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.
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