This paper describes two successful applications of ultrafiltration (UF) and reverse osmosis (RO) membrane technology used to reclaim water, reduce waste water discharge costs. The first case history covers oily waste water treatment at a major aluminum beverage can manufacturing plant. The second case describes treatment of waste water from a multi-plating bath section of a major wheelchair manufacturing facility. The mechanisms of separation effected by both UF and RO are described at the molecular level and descriptions of the membranes and hardware design employed in thislow-energy process are included. Ultrafiltration is discussed as a means of removing oil wastes from the waste stream in the first case study. Reverse Osmosis is presented as a means of removing dissolved inorganic contaminents from the waste stream in both cases to provide water for reuse in the manufacturing processes. Long term successful installations are resulting in rapid acceptance of this emerging technology and virtually all areas of industry which use water for processing are presently, or soon will be utilizing membrane for fluid separations.
Crossflow Membrane Filtration
Membrane filtration is the separation of the components of a pressurized fluid performed by polymeric membranes. The openings in the membrane matrices (pores) are so small that significant fluid pressure is required to drive liquid through them; the pressure required varies depending on the size of the pores. Reverse osmosis (RO) membranes have the smallest pores, while microfiltration (MF) membranes have the largest pores, and hence require the least pressure.
Normal particle filtration has historically not been run in a crossflow design, 'perpendicular flow' may be the most appropriate term, with the solution to be filtered approaching the filter media in a perpendicular direction. The entire influent stream passes through the filter media (except for the particulate matter filtered out). Filter media can be either of the 'depth' type or the 'surface' or 'barrier' type. In this perpendicular flow design, there are only two streams, the influent and the effluent. separation is effected in the micron range or greater with certain depth filter media achieving as low as a nominal one micron separation.
Crossflow membrane filtration is fundamentally different in design, in that the influent stream is separated into two effluent streams, known as the permeate and concentrate. Permeate is that fraction which has passed through the 'semi-permeable' membrane. The concentrate is that stream which has been enriched in the solutes or suspended solids, which have not passed through the membrane. Membrane is a surface filtration media, which affects separation in the ionic and molecular range as well as the macromolecular and particle range. The advantage of this design approach is that the membrane media is operated in a continuously self-cleaning mode, with solutes and solids swept away by the concentrate stream, which is running parallel to the membrane. Hence the term 'crossflow,' and the advantage of this approach for separation in the micron, sub-micron, molecular and ionic range.
Figure 1 shows an artist’s conceptual view of crossflow membrane filtration. The feed solution flows under pressure between the two membranes depicted, and as it flows over the membranes, permeate passes through. The rate of permeation is known as flux. The concentrated fraction of the stream exits through the same flow channel as the feed enters, carrying away concentrated solutes and particles.
Mechanism of Separation
Figure 2 shows the mechanism of RO. Pores in this membrane class are so small they have not been resolved even by scanning electron microscopy. The pore diameter is theorized to be somewhat smaller than twice the thickness of the pure water layer which occurs over the membrane.(1) This pure water layer is void of ions, and through this exclusion 'rejects' the ions, preventing them from permeating the membrane. Note that very small molecular weight organics and uncharged solutes may pass through the membrane along with the solvent (usually water).
Reverse osmosis, sometimes called hyperfiltration, is perhaps the most technically complex class of membranes, affecting separation at the micromolecular and ionic size range. Pore sizes ranging from 5 to 15 Angstroms affect separation of the solutes down to 150 molecular weight (MW) and often lower. As with ultrafiltration (UF), RO has rarely been viable unless in a crossflow mode.
Ultrafiltration is perhaps easier to understand, and as Figure 3 shows, the mechanism of separation is largely based on size, (sieving). The pores are too large to affect desalting, and ions readily pass through. For the UF membrane shown, organic molecules under 1,000 molecular weight may permeate the membrane. It should be noted that all discussions where molecular weights are used as an indicator of size assume that the molecule is a polysaccharide of that molecular weight. Ultrafiltration is generally defined as affecting separation in the 0.002 to 0.2 micron range. This is perhaps more usefully described as the 500 to 300,000 molecular weight cut-off (MWCO) range, requiring pore sizes of from 15 to 1,000 Angstroms.
Microfiltration membranes in the crossflow mode work on the same principle as UF, although materials small enough to be present in dissolved form will generally permeate the MF membrane, while suspended solids are rejected. Microfiltration has generally come to be regarded as affecting separation in the 0.02-2.0 micron range. Microfiltration has historically been run in the perpendicular flow mode, requiring disposal of the membrane media due to blinding by the retained material. Interest in MF crossflow has been increasing lately and development of the use of this membrane class in various crossflow configurations is also increasing.
Figure 4 shows a chart of separation from the ionic to the particulate range. You will notice that the membrane classes overlap each other somewhat, since neither the marketplace nor the technologies have been able to develop sharp distinctions. To give one an idea of the wide range of membranes available, Osmonics® has over 30 different membrane types made from over six different polymers spanning the RO to MF range.
Membrane Systems for Discharge and Water Reclamation
As water resources become more limited and waste discharge becomes increasingly expensive, the concept of water reclamation or water 'reuse' is gaining acceptance in industry. Depending on the cost of water and sewer, and even more expensive costs such as surcharges and hauling costs, the concept of water reuse is often already economically justified. This is especially true in cases where the waste is considered 'hazardous,' requiring hauling and disposal at specifically classified hazardous waste disposal sites.
Conventional waste process equipment has been applied for purposes of precipitation/filtration, flocculation/filtration, clarification, etc. to remove particular dissolved solids and/or insoluble, filterable solids In order to remove dissolved solids (e.g., most sodium salts), ion exchange or crossflow membrane technologies such as reverse osmosis or ultrafiltration are necessary.
There are often cases where the waste stream can be concentrated to a point where the material can actually be recovered for reuse. In such cases, depending on the value of the recovered material, the economics of water reuse technologies become quite attractive.
Ultrafiltration can be used to concentrate or separate larger organic compounds. Reverse osmosis can be used to concentrate inorganic and small MW organics wastes. Ion exchange can only be used as a less effective alternative to reverse osmosis due to its inability to remove organic compounds. Ion exchange is usually also significantly more costly to operate than an RO system on organic streams with relatively high salt levels.
A brief capsule of two case histories in which reverse osmosis or reverse osmosis/ultrafiltration has been successfully utilized to reclaim water and minimize waste water follows.
A major West Coast wheelchair manufacturer generates a nickel and chrome plating rinse waste stream. Their manufacturing facility was located in an industrial area which had waste water discharge limitations on total dissolved solids as well as some of the particular inorganic compounds, most notably boron. Since boron (as boric acid) is a major component in the nickel plating process, its presence in the waste stream caused major concern. The limitations on boron were set due to the fact that the waste water treatment facility was intended for agricultural irrigation, and boron can be detrimental to the growth of plants. Stringent discharge standards had to be met for any effluent streams discharged from the plating process. (2)
A conventional wastewater treatment system was designed to remove nickel, chrome, etc. as a precipitated sludge, leaving a relatively high total dissolved solids stream containing mostly sodium sulfate from the various pH adjustments in the conventional process with NaOH and H2S04. Since this total dissolved solids level was too high for water reuse in the rinses, reverse osmosis was identified as the best approach to reduce the total dissolved solids, recover the water for reuse in the manufacturing process and totally eliminate the discharge.
The RO unit concentrates the dissolved solids by a factor of ten and sends one-tenth of the flow to an evaporation system, greatly reducing energy consumption and operating expense compared to a single large evaporator to handle the total stream. Figure 5 shows the RO unit.
The RO machine processes the waste stream at 55 gallons per minute (gpm), returning 50 gpm of permeate (pure water) to the plant for reuse and sending 5 gpm to the evaporator for further concentration. The salt concentration in the feed to the RO unit ranges from 2,000 to 6,000 micromhos and the permeate produced for return to the plating rinse tanks ranges from 200 to 500 micromhos. Figure 6 is a schematic illustrating where the RO unit is located in the overall waste treatment system.
Since discharge of this particular waste stream was completely prohibited, the economics of the installed waste water treatment system can only be compared to alternate disposal methods associated with waste water treatment equipment to accomplish the necessary objectives. However, simply taking into consideration the value of the water recovered, the RO system pays for itself. The operational costs of the RO machine are approximately $3 per thousand gallons of water (3.8 m3) which is quite comparable to the cost of water and water disposal. Discharge of the effluent stream from the conventional waste treatment system would have been banned without the use of RO, evaporation or ion exchange to polish the effluent. The most economical method to produce a closed-loop system was to couple RO with evaporation to produce a sludge for disposal and 50 gpm of water for reuse, saving the company over $50,000/year in water and sewer cost alone.
The closed-loop system is completely operational and has been performing well for over 12 months.
An aluminum can manufacturer has placed a facility in an area, which is in close proximity to their major customer, a national brewer. Since the brewery was located in a relatively remote area, the can manufacturing facility was built on a site which lacks a sewer access, and therefore requires the hauling of all wastes.
The various rinsing stages that the aluminum cans must go through during the manufacturing process are major contributors to the total waste stream. This stream ends up with metal fines, cutting oils and dissolved salts, which must be dealt with as pollutants.
The waste treatment system design includes a bag filter system to remove the metal fines, followed by an ultrafiltration unit to concentrate the oily waste to a point where it can be sent to an oil stripper. Utilizing this method allows the oil to be removed from the system and recovered for use as a fuel source, aiding the overall economics of the waste water treatment system.
The permeate from the ultrafiltration unit, containing the small MW organics and dissolved salts which pass through the larger pore UF membrane, is sent to the reverse osmosis unit for concentration. The permeate from the RO unit is reused in the process as can rinse water while the concentrate is hauled away. Figure 7 is a system schematic showing this process.
Initial testing showed that the Sepa®-CA polymer UF membrane was superior to Sepa-PS polymer (the two most common UF membranes) in that it resisted fouling and therefore maintained its permeate rate (flux) better. A custom manufactured CA membrane with optimum pore size for both flux and the required separation proved to be well suited for the job. After several years of successful operation, a new UF membrane, Sepa-O (VF) was developed and tested in the system. This membrane proved to be superior in both flux maintenance and separation (3). The membrane in the entire machine was gradually changed over to Sepa-O (VF) and even more efficient operation resulted.
The total system is designed to handle 20 gpm on a 20 hour per day basis. Overall, only 10% of the waste stream must be hauled. Based on the original cost of $0.075 per gallon hauling charges, the UF/RO system saves over $400,000/yr. in hauling costs. The system is extremely cost-effective since operating the total RO/UF system costs only $0.005 per gallon, or less than 10% of the hauling charges. Total installed cost of the waste treatment system was under $300,000, showing a payback within one year.
Large potential savings exist where process disposal problems must be alleviated. Crossflow membrane is a proven technology well suited to not only solve disposal problems, but reclaim water for reuse in the process. In many cases, membrane technology is much more cost-effective than alternative methods.
For a more in-depth study of the application of crossflow membrane processing, see the paper entitled, 'Application of Membrane Technology for the Recovery and Reuse of Water' in these same proceedings.