Consumers and their suppliers have realized that while five-gallon bottles of water taste great, there are shortcomings:
The market’s response was a family of systems that filtered water as needed by connecting a water cooler with multiple filtration steps to the water supply coming into the building. The point of use (POU) water business is divided into two classes: bottleless coolers (a.k.a flat-tops and automatically replenished bottle systems (see Figures 1 and 2).
Both designs follow similar patterns of water treatment: microporous filtration to remove silt, dirt and bacteria and carbon filtration to remove chlorine and volatiles. Depending on the salt content and taste of the water, most companies also offer reverse osmosis (RO). This article looks at filtration and its impact on consumer perceptions of purity and taste.
The filtration spectrum
Webster’s Dictionary defines filtration as a process of separating solids, liquids or gases using a perforated or porous membrane. Membrane filtration covers a spectrum that ranges from removing materials of 100 µm to 0.0002 µm. The processes are summarized in Table 1.
Typical photomicrographs of membranes are shown in Figures 3 and 4. The UF/NF/RO membranes at the right are manufactured differently than the microporous membranes. The Figure 4 example could be any one of the three process membranes. Since they represent a continuum of pore size, there are not definitive cut-offs where one technology ends and the next begins.
When membranes are used in process, water normally flows into the microporous membrane (dead-ended). For the other processes, water flows along the membrane surface (crossflow) although there are some applications within the pharmaceutical and biotechnology industries where microporous filtration is also used in crossflow (see Figure 5).
When used in the dead-ended mode, water flows into the membrane. Silt, debris and bacteria (depending on the cut-off of the membrane) are removed, captured on the surface or within the interstices of the membrane structure. Water and all dissolved solids pass through the membrane.
The other membrane processes are used in a crossflow or tangential flow mode. The membranes are very thin with a spongy layer integral with a very tight skin. Any material that passes through a pore in the membrane surface freely passes out as product. The pores are very fine and limited in number. Attempts to flow dead-ended would cause a rapid accumulation of material too large for the pores. Filtration ceases quickly. This crossflow mode requires a continuous flow to drain (for water applications) of the rejected species.
Microporous filtration (Figure 6) is a process for suspended solids, emulsified oils and bacteria in liquids like water or organic solvents. Separation takes place based on size exclusion only; is independent of temperature and the specific weight of components. The process normally operates at low pressures; i.e., 10-50 psig, with high fluxes (liquid flow per unit area).
Absolute versus nominal filters
Microporous filters are typically rated in microns, ranging from 100 µm down to 0.1 µm. This reflects the particle size to be retained by the membrane. All filtration ratings are not equal, however. The industry has numerous definitions of membrane cut-off. Membranes are usually classified by absolute or nominal ratings. According to the Filters Manufacturing Council’s Technical Service Bulletin 89-5R2:
“The two most popular reported media ranges are a nominal micron rating (50 percent) and an absolute rating (98.7 percent). A nominal rating usually means the filter’s media can capture a given percentage of particles of a stated size…An absolute micron rating is a single-pass test and is usually obtained by passing a test fluid containing particles of a known size through a small, flat sheet of filter media. Any particles that pass through the media are captured and measured.”
Since all POU water systems have a minimum of microporous filtration, it is essential for the user to understand and question his supplier about the filtration capabilities of the filtration being used. A one µm nominal microporous filter will pass far more particles of that size than a one µm absolute filter. Typically, a one µm nominal microporous filter will be the equivalent of a five-10 µm absolute filter.
Ultrafiltration and nanofiltration
Ultrafiltration (Figure 7) is the next-more selective membrane process. It is a selective fractionation process that uses pressures up to 150 psig. Separation takes place as a function of size and/or shape of the species. Ultrafiltration flux is dependent on the velocity of the liquid along the surface of the membrane, the pressure drop from the process side (incoming) to the permeate side (product), temperature and sometimes pH and ionic strength. Ultrafiltration is used to concentrate and/or remove suspended solids, colloids, bacteria, viruses and solutes of molecular weights greater than 1,000 Daltons. Ultrafiltration was developed for (and is used primarily in) the pharmaceutical and biotechnology markets and the food industry to concentrate, separate and purify proteins in solution. While the process is seeing new uses in large-scale water treatment facilities, it currently has very limited application to POU.
Nanofiltration (Figure 8) is normally selected when ultrafiltration and RO are not the correct choice for separation. Nanofiltration can perform in separation applications such as demineralization, color removal and desalination. In the concentration of organic solutes, suspended solids and polyvalent ions, the permeate (what comes through the membrane) contains monovalent ions and low molecular weight organic solutions like alcohol. Like ultrafiltration, nanofiltration has seen limited use in POU.
The key crossflow membrane process for use in POU is RO (Figure 9), a highly efficient technique for dewatering process streams, concentrating/separating low molecular weight substances in solution or cleaning wastewater. It has the ability to concentrate/remove all dissolved and suspended solids from water. While typically used in large scale desalination of brackish and sea water, RO has extensive use in POU where total dissolved solids (TDS) are high, typically less than 150 ppm.
Table 2 shows typical rejections by RO membranes and the percentage of those anions and cations in the water that will be consumed. In addition, RO will typically remove virtually all bacteria, dyes and sugars, though vendors normally do not guarantee a sterile permeate for POU applications.
Purity versus taste
The U.S. Food and Drug Administration (FDA) defines purified water as, “Water that has been produced by distillation, deionization, reverse osmosis or another process that meets the definition of purified water in the U.S. Pharmacopoeia (USP) 23.” Therefore, by definition, water from an RO system is more purified than water from a microporous filtration system. However, this does not mean that water produced from these processes is more pure. A dictionary definition of purity is being undiluted with extraneous material. That is normally taken as free of impurities such as bacteria, chlorine and volatile off-taste or odorous materials. Here, both the basic filtration system and the RO systems will produce comparable products. The FDA specification for purified water limits is shown in Table 3.
Ultimately it all comes down to the individual consumer and their taste preference. Individuals who usually drink water produced by RO find filtered water salty even with low levels of TDS. Conversely, individuals who drink and enjoy water with low to moderate levels of TDS find water produced by reverse osmosis flat. Whether one water is better for one’s health than the other is the subject of numerous studies.
Who needs RO?
One of the most frequently asked questions by consumers is whether they need RO. The answer is largely one of personal taste. When the TDS are below 150 ppm, most consumers will choose basic filtration. The salts in the water increase the mouth feel and give water its taste. Some waters (e.g., mineral water) have very high TDS levels, yet taste good.
There will always be a value to the consumer in bottled water, especially for hand-held bottles. However, POU filtration, whether a bottle-less or a replenished bottled system, offers the consumer limitless quantities of pure, cool (or hot) water that will taste every bit as good as the bottled water.
- Cheryan, M. Ultrafiltration and Microfiltra-tion Handbook. Second Edition.
- Rozelle, L. T. Reverse Osmosis Process, Theory and Membranes, Parts I and II. Culligan Technology 1
- Lykins, B.W. Jr., R. M. Clark and J. A. Goodrich. Point-of-Use/Point-of-Entry for Drinking Water Treatment. Chelsea, Mich: Lewis. 1992.
- Filters Manufacturing Council. Technical Service Bulletin 89-5R2.
About the author
Stephen Messinger joined Pure 1 after over 30 years marketing and operational experience in membrane filtration. That included Romicon, Inc., which he helped found, Amicon, Millipore and Ionics. Messinger is a graduate of Columbia University, where he attained bachelor’s and master’s degrees in chemical engineering. He may be reached via email at Messinger@Pure1.com, phone (978) 975-1800 or fax (978) 975-1400.
About the company
Pure 1 Systems was started in 1988 to develop answers to the problems with five-gallon bottles of water: lifting, spilling, storage, reservoir contamination and expense. The firm has sold over 60,000 systems for offices and homes around the world. Pure 1 is headquarted in Lawrence, Massachusetts. Visit the website www.pure1.com or call (978) 975-1800 for more information.