Preengineered Systems Don't Always 'Fit'
Custom-designed nanofiltration system solves space and process challenges
Whether it is an issue of limited space, material compatibility or just challenging feed water, there always will be a need for custom-engineered solutions in the water treatment industry. Lee Co. of Nashville, Tenn., first looked at preengineered reverse osmosis (RO) systems from its local water treatment equipment dealers. The company quickly realized that this approach would not fit a particular situation.
We can all acknowledge the benefits of preengineered equipment, which can be ordered directly from a catalog. Generally, they offer economical prices and quick deliveries. The trend that has developed over last several years is for manufacturers to focus their sales and marketing efforts on this type of "standard" equipment. The benefits of such a product line also include being able to process orders quickly and efficiently and often it is easier to sell from a technical standpoint because they have been preengineered.
In the rush to standardize, some projects that require a more custom solution are finding it more difficult to source equipment that meets their needs. This is exactly what a nationally recognized design/build firm, Lee Co. of Nashville, Tenn., recently encountered while designing Alliance Hospital, a state-of-the-art cardiac specialty hospital to be built in Odessa, Texas.
In most cases, hospitals do not require "ultrapure" water. A filtered and softened potable water supply usually is sufficient. Therefore, architects may design a water treatment utility room that is sized just large enough to house a minimal amount of equipment; typically, a set of multimedia filters and water softeners. This assumes that there is a relatively good feed water source available. Unfortunately, feed waters that are common to this area of Texas can have a total dissolved solids (TDS) level that is nearly three times higher than the 500 mg/L maximum for potable water and may include more than 28 grains of total hardness.
Having identified the need to reduce the TDS and hardness in the feed water, Lee Co. first looked at preengineered reverse osmosis (RO) systems from its local water treatment equipment dealers. The company quickly realized that this approach would not fit this particular situation.
The two main issues were:
- The preengineered RO system would not fit into the available space. Most preengineered RO systems tend to be long and narrow. This type of design is economical, using fewer--but longer--membrane element housings, which minimizes the number of costly housing end cap assemblies and manifold connections. For example, a typical preengineered 70-gpm RO system (the size they required) will use five-element long housings, resulting in an overall system length of more than 18 feet (26 feet with membrane removal clearance area added). When the space is not available for these long preengineered systems, a custom engineered solution is required.
- RO product water would be too corrosive for the distribution piping system. RO membranes reject a high percentage of organic matter and inorganic ions, and thus produce a highly purified product water (permeate) that generally is considered "aggressive" due to its low TDS level and acidic pH. As a result, it is important to scrutinize the materials that the RO permeate comes in contact with, not only within the water treatment equipment but also throughout the distribution equipment and piping system. For example, RO permeate can leach copper from copper piping systems. This is undesirable, possibly causing higher than normal heavy metal levels in the supplied water and the premature deterioration of the piping material. A different process that provided a non-corrosive purified product water was needed.
The solution was to engineer a custom-designed, compact nanofiltration (NF) membrane treatment system, which was pretreated with multimedia filtration and water softeners. While similar to RO, NF treatment--a relatively new niche membrane technology--is unique because it has special rejection characteristics that allow more of the dissolved solids from the feed water to go through to the product side of the membrane. An NF system is capable of producing a high-quality product water that is not over-purified. This technology typically is used for water softening and general TDS reduction such as the Alliance Hospital application required.
Figure 1 details the complete system design. Sodium bisulfite is injected before the NF system for chlorine removal, and sodium hypochlorite is injected into the NF permeate line to provide a chlorine residual in the distribution storage tank. The dechlorination/rechlorination is required because, just like most RO membranes in use today, the PA-type NF membrane elements used in this design are not chlorine tolerant.
The resulting NF-based system provided a projected final product water quality that had virtually no hardness, near neutral pH, good chlorine residual and a TDS level that was well within the potable water limits--but not too low to cause corrosion and/or leaching issues. A summary of the design flow and water quality is shown in Table 1.
The key to fitting all the equipment into the available space was to engineer a compact NF skid, because all of the other component sizes were somewhat fixed. Taking into account the prefilter housing, control panel and the high-pressure pump that are all mounted on the front of the NF skid, the minimum length possible for this type of equipment is about 10 feet. Using that as the basis of design, two-long membrane element housings were used to maintain an overall length of just less than 10 feet (which is about half the preengineered RO length). This unique NF skid design utilizes 10 membrane element housings, as shown in Figure 2.
As Figure 3 illustrates, all the required clearances and walkways were maintained by using a portable Clean-In-Place (CIP) skid (Figure 4), in lieu of a permanently mounted CIP system. The portable CIP skid easily can be moved when necessary to gain access.
Whether it is an issue of limited space, material compatibility or just challenging feed water, there always will be a need for custom-engineered solutions in the water treatment industry. While it is important for equipment suppliers to keep their focus on providing value to the marketplace with standard preengineered products, they also need to maintain the ability to support the more demanding opportunities to be considered a full-service company.