By Peter S. Cartwright, P.E., CWS-VI
For the last 30 years, protection of the environment has been steadily moving up on the priority list of concerned citizens. Issues such as the hole in the ozone layer, 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.
Responsible manufacturers are beginning to realize the potential economic benefits of resource recovery, as well as the somewhat intangible rewards accrued by an image of responsible environmental stewardship.
The membrane separation technologies of microfiltration, ultrafiltration, nanofiltration and reverse osmosis possess characteristics that make them attractive as recycle and recovery processes, including:
- Continuous process, resulting in automatic and uninterrupted operation;
- Low energy utilization involving neither phase nor temperature changes;
- Modular design with no significant size limitations;
- Minimum of moving parts, with low maintenance requirements;
- No effect on form or chemistry of the contaminant;
- Discrete membrane barriers to ensure physical separation of contaminants;
- Separation effected with no chemical additions required.
It is virtually impossible to accurately design a wastewater reuse system utilizing membrane technologies without a complete and comprehensive testing program. This is necessary to identify the best membrane polymer and element configuration and to optimize the system design and operating conditions.
In this article, fundamentals of these technologies are introduced, engineering design requirements addressed and testing details described.
Membrane technologies are based on a process known as crossflow or tangential-flow filtration which allows for continuous processing of liquid streams. In this process, the bulk solution flows over and parallel to the membrane surface and, because this system is pressurized, water is forced through the membrane. The turbulent flow of the bulk solution over the surface minimizes the accumulation of particulate matter.
The crossflow membrane separation technologies of Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse Osmosis (RO) are defined by some membrologists on the basis of pore size. Other experts prefer to use definitions based on the removal function, as follows:
- MF is utilized to remove submicron suspended materials on a continuous basis. The size range is from approximately 0.01 to 1 micron (100 to 10,000 angstroms). By definition, micro-filtration does not remove dissolved materials.
- UF is the membrane process which removes dissolved non-ionic solute, typically organic materials (macromolecules). Ultrafiltration membranes are usually rated by Molecular Weight Cut-Off (MWCO), the maximum molecular weight of the dissolved organic compound that will pass through the membrane into the permeate stream. Ultrafiltration pore sizes are usually smaller than 0.01 micron (100 angstroms).
- The above processes (MF and UF) separate contaminants on the basis of a sieving process; that is, any contaminant too large to pass through the pore is rejected and exits in the concentrate stream.
- NF can be considered loose reverse osmosis. It rejects dissolved ionic contaminants but to a lesser degree than RO. NF membranes reject multivalent salts to a higher degree than monovalent salts (for example, 99 percent vs. 20 percent). These membranes have molecular weight cut-offs for non-ionic solids below 1,000 daltons.
- RO produces the highest quality permeate of any pressure driven membrane technology. Certain polymers will reject over 99 percent of all ionic solids, and have molecular weight cut-offs in the range of 50 to 100 daltons.
Both NF and RO membranes reject salts utilizing a mechanism that is not fully understood. Some experts endorse the theory of pure water preferentially passing through the membrane; others attribute it to the effect of surface charges of the membrane polymer on the polarity of the water. Monovalent salts are not as highly rejected from the membrane surface as multivalent salts; however, the high rejection properties of the newer thin film composite RO membranes exhibit very little differences in salt rejection characteristics. As indicated earlier, this difference is significant with NF membranes.
In all cases, the greater the range of contaminant removal, the higher the pressure requirement to effect this separation. In other words, reverse osmosis, which separates the widest range of contaminants, requires an operating pressure typically on an order of magnitude higher than microfiltration, which removes only suspended solids.
The water passage rate through membrane (to generate permeate) is known as the flux rate. It is a function of applied pressure, water temperature and in the case of NF and RO (and to a limited extent UF), the osmotic pressure of the solution under treatment. Flux rate is usually measured as GFD (gallons per square foot per day) or LMD (liters per square meter per day).
Increasing the applied pressure will increase the permeate rate; however, a high flow of water through the membrane will promote more rapid fouling. Membrane element manufacturers usually provide limits with regard to the maximum applied pressures to be used as a function of feed water quality. Heating the water will also increase the permeate rate, but this requires significant energy and is generally not considered practical.Table 1 summarizes the various properties of these technologies.
To be effective, membrane polymers must be packaged into a configuration commonly called a device or element. The most common element configurations are tubular, capillary fiber, spiral wound and plate and frame. The element configurations are described below and illustrated in Figure 1.
Manufactured from ceramics, carbon, stainless steel, or a number of thermoplastics, these tubes have inside diameters ranging from ¼ inch up to approximately one inch (6 to 25 mm). The membrane is typically coated on the inside of the tube and the feed solution flows through the interior (lumen) from one end to the other, with the permeate passing through the wall and collected on the outside of the tube.
Capillary (Hollow Fiber)
These elements are similar to the tubular element in design, but are smaller in diameter, are usually unsupported membrane polymers and require rigid support on each end provided by an epoxy “potting” of a bundle of the fibers inside a cylinder. Feed flow is either down the interior of the fiber (lumen feed) or around the outside of the fiber.
This element is constructed from an envelope of sheet membrane wound around a permeate tube that is perforated to allow collection of the permeate. Water is purified by passing through one layer of the membrane and flowing into the permeate tube. It is by far the most common configuration in water purification applications.
Plate and Frame
This element incorporates sheet membrane stretched over a frame to separate the layers and facilitate collection of the permeate which is directed to a center tube.
From the perspective of cost and convenience, it is beneficial to pack as much membrane area into as small a volume as possible. This is known as packing density. The greater the packing density, the greater the membrane area enclosed in a certain sized device and generally the lower the cost of the membrane element. The downside of the high packing density membrane elements is their greater propensity for fouling. Table 2 compares the element configurations with regard to their packing densities.
The vast majority of membrane system failures occur as the result of membrane fouling. This fouling is usually caused by one or more of the following mechanisms:
- Suspended solids in the feed stream resulting from incomplete feed water filtration.
- Precipitation of insoluble salts or oxides resulting from concentration effects within the membrane device; and/or
- Biofilms resulting from microbiological activity.
These mechanisms cause the membrane surface to become coated with fouling materials that build up in layers. As the layer thickness increases, the flow rate across the membrane surface and immediately adjacent to it is reduced, thereby encouraging more settling of suspended solids and increasing the fouling layer thickness—a vicious cycle.
With nanofiltration and reverse osmosis membranes which reject ionic contaminants, fouling usually creates a phenomenon known as concentration polarization. The fouling layers inhibit the free movement of the feed stream away from the membrane surface and as salts are rejected from the membrane their concentration at the surface is higher than in the bulk solution (that portion above the fouling layer). Since ionic rejection is always a percentage of the salts concentration at the surface of the membrane, the permeate quality decreases as a result of concentration polarization and this phenomenon may actually indicate the presence of foulants before a reduction in permeate rate is detected. The increased salts concentration at the membrane surface also promotes precipitation of those salts whose solubility limit is exceeded.
Note that the feed stream enters the system (or membrane element) and as the stream passes along and parallel to the surface of the membrane under pressure, a percentage of the water is forced through the membrane polymer producing the permeate stream (see Figure 2).
The percentage of feed stream flow that becomes permeate is defined as recovery. Contaminants are prevented from passing through the membrane based on the polymer characteristics. This contaminant-laden stream exits the membrane system (or element) as the concentrate stream, also known as brine or reject.
The permeate rate of a given membrane element cannot be changed without varying the applied pressure or temperature. Recovery, however, can be easily changed by varying the feed flow rate to the element and this is one of the variables that can be controlled by the system designer.
The effect of recovery on system performance is important. As recovery is increased, the flow rate of the concentrate stream diminishes; all contaminants that are rejected by the membrane and concentrated in the concentrate stream become more concentrated. The effect of increasing recovery upon the concentration of contaminants in the concentrate stream is illustrated in Table 3 and Figure 3. For wastewater treatment and water reuse applications, the minimum recovery is usually at least 80 percent.
The advantage of operating systems at high recoveries is that the volume of concentrate is small and the flow rate of the feed pump is smaller; the potential disadvantages are numerous:
- Higher concentration of contaminants is more likely to result in fouling.
- In nanofiltration and reverse osmosis applications, the concentrated salts solution results in high osmotic pressure, requiring a higher-pressure pump and a more pressure resistant system.
- As higher recoveries reduce the quantity of concentrate to be discharged, the higher concentration of the concentrate stream may present discharge problems.
The issue of recovery is definitely application specific: most water purification applications—those treating water to be purified for some down-stream application (drinking, product manufacturing, rinsing, etc.)—generally operate at relatively low recoveries, not exceeding 85 percent even for the largest applications. In general, most water purification applications involve feed water conductivities that are relatively low; the one exception is seawater desalination. Usually, the larger the system, the higher its recovery.
It is possible to completely close off the concentrate line, through the use of a valve, thereby using the membrane as a conventional or dead-end filter, forcing 100 percent of the water through the membrane with occasional periods when the concentrate valve is opened to allow the crossflow feature to reduce the concentration of contaminants at the surface of the membrane. Some membrane elements of tubular, capillary fiber and plate and frame configuration can also be backwashed, which involves running permeate (or another high quality water source) backwards into the element to dislodge suspended materials from the surface of the membrane.
As the newest membrane technology application, and one with huge potential, Membrane BioReactor (MBR) technology merits special mention.
For wastewaters containing biodegradable contaminants, the traditional treatment method is to encourage the application of bacteria to break down the contaminant. This encouragement can take the form of adding oxygen (in the case of aerobic treatment), some sort of a mechanical matrix (for bacterial attachment), mixing and other approaches intended to maximize the metabolic activity of these microorganisms. MBR involves utilizing an MF or UF membrane to filter the treated water to remove particles and microorganisms. Compared to other biological processes, MBR offers the following advantages:
- High-quality effluent, almost free from solids;
- The ability to disinfect without the need for chemicals;
- Complete independent control of HRT (Hydraulic Retention Time) and SRT (Sludge Retention Time);
- Reduced sludge production;
- Process intensification through high biomass concentrations;
- Treatment of recalcitrant organic fractions and improved stability of processes such as nitrification;
- Ability to treat high strength wastes;
- Smaller system footprint.
The device configurations most commonly used today are capillary fiber and plate and frame, although tubular devices are becoming more widely used. The most common biological treatment is aerobic and, typically, air is bubbled into the treatment tank. A very popular approach is to immerse the membrane element in the treatment tank and either allow the hydrostatic head of the solution to provide the driving force or to use a pump to pull the permeate through the membrane (or both). In this case, air bubbles are also directed over the surface of the membrane in an effort to reduce fouling.
Another design involves pumping the water through the membrane system external to the treatment tank, and yet another uses a separate tank for membrane processing downstream of the biological treatment tank. Additional designs and configurations are sure to appear as MBR technology becomes more widely used (see Figure 4).
Slowly but surely, the mindset of water recovery and reuse is reaching global proportions. Membrane technologies represent some of the most successful processes to realize this mindset. As the need increases and the costs become more acceptable, these technologies will become the lynchpins of water reuse and recovery.
About the author
Peter S. Cartwright, president of Cartwright Consulting Co., Minneapolis, is a registered professional engineer in several states. He has been in the water treatment industry since 1974 and has published more than 100 papers and articles on related issues. Cartwright has been chairman of several WQA committees and task forces and has received the organization’s Award of Merit. A member of the WC&P Technical Review Committee since 1996, his expertise includes such high technology separation processes as reverse osmosis, ultrafiltration, microfiltration, electrodialysis, deionization, carbon adsorption, ozonation and distillation. He can be reached at (952) 854-4911, (952) 854-6964 (fax), email: CartwrightConsul@cs.com or website: www.cartwright-consulting.com