Abstract:
A method and system for processing fluoride-containing wastewater includes treating the wastewater with brine (waste) created by the regeneration process implemented by in ion exchanging water softener. The brine, which is typically disposed of, contains both calcium and magnesium salts, in varying concentrations and ratios. The regeneration process brine is added to the fluoride-containing wastewater within a reaction tank, and the fluoride ion concentration is monitored. When the fluoride ion concentration falls below a predetermined level (e.g., 15 ppm), the flow of regeneration process brine is stopped. A pH controller monitors the pH within the reaction tank, and adds a basic agent to ensure that the pH remains above a predetermined level (e.g., pH&gt;9). The pH control results in a clear effluent, and a sludge having a high settling rate and a high dewater ability.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to methods for efficiently treating fluoride-containing waste. 
     
    
     RELATED ART 
       [0002]    Fluoride found in wastewater generated by semiconductor fabrication plants (or other industrial plants) must be removed before the wastewater may be safely disposed. In many cases fluoride-containing wastewater is treated by a calcium salt addition process, followed by precipitation of calcium fluoride and further dewatering by filter press. The cost of chemicals (e.g., calcium salt) is a significant part of total cost of waste treatment. Precise and complicated control of the chemical dosing is typically required due to variations in fluoride concentration in the wastewater feed. 
         [0003]    Application of adding calcium salt for the removal of fluoride is known in waste treatment. For example, a system and method for removing fluoride from wastewater by the addition of calcium salts is described by G. A. Krulik, et al., in U.S. Pat. No. 6,645,385. Krulik et al. teach a single fluoride sensing electrode disposed at the reaction tank for measuring a concentration of fluoride in the influent wastewater, and a programmable controller that defines a setpoint of fluoride concentration in the reaction tank, and automatically controls the addition of calcium salts based on the setpoint and an output signal provided by the single fluoride sensing electrode. 
         [0004]    Another method of treating fluoride-containing wastewater is described by Hsein et al., in U.S. Pat. No. 7,182,873. Hsein et al. teach that a primary fluoric ion concentration detection process is initially performed upon the wastewater. The wastewater is then introduced into a first reaction tank, and a primary calcium salt addition process is performed to add calcium salt into the first reaction tank, wherein the dosage of the calcium salt is determined according the fluoric ion concentration detected during the primary fluoric ion concentration detection process. The wastewater and calcium fluoride are then delivered into a second reaction tank, and a secondary calcium salt addition process is performed. A solid-liquid separation process is then performed, and a secondary fluoric ion concentration detection process is then performed upon the wastewater. The dosage of the calcium salt in the secondary calcium salt addition process is determined in a feed back control manner according to a fluoric ion concentration detected in the secondary fluoric ion concentration detection process. 
         [0005]    Both Krulik et al. and Hsein et al. only consider the use of calcium salts for use in fluoride waste treatment. Moreover, both Krulik et al. and Hsein et al. require the measuring of fluoride concentration in the influent wastewater, and dosing with calcium salt with a known concentration based on the measured fluoride concentration of the influent wastewater. This undesirably results in relatively complicated and costly fluoride treatment systems. 
         [0006]    It would therefore be desirable to have an improved system and method for treating fluoride-containing wastewater, which does not exhibit the above-described deficiencies of conventional fluoride treatment systems. 
       SUMMARY 
       [0007]    Accordingly, the present invention provides an efficient system for treating fluoride-containing wastewater that uses the waste produced by a regeneration cycle in an ion exchange water softener, instead of calcium salts. The waste (brine) produced by the regeneration cycle of an ion exchange water softener contains both calcium and magnesium salts, which react with fluoride present in the fluoride-containing wastewater. The brine produced by the regeneration cycle of an ion exchange water softener (hereinafter referred to as regeneration process brine) is readily available and inexpensive. For example, regeneration process brine is typically available from an ultrapure water (UPW) plant that softens raw water at a semiconductor fabrication facility. 
         [0008]    In accordance with one embodiment, the regeneration process brine is initially neutralized to a pH up to about 7. The fluoride-containing wastewater is pumped into a reaction tank, and the regeneration process brine is then added to the reaction tank. The regeneration process brine has varying concentrations and ratios of calcium and magnesium salts. As a result, the dose of the regeneration process brine cannot be predetermined based on the fluoride concentration of the influent fluoride-containing wastewater. Consequently, the fluoride ion concentration of the influent fluoride-containing wastewater is not measured in accordance with the present invention. 
         [0009]    Rather, the dose of the regeneration process brine is defined by a predetermined setpoint of the residual concentration of fluoride in treated effluent only. That is, regeneration process brine is added to the reaction tank until a predetermined setpoint of residual fluoride concentration is achieved in the reaction tank. 
         [0010]    The pH of the contents of the reaction tank is also adjusted to have a value greater than 9, thereby providing efficient clarification (i.e., low turbidity) of the effluent, a high settling rate of the resulting sludge, and a high dewater ability of the resulting sludge. 
         [0011]    The present invention results in cost savings associated with the purchase of calcium salts, as well as the ability to eliminate the system required for the storage and dosing of these calcium salts. In addition, cost savings are realized because there is no need to dispose of regeneration process brine as a waste product. Moreover, the dosing system is simplified, as there is no need to measure the fluoride concentration of the influent wastewater. 
         [0012]    The present invention will be more fully understood in view of the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a flow diagram illustrating a system and method for treating fluoride-containing wastewater in accordance with a first embodiment of the present invention. 
           [0014]      FIG. 2  is a flow diagram illustrating a system and method for treating fluoride-containing wastewater in accordance with a second embodiment of the present invention. 
           [0015]      FIG. 3  is a flow diagram illustrating a system and method for treating fluoride-containing wastewater in accordance with a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The ion exchange water softener is one of the most common tools used in water treatment. The function of an ion exchange water softener is to remove scale-forming calcium and magnesium ions from hard water, thereby ‘softening’ the water. An ion exchange water softener typically includes a tank that contains small beads of synthetic treated resin. The resin is initially treated to adsorb hydrogen or sodium ions. Hard water containing calcium and magnesium ions are passed through the resin. The resin has a greater affinity for multi-valent ions, such as calcium and magnesium ions, than it does for hydrogen or sodium ions. As a result, the calcium and magnesium ions adhere to the resin, releasing the hydrogen or sodium ions. In this manner, the water softener exchanges the hydrogen or sodium ions for the calcium and magnesium ions present in the water. 
         [0017]    After equilibrium has been reached (i.e., after the quantity of calcium and magnesium ions adsorbed by the resin is large enough that ion exchange no longer takes place), the resin can be regenerated. During the regeneration process, HCl or NaCl solution is passed through the resin, exchanging the calcium and magnesium ions previously adsorbed by the resin with the hydrogen or sodium ions. The resin&#39;s affinity for the calcium and magnesium ions is overcome by using a highly concentrated HCl or NaCl solution. At the end of the regeneration process, the resin has adsorbed hydrogen or sodium ions, and may be re-used to treat hard water in the manner described above. The waste product of the regeneration process is brine (hereinafter referred to as “regeneration process brine”) that includes both calcium and magnesium salts. Regeneration process brine is typically generated at a UPW plant that softens raw water at a semiconductor fabrication facility. Regeneration process brine can also be obtained inexpensively from other industrial plants that implement water softening. 
         [0018]    The present invention implements fluoride waste treatment without use of costly chemicals and complicated control systems. Regeneration process brine is used as a chemical for precipitation of fluoride from fluoride-containing wastewater. Process control is based on measurement of residual fluoride concentration and pH in a reaction tank. Regeneration process brine is added to fluoride-containing wastewater until achieving a setpoint of residual fluoride concentration in a reaction tank. The pH is then adjusted to an optimal range of greater than 9 to provide efficient separation of solids from effluent and for obtaining sludge with a high dewater ability. 
         [0019]    In accordance with the present invention, the regeneration process brine has varying concentrations of both calcium and magnesium salts. Moreover, the ratio of calcium salts to magnesium salts within the regeneration process brine is variable. As a result, the dose of the regeneration process brine cannot be predetermined based on the concentration of fluoride in the influent fluoride-containing wastewater. Instead the dose of the regeneration process brine is defined only by a setpoint of residual concentration of fluoride in the treated effluent wastewater. Optimizing the pH range assures a high settling rate of the sludge, low turbidity of the effluent, and high de-water ability of the sludge. 
         [0020]    By treating the fluoride containing wastewater with regeneration process brine, it is unnecessary to purchase costly calcium salts. Moreover, it is unnecessary to provide a system for storage and dosing of these calcium salts. In addition, cost savings are realized because there is no need to dispose of the already available regeneration process brine. 
         [0021]    Furthermore, maintaining an optimal pH range (pH&gt;9) provides additional savings because there is no need to provide additional chemicals for coagulation and flocculation of solids, or a control system for introducing such additional chemicals. 
         [0022]    Several specific embodiments of the present invention will now be described in detail. 
         [0023]      FIG. 1  is a block diagram of a fluoride wastewater treatment system  100  in accordance with a first embodiment of the present invention. As illustrated in  FIG. 1 , regeneration process brine (obtained from the regeneration process of an ion exchange softener used for pretreatment of raw water in a UPW plant of a semiconductor fabrication facility) is added to accumulation and neutralization tank  101 . Hydrochloric acid, which is inherently present in the regeneration process brine, causes this brine to have a relatively low pH. The regeneration process brine is neutralized with a basic agent to create a neutralized brine solution having a pH of up to about 7. In accordance with one embodiment, the basic agent added to tank  101  is NaOH. However, it is understood that other basic agents can be used in other embodiments. 
         [0024]    Influent fluoride wastewater is pumped into reaction and settling tank  102 . In the described embodiment, this fluoride wastewater contains about 30,000 ppm of fluoride, mostly in sodium form, and the pH of this fluoride wastewater is about 10. The neutralized brine solution is then added to the reaction and settling tank  102  through a flow control device  110 , while a mixer is controlled to mix the contents of this tank  102 . During this process, pH controller  115  monitors the pH level of the mixture in the tank  102 . PH controller  115  causes a basic agent (e.g., NaOH) to be added to the reaction and settling tank  102 , as necessary, to maintain a pH greater than 9. Note that because the regeneration process brine is initially neutralized to a pH of about 7 (in tank  101 ), the regeneration process brine added to the reaction and settling tank  102  does not drastically reduce the pH of the influent fluoride wastewater. As a result, it becomes easier for pH controller  115  to maintain a pH greater than 9 within tank  102 . 
         [0025]    During the above-described process, a fluoride monitor  120  detects the residual fluoride ion concentration of the contents of the reaction and settling tank  102 . In response to detecting that the residual fluoride ion concentration of the mixture in tank  102  has been reduced to a predetermined level (for example 20 ppm), fluoride monitor  120  activates a control signal (STOP), which causes flow control device  110  to stop the flow of neutralized brine solution to the reaction and settling tank  102  (i.e., to stop the dosing of the neutralized brine solution). At this time, the mixer within the reaction and settling tank  102  is switched off, and sludge, comprising mostly of calcium fluoride (CaF 2 ) and magnesium fluoride (MgF 2 ), is separated from the effluent by sedimentation. The separated effluent can be safely discarded from the tank  102  into the sewer system  190 . 
         [0026]    After the separated effluent has been removed from the reaction and settling tank  102 , the remaining sludge is transferred from tank  102  into a thickener tank  103 , wherein further concentration of the sludge occurs. Liquid removed from the sludge within the thickener tank  103  can be safely discarded into the sewer system  190 . The sediment remaining in the thickener tank  103  is transferred from the thickener tank  103  to a filter press  104 , wherein de-watering of the sludge is performed. The filtrate extracted from the sludge within the filter press  104  can be safely discarded into the sewer system  190 . The de-watered sludge remaining in the filter press  104  is disposed of in an appropriate manner. For example, the de-watered sludge can be used in the manufacturing of cement or disposed of according to environmental requirements. 
         [0027]    In accordance with one embodiment of the present invention, maintaining a pH greater than 9 within the reaction and settling tank  102  advantageously provides a clear effluent, a high settling rate, and a sludge with a high de-water ability, without requiring the use of coagulants and/or flocculants. 
         [0028]      FIG. 2  is a block diagram of a fluoride wastewater treatment system  200  in accordance with a second embodiment of the present invention. Because system  200  is similar to system  100 , similar elements in  FIGS. 1 and 2  are labeled with similar reference numbers. System  200  replaces the reaction and settling tank  102  of system  100  with two separate tanks. Thus, system  200  includes reaction tank  201  and settling tank  202 . Processing proceeds in the manner described above in connection with  FIG. 1 , wherein the influent fluoride-containing wastewater is pumped into reaction tank  201 , and the neutralized regeneration process brine is then added to the reaction tank  201 , while a mixer is controlled to mix the contents of reaction tank  201 . During this process, pH controller  115  monitors the pH level of the mixture in the reaction tank  201 . Again, pH controller  115  adds a basic agent (e.g., NaOH) to the reaction  201 , as necessary, to maintain a pH greater than 9. 
         [0029]    During the above-described process, the fluoride monitor  120  detects the residual fluoride ion concentration of the contents of the reaction tank  201 . In response to detecting that the residual fluoride ion concentration of the mixture in the reaction tank  201  has been reduced to a predetermined level (for example 15 ppm), fluoride monitor  120  activates the control signal (STOP) to stop the flow of neutralized brine solution to the reaction tank  201 . At this time, the mixer within the reaction tank  201  is switched off, and the suspension of CaF 2  and MgF 2  within the reaction tank  201  is transferred to settling tank  202 . Within the settling tank  202 , the sludge (CaF 2  and MgF 2 ) is separated from the effluent by sedimentation. The separated effluent is safely discarded from the settling tank  202  into the sewer system  190 , and the sludge is processed in thickener tank  103  and filter press  104  in the manner described above in connection with  FIG. 1 . If the available capacity of the settling tank  202  and/or the filter press  104  is limited, coagulants and/or flocculants can be added to the suspension to facilitate the separation of the sludge from the effluent. By separating the reaction tank  201  and the settling tank  202  as set forth in system  200 , the capacity of system  200  is advantageously increased (with respect to system  100 ). 
         [0030]      FIG. 3  is a block diagram of a fluoride wastewater treatment system  300  in accordance with a third embodiment of the present invention. Because system  300  is similar to systems  100  and  200 , similar elements in  FIGS. 1 ,  2  and  3  are labeled with similar reference numbers. System  300  eliminates the thickener tank  103  and the settling tank  202  from system  200 . Processing proceeds in the manner described above in connection with  FIG. 2 , wherein the suspension of CaF 2  and MgF 2  from the reaction tank  201  is transferred directly to the filter press  104 . The filtrate from the filter press  104  is safely disposed into the sewer system  190 , while the dewatered sludge from the filter press  104  is properly disposed. 
         [0031]    If the available capacity of the filter press  104  is limited, coagulants and/or flocculants can be applied to the suspension to facilitate the separation of the sludge from the filtrate. Eliminating the settling tank  202  and the thickener tank  103  from system  300  advantageously allows system  300  to simplify batch treatment process or, if required, continuously treat the fluoride wastewater. That is, there is no need to wait for sedimentation or thickening of the suspension, so the process steps can be performed with fewer delays for a more continuous process flow. 
         [0032]    Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. Thus, the invention is limited only by the following claims.