WO2006031998A1

WO2006031998A1 - Method and system for removing metallic contaminants from a pressurized liquid flow stream

Info

Publication number: WO2006031998A1
Application number: PCT/US2005/033064
Authority: WO
Inventors: Wesley Clifton Boerm; Joseph F. Sarro
Original assignee: INTELLIGENT SOLUTIONS Inc
Current assignee: INTELLIGENT SOLUTIONS Inc
Priority date: 2004-09-14
Filing date: 2005-09-14
Publication date: 2006-03-23
Anticipated expiration: 2007-03-14

Abstract

A prefabricated process system to transform dissolved metallic contaminants into an insoluble solid facilitating the contaminant's removal from the liquid phase is presented. The metallic ions dissolved in the flow stream are oxidized to a desirable valence state, accelerating the formation of floc particles by chemical precipitation following the addition of one or more reagents. The precipitation of the metallic ions, along with reactive organic and/or inorganic constituents in the flow stream, occurs within a pressurized reactor vessel. Flow from the reactor passes through a <0.2 micron cross-flow membrane microfiltration system thus providing a physical barrier for discharge of residual floc particles from the system. The oxidation-precipitation-membrane microfiltration process is capable of removing >90% of the targeted metallic ions regardless of influent concentration. The cleansed flow stream can be recycled for industrial process applications or discharged directly for its intended use such as for a municipal potable water supply.

Description

Method and System for Removing Metallic Contaminants from a Pressurized Liquid Flow Stream

RELATED APPLICATION DATA

[0001] This application is based on and claims the benefit of U.S. Provisional

Patent Application No. 60/609,797, filed on September 14, 2004, the disclosure of which is incorporated herein in its entirety by this reference.

BACKGROUND

[0002] This invention pertains generally to treatment of contaminated liquids.

More specifically it relates to a method and system for removal of various undesirable constituents, including dissolved heavy metal constituents, from a confined liquid flowing under pressure, such as an industrial waste stream or groundwater for municipal use.

[0003] The potentially harmful effect to human beings by contaminating impurities in liquids, particularly in surface waters, has long been widely recognized. The types of harmful impurities, or contaminants, are as numerous and diverse as their various sources. Of the many potentially harmful contaminants, dissolved inorganics like metal pollutants, especially heavy metal pollutants, are known to be particularly pervasive, both in terms of their apparent toxicity to surrounding life forms and in terms of the quantities and concentrations in which they are found. Toxic metal pollutants, however, are not subject to destruction via biological or thermal oxidation as organic industrial pollutants often are.

[0004] Devices used to filter and remove dissolved and/or suspended substances from a pressured liquid can generally be classified into three broad categories-- those methods wherein a dissolved metal contaminant is precipitated by the introduction of an activating composition; methods wherein removal of dissolved species of contaminants is attempted by use of membrane processes, adsorption, chelation, ion-exchange, and sequestration employing various materials; and, those methods wherein the removal of non- dissolved solid contaminants suspended within a liquid is performed through the use of solid/liquid separation techniques, such as gravity separation, centrifuging, magnetic separation, and the like. However, significant drawbacks exist for each category, which severely curtail their practicability for many applications. Moreover, many of these systems employ treatment techniques that are not cost effective. Traditional gravity flocculation- sedimentation-filtration systems currently employed in most municipal and industrial wastewater treatment systems and large and extremely expensive. [0005] Additionally, there exists a present-day demand for a high capacity pump and treatment arsenic remediation system for groundwater supplies serving municipal water systems in order to meet the new water quality standard for arsenic imposed by the United States Environmental Protection Agency (USEPA).

[0006] Thus, there is a need for an improved method and system that can facilitate the removal of heavy metallic elements from a pressurize flow stream. It is an object of the present invention to provide such a method and system.

[0007] Another object of the present invention is to provide a compact, high- rate filtration system offering the benefits of process reliability while operating within an enclosed (pressurized) environment.

[0008] Still another object of the present invention is to provide a high capacity, compact, and reliable prefabricated treatment unit having further improved chemical precipitation and filtration capabilities.

[0009] Yet another object of the present invention is to provide a method and system that can operate dependably with minimal maintenance while consistently providing a high removal rate (>90% in most instances) of the targeted metallic ion.

[0010] Still another object of the present invention is to provide a system that can be prefabricated and can replace the large traditional gravity flocculation-sedimentation- filtration systems currently employed in most municipal and industrial wastewater treatment systems.

[0011] Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods and apparatus pointed out in the appended claims.

SUMMARY

[0012] To achieve the foregoing objects, and in accordance with the purposes of the invention as embodied and broadly described in this document, there is provided a method and system for removing metal contaminants from a pressurized flow stream.

[0013] A process according to the present invention includes the steps of introducing an oxidizing agent into the flow stream, introducing a flocculant into the flow stream, removing inorganic and organic constituents from the flow stream by co-precipitation to provide a settled, flow stream, and filtering the settled flow stream to remove residual particulate matter from the settled flow stream. The method preferably also includes introducing a polymer into the flow stream for aiding the filtering of the settled flow stream.

[0014] According a preferred method of the invention, the flow stream is retained a reactor vessel, providing sufficient retention time (e.g., 5-10 minutes) to allow co- precipitation of inorganic and organic constituents contained in the flow stream to precipitate out, leaving a settled flow stream. The settled flow stream is discharged from the top of the reactor vessel and the solid particulates settled within the reactor vessel are continuously discharged from the bottom of the reactor vessel. The settled flow stream is filtered through a membrane microfiltration system with a pore opening less than 0.2 microns so as to filter the process flow steam while providing a physical barrier to residual particulate matter being discharged from the prefabricated treatment unit.

[0015] A system according to the invention includes means for introducing an oxidizing agent into the flow stream, means for introducing a flocculant into the flow stream a reactor vessel for retaining the flow stream for a predetermined retention time and a filtration unit for filtering the settled flow stream to remove residual particulate matter from the settled flow stream. The reactor is adapted to retain the flow stream for a time during which the process stream and chemical additives react and a co-precipitation reaction occurs, thereby creating particulate material. In one embodiment, the reactor vessel includes an inlet for receiving an inlet feed flow of the pressurized flow stream and a discharge outlet for discharging a settled flow stream from the reactor vessel. The reactor vessel also includes a perforated baffle plate is for creating a controlled flow pattern and upward flow velocity of the flow stream within the reactor vessel. A settling area in the reactor vessel is provided for collecting precipitant that has precipitated from the flow stream. A solids removal outlet is provided for discharging the collected precipitant from the reactor vessel. A settling device comprising a plurality of tubes is disposed within the reactor vessel settling area.

[0016] The present invention can be utilized in water treatment systems and a wide variety of industrial process applications requiring the removal of dissolved substances, especially heavy metals, along with a limited amount of suspended particles from a flow stream. As an illustration of the process's applications, the removal of dissolved arsenic from a groundwater source to be used as a municipal potable water supply will be provided as an example within this patent presentation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate the presently preferred embodiments and methods of the invention and, together with the general description given above and the detailed description of the preferred methods and embodiments given below, serve to explain the principles of the invention.

[0018] FIG. 1 shows a schematic diagram of an exemplary system for removing a metallic (e.g., arsenic) ion from a groundwater well used for municipal water supply, according to the present invention.

[0019] FIG. 2 shows a side view, in partial cross-section, of an exemplary reactor vessel for the treatment of a 200 gpm process inlet feed flow, according to the present invention.

[0020] FIG. 3 shows a top yiew of the reactor vessel of FIG. 2.

[0021] FIG. 4 shows the internal baffle plate configuration for the reactor vessel of FIG. 2.

[0022] FIG. 5 shows a metal hydroxide solubility curve showing the solubility of the common heavy metal ions and their respective solubility versus pH.

[0023] FIG. 6 shows a graph of the percentage or arsenic removal vs. coagulant dosage with ferric sulfate and aluminum sulfate used as a coagulant.

[0024] FIG. 7 shows a graph of the typical distribution of floe particles by size for a flow stream treated with ferric chloride used as a coagulant.

[0025] FIG. 8 shows the effect of adding a cationic polymeric flocculant as a filter aid.

[0026] FIG. 9 shows graphs of the percent arsenic removed, with a ferric ion dose added as ferric chloride, after flocculation and filtering of a feed stream using a 0.22 μM pore size membrane.

[0027] FIG. 10 shows graphs of the percent arsenic removed, with a ferric ion dose added as ferric chloride, after flocculation and filtering of a feed stream using a 1.2 μM pore size membrane. DESCRIPTION

[0028] The presently preferred methods and embodiments of the invention will be described in more detail with reference to the accompanying drawings, in which like reference characters designate like or corresponding parts throughout the drawings. In the following description, methods and embodiments of the invention have been shown and described simply by way of illustration of the best mode contemplated by the inventor of carrying out the invention. As will be realized, the invention is capable of modification in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.

[0029] FIGs. 1-4 illustrate one embodiment of a system 10 according to the present invention, which can transform dissolved metallic contaminants into an insoluble solid, thereby facilitating the contaminant's removal from the liquid phase. The metallic ions dissolved in the flow stream are oxidized to a desirable valence state, accelerating the formation of floe particles by chemical precipitation following the addition of one or more reagents. Chemical precipitation is the formation of a separable solid substance from a solution, either by converting the substance into an insoluble form or by changing the composition of the solvent to diminish the solubility of the substance in it. Metals precipitation from contaminated water involves the conversion of soluble heavy metal salts to insoluble salts that will precipitate. The precipitation of the metallic ions, along with reactive organic and/or inorganic constituents in the flow stream, occurs within a pressurized reactor vessel. Flow from the reactor vessel passes through a microfiltration system thus providing a physical barrier for discharge of residual floe particles from the system. In a preferred embodiment, the microfiltration system is a <0.2 micron cross-flow membrane microfiltration system. The oxidation-precipitation-membrane microfiltration process performed by the system is capable of removing >90% of the targeted metallic ions regardless of influent concentration. The cleansed flow stream can be recycled for industrial process applications or discharged directly for its intended use such as for a municipal potable water supply.

[0030] FIG. 1 shows a schematic diagram of a system according to the present invention for removing a metallic ion from a pressurized flow stream. The system performs an oxidation-precipitation-filtration process by combining various chemical feed systems to initiate particulate formation within one or more pressurized reactor vessels 14. The reactor vessel 14 functions as an up-flow pressurized clarifier. The reactor vessel 14 includes an inlet nozzle 18 connected to incoming process piping to provide a means by which the feed flow 12 enters the reactor vessel 14. A plate or tube settler system 22 is supported by a retainer screen in a lower section of reactor vessel 14. This enhances and accelerates settling of floe particles produced by process reaction. In addition, it provides a means of isolating floe particles within the reactor vessel and concentrates floe particles at the bottom of the reactor vessel. Floe collected in bottom of reactor vessel is continually discharged and is isolated by the tube settlers from the process flow in mixing zone of reactor.

[0031] An ozone generator 16 injects ozone into the flow stream 12 prior to the flow entering the reactor vessel 14. The ozone serves to oxidize the targeted metallic ion, thus converting it to a valence state which enhances the effectiveness of the co-precipitation process. A coagulant, usually ferric chlorine, and a cationic polymer are injected into the reactor vessel 14 through piping connections to the vessel. The coagulant and type of polymer varies with the metallic ion targeted for removal. Multiple piping connections provide for a variety of injection locations to optimize the process.

[0032] The reactor vessel 14 provides a mixing zone and provides detention time to allow the desired process to occur. The size and configuration of the reactor vessel can be varied to create the conditions necessary to optimize the process of precipitating the target ion. Parameters that affect the size and configuration of the reactor vessel and its associated piping connections include the volume, temperature, pressure, pH, viscosity, etc. The feed flow 12 is received by the reactor vessel 14 where the turbulent inlet flow stream entering the reactor vessel 14 provides additional energy and mixing of the chemicals injected to initiate the co-precipitation process. The reactor vessel 14 provides for a predetermined retention time within the vessel, during which the process stream and chemical additives react and the co-precipitation reaction occurs, thereby creating particulate material. The particulate matter created within the reactor vessel 14 settles towards the vessel bottom 20 where it is collected by the tube settler system 22 in the lower section of the vessel. The collected precipitant is continuously discharged from the reactor vessel 14 through a solids removal outlet 24 at a variable flow rate, thereby providing a process waste stream 26.

[0033] A quantity of perforated buoyant media 32 (e.g., perforated, hollow buoyant balls) is provided in the upper section of the reactor vessel 14. A retainer screen or perforated baffle plate 28 installed in the upper potion of the reactor prevents the media from being discharged from the reactor. Thus, the buoyant media 32 placed in the main body of the vessel reactor 14 is retained on the downstream face of the baffle plate 28. The buoyant media layer 32 prevents "short circuiting" of the process flow through the upward reactor and creates a laminar flow condition within the mixing zone and top discharge area of the reactor. The buoyant media 32 eliminates short circuiting of the flow through the baffle plate 28 and minimizes the passage of precipitants through the baffle plate 28. The perforated baffle plate 28 and buoyant media 32 create a controlled flow pattern and upward flow velocity within the reactor vessel 14 for the process fluid to exit the reactor vessel 14. The buoyant media 32 block and retain small floe particles, thereby preventing them from being discharged from the reactor. The buoyant media layer 32 provides an additional mechanism by which the floe particles can combine with other particles to create a floe with sufficient mass to settle by gravity.

[0034] The flow exits a discharge outlet 30 located at or near the top of the reactor vessel 14 as a reactor exit flow stream 34. The reactor exit flow stream 34 is piped to one or more membrane micro filtration units 36 to filter the process flow, preventing the discharge of precipitants into one or more filtered flow streams 38. The filtered flow streams exit 38 the membrane micro filtration units 36 to be used for recycling or safe disposal as desired. One embodiment of a microfiltration unit 36 suitable for use with the system 10 is described in detail in PCT Patent Application No. PCT/US 05/28577, entitled "Bundled Element Filtration System and Method, and filed on 10 August 2005 by Wesley Clifton Boerm and Joseph F. Sarro, which application is incorporated in its entirety herein by this reference.

[0035] The system 10 is capable of removing metallic ion contaminants (such as arsenic, cadmium, cyanide, chromium, copper, lead, molybdenum, nickel, selenium, uranium, and zinc) from solution within a pressurized liquid flow stream. Commonly used precipitants can include carbonates, sulfates, ferric salts, lime, and other hydroxides. The system can make use of on-site ozone generation for the oxidation step, followed by the addition of ferric chloride and a polymer to accelerate precipitation within the reactor. The flow stream exiting the reactor passes through a spiral wound membrane microfiltration unit in the final step of the system. The system can be prefabricated. The necessary piping, electrical, and instrumentation components can be provided to automate the process flow in and through the reactor vessel 14.

[0036] Referring to FIGs. 2-3, an exemplary reactor vessel 14 is shown in more detail. The reactor vessel shell is made of carbon steel and is designed to operate at pressures up to about 75 psi. In a treatment system according to the invention, it is desirable to minimize the size of the tankage, and thus, the total area required for installation of the system. To maximize the removal rate of precipitants from the flow stream retained within the reactor vessel 14, a settling area 40 is provided in the reactor vessel lower portion (e.g., 1- 2 feet of the reactor vessel). The plate and tube settler 22 comprises shallow (short) settling devices such as stacked, off-set trays of flat metal or plastic plates, or bundles of small plastic tubes, of various geometries. Both lamella plate and tube style settlers can be configured in rectangular or circular arrangements and can be utilized for this application. Two important aspects of the tube or plate type settler 22 are the height and angle of inclination of the tubes or plates. Preferably, the height of the settling area should be a minimum of 18" and set at a 45°- 60° incline. If the angle > 60°, then efficiency decreases. If the angle < 45°, then sludge tends to accumulate within the plates and tubes. The removal efficiency of the settling device 22 is directly related to the settling velocity of the floe particles and not the depth of the settling area. Therefore, the settling / solids reactor vessel settling area 40 optimizes the efficiency of the precipitant removal process within the vessel 14.

[0037] Referring to FIGs. 2-3, the baffle plate 28 is installed in a fixed position, such as by welding, in the upper portion of the reactor vessel 14. The baffle plate 28 includes multiple ports 42 and serves as multi-port orifice to control the flow velocity of the settled water exiting the reactor vessel 14 via the reactor outlet 14. The ports in the baffle plate are sized and positioned to provide a controlled, upward, laminar flow through the reactor vessel 14. The baffle plate 28 thus creates the flow conditions within the reactor vessel 14 necessary for the precipitants to settle within the reactor vessel 14. The floating media 32 is retained on the downstream side of the baffle plate 28. The floating media 32 is evenly distributed across the width of the baffle plate 28 by the process flow exiting the reactor vessel 14. The floating media 32 disrupts the flow through the baffle plate ports 28 and prevents the creation of localized high velocity currents near the baffle plate ports 28, which would allow precipitants to be carried out of the reactor to downstream processes.

[0038] Still referring to FIGs. 2-3, the reactor vessel 14 also preferably includes a pressure relief valve 50 and an air/vacuum valve 52, which provides a means for displacing air within the reactor vessel 14 when filling and draining the reactor vessel 14 with liquid. When filling the reactor vessel with fluid, excess air is discharged through the air/vacuum valve 52. When draining the reactor vessel 14, the air/vacuum valve provides a means for air to enter the vessel as the liquid is removed. This keeps the vessel from collapsing inward due to the forces created by the vacuum pressure applied to the reactor. In addition, the reactor vessel 14 can include a manway 54 for accessing the internal components of the reactor vessel 14 during fabrication and maintenance operations. A chemical injection port 56 allows for introducing into the reactor vessel 14 different chemicals or substances that may otherwise react in an undesirable or explosive manner if not separated into individual flow streams. A sample port 58 also is provided for removing a small portion of the process fluid at various areas within the reactor vessel 14 to verify and optimize the efficiency of the desired process reactions.

Precipitation Process

[0039] Precipitation is a physical-chemical process, in which soluble metals and inorganics are converted to relatively insoluble metal and inorganic salts precipitate by the addition of a precipitating agent. When the presence and precipitation of other metals in solution aid in the removal of target metals through surface adsorption, it is called coprecipitation. An example of this is improved cadmium removal by adsorption onto calcium carbonate precipitates (Anderson, 1994).

[0040] Certain metals may require oxidation (e.g., Fe⁺² to Fe⁺³) or chemical reduction (e.g., Cr⁺⁶ to Cr⁺³) to change the valence state so that a particular precipitation method can be effective. Commonly used oxidizing agents are oxygen, ozone, chlorine, and potassium permanganate (KMnO₄). Oxidation rates depend on the oxidant used, pH, the alkalinity of the waste stream, and the presence of organic matter.

[0041] According to a preferred method of the present invention, ozone is used as an oxidation agent because it is the most powerful and rapid-acting oxidizer produced. It is created by exposing oxygen, either in air or pure oxygen, to high energy such as an electric discharge field (i.e., corona discharge) or to UV radiation. This causes the oxygen molecules to react to form an unstable configuration of three oxygen atoms - the oxygen molecule contains only two. Because of its instability, ozone is very reactive and is a very efficient oxidant. The only byproduct from oxidation with ozone is oxygen, which is dissolved in aqueous systems. The stoichiometric oxidant demands and the oxidation- reduction reactions for ozone to oxidize iron, manganese, and sulfide are provided below (USEPA Arsenic Treatment Technology Evaluation Handbook for Small Systems, July 2003).

[0042] The stoichiometric ratio for the oxidation of Fe(II) is 0.43 mg O₃ per mg Fe⁺² :

2Fe⁺² + O₃ + 5H₂O → 2Fe(OH)₃ + O₂ + 4H⁺ :

The stoichiometric ratio for oxidation of Mn(II) is 0.88 mg O₃ per mg Mn⁺² Mn⁺² + O₃ + H₂O → MnO₂ + O₂ + 2H⁺

The stoichiometric ratio for oxidation of sulfide is 1.50 mg O₃ per mg HS^" : HS- + O₃ + H+ → SO + O₂ + H₂O

[0043] As a powerful oxidizer, ozone is capable of improving the oxidation of metal ions. Therefore, it's effective in the removal of metal ions through the resulting precipitation. Metal ions that can be removed this way include Lead, Iron, Zinc, Cadmium, and Nickel. Other inorganics that are very reactive to ozone include Cyanide, Thiocyanate, Sulfite, Bromide, Nitrite, Iodide, Aluminum, Arsenic, Cadmium, Chromium, Cobalt, Copper, and Manganese.

[0044] The measure of an oxidizer and its ability to oxidize organic and inorganic material is its oxidation potential (measured in volts of electrical energy). Oxidation potential indicates the degree of chemical transformation to be expected when using various oxidants. It gauges the ease with which a substance loses electrons and is converted to a higher state of oxidation (USEPA 1990). Theoretically the substance with the lower oxidation potential will be oxidized by the substance with the higher oxidation potential. A substance can only be oxidized by an oxidizer with a higher potential (USEPA 1978). There is only one element with a higher oxidation potential than ozone and that is fluorine.

[0045] Oxidation potential does not indicate the relative speed of oxidation nor how complete the oxidation reactions will be. Complete oxidation converts a specific organic compound to carbon dioxide and water. Organic compounds treated with a powerful oxidant as ozone will not always be converted totally to carbon dioxide and water, especially under abnormal industrial wastewater conditions. Therefore, no other commonly employed and less powerful water treatment oxidant (i.e. chlorine, bromine, chlorine dioxide), all of which have lower oxidation potentials than ozone, will not completely oxidize an organic material if ozone will not.

[0046] Ozone has been reported by some to improve coagulation and filtration efficiency (Gurol and Pidatella, 1993; Farvardin and Collins, 1990; Reckhow et al., 1993; Stolarik and Christie, 1997). Ozone oxidation of iron and manganese generates insoluble oxides that can be removed by sedimentation or filtration (USEPA Guidance Manual Alternative Disinfectants and Oxidants, April 1999). [0047] Studies have shown that low alkalinity levels can lessen oxidation rates because the buffer system is slow to respond to acidity changes induced by the oxidation reaction. Further studies have shown that the presence of organic matter may also lessen the oxidation rates as organics create additional demand and must be accounted for when calculating the total ozone feed rates. The presence of complexing and chelating agents can also slow oxidation rates. Organic iron and manganese are not effectively removed by oxidation alone, but in these cases, studies have shown that coagulation followed by sedimentation can be effective. (Precipitation/Coagulation/Flocculation, US Corps of Engineers, 2001).

Coagulation and Flocculation

[0048] Coagulation and flocculation are used to remove the insoluble and colloidal heavy metal precipitates formed during the precipitation step. Colloidal heavy metal precipitates are tiny particles that possess electrical properties, which create repelling forces and prevent agglomeration and settling. Coagulation is the process of making the particle less stable by neutralizing its charge, thus encouraging initial aggregation of colloidal and finely divided suspended matter. Particles no longer repel each other, and can be brought together.

[0049] When suspended in water, the charge on organic and inorganic colloids is typically negative. Because of electrostatic forces, the negative colloid charge attracts positive ions. Flocculation is the process of bringing together the destabilized or "coagulated" particles to form a larger agglomeration of floe by physical mixing or addition of chemical coagulant aids, or both.

[0050] Zeta potential is a measurable quantity and is sometimes used to predict the potential for coagulation. Effective coagulation has been found experimentally to occur at zeta potential values ranging from ± 0.5 mV. Inorganic compounds (typically iron and aluminum derivatives) are commonly used as coagulants. During dissolution, the cations serve to neutralize the particle charge and the effective distance of the double layer, thereby reducing the zeta potential. , In inorganic coagulants, a trivalent ion can be as much as 1000 times more effective than a monovalent ion. This is the reason that alum and iron salts are extremely efficient coagulants. Iron coagulants include ferric sulfate, ferric chloride, and ferrous sulfate. Compared to aluminum derivatives, iron coagulants can be used successfully over a much broader pH range of 5.0 to 11.0. The ferric hydroxide floe is heavier than alum floe and therefore settles more rapidly. Soluble iron and manganese are effectively precipitated through simple oxidation techniques.

Co-precipitation

[0051] Co-precipitation with ferric salts offers a robust treatment option capable of removing heavy metals to the "parts per billion" (ppb) levels that have been unattainable with more conventional methods. Metal coagulants such as alum and ferric chloride are absorbed almost immediately and the dispersion should therefore be completed within 1 or 2 seconds. In-line static mixing of the injected chemical has the advantages of no moving parts, no external energy, and fewer clogging problems. The process system presented herein will use ferric chloride as the primary coagulant in the prefabricated treatment units. Depending on the water chemistry and the element targeted for removal, an alternate primary coagulant may be used with or without the addition of a polymer flocculant.

[0052] Co-precipitation generally describes a single-stage process that combines two precipitants in a reaction vessel that serves to increase metal precipitation efficiency greater than the use of either precipitants individually. Chemical precipitants that have been used for mining wastewaters include sulfides, hydroxide, and ferric ions. Ferric salts are recognized as an effective scavenger of heavy metals and extensive treatment has been given to their application and underlying removal mechanism {Trace Heavy Metals Removal with Ferric Chloride, Patoczka, Johnson, Scheri, 1998). Additionally, ferric chloride is capable of removing heavy metals from a complex fluid matrix within which the metals may be present in various forms, such as dissolved, colloidal, emulsified, and particulate.

[0053] Ferric salts are themselves ubiquitous in waste streams where heavy metals are present and are likely responsible for better than theoretically expected removal of heavy metals from waste streams by controlled pH induced precipitation (Boiling, 1991). Many alternative chemical/processes have been proposed and/or used for heavy metal removal. Some of the examples are organic precipitants such as DTC, (alkyl dithiocarbamates) which is used in the treatment of printed circuit board wastewater (Choo, 1992), modified natural zeolites (Groffman, 1992), and the membrane filtration processes (micro-, ultra- and nano-filtration) (Capaccio, 1996). The application of organic precipitants in many practical situations is limited however, due to various ionic interferences, development and control costs, maintenance problems and process limitations. [0054] The ability of the ferric hydroxide precipitate to absorb ions with heavy metals is characterized in single and multi-adsorbate systems. Heavy metals could be absorbed both as cations (Cr⁺³, Pb⁺, Cu⁺², Zn⁺², Ni⁺², Cd⁺²) in neutral to high pH, and as anions (SeO₄ ^'2, CrO₄ ^"2, VO₃(OH)^"2, AsO₄ ^"3) in neutral to mildly acidic pH. A summary of the impact of pH on absorption efficiency for a number of ions can be found in a paper by Manzione et al., (1994). For heavy metals present in cationic form, the absorption efficiency increases with pH, while concentration of both sorbate and ferric hydroxide play a secondary role (Farley et al., 1985).

[0055] Co-precipitation occurs when ferrous iron is added to metallic waste streams and subsequently oxidized in an aerated reactor. The oxidized iron, which is insoluble, precipitates with other metallic contaminants present in the waste stream, thereby enhancing metals removal. By adjusting the pH value of a solution, the corresponding metallic hydroxide compounds become insoluble and precipitate from solution. The theoretical solubility usually does not exist in practice so a metallic coagulant such as ferric chloride is often used to accelerate the coagulation and precipitation process. A voluminous precipitate can capture ions and particles during formation and settling, in effect "sweeping" ions and particles from the wastewater. (Tchobanoglous and Burton, 1991). FIG. 5 depicts a metal hydroxide solubility curve showing the solubility of the common heavy metal ions and their respective solubility versus pH. The typical dosing rates for ferric chloride in precipitation reactions are 5 - 30 mg/ml.

[0056] In summary, ferric salts have proved to be the method of choice, if not the only feasible method, for removing a variety of heavy metals to sub-mg/1 concentration levels from complex industrial wastewater. Due to several different removal mechanisms, ferric salts are capable of removing heavy metals present in soluble, complexed, chelated, colloidal, emulsified and particulate form. This method is particularly applicable for treatment of low-volume industrial steams, where disposal of relatively large quantities of sludge generated, is still an economically competitive solution.

[0057] When added to water containing colloidal substance with a negative electric charge, ferric chloride hydrolyzes into ferric hydroxide, an electropositive compound. It thus causes the rapid precipitation of the colloids by: , the formation of electrically neutral floes; the grouping together of these floes under the effect of the turbulence; and the absorption of impurities on the ferric hydroxide floes.

Ferric chloride not only functions as a reactant to remove water impurities but it also functions as both a coagulant and a flocculant. The reactions of ferric chloride in water include an ability to form precipitates with hydrogen sulfide (H₂S), phosphate (PO₄), arsenic as arsenate (AsO₄) and hydroxide alkalinity (OH). Ferric chloride reacts in water with hydroxide alkalinity to form various hydrolysis products that incorporate Fe(OH)₃. These compounds possess high cationic charge which allows them to neutralize the electrostatic charges found on colloidal compounds and also to bind to negatively charged particles, including the ferric hydroxide itself. This ability to bind to itself is the mechanism for the formation of floe aggregates and the basis for ferric chloride's flocculation abilities.

[0058] The hydrolysis products from ferric chloride, nominally ferric hydroxide, are different from those of sulfate based ferric sulfate and aluminum sulfate (alum). Ferric chloride forms a more discrete, dense floe that is less easily broken up than alum floe and promotes quicker settling and specifically, better sedimentation in cold water. This dense floe has more available cationic charge that allows higher reactivity with colloidal solids. In contrast, the floe aggregates of ferric sulfate and aluminum sulfate tend to be less discrete and "fluffy" or cloud like, this apparently due to differences in the types of bonding of the hydrolysis products. The high ratio of cationic charge to total mass also makes the ferric chloride hydrolysis products more reactive and adsorptive with emulsified and semi- emulsified organic matter; such as oils, fats, and other natural and synthetic organic matter. This would explain the ability of ferric chloride to remove TOC and other disinfection by product precursors (DBP 's).

[0059] These differences translate into characteristics and abilities for ferric chloride that set it far apart from the sulfate based coagulants. In typical plant situations one can expect to use about 30% to 50% less ferric chloride than aluminum sulfate (on a dry weight basis) to achieve similar results. The settled sludge volume of the ferric (chloride) hydroxide typically ranges from 1/3 to 2/3 that of sulfate based coagulants. Additionally, the sludge developed through the use of ferric chloride is generally much more dewaterable. So, although the ferric hydroxide molecule itself is heavier than the aluminum hydroxide molecule, this does not translate into more sludge to be disposed of. Instead, because sludge is disposed of on a wet basis rather than on a dry basis, the use of ferric chloride produces fewer wet tons of sludge and yields significant solids handling and disposal savings. [0060] A degree of precipitant removal can be accomplished within the reactor vessel 14 by natural subsidence (settling), but it is advantageous to collect as much precipitate as possible within the reactor vessel 14. The suspended particles will settle given enough time, but it is not practical to provide this degree of retention within the vessel. The settling time is dependent on many factors, including the: Weight of the particle

Shape of the particle Size of the particle

Viscosity and/or frictional resistance of the water, which is a function of temperature

Therefore, the settling velocities of the precipitant particles may be calculated from Stokes' Law :

_F _ 2662(S₁ - S₂)D² z

Where: V = Velocity of fall (ft/sec)

D = Diameter of particle (in)

S₁= Density of particle (Ib/ft3)

S₂= Density of fluid (Ib/ft3) z = Viscosity (centipoises)

In this equation it is assumed that the particles are spherical, failing under viscous resistance, and that they have no electrostatic charges. This is, of course, never true under actual conditions. Therefore, chemicals are added to neutralize particle charge and enhance particle settling.

[0061] Although there is little formal data regarding the use of ferric chloride as a filtration aid, there is much operational data that speaks to its ability to greatly enhance turbidity removal with sand filter filtration systems. To ensure that the maximum amount of precipitation and settling occurs in the reactor vessel, a small amount of cationic polymer can also be added to the flow stream into the reactor vessel 14.

[0062] The use of cationic polymeric flocculants as flocculation aids has been described and well researched in previous studies. The addition of a small amount of cationic polymeric flocculant can lead to significant improvements in membrane flux rates. Not only is the permeate flux higher, the addition of a small amount of polymeric flocculant is also likely to increase the run time before the membrane cleaning is necessary. Polymer dosages are typically less than 1 mg/ml to avoid plugging and build-up of excess polymer on the surface of the microfiltration membrane elements. Monitoring of the polymer dosing and activation rates becomes even more critical as the pore size of the filtration elements decreases.

[0063] When ferric chloride is added to water as a primary coagulant, the following reaction occurs in addition to the coprecipitation of the metallic ion:

2 FeCl₃ + 3 Ca(HCO₃)₂ → 2 Fe(OH)₃ + 3CaCl₂ + 6CO₂ Ferric + Calcium gives Ferric + Calcium + Carbon

Chloride Bicarbonate Hydroxide Chloride Dioxide

(present in the water to treat)

Therefore, the addition of ferric chloride as a coagulate will decrease the alkalinity of the flow stream by removal of dissolved carbonate ions. This reaction thus provides a softening function for the flow stream during treatment.

Example - Removal of Arsenic

[0064] The following example will help to further explain the invention. It will be understood, however, that the example is illustrative of the invention and that the invention is not limited only to the example.

[0065] We have implemented an exemplary system according to the present invention for removing arsenic from groundwater. Arsenic is a well-known toxic metal and is present mainly as oxyanion compounds in groundwater. The World Health Organization's current provisional guideline for arsenic in drinking water is 10 μg/L, but all developing countries affected with contaminated groundwater are still struggling to keep up with the previous WHO guideline value of 50 μg/L. The USEPA recently approved new guidelines which will mandate a MCL of 10 μg/L as of January 2006. The USEPA estimates that the lower arsenic standard will require 3,000 community water systems, serving 11 million people, to take corrective action to lower the current level of arsenic in the drinking water. There is therefore a great need to develop cost efficient methods for arsenic removal from drinking water.

[0066] Background concentrations of arsenic in groundwater are in most countries less than 10 μg (Welch et al., 2000) and sometimes substantially lower. However, values quoted in the literature show a very large concentration range exists from <0.5-5,000 μg (i.e. four orders of magnitude). High concentrations of arsenic are found in groundwater in a variety of environments. This includes both oxidizing (under conditions of high pH) and reducing aquifers and in areas affected by geothermal, mining, and industrial activity.

[0067] Arsenic is perhaps unique among the heavy metalloids and oxyanion- forming elements (e.g. antimony, arsenic, chromium, rhenium selenium, molybdenum, uranium, vanadium) in its sensitivity to mobilization at the pH values typically found in groundwaters (pH 6.5-8.5) and under both oxidizing and reducing conditions. Therefore the oxyanion-forming elements such as chromium, arsenic, uranium, and selenium are some of the most common naturally occurring trace contaminants found in groundwaters.

[0068] Unlike other toxic trace metals whose solubilities tend to decrease as pH increases, most oxyanions, including arsenate (As⁺⁵), tend to become more soluble as pH increases. When most other metals become insoluble within the neutral pH range, arsenic is soluble at even near-neutral pH in relatively high concentrations. That is why groundwaters are easily contaminated with arsenic and other oxyanions (Review of Arsenic Removal Technologies for Contaminated Groundwaters, Argonne National Laboratory, April 2003).

[0069] Total inorganic arsenic is the sum of particulate and soluble arsenic. A

0.45-micron filter can generally remove particulate arsenic. Soluble, inorganic arsenic exists in either one of two valence states depending on local oxidation reduction conditions. Typically groundwater has anoxic conditions and arsenic is found in its arsenite or reduced trivalent form As⁺³. Surface water generally has aerobic conditions and arsenic is found in its arsenate or oxidized pentavalent form As⁺⁵ (USEPA Arsenic Treatment Technology Evaluation Handbook for Small Systems, July 2003).

[0070] As early as 1972, prior to the start of the USEPA review of the arsenic standard, arsenic removal studies (Gulledge and O'Connor) demonstrated that following oxidation to arsenate ion, As⁺⁵, arsenic could be readily removed (90 to 98 percent) by conventional water treatment processes utilizing chemical coagulants, such as iron and aluminum (see FIG. 6).

[0071] While a great deal of research has been done in recent years on alternate processes for arsenic removal, little attention was previously focused on the potential capital and operating costs of these alternatives (Arsenic Removal Methods, O'Conner and O'Conner, 2001). There are currently many methods for removing arsenic from drinking water, with flocculation using ferric chloride followed by filtration commonly employed. Most arsenic treatments fall into four process categories: ion exchange, membrane process, adsorption, or chemical precipitation. Ion-exchange treatments are very limited in their ability to remove arsenic because of exchange competition from other anions found in grόundwater. Membrane processes are very effective at removing arsenic from groundwater, but the cost is high.

[0072] Arsenic removal by microfiltration depends on the entrapment of the floe particles by the microfiltration membrane. Removal of arsenic by flocculation and microfiltration depends on the effectiveness of arsenic adsorption onto the ferric complexes present and on the rejection of the arsenic containing floes formed by the membrane. The larger the membrane pore size, the lower the resistance to filtrate flow and therefore, the lower the pumping costs. However, the larger the pore size, the greater the number of small floe particles that are able to pass through the membrane into the filtrate. The pH of the water has a strong effect on the percent arsenic removed at a given ferric ion dose. Further, it is the total ferric ion present. (Arsenic Removal from Drinking Water by Flocculation and Microfiltration, Hana, Runnellsb, Zimbronb, Wickramasinghea, 2002).

[0073] Chemical speciation is a critical element of arsenic treatability.

Negative surface charges facilitate removal by adsorption, anion exchange, and coprecipitation processes. Since the net charge of arsenite As⁺³ is neutral at natural pH levels (6-9), this form is not easily removed. However, the net molecular charge of arsenate As⁺⁵ is negative (-1 or -2) at natural pH levels, enabling it to be removed with greater efficiency. Although both forms have strong affinities for iron complexes, they behave oppositely with respect to pH. In general in the pH range 3- 10, adsorption of arsenate decreases with increasing pH while the adsorption of arsenite increases. Conversion to As⁺⁵ is a critical element of most arsenic treatment processes. This conversion can be accomplished by adding an oxidizing agent such as chlorine, ozone, or permanganate. (USEPA Arsenic Treatment Technology Evaluation Handbook for Small Systems, July 2003).

[0074] Ozone is the oxidizer of choice for use in method and system according to the present invention. Ozone is extremely effective at the oxidation of As⁺³ to As⁺⁵, with complete oxidation achieved in less than 15 seconds. No adverse effect was observed in the presence of dissolved manganese or iron, but sulfide slowed the oxidation of As⁺³ considerably (University of Houston, Ghurye and Clifford, 2000). Ozone's half-life in water is only several minutes, but since ozone is very reactive in an aqueous environment, ozone can oxidize material between 10 to 1,000 times faster than most oxidants used in water treatment (Hoishe and Bader, 1983). The presence of carbonate ions in the groundwater can provide stabilization and double the half life of dissolved ozone at a pH of 8.0 (Staehelin and Holgne, 1982).

[0075] The dosing rate for ozone within the treatment process varies with water chemistry, presence of reactive ions, and organic material. Although organic matter can greatly increase ozone demand, organics typically are not present in the deep groundwater wells utilized for municipal public water supply. High levels of sulfides typically are not present when elevated levels of arsenic are found in groundwater. This is often due to differences in the geological formations which generally contain these elements. Therefore, without significant load contributions from iron or manganese in the groundwater, approximately 12 pounds of ozone is applied per 1.0 million gallons of water treated for arsenic removal.

[0076] FIG. 7 shows a graph of the typical distribution of floe particles by size for a flow stream treated with ferric chloride used as a coagulant, with the percent by volume of a given particle size plotted against the particle size. The effect of adding a cationic polymeric flocculants for a filter aid is shown in FIG. 8. The raw water with an adjusted pH 6.8 was treated with 10 mg/ml OfFeCl₃ and filtered using a 0.2 μm microfiltration system. In the absence of a polymer, the permeate flux drops to about 50% of the clean water flux (water flow per square foot of surface area of the clean membrane) in less than 1 hour. Upon the addition of 0.3 mg/ml of a cationic polyacrylamide, MW 10⁷, (Cytec Industries #2461) the permeate flux remains constant at the rated clean water flux rate for over 3 hours before a decrease in performance is observed.

[0077] FIG. 8 shows that flocculation with the addition of a polymer followed by microfiltration is significantly better than without the use of a small amount of polymer for arsenic removal. Tangential flow microfiltration has a number of advantages since the membrane pores are large (0.22 μm or larger), the pumping costs are much lower and fluxes much higher than other membrane based processes such nanofiltration and reverse osmosis. Additionally, microfiltration modules are easy to scale up and membrane cleaning is simple.

[0078] Flocculation using ferric ions can lead to significant arsenic adsorption onto the ferric complexes present. However, the efficiency of this adsorption depends on the pH of the water and the presence of other ions (see FIGs. 9-10). Microfiltration of the flocculated water results in rejection of the floes formed on the membrane, thus leading to low turbidity and arsenic removal in the filtrate. Addition of small amounts of cationic polymeric flocculant can greatly increase the permeate flux during microfiltration. (Arsenic Removal from Drinking Water by Flocculation and Microfiltration, Hana, Runnellsb, Zimbronb, Wickramasinghea, 2002).

[0079] For groundwater treatment systems, optimized coagulation-filtration systems are capable of achieving over 95% removal of arsenic and producing water with less than 5 μg/L of arsenic. Influent arsenic levels do not appear to impact the effectiveness of this treatment process. The use of pre-packaged coagulation-assisted microfiltration systems is a realistic possibility for new installations where water quality precludes the use of sorption treatment (EPA, Arsenic Design Manual for Small Systems, 2002). Due to limited performance research, it has not yet been designated as a "Best Available Technology" by the USEPA.

[0080] From the foregoing disclosure, it will be understood by those having skill in the art that the method and system of present invention have a number of advantages. They are capable of removing metallic ions contaminants (such as arsenic, cadmium, cyanide, chromium, copper, lead, molybdenum, nickel, selenium, uranium, and zinc) from solution within a pressurized liquid flow stream. In comparison to previous methods and systems, they offer decreased chemical consumption with a corresponding decrease in the amount of sludge generated. This feature leads to significant cost savings in the total operational cost of the system. Because of the simplicity in operating and maintaining the system of the invention, the time devoted to process control and maintenance is significantly reduced, which results in increased operator utilization. Therefore, the process and system of the present invention offer reliable process results at a lower capital and operating cost than any other known system currently available. Not only is the estimated capital cost of an iron precipitation facility lower, but the estimated annual operation and maintenance costs would be 60% of most other alternatives.

[0081] The system of the present invention addresses the problem of fouling at the filter elements so to maximize the operational availability of the unit and reduce the volume of liquid and solid waste streams generated by the removal of metallic elements from the flow stream. The system thereby provides a prefabricated process treatment unit wherein one or more metal ions can be removed from a flowing, pressurized liquid flow stream. The basic prefabricated treatment unit provides a high degree of reliability and operational flexibility in the removal of a particular metallic element, often irregardless of the initial influent concentrations of the targeted element. Conclusion

[0082] While the particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the teachings of the present invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the claims when viewed in their proper perspective based on the prior art.

Claims

What is claimed is:

1. A method for removing metallic ion contaminants from a pressurized flow stream, the method comprising: introducing an oxidizing agent into the flow stream; introducing a flocculant into the flow stream; removing inorganic and organic constituents from the flow stream by co-precipitation to provide a settled flow stream; and filtering the settled flow stream to remove residual particulate matter from the settled flow stream.

2. The method of claim 1 further comprising introducing a polymer into the flow stream for aiding the filtering of the settled flow stream.

3. The method of claim 1 wherein the oxidizing agent comprises ozone.

4. The method of claim 1 wherein the filtering of the settled flow stream comprises a microfiltering process.

5. A system for removing metallic ion contaminants from a pressurized flow stream, the system comprising: means for introducing an oxidizing agent into the flow stream; means for introducing a flocculant into the flow stream; a reactor vessel for retaining the flow stream for a predetermined retention time, during which the process stream and chemical additives react and the co-precipitation reaction occurs, thereby creating particulate material; and a filtration unit for filtering the settled flow stream to remove residual particulate matter from the settled flow stream.

6. The system of claim 5 wherein the reactor vessel includes: an inlet for receiving an inlet feed flow of the pressurized flow stream and a discharge outlet for discharging a settled flow stream from the reactor vessel; a perforated baffle plate for creating a controlled flow pattern and upward flow velocity of the flow stream within the reactor vessel; a settling area for collecting precipitant that has precipitated from the flow stream; a solids removal outlet for discharging collected precipitant from the reactor vessel; and a settling device comprising a plurality of tubes disposed within the reactaor vessel settling area.