Patent Publication Number: US-2013228457-A1

Title: Reactor for recovering phosphate salts from a liquid

Description:
Reactor for recovering phosphate salts from a liquid 
     The invention concerns a reactor for complete separation of phosphate from a liquid and recovery of phosphate salts, comprising a housing and two electrodes of different polarity. 
     Phosphate salts such as magnesium ammonium phosphate (in the following abbreviated as MAP) or potassium magnesium phosphate (in the following abbreviated as PMP) are high-value plant adjuvants for which there is high demand. The elements nitrogen, potassium, magnesium, and phosphate of which these plant adjuvants are comprised are typically contained in all solid or liquid organic waste materials. While potassium, magnesium and other ions are present in the form of water-soluble cations, nitrogen and phosphate are predominantly bound to or in organic material or cell mass. Accordingly, a major proportion of nitrogen and phosphate are not available for the production of plant adjuvants. For this reason it is necessary to convert nitrogen and phosphate into their inorganic form that is suitable for precipitation. 
     The spontaneous precipitation of MAP or PMP is limited by the usually very low magnesium concentration in wastewater. Known is the addition of magnesium hydroxide, magnesium oxide or soluble magnesium salts for MAP precipitation. The disadvantage in this context is the bad solubility of the oxides as well as of the salt-like hydroxides. Upon addition of magnesium hydroxide or magnesium oxide to the wastewater, these compounds dissolve only very slowly and with a minimal proportion. This has the result that it is necessary to continuously perform stirring or mixing which, however, causes extra expenditure in regard to technology and energy and thus also with respect to costs. Moreover, both compounds, because of their bad solubility, must be added in over-stoichiometric amounts because otherwise an incomplete precipitation of the desired plant adjuvants occurs and significant quantities of phosphate remain in the wastewater. When magnesium salts are beforehand transferred into a solution, the efficiency of the method decreases because of the dilution with water. 
     The optimal pH value for precipitation of MAP is at 9. Wastewater has usually pH values between 5 and 7. Therefore, for increasing the pH value, a base is added. By using a soluble base, for example, sodium hydroxide. When using a base that is sparingly soluble, for example, magnesium hydroxide, the latter will hardly dissolve in water and the aforementioned disadvantages will occur. 
     A further possibility for adjusting a pH value that is favorable for precipitation is disclosed in DE 101 12 934 B4. The aeration of primary sludge mentioned therein with subsequent CO 2  stripping is however very energy-intensive and causes therefore high additional costs. 
     WO 00200101019735 A1 discloses a reactor for removal of dissolved nitrogen and phosphate from the aqueous portion of liquid manure by means of electrochemical precipitation. 
     The reactor described therein requires relatively high electrical voltages and is therefore energy-intensive and cost-intensive. A disadvantage is also that nitrogen and phosphate that are present organically bound in the aqueous portion of the liquid manure cannot be removed by the disclosed method. As a result of this, this wastewater must therefore be subjected to a subsequent purification in a water treatment plant. The construction of the reactor is disadvantageous in that, due to the arrangement of the electrodes, large areas are generated in which the liquid to be purified has no direct contact with the electrodes. Moreover, as a result of the geometry of the sacrificial anode more magnesium than required is released. Both effects reduce the efficiency of the reactor significantly. Moreover, in this method, due to the use of aluminum-containing electrodes, the plant poison aluminum ends up in the precipitation product. When this product is applied to the soil, the aluminum can be released and the plant growth can be affected negatively. 
     An electrochemical precipitation reactor for MAP is disclosed in WO 2007/009749 A1. The reactor is however not suitable for the precipitation of other phosphate salts. Also, the construction of the reactor does not allow for automatic separation between purified wastewater and precipitated MAP so that it is necessary to arranged downstream of the reactor a further apparatus for solid/liquid separation. Also, a significantly higher apparatus expenditure is required because, as a result of the construction, the housing of the reactor cannot be operated as a cathode. In addition, the magnesium is dissolved non-uniformly because, due to the spatial arrangement of the electrodes in the reactor, only one side of the anode is participating in the reaction. 
     The invention has the object to provide a reactor by means of which phosphate-containing wastewater can be treated and supplied to a further use. Moreover, the invention has the object to provide a reactor for recovery of phosphate salts as plant adjuvants which overcomes the aforementioned disadvantages of the prior art. 
     The object is solved according to the invention by a reactor with a housing. At the center of the housing a sacrificial anode of magnesium or a magnesium-containing material is arranged. An inert cathode is arranged concentrically about the sacrificial anode. In this way, the arrangement of the electrodes in accordance with the invention provides for a best possible control of the release of magnesium ions. Since the spacing between magnesium anode and cathode is kept as minimal as possible, the ions that are required for the precipitation of the phosphate salts are immediately in contact with each other and a constantly high concentration of magnesium ions in the reaction space is ensured. At the same time, because of the geometry of the sacrificial anode, its surface is very small so that only little magnesium is spontaneously released. In this way, advantageously an unnecessary excess of magnesium ions is prevented. 
     By means of the reactor according to the invention, it is possible by application of a minimal electrical direct current, smaller than 1 V with current strengths below 1 A, to supply magnesium ions to the phosphate-containing and ammonium-containing liquid and to split the water that is contained in the liquid to OH −  and H +  ions so that the pH value is increased and the reactions required for precipitation can take place. Due to the minimal energy demand, the costs for operating the device drop in comparison to methods known from the prior art. It is even possible to operate the reactor by galvanic operation. Electrical current is produced thereby. 
     Moreover, it is proposed that an anaerobic fermentation process is arranged upstream of the reactor according to the invention. In this fermentation process nitrogen and phosphorus that are bound organically are decomposed to inorganic water-soluble ions. From these ions, ammonium (NH 4   + ) and phosphate (PO 4   3− ), the phosphate salts, in particular MAP and PMP, can be formed. In this way, nitrogen and phosphate that are bound predominantly to or in organic material or cell mass are converted advantageously into a water-soluble form and are thus available for the production of plant adjuvants. Moreover, in this process biogas is produced which has a significant market value as an energy source. 
     An advantageous embodiment of the invention provides that the housing is manufactured of an electrically conductive material, for example, metal and therefore serves as an inert cathode. With the geometry of the reactor according to the invention and the concentric arrangement of the sacrificial anode the reaction space is limited to the space between the two electrodes. In this way, dead space is avoided and thus material costs are lowered. 
     A further advantage of the invention provides that the sacrificial anode is comprised substantially of magnesium. Of course, this includes also electrode materials that are comprised of a magnesium alloy or magnesium with minimal additions of other components. 
     In order to be able to advantageously perform the process of phosphate salt recovery continuously, the reactor according to the present invention has an inlet for the phosphate-containing liquid, an outlet for the purified liquid, as well as a removal device for the precipitated phosphate salts. The crystals can be removed via the removal device by means of a shut-off valve, for example, a seat valve or disk valve or ball valve, from the reactor without the inlet or outlet being changed and thereby the purification performance of the reactor being negatively affected. 
     An advantageous embodiment of the reactor according to the invention provides for the operation of the reactor as an upflow reactor. In this context, the inlet is located laterally at the bottom end of the reactor. The wastewater flows upward and escapes laterally at the top. This arrangement has the advantage that an automatic separation of liquid that flows upwardly and precipitated salts that sink to the bottom is taking place. 
     In principle, the reactor can be operated also as a downflow reactor wherein the liquid and solid move in the same direction. In this way, the sedimentation rate of the precipitated phosphate salts is accelerated. This means that the reactor can be made smaller for the same throughput. 
     In supplementing this, it is proposed that the crystals are separated in a filter from the liquid. In this way, in the reactor that is flowed through from top to bottom the precipitated phosphate salts can be removed together with the liquid from the reactor. Accordingly, additional fixtures or devices for separate solids removal are saved. Also, in case of common removal of phosphate salts and purified wastewater, a turbulent flow is generated in the conduit and prevents clogging of the conduit by the crystals. 
     It is particularly beneficial when the housing of the reactor is closed. The electrolytic reactions that occur in the reactor produce large quantities of foam which in case of a closed housing cannot overflow. Accordingly, in an advantageous way, a product loss is avoided. In principle, the housing of the reactor can also be open partially or entirely. 
     The reactor according to the invention operates even better when it has a slanted bottom. In this way, it is possible that the precipitated crystals glide along the slanted surface downwardly and collect at the removal device. In this way, the crystals can be removed from the reactor without its continuous operation having to be interrupted. The formation of the slanted surface is realized by a preferably conical bottom. Conceivable is also the shape of a pyramid. 
     An advantageous embodiment of the reactor according to the invention provides that to the sacrificial anode a positive pole and to the cathode a negative pole of a direct current source are connected. The supply of electrical current prevents deposits on the electrode which are not stable in the electrical field. When the reactor, on the other hand, is operated without a direct current source, we by the process of magnesium release electrons are released. This means that the reactor requires no electrical current but even produces electrical current. 
    
    
     
       Further advantages and advantageous embodiments of the invention can be taken from the following Figures, their description, and the claims. In this context, all features disclosed in the Figures, their description, and the claims can be important for the invention individually as well as in any combination with each other. 
       It is shown in: 
         FIG. 1  a schematic illustration of the reactor according to the invention 
         FIG. 2  a schematic illustration of a first embodiment of the reactor according to the invention 
         FIG. 3  a schematic illustration of a second embodiment of the reactor according to the invention 
         FIG. 4  a schematic illustration of a third embodiment of the reactor according to the invention 
         FIG. 5  a schematic illustration of a third embodiment of the reactor according to the invention for recovery of phosphate salts and 
         FIG. 6  a schematic illustration of the use of the reactor according to the invention with upstream fermentation process 
     
    
    
       FIG. 1  shows a schematic illustration of a reactor  10  according to the invention. The reactor  10  has a housing  12 . The housing  12  serves for receiving a phosphate-containing liquid  14 . At the center of the housing  12  an electrode  16  is arranged. 
     The electrode  16  is a so-called sacrificial anode which is connected to the positive pole of a direct current source, not illustrated in the drawing, while the housing  12  forms the cathode  18  which is connected to a negative pole of the direct current source. 
     The sacrificial anode  16  is comprised of a magnesium-containing material so that magnesium ions are transferred into the solution  14  as soon as electrical voltage is applied to the electrodes  16  and  18 . 
     Reaction equation for formation of MAP: 
       Mg 2+ +NH 4   + +PO 4   3− +6 H 2 O→MgNH 4 PO 4 .6 H 2 O
 
     Reaction equation for formation of PMP: 
       Mg 2+ +K + +PO 4   3− +6 H 2 O→MgKPO 4 .6 H 2 O
 
     Reaction equation for release of magnesium: 
       Mg(s)→Mg 2   + +2 e   − 
 
     Reaction equation for formation of hydroxide ions: 
       2 H 2 O+2 e   − →2 OH − +H 2  
 
     The formed phosphate salts are sparingly soluble in aqueous solution and precipitate as crystals which deposit at a bottom of the reactor  10 . 
     One configuration of the reactor  10  according to the invention provides for galvanic operation. For this purpose, the two electrodes  16 ,  18  are not connected to the external direct current source. The magnesium ions are transferred into solution by galvanic operation. 
       FIG. 2  shows a first embodiment variant of the reactor  10  according to the invention. Illustrated is the housing  12  of the reactor and centrally arranged therein is the electrode  16  that is embodied as a sacrificial anode. Concentric between housing  12  and electrode  16  there is the cathode  18 . In this special arrangement the spacing between sacrificial anode  16  and inert cathode  18  is minimal so that the ions that are participating in the precipitation are immediately in contact with each other. 
     In  FIG. 3  the reactor  10  is illustrated. Laterally at a preferably conical bottom  22  there is an inlet  24 . An outlet  26  is located at the top laterally at the housing  12  of the reactor  10 . An optional return line  28  connects the outlet  26  with the inlet  24 . At the bottom end of the preferably conical bottom  22  there is the removal device  30 . 
     The phosphate-containing liquid  14  flows through the inlet  24  from the bottom to the top through the reactor  10  and exits from it through the outlet  26 . The precipitated phosphate salts glide along the slanted plane of the conical bottom  22  in downward direction and are removed via the removal device  30 . In this way, the reactor  10  can be operated continuously and the precipitated and deposited crystals can be removed at any time without changing a throughput of the reactor. By means of the optional return line  28 , already purified liquid  14  is returned to the reactor  10  as circulating water. 
       FIG. 4  shows a third embodiment of the reactor  10  according to the invention. Here, the reactor  10  is flowed through in downward direction. The inlet  24  is located laterally at the top on the housing  12 . The outlet  26  is located laterally at the conical bottom  22 . The optional return line  28  connects the outlet  26  with the inlet  24 . At the conical bottom  22  the removal device  30  is arranged. 
     The phosphate-containing liquid  14  flows through the inlet  24  from the top to the bottom through the reactor  10  and exits from it through the outlet  26 . Precipitated phosphate salts are removed via the removal device  30  without affecting the operation of the reactor. By means of return line  28  the already purified liquid  14  is supplied again to the reactor  10  as circulating water. 
       FIG. 5  shows a further embodiment of the reactor  10  according to the invention. In this way, the reactor  10  is flowed through in downward direction. The inlet  24  is located laterally at the top at the housing  12 . The outlet  26  is located at the bottom end at the conical bottom  22  and extends from there to a downstream filter  31 . The optional return line  28  connects the outlet  26  with the inlet  24 . 
     In this fourth embodiment according to the invention, the precipitated phosphate salts are removed together with the purified liquid  14  from the reactor  10 . In the downstream filter  31  the phosphate salts are separated from the liquid  14 . In this context there is the possibility to supply seed crystals to the reactor  10  via the return line  28 . 
     In  FIG. 6 , an application of the reactor  10  according to the invention in connection with producing biogas from phosphorus-containing wastewater is schematically illustrated. 
     A wastewater flow  32  of organic origin is supplied to a bioreactor  34 . Herein, by anaerobic fermentation processes, the organic carbon compounds that are contained in the solids are converted into biogas and mineral residual substances. In this process, ammonium-containing and phosphate-containing process water  36  is produced. Before the process water  36  is supplied through inlet  24  into the reactor  10 , possibly contained solids  40  are separated in a filter  38 . The solids  40  which are retained in the filter  38  are returned into the bioreactor  34 . In the afore described way, in the reactor  10  according to the invention the phosphate salts are separated. The ammonium-containing and phosphate-containing outflow  26  can be returned partially into the bioreactor  34 . In this way, an impairment of the fermentation process, caused by a high ammonium concentration, is prevented.