insoluble or actually pass through the membrane to a high degree. The net result is a product water quality that has been degraded as a direct result of the fouling layer.
Although the particular foulant is a bacterium, any of the materials described above can produce a fouling layer and cause concentration polarization.
In general, most fouling materials fall into one of the following categories.
- Suspended solids
- Precipitated solids
- Biological materials
Suspended solids fouling results from an accumulation of particulate material on the membrane surface. The particles enter the membrane system in the feed water and, due to either improper prefiltration or inadequate system design (or both), settle out on the membrane surface.
Precipitated solids are suspended solids that are so small they will not normally settle out of solution. They also are negatively charged and, therefore, resist agglomeration (clumping). The concentrating caused by membrane processing can result in colloidal materials depositing on the membrane surface. As the colloids are forced closer together, they tend to agglomerate and precipitate out.
Scale formation results from the precipitation of certain sparingly soluble salts whose solubility limits are exceeded during the concentration process of the membrane system. These include calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate and calcium fluoride. With membrane technologies, this phenomenon only occurs in RO where ionic materials are concentrated.
Metal oxide deposition is usually in the form of iron, aluminum and, to a lesser extent, manganese. Insoluble iron hydroxide can result from colloidal iron, oxidation of ferrous iron in the feed stream, iron corrosion products in the feed water or components of the system itself. Aluminum hydroxide has a minimum solubility at a pH of 6.6 and often occurs in water supplies as a result of the addition of alum (aluminum sulfate) added by the municipal treatment plant. Manganese hydroxide often is found in very small quantities in feed water supplies.
Oil/grease are organic contaminants that are insoluble in water but soluble in hexane, chloroform or Freon TF solvent. They often are found in water as an emulsion; certain surface-active chemicals react with the oil or grease to form colloid-size droplets that are typically very stable in water. Oil/grease foulants result from these materials coating the membrane surface. Often the selective permeability of UF and RO membranes "breaks" the oil/water emulsion, and the resulting free oil is attracted to the membrane surface.
Biological fouling is a result of microorganisms literally growing on the membrane surface and forming a biofilm, discrete films formed by microorganisms as a result of their metabolic activity. The "polymer" film is a matrix of "glycocalyx," a capsular material of extracellular polysaccharides that forms during the growth and reproduction of the microorganisms. This matrix is attached to a surface and the microorganisms colonize in the biofilm.
In addition to serving as structures to stabilize the colonies, biofilms protect the microorganism from disinfectants and from being removed by the moving water. They also help capture food from the stream. Biofilm growth creates a layer that entraps salts and prevents the turbulent flow from thoroughly mixing the solutes in the feed stream.
As pieces of the biofilm slough off into the water stream, they release bacteria, detritus and organic polysaccharide materials known as "pyrogens" (fragments of bacteria cell walls). Some general characteristics of biofilm formation include the following.
- After attachment of bacteria to a surface, formation of the biofilm will start almost immediately (within hours) and is very rapid, particularly in the absence of a biocide such as chlorine.
- The type and growth rate of bacteria forming the biofilm is a function of feedwater composition, available food source in the water and the presence or absence of a biocide in the water.
- Extent of biofilm formation is a function of the surface material, smoothness, nutrients in the feedwater, water properties such as temperature and pH, and system design characteristics such as flow and pressure.
- A biofilm will invariably form on piping walls and other surfaces of system components in contact with the water. Turbulent flow velocities may inhibit but will not prevent the formation of biofilms.
- With membrane systems, biofilm growth creates a layer that entraps salts and prevents the turbulent flow from thoroughly mixing the solutes in the feed stream. This concentration gradient produces the "concentration polarization" effect. In addition, under conditions of no (or low) flow, grow-through of bacteria likely will occur, contaminating the pure water on the permeate side.
Any truly effective disinfectant must not only kill bacteria but remove all traces of the biofilm-a truly daunting challenge.
Minimization of Fouling
System design. Because each type of foulant has its own particular characteristics, no one system design feature will reduce the potential for all types of fouling. In general, however, keeping the "recovery" of the system relatively low will help minimize fouling. (Recovery is defined as that percentage of the feed stream that passes through the membrane and comes out as product water.) Obviously, the higher the percentage of feedwater that is forced through the membrane, the greater the danger that suspended materials or those contaminants that become insoluble at higher concentrations will foul the surface of the membrane.
Membrane element configuration. Membrane devices such as spiral wound with higher packing densities exhibit less resistance to fouling. In other words, the close spacing of membrane layers required to effect a high packing density creates areas of high friction and impeded flow that tend to cause suspended and precipitated material to drop out of the bulk water stream and coat the membrane surface.
Although almost all of the membrane devices are designed to operate in the turbulent flow range (Reynolds No. >4,000), the close spacing of high packing density membrane devices creates areas of high water friction, resulting in laminar flow conditions in certain areas of the membrane device.
Even at turbulent flow, spiral wound elements are generally less tolerant to high concentrations of suspended solids than configurations such as capillary fiber or tubular. On the other hand, to maintain turbulent flow through these latter devices, much higher flow rates are required than with the more closely packed configurations such as spiral wound.
In other words, for those membrane devices that require less maintenance to overcome the effects of fouling (lower operating costs), a higher capital cost is required because more membrane elements are needed (less membrane area per element) as well as larger pumps (higher flow rate). The alternative approach is to use a spiral wound membrane with more pretreatment to reduce the suspended solids.
Because membrane fouling is the most common cause of system failure, most membrane device configurations are based on designs intended to minimize membrane fouling. Plate and frame (flat sheet), tubular and capillary fiber devices all sacrifice a certain amount of packing density or fouling resistance.
With the significantly high flux rates provided by MF and UF membranes, they can be more economically manufactured in the more fouling resistant configurations. All of these membrane technologies require pumping energy to effect the separation. Pressure requirements are dictated by the characteristics of the membrane-if the pressure is too high, it may force water through at such a high flux rate that the fouling rate becomes excessive; if the pressure is too low, it will result in insufficient permeate flow and possibly poor membrane rejection. In the case of RO and NF membranes, osmotic pressure can play a significant role. This phenomenon is a characteristic of all ionic solute and some organic materials. It is a function of both the solute species and concentration in solution and represents backpressure, resisting the passage of permeate through the membrane. Osmotic pressure can be significant as evidenced by the fact that normal seawater (35,000 ppm TDS) requires 400 psi pressure simply to overcome its osmotic pressure. (Osmotic pressure plays almost no role in MF and UF technologies.)
Flow rate requirements for pumps are dictated by the characteristic of the membrane device. In most cases, a portion of the concentrate stream exiting the membrane device is recycled back to the feed pump in order to maintain turbulent flow. These recycle flow rates are typically at least as high as the permeate rates for spiral wound configurations and often at least an order of magnitude greater in the case of capillary fiber and tubular devices. These latter membrane elements require relatively high flow rates to maintain turbulence as a result of the more open area required to minimize fouling.
For each type of foulant, the optimum form of pretreatment may be different. So, it is extremely important that the feedwater be thoroughly analyzed to identify prime candidates that may cause fouling. In many cases, on-site testing is recommended to optimize the pretreatment technologies.
Figure 3 lists generic pretreatment technologies that are appropriate for the various categories of foulants.
Because Murphy's Law always prevails and no amount of pretreatment will completely eliminate fouling, cleaning ultimately will become necessary. As a rule of thumb, the time to clean is dictated by a 10 percent reduction either in product water flow or product water quality. As a result of concentration polarization with NF and RO membranes, a drop in product water quality may signal the onset of fouling before a drop in permeate flow. As an interim procedure, it is possible to temporarily reduce the back pressure on the system to increase the velocity of feed flow over the surface in order to scrub the fouling materials off the membrane surface. Most large RO systems are equipped with automatic units that accomplish this at a preset frequency such as once a day for 10 minutes.
A well-designed membrane processing system includes a clean-in-place system. An appropriately sized tank containing the cleaning solution is equipped with a pump to direct the cleaning solution into the membrane system. The permeate and concentrate flows are returned to the cleaning system storage tank to accomplish recirculation of the solution. Depending on the nature of the foulant and type of cleaner used, the solution may be heated and the membrane exposed to high velocities, soaking, etc. In addition, the cleaning solution may be re-used several times before discarding.
Cleaning chemicals are as varied as there are kinds of foulants. Figure 4 lists examples of various chemical systems used to remove specific foulants.
The increased applications of membrane processing technology to new areas of water purification, waste treatment and food and chemical processing have presented new challenges with regard to the design and operation of these systems to minimize the potential for fouling. An understanding of the nature of fouling as well as the effects of system design and cleaning schemes is critical. Additionally, testing is recommended to optimize chemical cleaning systems and procedures for every new application.