By Peter S. Cartwright
For the last 30-plus 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, overpopulation and looming shortages of water supplies of acceptable quality have created an attitude of conservation and environmental responsibility throughout the world.
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 stewardship.
The membrane separation technologies of microfiltration, ultrafiltration, nanofiltration and reverse osmosis possess characteristics which make them attractive as water reuse processes. These include:
- Continuous process, resulting in automatic and uninterrupted operation;
- Low-energy utilization involving neither phase nor temperature changes;
- Modular design—no significant size limitations;
- Minimum of moving parts, with low maintenance requirements;
- No effect on form or chemistry of the contaminant;
- Discrete membrane barrier to ensure physical separation of contaminants;
- No chemical additions required to effect separation.
It is virtually impossible to accurately design an industrial wastewater treatment system utilizing membrane technologies without a complete and comprehensive testing program. This is required to identify the best membrane polymer and element configuration and to optimize the system design and operating conditions. In this paper, fundamentals of these technologies are introduced; then engineering design requirements and testing details are 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. illustrates crossflow filtration compared to conventional filtration.
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 one micron (100 to 10,000 angstroms). By definition, MF does not remove dissolved materials. MF is illustrated in Figure 2.
- UF is the membrane process which removes dissolved non-ionic solute, typically organic materials (macromolecules). UF 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. UF pore sizes are usually smaller than 0.01 micron (100 angstroms) in size. UF is depicted in Figure 3.
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 versus 20 percent). These membranes have MWCOs for non-ionic solids below 1,000 Daltons. NF is illustrated in Figure 4.
- 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 MWCOs in the range of 50-100 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 and the ionic characteristics of the contaminants. 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 difference in salt rejection characteristics as a function of ionic valance. As indicated earlier, this difference is significant with NF membranes.
In all cases, the greater the degree of contaminant removal, the higher the pressure requirement to effect this separation. In other words, RO, which separates the widest range of contaminants, requires an operating pressure typically on an order of magnitude higher than MF, which removes only suspended solids.
The water passage rate through membrane (to generate permeate) is known as 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 and other features 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. These element configurations are described below and illustrated in Figure 6.
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 (six to 25 mm). The membrane is typically coated on the inside of the tube and the feed solution flows under pressure through the interior (lumen) from one end to the other, with the permeate passing through the wall and collected outside of the tube.
Capillary (hollow) fiber
These elements are similar to the tubular element in design, but are smaller in diameter; are usually either unsupported membrane polymers or ceramics and require rigid support on each end provided by an epoxy ‘potting’ of a bundle of the fibers inside a cyclinder. Feed flow is either down the interior of the fiber (lumen feed) or around the outside of the fiber (outside-in).
This element is constructed from an envelope of sheet membrane wound around a permeate tube that is perforated to allow removal of the permeate. Water is purified by passing through one layer of the membrane and, following a spiral pattern, flowing into the permeate tube. It is by far the most common configuration in water purification applications.
Plate and frame
This device incorporates a sheet membrane stretched over a frame to separate the layers and facilitate collection of the permeate which is directed to a collection 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 its cost. 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.
(Part 2 in our June issue)
About the author
Peter S. Cartwright, President of Cartwright Consulting Co., Minneapolis, is a registered Professional Engineer in Minnesota. He has been in the water treatment industry since 1974, has authored almost 100 articles, presented over 125 lectures in conferences around the world and has been awarded three patents. Cartwright has chaired 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; fax (952) 854-6964; email: [email protected] or website: www.cartwright-consulting.com.