Abstract:
A process for treatment of waste containing organic material, phosphorous and water in suitable amounts to be a pumpable sludge and in order to be oxidizable through supercritical water oxidation, comprises the steps of: putting the sludge into conditions being supercritical for water; adding oxidant, particularly, oxygen to the sludge, wherein the organic material contained in the waste is substantially completely oxidized by means of supercritical water oxidation; separating the phosphorous from water and from carbon dioxide formed during the oxidation; and recovering the phosphorous by means of dissolving the phosphorous in an alkali.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to a process and to a plant, respectively, for treatment of waste such as sludge from water purification or wastewater treatment plants.  
         DESCRIPTION OF PRIOR ART AND BACKGROUND OF THE INVENTION  
         [0002]    As a result of purification of water and cleaning of wastewater, sludge is obtained due to chemical precipitation and flocculation using so called flocculants, which often are aluminum or iron based salts. The sludge comprises further typically organic material; particularly sewage sludge and digested comprise large amounts of organic material. Further, such sludge may contain phosphates and heavy metals.  
           [0003]    Recovery of precipitation chemicals and phosphor may be obtained in a plurality of manners, for instance by means of leaching using acid followed by a cleaning process, or instance precipitation, ion exchange or nano-filtration, but often the organic material imposes problems. Normally, the organic material in the waste is burned, wherein, however, an organic residue remains unburned due to incomplete burning. The high temperature that is obtained using combustion causes the residue waste or ash to be sintered together to larger, continuous solid material, which is difficultly soluble.  
           [0004]    A plant or recovery of matter in such residue product is described in or instance WO 00/50343. The plant comprises an apparatus, such as a beating or a crushing apparatus, for finally dividing the residue product, a mixing tank for mixing the finally divided residue product with an acid, wherein matter to be recovered is dissolved in the acid, and a filter for filtration of unsolved matter. Further, the plant comprises a plurality of ion exchanges and possibly nano-filters, for separation of the matter dissolved in the acid. It is disclosed that the organic content in the residue product ought to be less than 2% in order not to stop up the ion exchanges.  
           [0005]    Drawbacks of such an approach comprise that the residue product has to be treated into a very finally divided form in order to effectively dissolve matter therein, because otherwise the risk is large that some part of the matter is not dissolved and is thus filtered away together with the solid phase. Further only ashes having low organic content may be used in the recovery process.  
           [0006]    In EP 0 723 938 A2 is described how organic contaminants are separated from water by means of a precipitation chemical, and the resulting sludge is oxidized, whereafter the residue product is thickened and the precipitation chemical is recovered by means of leaching in sulphuric acid. The oxidation is preferably conducted through ozone oxidation, typically at room temperature and atmospheric pressure, but may be performed with oxidation under 375° C. and 22 MPa or through supercritical oxidation above 375° C. and 22 MPa.  
         SUMMARY OF THE INVENTION  
         [0007]    In said. EP 0 723 938 A2 is disclosed supercritical oxidation incidentally as one of many alternatives, but nothing further is disclosed about the technique.  
           [0008]    The inventors of the present invention have surprisingly discovered that the oxidized residue product created in supercritical water oxidation (SCWO) is made of a fine, very reactive powder and that this enables recovery of precipitation chemicals and phosphates to be made in manners, which were not earlier possible to use.  
           [0009]    It is therefore an object of the present invention to provide processes and plants for treatment of waste, which contains water, organic material and a precipitation chemical or phosphorous, where the organic material contained in the waste is oxidized and the precipitation chemical or the phosphorous is recovered, which may use recovery principles that are more effective than earlier known principles.  
           [0010]    It is in this context a particular object of the present invention to provide such processes and such plants for treatment of a slurry from a cleaning plant comprising organic material, an iron salt and phosphorous (in the form of phosphate), wherein the organic material contained in the slurry is oxidized and the phosphate is recovered by means of leaching in an alkali and preferably by means of subsequent precipitation as calcium phosphate.  
           [0011]    Another object of the present invention is to provide such processes and such plants for treatment of a slurry from a water purification plant comprising organic material, and an aluminum salt, where the organic material contained in the slurry is oxidized and the aluminum salt is recovered by means of leaching in hydrochloric acid, preferably using shortage of acid, for direct formation of polyaluminum chloride.  
           [0012]    Yet, a further object of the invention is to provide such processes and such plants, which are effective, uncomplicated, flexible and cost-efficient.  
           [0013]    Still a further object of the invention is to provide such processes and such plants, which are unobjectionable in environmental respects.  
           [0014]    According to the invention these objects among others are attained by processes and plants in accordance with the appended patent claims.  
           [0015]    By means of the present invention considerably improved performance is attained. The inventors of the present invention have realised that by oxidizing the waste using supercritical water oxidation and then recovering phosphates by effective dissolving of the oxidized waste in an alkali, for instance sodium hydroxide, which surprisingly has been shown to be possible as the supercritically oxidized waste is constituted by a fine, very reactive powder, where the phosphorous is well separated from any occurring metals in the waste, a technique is obtained which is considerably better than earlier known approaches. If an iron salt is used as precipitation chemical this is not dissolved in the alkali, but can be separated from the phosphorous and then be recovered through leaching in an acid.  
           [0016]    Furthermore, the inventors have realised that by oxidizing the waste using supercritical water oxidation and then recovering the precipitation chemical—if this is an aluminum salt—through effective leaching of the oxidized waste in hydrochloric acid, preferably with shortage of acid and preferably at atmospheric pressure and below the boiling point for water, in order to directly create polyaluminum chloride, which surprisingly has been shown possible as the supercritically oxidized waste is constituted by a fine, very reactive powder, a technique, which is considerably better than earlier known techniques, is obtained.  
           [0017]    If the waste contains a heavy metal, such as for instance copper, it may be precipitated in a reduction process by means of adding pure aluminum.  
           [0018]    Further, the invention may be applied for destruction of other organic waste, including for instance waste oil and other industrial waste, and foodstuff industry waste. By means of the present invention any phosphorous in the waste may be recovered and any possible heavy metals can be separated.  
           [0019]    The supercritical water oxidation is a completely closed process without any risk for production of unwanted bi-products. Toxic chlorine and fluorine based hydrocarbons such as PCB and dioxins are destructed. Only a minimum amount of gaseous products are formed, mainly carbon dioxide, and no flue gas cleaning is required.  
           [0020]    Further features of the invention, and advantages of the same, will be apparent from the detailed description of embodiments of the present invention herein below and the appended FIGS. 1-3. 
       
    
    
     SHORT DESCRIPTION OF THE FIGURES  
       [0021]    [0021]FIG. 1 shows a schematic block scheme of a plant for treatment of waste according to an embodiment of the present invention.  
         [0022]    [0022]FIG. 2 shows a schematic block scheme of a recovery system, which may comprise part of the plant of FIG. 1.  
         [0023]    [0023]FIG. 3 shows a schematic block scheme of an alternative recovery system, which may comprise part of the plant of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0024]    In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular applications, processes and plants, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to the man skilled in the art that the present invention may be practised in other embodiments that depart from these specific details.  
         [0025]    With reference to FIG. 1 a plant for treatment of waste according to an embodiment of the present invention will be described.  
         [0026]    The plant comprises one oxidation stage and one recovery stage.  
         [0027]    The oxidation stage is mainly a system or supercritical water oxidation (SCWO) and includes a supply conduit  11  for wastewater, a high-pressure pump  12 , a heat exchanger  13 , a heater  14 , and a supply conduit  15  for oxygen, a SCWO-reactor  16 , and output conduit  17 , a cooler  18  and a separator  19 . The SCWO-reactor  16  includes two inlets  16   a  and  16   b  for the supply conduits  11  and  15 , and an outlet  16   c,  to which the output conduit  17  is connected. The separator  19  comprises an inlet  19   a  for the output conduit  17 , and three outlets  19   b,    19   c  and  19   d  for separation of gases, for instance CO 2 , O 2  and N 2 , cleaned water and residue products.  
         [0028]    The recovery stage comprises a supply conduit  20  and a recovery tank  21  provided with *two inlets  21   a,    21   b  and two outlets  21   c,    21   d.  The supply conduit  20  is connected between the outlet  19   d  of the separator and the inlet  21   a  of the recovery tank. In operation, an alkali or an acid is supplied to the recovery tank via the inlet  21   b,  wherein the alkali or the acid dissolves the product that is be recovered and this is output from the tank  21  through the outlet  21   c,  whereas unsolved products or so called residue waster is exhausted from the tank  21  through the outlet  21   d.  The recovery stage may be provided with further devices for separating different species dissolved in the alkali or the acid. More about this will be closer discussed below with reference to FIGS. 2 and 3.  
         [0029]    The plant of FIG. 1 is in its simplest version intended to treat waste containing organic material and a precipitation chemical, for instance an iron or aluminum salt, and a brief. description of the operation of such a plant follows. The waste may typically be sludge from a cleaning or a water plant.  
         [0030]    If the waste does not already contain water in suitable amount in order to be a pumpable sludge and in order to be oxidizable via supercritical water oxidation the waste is put into such consistence by means of thickening or diluting. A suitable dry content ought to be 10-15%, and in order to obtain an effective plant the highest possible dry content is utilized.  
         [0031]    The sludge is pumped through the supply conduit  11  by means of a high-pressure pump, which increases the pressure to a supercritical pressure for water (22, 13 MPa). The sludge is further pumped through a heat exchanger and is preheated by hot, already reacted sludge, which is pumped from the SCWO-reactor  16  through the output conduit  17 . The preheated sludge is further pumped towards the-heater  14 , which heats the sludge to a supercritical temperature (374° C.) for water, or close to such a temperature.  
         [0032]    The sludge is further pumped into the SCWO-reactor  16  through the inlet  16   a.  Simultaneously herewith, oxygen or other oxidant is supplied to the SCWO-reactor  16  through the inlet  16   b.  As a result hereof the organic material contained in the waste is substantially completely oxidized through supercritical water oxidation and the precipitation chemical is converted into an oxide. If the sludge before the reaction has not really reached a supercritical temperature for water the sludge will anyhow react and the released reaction heat will increase the temperature to a supercritical temperature for water and the reaction velocity is increased.  
         [0033]    The reacted sludge is transported through the outlet  16   c  and is guided through the heat exchanger  13  in order to give off heat to the still non-reacted sludge in the supply conduit  11 . Then the reacted sludge is cooled in the cooler  18  or in other apparatus in order to take care of the heat, whereafter the sludge is introduced into the separator  19  through one inlet  19   a.  The separator separates the products mainly by means of the phase, such that gases are guided through the outlet  19   b  at the top, solid material, a so called residue product, is settled such that the main portion of the liquid phase, i.e. water may be exhausted through the outlet  19   c,  and finally the solid phase is exhausted through the bottom outlet  19   d.    
         [0034]    The process in the SCWO-reactor  16  converts rapidly and effectively organic material, which substantially comprises carbon and hydrogen, to carbon dioxide and water at a temperature and pressure above the critical point for water (374° C. and 22, 13 MPa) while releasing energy. The process is completely closed and the destruction efficiency is often higher than 99.9%.  
         [0035]    If the non-reacted sludge only contains water, organic waste and the precipitation chemical, clean carbon dioxide is emitted through the outlet  19   b,  optionally together with oxygen if a surplus of oxygen has been used in the reaction, while the residue product, which is exhausted at  19   d,  comprises an oxide of the precipitation chemical.  
         [0036]    Heavy metals present in the process are converted to their oxides, whereas the phosphorous is oxidized and is exhausted as a residue product at  19   d.    
         [0037]    Smaller amounts of nitrogen compounds, amines and ammonia, which exist in the waste, are converted to molecular nitrogen and not to NOx, which is an acidifying and fertilizing residue product, which is therefore not wanted in the gaseous phase. The nitrogen is outlet at  19   b  together with the other gases.  
         [0038]    The residue products exhausted through the outlet  19   d  exist, due to the nature of the SCWO-reaction, in a finally, reactive form and is well suited for treatment to recover, inter alia, the precipitation chemical.  
         [0039]    Thus, the residue products are transported through supply line  20  into the recovery tank  21  and alkali (if the residue products comprise phosphorous) or hydrochloric acid, particularly using shortage of acid (if the oxidized residue products comprise aluminum) is added through inlet  21   b,  wherein the phosphorous is dissolved or the aluminum is dissolved to directly form polyaluminum chloride. Insoluble material is settled and is exhausted through the outlet  21   d  as a residue waste, while the recovered phosphorous or polyaluminum chloride is exhausted through outlet  21   c,  possibly after filtration.  
         [0040]    With reference next to FIG. 2, which illustrates a recovery system  21  usable in the plant of FIG. 1, a first particular embodiment of the invention will be described.  
         [0041]    The residue product from the SCWO-reactor is in this embodiment a slurry with a fine, very reactive powder comprising iron oxide, phosphorous oxide (P 2 O 5 ), possibly oxides of heavy metals, and silicates and other insoluble residue products, which may originate for instance from sewage sludge from a cleaning plant. This residue product is collected in a thickener/storage tank  22  and Is then filtered through a filter  23  in order to increase the dry content. The thickened and filtered residue product is collected in a reactor  24 , to which caustic solution (NaOH) is added. It has surprisingly been seen that the phosphorous very effectively is dissolved in the caustic solution and forms sodium phosphate, which is believed to depend on the very finely divided and reactive nature of the residue product from the supercritical water oxidation, where the phosphorous and the iron are separated. The remaining matter iron oxide, possibly oxides of heavy metals, and silicates and other insoluble residue products remain in solid phase and can easily be filtered away in a filter  25 . As an alternative to caustic solution, ammonia may be used for formation of ammonium phosphate in solution.  
         [0042]    The solution is then transported to a second reactor  26 , to which lime (CaO or Ca(OH) 2 ) is added. Hereby calcium phosphate is precipitated, which can thus be separated from the solution in a third filter  27 . The caustic solution is then returned to the reactor  24 . A smaller amount of caustic solution has to be added due to losses in the system.  
         [0043]    Calcium phosphate with a dry content of about 70% is obtained at the third filter  27 , which is an excellent raw material for commercial fertilizers.  
         [0044]    Obviously, the filter cake from the second filter  25 , which contains iron oxide, possibly oxides of heavy metals, and silicates and other insoluble residue products may be further treated in order to recover the iron salt and separate the heavy metals. This may be performed for instance by leaching in an acid and reduction followed by a respective filtering step or by means of other processes known within the technical field.  
         [0045]    With reference now to FIG. 3, which illustrates a recovery system  21 , which may be used in the plant of FIG. 1, a second particular embodiment of the invention will be described.  
         [0046]    The residue product from the SCWO-reactor is in this embodiment a slurry with a fine, very reactive powder comprising an aluminum salt, possibly oxides of heavy metals, and silicate and other insoluble residue products, which may originate for instance from a water cleaning plant. This residue product is collected in a thickener/storage tank  30  and is then filtered through a filter  31  or other kind of dewatering device in order to increase the dry content. The thickened and filtered residue product is collected in a reactor  32  to which hydrochloric acid is added—preferably in stoichiometric shortage, particularly in 50% shortage. The reactor  32  operates preferably at atmospheric pressure and at a temperature below the boiling point of water. It has surprisingly been seen that the aluminum very effectively and directly forms polyaluminum chloride, which is believed to depend on the finely divided and very reactive nature of the residue product from the supercritical water oxidation. Thus, it is directly obtained  
         Al(OH) 1.5 Cl 1.5    
         [0047]    with 50% basicity without having to utilize pressure boiling or to produce the aluminum chloride by means of the stoichiometric reaction  
         Al(O) 3 +3HCl→AlCl 3 +3H 2 O  
         [0048]    and thereafter to produce highly basic polyaluminum chloride by means of adding an alkali, which may be performed according to conventional processes. By means of the process for forming polyaluminum chloride according to the invention less amounts of hydrochloric acids are wasted and no alkali is needed.  
         [0049]    Oxides of heavy metals will also be dissolved, but the heavy metals will be directly precipitated in a heavy metal reduction stage, in which pure aluminum is added in order to obtain the reduction  
         2Al+3Cu 2+ →2Al 3+ +3Cu(s)  
         [0050]    here exemplified with copper. The heavy metals may then be filtered away in a filter  34  and may be taken care of/deposited in a suitable manner. The solution is then transported to a storage tank for storage of polyaluminum chloride, which may be reused as a flocculant/precipitation chemical.  
         [0051]    The particular embodiments of the invention illustrated in FIGS. 2 and 3 may certainly be combined in case the residue product contains both phosphorous and aluminum.  
         [0052]    It shall be realised by the man skilled in the art that the invention is applicable for waste other than those containing precipitation chemicals, for instance industrial and food stuff industry waste comprising organic material and phosphorous or heavy metals. The phosphorous may be recovered and the heavy metals can be separated in the above-described manner.  
         [0053]    It is obvious that the invention can be varied in a plurality of manners. All such variations, which are obvious for a man skilled in the art, are intended to fall within the scope of the appended patent claims.