The key to a successful operating installation is quite straightforward. Choosing the correct membrane type and configuration, properly designing the machine, and providing adequate pretreatment is all that is required. Of course, that is easier said than done. Systems operating on process fluids have higher fouling tendencies than a typical pure water installation; therefore, they require a more conservative design. Proper application and pilot testing is the solution to determining the proper design and it is not wise to cut costs at these critical steps. Money spent on proper testing will result in the lowest overall expenditure and a final system designed for optimum performance.
The first step in proceeding with a process application is to perform an application test. This test is a feasibility study that is run on process samples with as little as 30 gallons of process solution. Prior to and during the test, the greatest challenge is to determine the best membrane type for the application. Normally, three to five different membrane configurations are tested. They are evaluated for chemical tolerance, mechanical suitability, cleanability, flux, separation, and price. The membrane type can usually be narrowed down to one or two types based on the performance data. This membrane type will then be used in the pilot study. Membrane usually accounts for 15 to 40 percent of the price of a crossflow filtration machine and, as it needs to be periodically replaced, careful selection is important.
Pilot studies are usually performed at the customer site using a slipstream of the actual process fluid. Doing so allows the pilot machine to be subjected to the normal variations in the make-up of the process fluid. Data obtained during this operation will be more realistic. Similar results can be expected from the full-sized production machine. Fouling, which directly affects the flux through the membrane can be determined as well as cleaning procedures for removing the foulant. All of this information is very important and is incorporated into the design of the final crossflow filtration unit assuring a custom unit properly designed for optimum performance with specific process fluids.
In the following case study, reverse osmosis proved to be the solution for one major US mid-western automotive manufacturer's waste problem. In the manufacturing of automobiles, the parts go through a rigorous series of steps prior to the electrodeposition of paint. These steps prepare the metal surfaces of the parts to ensure a proper coating of the paint. The parts are dipped into treatment solutions of a cleaner (phosphate and chromate). They are also submerged in various rinse baths and spray rinsed during the process. These rinsing steps avoid cross contamination between process steps.
As can be seen in Figure 1, these steps require a large quantity of pure water. The water in these rinse stages eventually makes its way to the on-site waste treatment facility. As production at the plant increased, so did the rinse flows to waste treatment. The flow rate finally grew to a point that the waste treatment facility could not effectively treat the volume of water it received. Based on the purity requirements of the various rinse streams and the cost involved to produce the rinse water, it was apparent that the best area to implement reverse osmosis to reduce the volume of water flowing to waste treatment was at the seventh stage. This stage made the most economic sense because the RO system will produce a high quality permeate that is suitable for rinse water, reducing the DI make-up water requirements by over 50 percent.
The project began with an application test on a 55-gallon drum of solution from rinse stage seven. The application test scanned three different membrane types to determine the best overall performer on flux, separation, fouling tendency, chemical compatibility, and price. This portion of the test took roughly three hours to complete. Osmonics®' Sepa®-ST10 membrane showed the best performance and was then put through a series of more rigorous tests for another three hours. These tests were performed to determine any potential fouling tendencies or any maximum concentration limitations prior to installing the pilot unit.
The next step was to create the pilot system ordered using the Sepa-ST10 membrane. The pilot unit that was installed was the Osmo®-80B PES (Process Evaluation System). It is a very compact skid-mounted system that holds up to eight 4-in x 40-in sepralators (membrane elements). Everything is included on the skid for proper operation. Installation only requires making the plumbing connections to the feed flange and to the two outlet flanges. Also, 230/460 VAC power must be hooked up to the main disconnect in the electrical enclosure. The PES unit should operate continuously for two to three months to obtain the necessary data to ensure proper operation of the full-scale production unit.
The objective of the RO machine was to purify the overflow of rinse water from rinse stage seven to less than ten fS. This stage is the first rinse stage directly following the chromate step and is contaminated mostly with chromate salts. Chromate salts can be quite difficult to treat because they have a tendency to foul the membrane under certain pH, temperature and concentration conditions. The permeate (purified water) will be sent to the final rinse stage. The RO has reduced the need of virgin DI water by 40 gpm (151.4 Lpm) and reduced the volume of rinse water from this rinse step going to waste treatment by over 40 percent. Figure 2 shows where the RO was incorporated into the system.
Crossflow filtration systems in general are very easy to operate and this RO machine was no exception. After start-up and operator training, the system requires minimal attention: fifteen minutes a day to take operational data; fifteen minutes a week to change prefilters; and two hours a month to chemically clean the system. The system actually reduced the overall labor and chemical usage at the plant because the labor and chemical-intensive steps of DI regeneration for the make-up water are less frequent.
DI water at this facility costs about $5/1000 gallons to produce; therefore, recovering the DI water provided a quick payback. In fact, it was estimated that the RO system is going to save the auto manufacturer over $100,000 per year just by reclaiming the DI water previously sent to waste treatment. The labor savings for the plant were not included in the justification for installing the system, but have been substantial. The on-site waste treatment facility is also saving money due to the reduced volume of water from the rinsing step as well as reduced loading from less frequent DI regenerations.
Crossflow filtration is no longer a treatment method that only provides a method to purify city or well water supplies. It provides unique separation and purification capabilities in the treatment of water for reuse and waste minimisation. The crossflow mode of filtration continuously 'sweeps' the membrane surface clean of impurities, which allows for many hours of trouble free and continuous operation between chemical cleanings.
Successful development of a process application requires thorough application testing and pilot testing on a slipstream of the process solution. The result of good development is a system that will operate reliably and consistently, meet application objectives, reclaim valuable resources, and in many cases provide excellent payback.