Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1999-09-28
2003-03-25
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S638000, C210S639000, C210S650000, C210S651000, C210S661000, C210S900000
Reexamination Certificate
active
06537456
ABSTRACT:
TECHNICAL FIELD
My invention relates to a method for the treatment of water in membrane based water treatment, purification, and concentration systems, and to apparatus for carrying out the method. In one embodiment, my invention relates to methods for feedwater pretreatment and for operation of reverse osmosis (“RO”) equipment, which achieve increased solute rejection, thereby producing very high purity (low solute containing) product water, while significantly increasing on the on-stream availability of the water treatment equipment.
BACKGROUND
A continuing demand exists for a simple, efficient and inexpensive process which can reliably provide water of a desired purity, in equipment which requires a minimum of maintenance. In particular, it would be desirable to improve efficiency of feed water usage, and lower both operating costs and capital costs for high purity water systems, as is required in various industries, such as semiconductors, pharmaceuticals, biotechnology, steam-electric power plants, and nuclear power plant operations.
In most water treatment systems for the aforementioned industries, the plant design and operational parameters generally are tied to final concentrations (usually expressed as total dissolved solids, or “TDS”) which are tolerable in selected equipment with respect to the solubility limits of the sparingly soluble species present. In particular, silica, calcium sulfate, and barium sulfate often limit final concentrations achievable. In many cases, including many nuclear power plants and many ultrapure water plant operations, boron or other compounds of similarly acting ampholytes have a relatively low rejection across membranes in conventionally operated RO systems, and may dictate design or operating limitations. More commonly, the presence of such compounds result in sufficiently poor reverse osmosis product water, known as permeate, that additional post RO treatment is required to produce an acceptably pure water. In any event, to avoid scale formation and resulting decreases in membrane thruput, as well as potential deleterious effects on membrane life, the design and operation of a membrane based water treatment plant must recognize the possibility of silica and other types of scale formation, and must limit water recovery rates and operational practices accordingly. In fact, typical RO plant experience has been that declines in permeate flow rates, or deterioration of permeate quality, or increasing pressure drop across the membrane, require chemical cleaning of the membrane at regular intervals. Such cleaning has been historically required because of membrane scaling, particulate fouling, or biofouling, or some combination thereof. Because of the cost, inconvenience, and production losses resulting from such membrane cleaning cycles, it would be advantageous to lengthen the time between required chemical cleaning events as long as possible, while nevertheless efficiently rejecting undesirable ionic species and reliably achieving production of high purity permeate.
Since the introduction and near universal adoption of thin film composite membranes in the mid to late 1980s, the improvements in RO technology have been evolutionary in nature. Operating pressure needed to achieve desired rejection and flux (permeate production rate per unit of membrane surface area, commonly expressed as gallons per square foot of membrane per day, or liters per square meter per day) has been slowly reduced, while average rejection of thin film composite membrane has improved incrementally.
Historically, brackish water RO systems have been limited in their allowable recovery and flux rates by the scaling and fouling tendencies of the feedwater. It would be desirable to reduce the scaling and fouling tendencies of brackish feedwater to the point where recovery limits would be dictated by osmotic pressure, and where flux rates can be increased substantially, compared to limits of conventional brackish water RO systems.
From a typical end user's point of view, several areas of improvement in RO technology—chlorine tolerance being one of them—are still sought. Thin film composite membranes, at least partly due to their surface charge and characteristics, are relatively prone to biological and particulate fouling. With certain feedwaters, particularly from surface water sources, membrane fouling and the frequent cleaning required to combat fouling can present some arduous, costly, and time-consuming operational challenges.
It is known that rejection of weakly ionized species, such as total organic carbon (“TOC”), silica, boron, and the like, is significantly lower than rejections for strongly ionized species as sodium, chloride, etc. Since the efficiency of post-RO ion exchange is largely determined by the level of the, weak anions present in the RO permeate, it would be advantageous to remove (reject) as many weak anions as possible in the RO unit operation. In other words, by removing (rejecting) more silica (and boron) in the RO step, a higher throughput is achievable in the ion-exchange unit operation that follows the RO unit.
With the exception of an RO process disclosed in U.S. Pat. No. 4,574,049, issued Mar. 4, 1986 to Pittner for a REVERSE OSMOSIS SYSTEM, which reveals a double pass (product staged) RO system design, carbon dioxide typically represents the largest fraction of the anion load in RO permeate. However, a multiple pass RO configuration provides very little benefit under conventional RO system operating conditions, since the carbon dioxide content of permeate stays at the same (absolute) level and represents an even bigger fraction of the anion load. High rejection of weak anions in a single pass RO system is, therefore, considered to be another area where significant improvement is still sought.
In addition to increasing the rejection of the weakly ionized species, the increased rejection of strongly ionized species is also desired.
Recovery rate, or volumetric efficiency, is another parameter where improvements in RO system performance would be advantageous. A typical RO system operates at about 75 percent recovery, where only 75 percent of the incoming feed to RO is used beneficially, and the rest (25 percent) is discharged. With water becoming both more scarce and more costly throughout the world, increasing the maximum achievable recovery rate in an RO system is an important goal.
Increasing the operating flux is always important to end users, as increased flux reduces capital costs.
Simplification and cost reduction for post-RO unit operations is also sought by end users. This is because allowable levels of impurities in ultrapure water have continually decreased with the ever tightening design rules in semiconductor device geometry. Thus, lower contaminant levels in the ultrapure water system are required. As a result, the cost and complexity of the post-RO system components have dramatically grown in recent years.
High purity water processing procedures and the hardware required for carrying them out are complex and expensive. In fact, the regenerable mixed bed ion, exchange system represents, by far, the most expensive (and complicated) single unit operation/process in the entire ultrapure water treatment system. Thus, significant improvement in the characteristics of the RO treated water would appreciably reduce the overall ultrapure water system cost and complexity.
I am aware of various attempts, some in high purity water treatment applications and some in wastewater treatment applications, in which an effort has been made to improve the efficiency of the rejection of certain ions which are sparingly soluble in aqueous solution at neutral or near neutral pH. Such attempts are largely characterized by conventional hardness removal and then raising the pH with chemical addition. One such method is shown in U.S. Pat. No. 5,250,185, issued Oct. 5, 1993 to Tao, et al., for REDUCING AQUEOUS BORON CONCENTRATIONS WITH REVERSE OSMOSIS MEMBRANES OPERATING AT HIGH PH. In a preferred embodiment, his invention provides use of a conventiona
Fortuna Ana
Goodloe, Jr. R. Reams
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