Patent Publication Number: US-2011062070-A1

Title: Apparatus for treating wastewater, particularly wastewater originating from a process for the production of photovoltaic cells

Description:
The present invention relates to an apparatus for the treatment of wastewater, particularly wastewater originating from a process for the production of photovoltaic cells or the like. 
     BACKGROUND OF THE INVENTION 
     Currently, a process for the production of photovoltaic cells based on crystalline silicon consumes extremely large amounts of water and generates wastewater with a high content of dangerous components, i.e., components that are harmful for people and the environment. 
     Currently known systems for treating wastewater in this field are aimed generally at converting wastewater with a lower level of acidity and wastewater with a very low level of acidity so as to lower their content of pollutants below the limit set by local laws for discharge into a sewage system. 
     This reduction of the content of pollutants, among which the most harmful are fluorides, must be even greater if, due to lack of sewage systems, discharge into surface water (river, ditch, or the like) is wanted or needed. 
     Experience teaches that possessing or not the requirements for discharging into a sewage system or into surface water can become a discriminating parameter between the survival and the failure of a company that manufactures photovoltaic cells, since the obligation to remove the stored wastewater that cannot be discharged, in order to have it disposed by specialized third-party companies, can cause cost items, as well as the substantial slowing of the production rates of a cell production line which may endanger the market strength of such manufacturing company. 
     For example, currently in a line for the production of photovoltaic cells equivalent to 30 MW/year, the wastewater in output to be treated has an average flow-rate of approximately 5 m 3  per hour. 
     This entails, in addition to an enormous expenditure of water resource, also enormous volumes of wastewater to be stored and disposed if the limits set by local applicable statutory provisions are not attained; taking also into account the fact that in currently known wastewater treatment apparatuses at least part of the wastewater that can be treated is processed with reverse-osmosis systems, which generally have an efficiency of 50%, i.e., require 2 cubic meters of mains or well water for the production of 1 cubic meter of osmotized water. 
     Moreover, any acid wastewater current with concentrations of salts higher than 200 ppm must be collected, stored and disposed at an authorized company and organized for the treatment of such wastewater. 
     Another limitation of currently known apparatuses for the treatment of wastewater that originates from a process for the production of photovoltaic cells is linked to the fact that traditional treatment systems work effectively when the wastewater is concentrated with pollutants; the wastewater that a cell production line produces instead has a relatively low pollutant load; this low concentration of free ions in the solution entails a low probability that they will bind and precipitate, separating out of the solution: accordingly, large quantities of chemical substances are required in order to ensure good suppression. 
     These known treatment apparatuses, therefore, entail a consumption of additional chemical substances useful for purifying the water, which are different from the ones already used for the cell production line, with a huge increase in the amount of sludge to be disposed. 
     These known treatment apparatuses entail, as mentioned, a further consumption of water in addition to the already high consumption needed for the operation of the cell production process, especially due to the reverse osmosis systems. 
     All this substantially entails, for the cell manufacturing company, that it is impossible to optimize the water resource, and also entails the need to dispose of a considerable quantity of toxic and noxious wastewater at dedicated external companies, with consequent environmental dangers due to the large volume of wastewater transported on road vehicles. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide an apparatus for treating wastewater, particularly wastewater originating from a process for the production of photovoltaic cells or the like, that is capable of obviating the above-mentioned drawbacks of known types of treatment apparatus. 
     Within this aim, an object of the present invention is to provide an apparatus that allows to save on mains water. 
     Another object of the present invention is to provide an apparatus that allows to reduce the environmental impact of the associated process for the production of photovoltaic cells, making it feasible in a manner that is substantially independent of any restrictive local laws or other environmental constraints. 
     Another object of the present invention is to provide an apparatus that reduces the toxic waste to be removed at companies specialized in disposal. 
     Another object of the present invention is to provide an apparatus that is capable of recovering water from wastewater for subsequent uses within the same production process or within the same treatment apparatus. 
     Another object of the present invention is to provide an apparatus for treating wastewater, particularly originating from a process for the production of photovoltaic cells or the like, that can be produced with known machines systems and technologies. 
     This aim and these and other objects, which will become better apparent hereinafter, are achieved by an apparatus for the treatment of wastewater, particularly wastewater originating from a process for the production of photovoltaic cells or the like, characterized in that it comprises:
         a first line for the treatment of wastewater originating from said process, with a high level of acidity (AWC),   a second line for the treatment of wastewater with a low level of acidity (AWD),   a third line for the treatment of wastewater with a very low level of acidity (AWDD),   a fourth line for the treatment of alkaline wastewater (CAWC),       

     the wastewater that exits from said second and third lines being adapted to be sent to a purification line designed for filtering, obtaining pure water and ultrapure water, said pure water and said ultrapure water being adapted to be reused in the same production process from which said wastewater originates, in the associated exhaust gas suppression systems, and in other similar systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the invention will become better apparent from the following detailed description of a preferred but not exclusive embodiment thereof, illustrated by way of non-limiting example in the accompanying drawings, wherein: 
         FIG. 1  is a diagram of an apparatus according to the invention; 
         FIG. 2  is a detailed diagram of a first line of the apparatus according to the invention; 
         FIG. 3  is a detailed diagram of a second line of the apparatus according to the invention; 
         FIG. 4  is a detailed diagram of a third line of the apparatus according to the invention; 
         FIG. 5  is a more detailed diagram of a fourth line of the apparatus according to the invention; 
         FIG. 6  is a more detailed diagram of a purification line of the apparatus according to the invention; 
         FIG. 7  is a diagram of an associated distillation system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the figures, an apparatus for the treatment of wastewater, particularly wastewater originating from a process for the production of photovoltaic cells or the like according to the invention, is generally designated by the reference numeral  10 . 
     The apparatus  10  comprises:
         a first line, shown schematically by the block  12  in  FIG. 1 , for the treatment of wastewater with a higher level of acidity, designated in  FIG. 1  as AWC (Acid Waste Concentrated), originating from a process  11  for example but not exclusively for the production of photovoltaic cells,   a second line  13  for the treatment of wastewater with a low level of acidity AWD (Acid Waste Diluted),   a third line  14  for the treatment of wastewater with a very low level of acidity AWDD (Acid Waste Double Diluted),   a fourth line  15  for the treatment of alkaline wastewater CAWC (Caustic Waste Concentrated).       

     The wastewater that exits from the second line  13  and the third line  14  is adapted to be sent to a purification line  16 , which is designed for filtration, obtaining pure and ultrapure water. 
     Such pure water (PW) and such ultrapure water (UPW) are reused in the same production process  11  from which such wastewater originates, in the associated exhaust gas suppression systems  17  and in other similar systems. 
     The exhaust gases that exit from the production process  11  are designated by the reference numeral  17   a  in  FIG. 1 . 
     Part of the pure and ultrapure water is reused in the same purification line  16 ; these flows are shown schematically by the arrow  16   a.    
     The apparatus  10  according to the invention advantageously also comprises means for the disposal of solid waste  18 . 
     Such means for disposal of solid waste  18  are constituted preferably by a filter press of a per se known type. 
     The first line  12  for the treatment of AWC wastewater with a high level of acidity is shown schematically in greater detail in  FIG. 2 . 
     The first line  12  comprises:
         first means  19  for collecting such wastewater in a first recirculation trap  20 ,   first means  21  for pumping toward first storage tanks  22 ,   means for transferring such current with a high level of acidity AWC toward at least one of the other lines, the second one  13 , the third one  14 , and the fourth one  15 .       

     The first collection means  19  are constituted by the pumps that are dedicated to such wastewater and are associated with each machine of the cell production line; such pumps are powerful enough to recirculate the current within discharge ducts arranged advantageously above the false ceiling of the enclosed space and just below the roof of the factory. 
     The wastewater is sent into a large-diameter duct (90 mm), which is conveniently inclined toward the first recirculation trap  20 , so that the line is never pressurized: only the force of gravity is used to transport the wastewater. 
     The first pumping means  21  are constituted by two pumps  21   a  and  21   b , so as to ensure redundancy in cases of anomaly or maintenance. 
     The first line  12  is designed both for storing an AWC current in case of external disposal and for reuse within the same apparatus  10 . In any case, therefore, it is possible to continue production activity even if the system is unable to reuse such current. 
     Thanks to its high level of residual acidity (&gt;0.5%), such AWC wastewater current can be reused in the apparatus  10  in two different fields:
         for steps for acidification of the reactor for a chemical physical treatment, described hereafter: instead of fresh sulphuric acid, such acid wastewater is used as it arrives from the first line  12 ;   for steps for regeneration of the demineralization means, also described hereafter (in particular only for cationic resins) as a replacement of fresh hydrochloric acid: in this case, the AWC current must be filtered appropriately in order to eliminate any silicates that might be present in the wastewater.       

     The second line  13  for the treatment of AWD wastewater with a low level of acidity, shown schematically in  FIG. 3 , comprises
         second means  23  for collecting such wastewater in a second recirculation trap  24 ,   second means  25  for pumping toward first clarification means  26 ,   first filtering means  27 ,   first demineralization means  28 .       

     The second collection means  23 , in a manner similar to what has been described above for the first line  12 , are constituted by pumps dedicated to such AWD wastewater, designed to recirculate the current inside the discharge ducts arranged above the false ceiling and just below the roof of the factory. 
     The AWD wastewater is sent into a large-diameter duct (90 mm), which is appropriately inclined toward the second recirculation trap  24 , so that the line is never pressurized; the duct is made of PVC and does not have a containment channel. 
     The second trap  24  is part of the system for recirculating the wastewater toward the first step of the treatment, i.e., the clarification means  26 . 
     The second line  13  has a double recirculation pump  25   a  and  25   b  so as to ensure redundancy in case of anomaly or maintenance. 
     Thanks to a twin-scale conductivity meter arranged directly after the pumps  25   a  and  25   b , the salt content of the wastewater produced by the second line  13  can be controlled: the system is therefore capable of giving continuous feedback also to production. 
     The third line  14  for the treatment of AWDD wastewater with a very low level of acidity, shown schematically in detail in  FIG. 4 , comprises:
         third means  29  for collecting such wastewater in a third recirculation trap  30 , fully similar to the ones described above for the first line  12  and the second line  13 ,   third pumping means  31 , which are similar to the corresponding ones described above, for pumping toward second clarification means  32 ,   second filtration means  33 ,   second demineralization means  34 .       

     The fourth line  15  for the treatment of CAWC alkaline wastewater, shown schematically in  FIG. 5 , comprises:
         fourth means  35  for collecting such wastewater in a fourth recirculation trap  36 ,   fourth means  37  for pumping toward at least one storage tank  38 ,   means  39  for a chemical physical treatment for purifying the stored wastewater.       

     The wastewater treated by the fourth line  15  also comprises the wastewater that arrives from all the auxiliary apparatuses, designated by the reference numeral  11   a  in  FIG. 5 . 
     In view of the impurity and variability in terms of pollutants of such wastewater, the decision has been made to keep the CAWC wastewater discharge pipes separate with respect to the current that arrives from the so-called SDE line: it is possible to reuse the current that arrives only from the SDE line as an alkaline additive (instead of soda) in the wet scrubber for exhaust gas suppression. 
     The purification line  16  of the water that exits from such demineralization means  28  and  34  comprises a regenerable mixed-bed system  40  with which a non-regenerable mixed-bed system  41  is associated in series, such systems being both of a per se known type. 
     The regenerable mixed-bed system  40  is termed in this way because it is possible to reconstitute the resins by means of the regeneration process. 
     For the sake of redundancy, there are two columns, one in use and one in standby (or in regeneration). 
     Part of the pure water that exits from the regenerable mixed-bed system  40  goes toward the non-regenerable mixed bed  41  and part goes to keeping wet both the resins of the regenerable mixed bed and the resins of the demineralization means that are in standby (already regenerated). 
     The pure water in output from the regenerable mixed beds  40  is stored in a first tank  40   a  and kept under nitrogen blanketing. 
     The water in output from the regenerable mixed bed  40  has an average conductivity of 0.06-0.15 μS/cm and it is referenced with the term “pure water” (PW). 
     The water that exits from the regenerable mixed-bed system  40  is sent to the non-regenerable mixed-bed system  41 . 
     Typically, a non-regenerable mixed bed has an efficiency similar to a regenerable one in terms of mass exchange but higher in terms of final purity. 
     For the sake of redundancy, there are two columns, one in use and one in standby or in regeneration. 
     Downstream of the non-regenerable mixed beds  41  there is a second tank  41   a  made of stainless steel with nitrogen blanketing, whereas all the pipes where the ultra pure water (UPW) flows are made of PVDF. 
     The water in output from the regenerable mixed bed has an average resistivity of 15-18 MOhm/cm. 
     This water is referenced by the term “ultrapure water”. The circuit for recirculation of the ultrapure water is always kept in motion: stagnant water tends to lose its purity properties. 
     Such circuit is kept cold by means of a heat exchanger  41   b . The apparatus  10  according to the invention advantageously comprises means, not shown for the sake of simplicity, for preparing and distributing a polyelectrolyte, means for preparing and distributing milk of lime, and distillation means  43 , these last being shown schematically in  FIG. 7 . 
     The distillation means  43  are constituted by a series of evaporators  43   a ,  43   b ,  43   c  with which a discharge tank  44  is associated for the concentrates that exit from the evaporators  43   a ,  43   b ,  43   c , a system for controlling the alkalinity of the solution in input to the distillation means  43 , and an additional tank for storing the distilled water  45 . The distilled water  45  that exits from the distillation means  43  is designed to be sent advantageously:
         upstream of the clarification means  26 ,  32  of the low-acidity wastewater AWD and very low-acidity wastewater AWDD,   to the means for preparing a polyelectrolyte,   to the means for preparing milk of lime.       

     The first clarification means  26 , whose purpose is to make some of the salts that are present in the current settle to the bottom, in addition to reducing the TSS of the current before the filters, are provided by a first static mixer  46 , which has in series in output a first settling tank  47 , followed by a storage tank  48  for the clarified fluid that exits from the settling tank, the sediment deposited on the bottom of such settling tank being adapted to be sent to such filter press. 
     The first filtration means  27 , downstream of the first settling tank  47 , are constituted by a group of at least two filters  27   a ,  27   b , each of which can be bypassed autonomously with respect to the other, with 20-μ and 5-μ cartridges in series. 
     The first demineralization means  28  for such low-acidity AWD current, clarified and filtered, are constituted by two assemblies with four transit stages in series  28   a ,  28   b , of which one is active and the other one is in regeneration or standby alternately. 
     Each assembly  28   a ,  28   b  comprises a first stage, which is constituted by a first column with activated charcoal  49 , followed by a second stage formed by a second column with cationic resins  50 , followed by a third stage constituted by a third column with weak anionic resins  51 , followed by a fourth stage constituted by a fourth column  52  that contains strong anionic resins. 
     Likewise, the second clarification means  32  of the third line  14  are constituted by a second static mixer  53 , which has in series in output a second settling tank  54 , followed by a tank  55  for storing the clarified fluid that exits from the settling tank  54 , the sediment deposited on the bottom of such settling tank being adapted to be sent to such filter press. 
     The second filtration means  33 , downstream of the second settling tank  54 , are constituted by an assembly of at least two filters  33   a ,  33   b , each of which can be bypassed autonomously with respect to the other, with 20-μ and 5-μ cartridges in series. 
     The second demineralization means  34  for the current with very low acidity AWDD, clarified and filtered, are constituted by two assemblies with three transit stages in series  34   a ,  34   b , of which one operates and the other one is in regeneration or standby alternately. 
     Each one of the two assemblies  34   a  and  34   b  comprises a first stage constituted by a first column  56  with an activated charcoal bed, which can be modified with activated alumina, followed by a second stage constituted by a second column with cationic resins  57 , followed by a third stage constituted by a third column with weak anionic resins  58 . 
     The means  39  for chemical physical treatment for purifying the stored wastewater are constituted by a reactor or a tank with a frustum-shaped bottom, provided with a mechanical agitator that is designed to mix uniformly the solution to be treated. 
     The chemical physical treatment system utilizes the tendency of the components within the solution to bond with others, producing solid compounds that tend to settle. 
     Purification with such chemical physical treatment means  39  of a per se known type occurs by binding the components that are harmful for human health and for the environment with other substances that are capable of producing a salt that is inert with respect to the original component and is heavier than water, so as to collect it in sludge. 
     The filter press, which forms the solid waste disposal means  18 , is therefore preset to receive sediments from:
         the bottom of the first settling tank  47  of the line for treatment of the low-acidity current AWD,   the bottom of the first storage tank  48  of the clarified fluid of the line for treatment of the low-acidity current AWD,   the bottom of the second settling tank  54  of the line for treatment of the very low-acidity current AWDD,   the bottom of the reactor for chemical physical treatment  39 .       

     In practice it has been found that the invention thus described solves the drawbacks noted in known types of apparatus for the treatment of wastewater originating from a process for production of photovoltaic cells and achieves the intended aim and objects. 
     In particular, the present invention provides an apparatus  10  which, by way of the redefinition of the wastewater to optimize their subsequent treatment, is based on the choice of the processes for the production of pure water with low water consumption (ion exchange resins instead of reverse osmosis) and suitable for reuse of all the diluted wastewater. 
     This choice is well-grounded, since the osmosis process generates, as mentioned above, a higher quantity of wastewater to be treated and has a much higher energy consumption; vice versa, the demineralization as described above, in steps of successive purity, optimizes both the volume of wastewater generated and the distillation. 
     Moreover, the present invention provides an apparatus  10  that allows optimum reuse of concentrated wastewater: the concentrated acid wastewater (AWC current) is used instead of sulfuric acid for the acidification process and the concentrated alkaline wastewater (CAWC current) is used instead of caustic soda for the alkalinization process. 
     In order to use all of the AWC current produced by the line, the system has an alkaline current deficit. 
     Further, all the CAWC current that arrives from the SDE machine is used in the scrubbers to suppress the acid exhaust gases; the choice has been made therefore to increase the pH of the solution that is present in the scrubbers and to change it more often, so as to:
         consume all the AWC current, avoiding expensive disposal at an external company that is structured and authorized for this purpose,   reduce atmospheric emissions significantly,   reducing drastically the environmental impact of the company where the apparatus  10  is installed and operates;   generating a solid waste whose components are the only components that are present in the production process.       

     Moreover, the present invention provides an apparatus  10  that allows reuse of all the liquid wastewater produced by the auxiliary apparatuses. 
     In brief, therefore, the invention provides an apparatus  10  that is capable of reducing considerably the consumption of mains water, eliminate almost entirely the environmental impact of the production process of photovoltaic cells, thus freeing the company that adopts it from the strictest limitations set by local laws and from the requirement to rely on toxic waste disposal companies. 
     Moreover, the present invention provides an apparatus  10  for treating wastewater, particularly originating from a process  11  for producing photovoltaic cells or the like, that can be provided by means of known technologies. 
     The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may further be replaced with other technically equivalent elements. 
     The apparatus  10  is to be understood to be applicable also to processes for the production of cells from monocrystalline silicon as well as polycrystalline silicon. 
     Constructive variations of the hydraulic solutions of the components of the apparatus must be considered part of the patent. 
     The possibility to integrate all the auxiliary wastewater of the production site is considered part of the patent. 
     The possibility to apply the integrated system also in the production of cells from monocrystalline silicon must be considered part of the patent. 
     Other systems for filtration/dehydration of the produced sludge must be considered part of the patent. 
     The use of regenerating agents for the resin apparatuses of a different kind must be considered part of the patent. 
     Resins other than the ones used but employed with the same intention must be considered part of the patent. 
     In practice, the materials used, so long as they are compatible with the specific use, as well as the dimensions, may be any according to requirements and to the state of the art. 
     The disclosures in Italian Patent Application No. PD2008A000143 from which this application claims priority are incorporated herein by reference. 
     Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.