WO2008091577A1

WO2008091577A1 - Systems and methods to treat wastewater

Info

Publication number: WO2008091577A1
Application number: PCT/US2008/000783
Authority: WO
Inventors: John F. Bossler; Todd A. Hook; Clarke D. Veach; Jeffrey Martin
Original assignee: Siemens Water Technologies Corp.
Priority date: 2007-01-22
Filing date: 2008-01-22
Publication date: 2008-07-31

Abstract

Systems and methods for treating wastewater are described. A wastewater treatment system includes a source of wastewater saturated with at least one target species and a separator in fluid communication with the source of wastewater capable of forming a secondary wastewater stream. A source of an alkali compound is disposed upstream of and in fluid communication with the separator which may facilitate the precipitation at least a portion of the at least one target species from solution in the wastewater. A source of acid is disposed downstream of and in fluid communication with the separator and in fluid communication with the secondary wastewater stream, which may facilitate the solubilization of at least a portion of the at least one target species into solution. A reverse osmosis unit may be disposed downstream of and in fluid communication with the source of acid.

Description

SYSTEMS AND METHODS TO TREAT WASTEWATER

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Serial No. 60/885963, entitled "A METHOD TO TREAT LOW PH PHOSPHATE POND WATERS BY REVERSE OSMOSIS," filed on January 22, 2007, which is herein incorporated by reference in its entirety for all purposes.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a wastewater treatment system and method, and more particularly, to a wastewater treatment system and method utilizing a membrane filtration unit.

2. Discussion of Related Art

A well known technique for removing contaminants from certain forms of wastewater is reverse osmosis. Reverse osmosis is a separation process that uses pressure to force a fluid through a semi-permeable membrane that may retain certain contaminants on one side and allow fluid with a reduced concentration of contaminants to pass to the other side. Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.

Waste process water produced by phosphate fertilizer or phosphoric acid manufacturing operations, known in the industry as process water or pond water, is typically highly acidic and typically contains various dissolved constituents, some of which may be at or above their saturation point.

Because pond water is often highly saturated with contaminants, early attempts at using reverse osmosis have resulted in rapid and irreversible fouling of the reverse osmosis membranes, rendering the technique, as practiced in these attempts, impractical for the treatment of pond water.

Methods of treatment involving double liming, followed by air stripping, have been used for this type of wastewater. This process adds lime in two stages, increasing the pH of the wastewater to levels as high as 10-12, to promote precipitation of fluoride species and phosphate species, among other things, followed by high pH air stripping to remove ammonia. Double liming processes often suffer from several disadvantages, including that treated water may often not meet discharge criteria, and the process can be very time consuming, in some cases requiring time periods on the order of days or weeks.

SUMMARY OF INVENTION

In accordance with one or more embodiments, the present invention relates to a system and method of treating wastewater. According to one embodiment, a wastewater treatment system comprises a source of wastewater saturated with at least one target species, a separator in fluid communication with the source of the wastewater, constructed and arranged to separate at least a portion of the target species from the wastewater, thereby forming a secondary wastewater, and a source of an alkali compound disposed upstream of and in fluid communication with the separator, constructed and arranged to supply an alkali compound to the wastewater. The system further includes a source of acid disposed downstream of and in fluid communication with the separator and in fluid communication with the secondary wastewater, configured and arranged to supply acid to the secondary wastewater, a membrane filtration unit disposed downstream of and in fluid communication with the source of acid, and a pH control system constructed and arranged to maintain a pH of the wastewater at the separator at below about 2.5.

In some embodiments, the pH control system is constructed and arranged to maintain pH of the wastewater at the separator in a range from about 0.1 to 0.2 pH units higher than a pH of the wastewater in the source of wastewater, and in some embodiments, the pH control system is constructed and arranged to further maintain a pH of the secondary wastewater at the membrane filtration unit at least at about 0.1 pH units below the pH level at the separator. In some embodiments, the target species may comprise at least one of a potassium salt and a sodium salt of fluorosilicic acid.

According to some embodiments, a wastewater treatment system comprises a source of wastewater saturated with at least one of a potassium and a sodium salt of fluorosilicic acid and containing at least one of calcium fluoride and calcium phosphate in solution and a separator in fluid communication with the source of wastewater, constructed and arranged to separate at least a portion of the at least one of a potassium and a sodium salt of fluorosilicic acid from the wastewater, thereby forming a secondary wastewater. The system further includes a source of an alkali compound disposed upstream of and in fluid communication with the separator, constructed and arranged to supply an alkali compound to the wastewater to precipitate at least a portion of the at least one of a potassium and a sodium salt of fluorosilicic acid in solution in the wastewater while maintaining at least one of calcium fluoride and calcium phosphate in solution, a source of acid disposed downstream of and in fluid communication with the separator and in fluid communication with the secondary wastewater, configured and arranged to supply acid to the secondary wastewater stream to solubilize at least a portion of the at least one of a potassium and a sodium salt of fluorosilicic acid, and a membrane filtration unit disposed downstream of and in fluid communication with the source of acid.

Some embodiments of a wastewater treatment system may comprise a source of wastewater saturated with at least one target species, a source of acid disposed downstream of and in fluid communication with the source of wastewater, a membrane filtration unit disposed downstream of and in fluid communication with the source of acid, and a pH control system constructed and arranged to maintain a pH level of the wastewater at an inlet of the membrane filtration unit in a range from about 0.1 to about 0.2 pH units below a pH level of the wastewater in the source of wastewater.

Another embodiment of the present invention is directed to a method for treating wastewater comprising providing wastewater saturated with an inorganic salt and having an initial pH level, precipitating at least a portion of the inorganic salt from the wastewater by adjusting the pH level of the wastewater to below about 2.5 and by a range of from about 0.1 to about 0.2 pH units above the initial pH level of the wastewater, thereby forming a secondary wastewater, adjusting the pH level of the secondary wastewater downward by at least about 0.1 pH units, and passing the secondary wastewater through a membrane filtration unit.

Some methods according to the present invention may comprise providing wastewater comprising calcium salts and saturated with at least one of a potassium and a sodium salt of fluorosilicic acid, precipitating at least a portion of the at least one of - A - the potassium and the sodium salt of fluorosilicic acid from the wastewater by adjusting a pH of the wastewater to a pH level sufficient to maintain the calcium salts in solution. The method may further comprise separating at least a portion of the precipitated portion of the at least one of the potassium and the sodium salts of fluorosilicic acid from the wastewater, thereby forming a secondary wastewater; solubilizing a remaining portion of the at least one of the potassium and the sodium salt of fluorosilicic acid by adjusting the pH of the secondary wastewater, and passing the secondary wastewater through a membrane filtration unit.

Some methods according to this invention may comprise providing wastewater saturated with at least one of a potassium and a sodium salt of fluorosilicic acid, maintaining the solubility of the at least one of a potassium and a sodium salt of fluorosilicic acid by adjusting a pH level of the wastewater, and passing the wastewater through a reverse osmosis unit.

The advantages, novel features, and objects of the invention will become apparent from the following detailed description with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing, nor is every embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the drawings:

FIG. 1 is a block diagram of a system in accordance with one embodiment of the invention;

FIG. 2 is a block diagram of a system in accordance with another embodiment of the invention;

FIG. 3 is a schematic diagram illustrating a computer system upon which one or more embodiments of the invention may be practiced; and FIG. 4 is a schematic illustration of a storage system that may be used with the computer system of FIG. 3 in accordance with one or more embodiment so the invention.

DETAILED DESCRIPTION

This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having," "containing," "involving," and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

The invention is directed generally to the treatment of wastewater. Wastewater treatment systems are generally used to treat wastewater from various sources, including agricultural and industrial sources, in order to remove contaminants from the wastewater, possibly to recover commercially valuable components and/or reduce the toxicity of the wastewater, possibly so that it may be released to the environment. For example, co-pending application, Publication Number 2007/0138093 Al to Bossier et al., incorporated herein by reference in its entirety for all purposes, discloses a system for treating industrial wastewater, comprising multiple reverse osmosis systems in series.

The treatment of wastewater saturated with dissolved contaminants, or supersaturated with these contaminants with membrane filtration systems is not typically done. Direct treatment of this type of wastewater with membrane filtration techniques is often not feasible because contaminants may precipitate out of the wastewater and form crystals that may clog the pores or form a scale on the membranes of a membrane filtration system, thereby reducing or sometimes interrupting the flow of wastewater through the system. Frequent cleaning and/or replacement of the membranes may then be required, which may drastically increase the operating costs of the system. Similarly, wastewater saturated with dissolved contaminants, or supersaturated with these contaminants, may be difficult to treat with other systems such as those utilizing ion exchange techniques, again due to the scaling that may occur on the ion exchange media and/or the rapid exhaustion of the ion exchange media that could make frequent regeneration of the ion exchange media necessary.

For the purposes of this specification, a fluid is understood to be "saturated" with a component when it contains a maximum amount of that component capable of being dissolved at its present temperature and pressure. A fluid is "supersaturated" with a certain component when the maintenance of that component in soluble form in the fluid system is thermodynamically un-favored, such that a fluid system with that component in solution is thermodynamically unstable or metastable. As used herein, a "saturated fluid" includes a fluid at or near its saturation point of a component as well as supersaturated with that component.

Some embodiments of the present invention are directed to the removal of contaminants and/or target species from wastewater such as those produced by ammonium phosphate or phosphoric acid manufacturing plants. Waste process water produced by phosphate fertilizer or phosphoric acid manufacturing operations, known in the industry as process water or pond water, is typically highly acidic and typically contains various dissolved constituents such as fluoride, ammonia, silica, sulfate, calcium, heavy metals, phosphate, magnesium, colloidal matter, organic carbon, phosphoric acid, sulfuric acid, hydrofluoric acid, and hydrofluorosilicic acid. It may be desirable to reduce or eliminate toxic or harmful contaminants prior to discharge of the wastewater into the environment.

The present invention encompasses methods and apparatus for removing contaminants from pond water that could reduce the efficiency of, or render inoperable, further purification operations such as separation of impurities from the pond water by a membrane filtration unit such as, for example, a reverse osmosis unit by clogging or forming scale on a membrane filter.

The present invention also encompasses methods and apparatus for preventing the precipitation of contaminants from pond water that could reduce the efficiency of, or render inoperable, further purification operations such as separation of impurities from the pond water by a filtration membrane unit such as, for example, a reverse osmosis unit by clogging or forming scale on a membrane filter. Pond water may in some instances be an acidic solution. In some instances, pond water may exhibit a pH level of less than about 2. A typical source of pond water may exhibit a pH of from about 1.8 to about 2. In some instances, pond water may exhibit a pH lower than 1.8. Pond water may be saturated or nearly saturated with at least one of sodium or potassium salts of hydrofluorosilicic acid. Attempts to separate impurities from such pond water using reverse osmosis can be inefficient, due to the scaling and fouling of the reverse osmosis membranes with salts which can precipitate from the pond water. Thus, if reverse osmosis is to be used to remediate such pond water, it may be desirable to first remove at least a portion of the salts which may precipitate and foul the reverse osmosis membranes, or at least to prevent these salts from precipitating. In some applications, it may be desirable to remove only a small amount of the salts which may precipitate and foul the reverse osmosis membranes, or only certain specific salts in order to limit the amount of solids or "sludge" that might be produced and might have to be disposed of or further treated.

One embodiment of a method for the treatment of wastewater according to the present invention involves directing wastewater which may be saturated or nearly saturated with one or more target species to a treatment system by means of, for example, a corrosive resistant pump and corrosive resistant piping. The wastewater may be subjected to one or more pre-treatment operations. These pre-treatment operations may include, for example, filtration to remove suspended solids or organic matter from the wastewater, the addition of chemical compounds which may facilitate downstream operations, or other pre-treatment operations known in the art of water purification.

The pH of the wastewater may be initially measured to establish a baseline initial pH level. The pH of the wastewater may then be adjusted upward by, in some embodiments, a range of from about 0.1 to about 0.2 pH units above the initial pH value. In some methods, where the wastewater may contain calcium salts, it is preferable to maintain the pH of the wastewater to below about 2.5 to avoid the precipitation of calcium salts such as calcium fluoride and calcium phosphate. Avoiding the precipitation of calcium salts such as calcium fluoride and calcium phosphate may limit the amount of sludge produced. This pH adjustment step may be performed by contacting the wastewater with an alkali compound. The alkali compound may be introduced to the wastewater in a liquid, slurry, or dry form. Non- limiting examples of alkali compounds which may be used include sodium hydroxide, ammonium hydroxide, potassium hydroxide, calcium hydroxide, and ammonia. In some methods, an agitator may be used to apply mechanical energy to a portion of the treatment system containing a portion of the wastewater stream in order to facilitate mixing of the alkali compound with the wastewater.

In some methods, the pH of the wastewater may be monitored by one or more pH sensors which may be arranged at various locations and may, but need not, provide a signal to a pH control system which may control an alkali compound addition system to provide sufficient alkali compound to the wastewater to attain a desired pH. The pH sensors may measure pH directly or indirectly, such as, for example, by titration, or by the measurement of the concentration of one or more target ions. The alkali compound may be added intermittently or continuously. The alkali compound may be added at various locations including, for example, at the inlet of the system, after a pre-treatment step, if any, before a > separation vessel that may be present in the system, or directly into a precipitation vessel. The target pH to which the wastewater may be adjusted may depend on the particular composition and initial pH level of the water.

Once the pH of the pond water has been adjusted, salts, such as, for example, potassium and/or sodium salts of fluorosalicic acid may precipitate. Precipitation may take place in any of various types of vessels, such as a separator vessel, or in the piping of the treatment system. The separator vessel may be, for example, a precipitation vessel, a clarifier, or a centrifuge. In some embodiments, mechanical agitation may be applied to fluid in the separator by a mechanical agitator mechanically coupled to the separator. Precipitation may take place at various temperatures, and at various times depending on a desired result. The time allotted for the precipitation to take place may be dependent on the temperature, the composition of the wastewater, and the amount of precipitate desired to be formed in a particular application. In some methods, pH adjusted wastewater may be held in a vessel for a time sufficient for a desired amount of precipitation to occur, and in other methods, precipitation may occur as the wastewater continuously flows through the treatment system.

At least a portion of any precipitate formed may be separated from the wastewater solution, thereby forming a secondary wastewater. In some methods, substantially all precipitate may be removed to form the secondary wastewater, while in others only a portion of the precipitate may be removed. The secondary wastewater may be separated from the precipitated solids according to separation techniques known in the art. Non-limiting examples include the utilization of a filter to remove precipitate from the wastewater stream, utilization of a precipitation vessel where precipitates settle and the secondary wastewater is formed from supernatant drawn from the vessel, or utilization of a centrifuge to separate precipitated solids from the liquid phase. In some methods the precipitated solids may be recovered and sent for further purification, or to be recycled for use in other operations.

The pH of the secondary wastewater may be further adjusted, for example, in some methods, to a level at least about 0.1 pH units below the pH level of the wastewater at the separator. In some methods, the pH of the secondary wastewater may adjusted to a level at least about 0.2 pH units below the pH level of the wastewater at the separator. In some methods, the pH of the secondary wastewater may be adjusted to a level substantially the same as the pH of wastewater present in the source of wastewater. This pH adjustment may result in at least a portion of any remaining precipitated solids in the secondary wastewater being solubilized (placed into solution). This pH adjustment step may be performed by contacting the secondary wastewater with an acid. The acid may be introduced to the wastewater in a liquid, slurry, or dry form. Non-limiting examples of acids which may be used include hydrochloric acid, hydrofluoric acid, and sulfuric acid. In some methods, an agitator may be used to apply mechanical energy to a portion of the treatment system containing a portion of the secondary wastewater stream in order to facilitate mixing of the acid with the secondary wastewater.

In some methods, the pH of the secondary wastewater may be measured and/or monitored by a pH sensor. The pH sensor may provide a signal to a pH control system which may control an acid addition system to provide sufficient acid to the secondary wastewater to attain the desired pH. The acid may be added intermittently or continuously. In some methods, the secondary wastewater may be held in a vessel for a time sufficient for a desired quantity of precipitates remaining in the secondary wastewater to solubilize, and in other methods the secondary wastewater may flow continuously through the treatment system after addition of acid. The target pH to which the secondary wastewater may be adjusted may depend on the particular composition and pH of the secondary wastewater.

In some methods, after the pH of the secondary wastewater is adjusted, it may be passed through a separation system such as a membrane filtration unit. Examples of membrane filtration units include microfϊltation units, nanofϊltration units, and reverse osmosis units. The membranes of any reverse osmosis system utilized are preferably compatible with solutions having a pH of approximately 2. An example of one such a reverse osmosis membranes is FILMTEC BW30-365 membrane, available from FilmTec, a subsidiary of The Dow™ Chemical Company, Midland, Michigan. Similarly, membranes utilized in nanofϊltration units, if utilized for filtering the secondary wastewater, should also be compatible with highly acidic solutions with pH levels of about 2. Non-limiting examples of such nanofϊltration membranes include FILMTEC NF-90-400 and FILMTEC NF-270-400, also available from FilmTec. If a reverse osmosis system is utilized, it may include a single, or in some methods, multiple stages. The reverse osmosis system may be operated at various temperatures and pressures according to the requirements of a particular application. Liquid that is rejected from the reverse osmosis system or systems, herein referred to as retentate, may be subject to further treatment, or may be returned to the source from which the pond water was drawn, or may be disposed of in accordance with typical procedures. In some methods, the retentate may be subject to further pH adjustments, for example, to pH levels above about 2.5, in order to facilitate precipitation and recovery of additional dissolved compounds. In some methods the retentate may be subjected to further separation, including separation by an additional reverse osmosis system.

Liquid passing through the membrane or membranes of the reverse osmosis system, herein referred to as permeate, may also in some methods be subject to further post-treatment steps. The post-treatment may include additional steps of pH adjustment and/or reverse osmosis treatment or other separation or purification operations in order to further remove remaining dissolved compounds from the permeate, such as, for example, those disclosed in co- pending application, Publication Number 2007/0138093 Al to Bossier et al, incorporated herein by reference in its entirety for all purposes. Additional separation operations may be performed at pH levels higher or lower than approximately 2.5 in order to facilitate the precipitation of different species of dissolved solids from the permeate. In some methods, the permeate may be treated in order to recover acid values, such as, for example, phosphoric acid, or in some cases, additional separated solids. In some methods, sufficient contaminants may be removed from the permeate in the post treatment step so that the permeate may be released to the environment, for example into a river, a lake, or a municipal drainage system. Adjustment of the pH of the permeate to approximately 7 may be performed in some methods before releasing the permeate to the environment.

In accordance with another method according to the present invention, the pH of the wastewater to be treated may be lowered prior to being passed through a membrane filtration system such as a reverse osmosis unit absent a precipitation step. In some embodiments, this may involve lowering the pH of the wastewater by an amount sufficient to convert salts present in the wastewater into their acidic forms. The target pH to which the wastewater may be adjusted may depend on the particular composition and pH of the wastewater. In some embodiments, the pH is lowered to prevent precipitation of salts onto the membranes of a membrane filtration system. The pH may be lowered to a level sufficient to maintain the solubility of salts present in the wastewater. Maintaining the solubility of salts present in the wastewater may be performed by adjusting the pH to a level at which the salts are converted to their acid forms. In some embodiments the pH may be lowered by a range of from about 0.1 to about 0.2 pH units. In some embodiments the pH may be lowered by an amount greater than about 0.1 pH units.

This may be accomplished through a method including the use of a pH sensor arranged to measure the pH of the wastewater. The pH sensor may provide a signal to a pH control system which may control an acid addition system to provide sufficient acid to the wastewater to attain the desired pH. The acid may be added intermittently or continuously. In some methods, the wastewater may be held in a vessel for a time sufficient for a desired quantity of precipitates that may be present in wastewater to solubilize, and in other methods the wastewater may flow continuously through the treatment system after addition of acid.

The present invention encompasses various embodiments of systems that may be utilized to perform methods of treatment of wastewater, including, but not limited to those methods described above. Systems in accordance with the present invention may include at least one source of wastewater, and a subsystem for pumping the wastewater through the treatment system. Pumps may be arranged at the inlet of the system, and/or throughout the system. Systems may include a pretreatment subsystem, a source of alkali and a source of acid and their associated dispensers, such as injectors, which may be arranged to inject acid and/or alkali compounds into the treatment system to contact wastewater at desired locations.

Wastewater treatment systems in accordance with the present invention may include one or more separator vessels where precipitation of solids from the wastewater may take place. Mechanical agitators may be disposed in locations throughout the system to aid in the mixing of acid or alkali compounds with the wastewater, and/or to aid in the precipitation or solubilization of target species.

The systems may also include one or more membrane filtration units, such as, but not limited to, microfiltration units, nanofiltration units, reverse osmosis units, and combinations thereof. These units may be located downstream of a separator, or downstream of an acid addition location or both.

Some systems in accordance with the present invention may include post treatment systems, which may include for example, additional pH adjustment subsystems, additional separators, precipitation subsystems, and/or further filtration systems. A pH control subsystem capable of controlling acid and alkali addition systems and coupled to pH sensors located at various locations throughout the treatment system may be present in some embodiments of systems in accordance with the present invention. In other embodiments, pH adjustments to the wastewater or secondary wastewater may be made by manually controlling the addition of acids or alkali compounds to the system.

FIG. 1 illustrates one embodiment of a system according to the present invention, generally referenced by reference number 200. The system 200 may include a wastewater source 202. This wastewater source may be a source of pond water as pond water is defined above. The wastewater may be saturated or nearly saturated with one or more target species. The one or more target species may include one or more inorganic salts, such as, for example, sodium and/or potassium salts of fluorosilicic acid. The pond water may also contain calcium salts such as calcium fluoride and calcium phosphate in solution. The pond water may be highly acidic. In some embodiments, the pond water may exhibit a pH level of less than about 2. A source of pond water in some embodiments may exhibit a pH of from about 1.8 to about 2. In some embodiments, the pond water may exhibit a pH lower than 1.8.

A separator 206 may be disposed downstream of and in fluid communication with the source of wastewater 202. The separator 206 may be constructed and arranged to separate at least a portion of one or more target species from the wastewater, thereby forming a secondary wastewater. The separator may be, for example, a precipitation vessel, a clarifier, or a centrifuge. A mechanical agitator (not shown) may be mechanically coupled to the separator 206 in some embodiments. Mechanical agitation systems may also be present in the lines upstream and downstream of the separator.

The separator 206 may separate at least a portion of solids which may precipitate from the wastewater, thereby forming a secondary wastewater. This secondary wastewater may be directed from the separator to a reverse osmosis system 208 disposed downstream of and in fluid communication with the separator 206. It is understood that other membrane filtration units as previously described may be substituted for reverse osmosis unit 208. Disposed between the separator 206 and the reverse osmosis unit 208 may be sensor 220C and/or acid source 216.

Reverse osmosis system 208 may be disposed downstream of and in fluid communication with the separator 206 and the source of acid 216. Reverse osmosis system 208 may include a single or multiple reverse osmosis units. Reverse osmosis system 208 may separate the secondary wastewater stream into a retentate, which may have a higher concentration of at least one impurity than the secondary wastewater and a permeate which may have a lower concentration of at least one impurity than the secondary wastewater.

A source of an alkali compound 214 may be disposed upstream of and in fluid communication with the separator 206. The source of alkali compound 214 may be constructed and arranged to supply an alkali compound to the wastewater stream through an addition system, such as, for example, an injection system, thereby causing at least a portion of at least one target species in solution in the wastewater to precipitate. The alkali compound may in some embodiments be in a liquid form, a slurry form, or a solid form. The alkali compound may in some embodiments include, for example, sodium hydroxide, ammonium hydroxide, potassium hydroxide, or ammonia.

The source of alkali compound 214 may be controlled by the pH control system 218. The pH control system 218 may control the source of alkali compound 214 in response to signals generated from sensors including in some embodiments, sensors 220A-220E. In alternate embodiments sensors 220A-220E may be all of one type, or may be different types of sensors including, for example, pH sensors or ion-specific concentration sensors. The pH control system 218 may control the source of alkali compound such that alkali is introduced into the wastewater stream intermittently or continuously. The pH control system 218 may receive signals from any one or more of the various sensors 220A-220E in order to maintain a predetermined pH at various locations in the system, such as in the separator 206 or in the reverse osmosis system 208. In some embodiments, sufficient alkali may be introduced into the wastewater stream such that wastewater in the separator may have a pH in a range of from about 0.1 to about 0.2 pH units higher than the incoming wastewater. In alternate embodiments, the source of alkali may introduce an alkali compound into a wastewater in a line leading to the separator 206, or may introduce an alkali compound directly into the separator 206, or both.

Acid source 216 may be disposed downstream of and in fluid communication with the separator 206 and in fluid communication with the secondary wastewater. Acid source 216 may be configured and arranged to supply acid through an addition system to the secondary wastewater, thereby solubilizing at least a portion of one target species which may be present in a solid phase and/or in suspended form in the secondary wastewater stream. The acid compound may in some embodiments be in a liquid form, a slurry form, or a solid form. The acid compound may in some embodiments include, for example, hydrochloric acid, phosphoric acid, hydrofluoric acid, or sulfuric acid. The source of acid 216 may be controlled by the pH control system 218. The pH control system 218 may control the source of acid 216 in response to inputs from sensors including, in some embodiments, sensors 220C and/or 220D. The pH control system 218 may control the source of acid such that acid is introduced into the secondary wastewater stream intermittently or continuously. Acid may be introduced into a line leading from the separator to the reverse osmosis unit or into an input of the reverse osmosis unit. In some embodiments, sufficient acid may be introduced into the secondary wastewater stream such that secondary wastewater entering the reverse osmosis system 208 may have a pH of at least about 0.1 pH units lower than the pH of the secondary wastewater exiting the separator. This pH may in some embodiments be approximately the same as an initial pH of the wastewater in the wastewater source.

Control system 218 may be manual or automatic. Control system 218 may respond to signals from timers (not shown) and/or sensors 220A-220E, and possibly additional sensors (not shown) positioned at any particular location within the system. For example, a sensor positioned in separator 206 may indicate less than optimum conditions in the separator. The sensors may monitor one or more operational parameters such as pressure, temperature, pH, one or more characteristics of the mixed liquor suspended solids, and/or one or more characteristics of the treated wastewater or effluent. Controller 218 may respond to conditions deemed unacceptable by generating a control signal causing an alarm to sound or causing all or a portion of the precipitate and/or wastewater to be removed from the separator. Similarly, a sensor (not shown) positioned in conduit 222 may indicate that contaminant levels remaining in the effluent from the reverse osmosis unit have reached an undesirable level. Controller 218 may again respond by generating a control signal causing an alarm to sound or to halt operation of the treatment system.

The system and controller of one or more embodiments of the invention provide a versatile unit having multiple modes of operation, which can respond to multiple inputs to increase the efficiency of the wastewater treatment system.

The controller 218 of the system of the invention 200 may be implemented using one or more computer systems 600 as exemplarily shown in FIG. 3. Computer system 600 may be, for example, a general-purpose computer such as those based on in Intel PENTIUM® -type processor, a Motorola PowerPC® processor, a Hewlett- Packard PA-RISC® processor, a Sun UltraSPARC® processor, or any other type of processor or combination thereof. Alternatively, the computer system may include specially-programmed, special-purpose hardware, for example, an application-specific integrated circuit (ASIC) or controllers intended for water treatment systems. Computer system 600 can include one or more processors 602 typically connected to one or more memory devices 604, which can comprise, for example, any one or more of a disk drive memory, a flash memory device, a RAM memory device, or other device for storing data. Memory 604 is typically used for storing programs and data during operation of the system 200 and/or computer system 600. For example, memory 604 may be used for storing historical data relating to the parameters over a period of time, as well as operating data. Software, including programming code that implements embodiments of the invention, can be stored on a computer readable and/or writeable nonvolatile recording medium (discussed further with respect to FIG. 4), and then typically copied into memory 604 wherein it can then be executed by processor 602. Such programming code may be written in any of a plurality of programming languages, for example, Java, Visual Basic, C, C#, or C++, Fortran, Pascal, Eiffel, Basic, COBAL, or any of a variety of combinations thereof.

Components of computer system 600 may be coupled by one or more interconnection mechanisms 606, which may include one or more busses (e.g., between components that are integrated within a same device) and/or a network (e.g., between components that reside on separate discrete devices). The interconnection mechanism typically enables communications (e.g., data, instructions) to be exchanged between components of system 600.

Computer system 600 can also include one or more input devices 608, for example, a keyboard, mouse, trackball, microphone, touch screen, and other man- machine interface devices as well as one or more output devices 610, for example, a printing device, display screen, or speaker. In addition, computer system 600 may contain one or more interfaces (not shown) that can connect computer system 600 to a communication network (in addition or as an alternative to the network that may be formed by one or more of the components of system 600).

According to one or more embodiments of the invention, the one or more input devices 608 may include sensors for measuring parameters of system 200 and/or components thereof. Alternatively, sensors, metering valves and/or pumps, or all of these components may be connected to a communication network (not shown) that is operatively coupled to computer system 600. Any one or more of the above may be coupled to another computer system or component to communicate with computer system 600 over one or more communication networks. Such a configuration permits any sensor or signal-generating device to be located at a significant distance from the computer system and/or allow any sensor to be located at a significant distance from any subsystem and/or the controller, while still providing data therebetween. Such communication mechanisms may be affected by utilizing any suitable technique including but not limited to those utilizing wireless protocols.

As exemplarily shown in FIG. 4, controller 218 can include one or more computer storage media such as readable and/or writeable nonvolatile recording medium 702 in which signals can be stored that define a program to be executed by one or more processors 602. Medium 702 may, for example, be a disk or flash memory. In typical operation, processor 602 can cause data, such as code that implements one or more embodiments of the invention, to be read from storage medium 702 into a memory 704 that allows for faster access to the information by the one or more processors than does medium 702. Memory 704 is typically a volatile, random access memory such as a dynamic random access memory (DRAM) or static memory (SRAM) or other suitable devices that facilitates information transfer to and from processor 602.

Although computer system 600 is shown by way of example as one type of computer system upon which various aspects of the invention may be practiced, it should be appreciated that the invention is not limited to being implemented in software, or on the computer system as exemplarily shown. Indeed, rather than implemented on, for example, a general purpose computer system, the controller, or components or subsections thereof, may alternatively be implemented as a dedicated system or as a dedicated programmable logic controller (PLC) or in a distributed control system. Further, it should be appreciated that one or more features or aspects of the invention may be implemented in software, hardware or firmware, or any combination thereof. For example, one or more segments of an algorithm executable by controller 218 can be performed in separate computers, which in turn, can be communication through one or more networks.

Referring back to FIG. 1, the wastewater source 202 may be fluidly connected with a pre-treatment system 204. The pre-treatment system 204 may include filtration subsystems, subsystems to add compounds to the wastewater in order to facilitate downstream operations, temperature or pH adjustment subsystems, or other pre- treatment sub-systems that may be known in the art.

Sensor 220A may be disposed between the wastewater source 202 and the pre- treatment system 204. Sensor 220A may be, for example, a pH sensor, an ionic concentration sensor specific to a particular ion of interest in a particular application, or a titration type sensor. Sensor 220A may provide a signal representative of a measured pH or ionic concentration to optional pH control system 218.

One or both of the retentate and permeate from the reverse osmosis system 208 may be subject to further treatment operations in order to, for example, precipitate and remove further dissolved compounds or to recover any desired acid or precipitated compounds. For example, permeate from reverse osmosis system 208 may be directed to post treatment system 210. Post treatment system 210 may include one or more additional separators and/or reverse osmosis systems. At least one additional reverse osmosis system may be disposed downstream of and in fluid communication with at least one additional separator. Post treatment system 210 may include a system such as that described in co- pending application, Publication Number 2007/0138093 Al to Bossier et al., incorporated herein by reference in its entirety for all purposes. Post treatment system 210 may include additional sources of alkali or acid which may be controlled by an additional pH control system in response to signals from at least sensors 220D and/or 220E. Post treatment system 210 may alternatively utilize pH control system 218, and in some embodiments may utilize one or both of alkali source 214 and acid source 216 for a supply of acid or alkali compounds. Post treatment system 210 may include separators that may operate at pH levels above or below 2.5, which pH may be controlled by a pH control system, and may be configured to remove different compounds from solution than separator 206.

In some embodiments, post treatment system 210 may render retentate and/or permeate from reverse osmosis system 208 in a sufficiently purified form so that the treated liquid may be released to the environment through discharge system 212.

In some embodiments, the system as illustrated in FIG. 1 may be modified to perform a method such as those discussed above which do not require the addition of an alkali or the precipitation of compounds from solution. In embodiments designed to run these methods, there may be no initial alkali compound addition, and no initial precipitation. Thus, source of alkali compound 214 and separator 206 and their associated pH sensors may be disabled, bypassed, or omitted, as is illustrated in FIG. 2 as system 300.

In systems such as system 300, the addition of small amounts of an acid compound, for example, sulfuric acid, to the pond water may significantly improve the performance of the reverse osmosis system 208 as measured by the rate of membrane fouling. Without being bound to any particular theory, it is believed that the reduced fouling is a result of the conversion of the potassium and sodium salts to free acid. This permits longer reverse osmosis run time between membrane cleanings or conversely running the reverse osmosis system at a higher recovery at the same membrane cleaning frequency.

EXAMPLES Example I

In one study, pond water was sampled after coagulation without any pH adjustment, coagulation with pH adjustment with sodium hydroxide and pH adjust with calcium hydroxide to compare the effect of pH on the precipitation of various constituents in the pond water. Approximately one liter of pond water was taken and a coagulation aid was added to induce the settling of suspended solids. The water was filtered and analyzed

Results of the study can be found in Table 1 below.

Table 1

Raw 10:1 Raw Coag Raw Coag Raw Coag

Raw Dilution no pH adjust pH -1.95 NaOH pH -1.8 Lime

Ca (ppm) 3120 348 3160 3020 4230

Mg (ppm) 1030 113 1060 1030 1220

Na (ppm) 5490 591 5450 10100 4370

Fl (ppm) 18200 1970 18700 8320 10900

SiO2 (ppm) 3329 370 3350 1559 2878

SO4 (ppm) 6920 722 7010 6510 3030

Conductivity Field

(μS/cm) 30050 29850 22630 22030

Conductivity Lab

(μS/cm) 30960 6096 30860 22600 19800 Turbidity Field (NTU) 40 3.9 2.9 1.3 Turbidity Lab (NTU) 55 3.6 15.8 47 15 pH Field 1.30 1.3 1.95 1.8 pH Lab 1.34 1.50 1.08 1.77 1.57

TDS (evap) (ppm) 53618 5065 53994 41560 37690

Coagulation and filtration without pH adjustment had little to no effect on the dissolved solids present. As shown in Table 1, total dissolved solids (TDS) remained approximately unchanged at about 54,000 ppm when there was no pH adjustment, but decreased to about 42,000 ppm when the pH was adjusted to 1.95 with NaOH and to about 38,000 ppm when the pH was adjusted to 1.8 with lime. pH adjustment with caustic precipitated out sodium fluorosilicate but not calcium sulfate. As shown in Table 1, fluoride concentration remained approximately unchanged at about 18,000 ppm when there was no pH adjustment, but decreased from 18,000 ppm to about 8,000 ppm when the pH was adjusted to 1.95 with NaOH and to about 11,000 ppm when the pH was adjusted to 1.8 with lime.

Calcium concentration levels remained at approximately 3,000 ppm when there was no pH adjustment, and when the pH was adjusted to 1.95 with NaOH, and increased to about 4,000 ppm when the pH was adjusted to 1.8 with lime (calcium hydroxide) which indicated a substantial amount of calcium selectively remained in solution and did not significantly contribute to solids formation. From this, it was concluded that pH adjustment with lime selectively precipitated sodium fluorosilicate. Both lime and caustic addition reduced conductivity to the target conductivity. Turbidity also decreased with pH adjustment, from approximately 4 NTU when there was no pH adjustment, to about 3 NTU when the pH was adjusted to 1.95 with NaOH and to about 1 NTU when the pH was adjusted to 1.8 with lime. These results indicated the selective precipitation of fluorosilicates from wastewater may result in reducing the load on a downstream membrane filtration unit as well as control the amount of sludge that may be generated in the system.

Example II A test was conducted utilizing a reverse osmosis unit equipped with a

FILMTEC BW30-365 membrane. Pond water was passed through the reverse osmosis unite at a feed rate of about 80 GPM (gallons per minute). 93% sulfuric acid was added to the pond water prior to the pond water entering the reverse osmosis unit at a feed rate of between 0 and about 4 GPH (gallons per hour). The results (not shown) indicated that an acid feed rate of 1 to 2 GPH was an optimal feed rate. Any rate beyond 4 GPH significantly increased the rate of membrane fouling as measured by the rate of increase of the pressure differential across the membranes.

Adding acid in the 1 to 2 GPH range permitted an increased recovery efficiency by about 5% to about 10% in the reverse osmosis unit without an increased rate of pressure rise when compared to a system run without an acid feed. These results showed that the addition of acid to the wastewater resulted in reducing the load on a downstream membrane filtration unit which may result in allowing the reverse osmosis unit to run at higher efficiencies.

Example El

A comparison test was performed to compare the rate of pressure increase across a reverse osmosis FILMTEC BW30-365 membrane with a pond water feed at 80 GPM with and without the addition of acid. The results of the test are shown below in Tables 2 and 3. Table 2 shows the results of a reverse osmosis unit run without pH adjustment of a pond water feed. Table 3 shows the results of a reverse osmosis unit run with pH adjustment of the pond water feed. The pH of the feed was adjusted with the addition of 1 GPH of 93% sulfuric acid to the pond water feed. With the exception of the ph adjustment, all process parameters in each test remained the same.

to

Table 3

As can be seen in column 6 (Membrane Feed Pressure) of Table 2, when pond water was fed through the reverse osmosis unit without the prior acid addition, the reverse osmosis membrane pressure increased by 14 psi in 11.5 hours which translates to about 1.22 psi/hr.

As can be seen in column 6 (Membrane Feed Pressure) of Table 3, when acid was added to the pond water before the pond water was fed to the reverse osmosis unit, the reverse osmosis membrane pressure increased by about 5 psi in 10.5 hours which translates to a pressure rate increase of about 0.48 psi/hr. Table 3 also shows that control of the pH to a desired range reduced the product conductivity to a greater extent than was exhibited without pH control.

The difference in the rate of pressure increase is significant, and shows that the addition of acid to the wastewater significantly reduced the load on the membrane filtration unit, which may allow the reverse osmosis unit to run for longer periods of time before being serviced.

Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

What is claimed is:

Claims

1. A wastewater treatment system comprising: a source of wastewater saturated with at least one target species; a separator in fluid communication with the source of the wastewater, constructed and arranged to separate at least a portion of the target species from the wastewater, thereby forming a secondary wastewater; a source of an alkali compound disposed upstream of and in fluid communication with the separator, constructed and arranged to supply an alkali compound to the wastewater; a source of acid disposed downstream of and in fluid communication with the separator and in fluid communication with the secondary wastewater, constructed and arranged to supply acid to the secondary wastewater; a membrane filtration unit disposed downstream of and in fluid communication with the source of acid; and a pH control system configured and arranged to maintain a pH of the wastewater at the separator at below about 2.5.

2. The wastewater treatment system of claim 1, wherein the membrane filtration unit is a reverse osmosis unit.

3. The wastewater treatment system of claim 1, wherein the pH control system is configured and arranged to further maintain a pH of the secondary wastewater at an inlet of the membrane filtration unit at least at about 0.1 pH units below the pH level at the separator.

4. The wastewater treatment system of claim 1, wherein the pH control system is configured and arranged to further maintain pH of the wastewater at the separator in a range from about 0.1 to 0.2 pH units higher than a pH of the wastewater in the source of wastewater.

5. The wastewater treatment system of claim 1 , wherein the wastewater in the source of wastewater has a pH in a range of from about 1.8 to about 2.

6. The wastewater treatment system of claim 1, wherein the pH control system comprises at least one pH sensor configured and arranged to provide a signal representing a pH level of at least one of the wastewater and the secondary wastewater, the at least one pH sensor located downstream of the source of wastewater.

7. The wastewater treatment system of claim 1, wherein the separator is a precipitation vessel.

8. The wastewater treatment system of claim 1, wherein the separator is a clarifier.

9. The wastewater treatment system of claim 1, wherein the separator is a centrifuge.

10. The wastewater treatment system of claim 1, further comprising an agitator mechanically coupled to the separator.

11. The wastewater treatment system of claim 1, further comprising at least one additional separator downstream of and in fluid communication with the membrane filtration unit.

12. The wastewater treatment system of claim 11, further comprising at least one additional membrane filtration unit downstream of and in fluid communication with the at least one additional separator.

13. The wastewater treatment system of claim 11, further comprising a pH control system configured and arranged to maintain a pH greater than about 2.5 at the at least one additional separator.

14. The wastewater treatment system of claim 1, wherein the target species comprises at least one salt of fluorosilicic acid.

15. The wastewater treatment system of claim 14, wherein the target species comprises at least one of a potassium salt and a sodium salt of fluorosilicic acid.

16. A wastewater treatment system comprising: a source of wastewater saturated with at least one of a potassium and a sodium salt of fluorosilicic acid and containing at least one of calcium fluoride and calcium phosphate in solution; a separator in fluid communication with the source of wastewater, constructed and arranged to separate at least a portion of the at least one of a potassium and a sodium salt of fluorosilicic acid from the wastewater, thereby forming a secondary wastewater; a source of an alkali compound disposed upstream of and in fluid communication with the separator, constructed and arranged to supply an alkali compound to the wastewater to precipitate at least a portion of the at least one of a potassium and a sodium salt of fluorosilicic acid in solution in the wastewater while maintaining at least one of calcium fluoride and calcium phosphate in solution; a source of acid disposed downstream of and in fluid communication with the separator and in fluid communication with the secondary wastewater, constructed and arranged to supply acid to the secondary wastewater to solubilize at least a portion of the at least one of a potassium and a sodium salt of fluorosilicic acid; and a membrane filtration unit disposed downstream of and in fluid communication with the source of acid.

17. The wastewater treatment system of claim 16, wherein the membrane filtration unit is a reverse osmosis unit.

18. The wastewater treatment system of claim 16, wherein the pH of the wastewater is substantially the same as the pH of the secondary wastewater.

19. A method of treating wastewater comprising: providing wastewater saturated with an inorganic salt and having an initial pH level; precipitating at least a portion of the inorganic salt from the wastewater by adjusting the pH level of the wastewater to below about 2.5 and by a range of from about 0.1 to about 0.2 pH units above the initial pH level of the wastewater, thereby forming a secondary wastewater; adjusting the pH level of the secondary wastewater downward by at least about 0.1 pH units; and passing the secondary wastewater through a membrane filtration unit.

20. The method of treating wastewater of claim 19, wherein adjusting the pH level of at least one of the wastewater and the secondary wastewater comprises controlling at least one of a source of an alkali compound and a source of an acid.

21. The method of treating wastewater of claim 19, wherein adjusting the pH level of the wastewater comprises introducing an alkali compound to the wastewater.

22. The method of treating wastewater of claim 19, wherein adjusting the pH level of the secondary wastewater comprises introducing an acid to the secondary wastewater.

23. The method of treating wastewater of claim 19, further comprising mechanically agitating at least one of the wastewater and the secondary wastewater.

24. The method of treating wastewater of claim 19, further comprising adjusting the pH level of a permeate of the membrane filtration unit to approximately 7.

25. The method of treating wastewater of claim 19, further comprising precipitating dissolved compounds from the secondary wastewater at a pH level greater than about 2.5.

26. The method of treating wastewater of claim 19, further comprising at least one of removal of solids from the secondary wastewater and additional membrane filtration of the secondary wastewater.

27. The method of treating wastewater of claim 19, further comprising recovering precipitated solids.

28. The method of treating wastewater of claim 19, further comprising recovering acid values.

29. A method of treating wastewater comprising: providing wastewater comprising calcium salts and saturated with at least one of a potassium and a sodium salt of fluorosilicic acid; precipitating at least a portion of the at least one of the potassium and the sodium salt of fluorosilicic acid from the wastewater by adjusting a pH level of the wastewater to a pH level sufficient to maintain the calcium salts in solution; separating at least a portion of the precipitated portion of the at least one of the potassium and the sodium salts of fluorosilicic acid from the wastewater, thereby forming a secondary wastewater; solubilizing a remaining portion of the at least one of the potassium and the sodium salt of fluorosilicic acid by adjusting the pH level of the secondary wastewater; and passing the secondary wastewater through a membrane filtration unit.

30. The method of treating wastewater of claim 29, wherein adjusting the pH level of the wastewater comprises raising the pH level of the wastewater by a range from about 0.1 to about 0.2 pH units.

31. The method of treating wastewater of claim 30, wherein adjusting the pH of the wastewater comprises raising the pH level of the wastewater to a pH level below about

2.5.

32. The method of treating wastewater of claim 29, wherein adjusting the pH level of the secondary wastewater comprises lowering the pH level by at least 0.1 pH units.

33. A wastewater treatment system comprising: a source of wastewater saturated with at least one target species; a source of acid disposed downstream of and in fluid communication with the source of wastewater; a membrane filtration unit disposed downstream of and in fluid communication with the source of acid; and a pH control system configured and arranged to maintain a pH level of the wastewater at an inlet of the membrane filtration unit in a range from about 0.1 to about 0.2 pH units below a pH level of the wastewater in the source of wastewater.

34. A method of treating wastewater comprising: providing wastewater saturated with at least one of a potassium and a sodium salt of fluorosilicic acid; maintaining the solubility of the at least one of a potassium and a sodium salt of fluorosilicic acid by adjusting a pH level of the wastewater; and passing the wastewater through a reverse osmosis unit.