JP4688842B2 - Wastewater treatment method - Google Patents

Description

  The present invention relates to a wastewater treatment method for treating and recycling wastewater containing heavy metals such as treated water in a cleaning factory and leachate in a waste disposal site.

Wastewater such as wastewater from a wastewater treatment plant and leachate from a waste disposal site contains harmful substances such as heavy metals, and thus treatment for detoxifying them has been performed.

For example, in the waste landfill leachate treatment method disclosed in Patent Document 1, leachate is first introduced into a pretreatment device to remove some heavy metals and calcium, and then reverse osmosis membrane filtration. By filtering with an apparatus and an NF filtration apparatus, calcium, heavy metal, and the like are separated to the concentrate side. The concentrated solution of the NF filtration device is subjected to treatment such as coagulation sedimentation, sand filtration treatment, activated carbon adsorption treatment, etc. in the wastewater treatment device, and then returned to the incinerator or ash melting furnace to thermally decompose harmful substances. About a liquid, after introducing into a chelate adsorption tower and removing a trace amount heavy metal etc., salt is collect | recovered by performing a crystallization process.
JP 2001-321768 A

  The treatment method disclosed in Patent Document 1 includes heavy metals contained in the concentrated liquid separated by the filtration device, and thus cannot be discharged as it is, and requires a heavy metal removal step such as coagulation sedimentation, and the treatment is complicated. There was a problem of high cost. Moreover, since the heavy metal is slightly contained also in the filtrate which passed through the filtration apparatus, in order to remove this, a chelate process was needed and the process became complicated also in this point.

  Therefore, an object of the present invention is to provide a wastewater treatment method capable of collecting and recycling a large amount of water while efficiently removing heavy metals contained in wastewater.

  The object of the present invention is to concentrate and recover the waste water containing heavy metal in a concentrating device and recover the generated steam, and to further concentrate the concentrated waste water generated by the concentrating device in a crystallization device. The step of recovering the generated steam and precipitating the salt contained in the concentrated waste water, the salt separation step of separating the precipitated salt crystals by a salt separation device, and taking out as a solid matter, and the salt separation device This is achieved by a wastewater treatment method comprising a drying step of drying the filtrate to evaporate excess water and leave solids.

  In this wastewater treatment method, the concentration and recovery step can include a step of evaporating and concentrating the wastewater with a two-effect concentrator, and further performing a vaporization and concentrating with a single-effect concentrator. It is effective in the case.

  As the heat source used in the concentration recovery step, exhaust steam of a steam turbine that uses the thermal energy of the combustion exhaust gas generated in the combustion facility as a drive source can be preferably used.

  In the case where calcium and heavy metal are contained in the wastewater, it is preferable to provide a pretreatment step for generating calcium carbonate by bringing the wastewater into contact with a carbon dioxide-containing gas before the concentration and recovery step. Preferably, a calcium separation step for separating calcium carbonate solids is provided between the step and the precipitation step.

  In this case, the combustion exhaust gas generated in the combustion facility is used as the carbon dioxide-containing gas used in the pretreatment step, and the heat energy of the combustion exhaust gas generated in the combustion facility is used as the drive source as the heat source used in the concentration recovery step. The steam extracted from the middle of the steam turbine can be used as a heat source used in the precipitation step and the drying step using the exhaust steam of the steam turbine.

  According to the waste water treatment method of the present invention, it is possible to provide a waste water treatment method capable of recovering and recycling a large amount of water while efficiently removing heavy metals contained in waste water. Moreover, the combustion exhaust gas generated in the combustion facility can be used effectively, and the amount of carbon dioxide emitted from the factory can be effectively reduced.

  Hereinafter, actual forms of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram of a wastewater treatment apparatus used in a wastewater treatment method according to an embodiment of the present invention. The waste water treatment apparatus shown in FIG. 1 shows an example installed in a cleaning factory equipped with a waste incinerator 90, and is particularly effective when heavy metals and calcium are included in the waste water.

  As shown in FIG. 1, the wastewater treatment apparatus 1 includes a pretreatment tank 10 that stores wastewater containing calcium ions generated in a cleaning factory, and a first concentration apparatus that evaporates and concentrates the wastewater supplied from the pretreatment tank 10. 20, a solid-liquid separation device 30 that performs solid-liquid separation of the concentrated wastewater generated by the first concentration device 20, a second concentration device 40 that evaporates and concentrates the concentrated wastewater that has passed through the solid-liquid separation device 30, The crystallizer 50 for precipitating salts contained in the reconcentrated wastewater generated by the second concentrator 40, the salt separator 60 for separating the crystals of the precipitated salts, and the reconcentrated wastewater after salt separation are dried. And a drying device 70 to be used.

  The pretreatment tank 10 includes an air diffuser 11 that introduces carbon dioxide-containing gas into the waste water. The wastewater stored in the pretreatment tank 10 can be adjusted in pH value (hydrogen ion concentration) by supplying an alkali such as sodium hydroxide, and the pH value of the wastewater can be measured by a pH meter 12. . Further, the pretreatment tank 10 is provided with a stirring device 13 for stirring the waste water.

The first concentrating device 20 includes a plurality of heat transfer tubes 21 through which a heating fluid passes, and a nozzle 22 for spraying the stored wastewater on the surface of the heat transfer tubes 21, and a vacuum pump (not shown) inside. In such a state, the waste water is circulated through the circulation line 23 in a state where the pressure is reduced, so that the waste water is heated and evaporated on the surface of the heat transfer tube 21. The steam generated in the first concentrator 20 is guided to the condenser 2 to become condensed water, and is stored in the condensed water tank 4. In FIG. 1, the first concentrating device 20 is a single effect type. However, when the amount of wastewater treatment is large (for example, 250 m ³ / day or more), a two-effect type concentrating device is considered in consideration of the evaporation load. Is preferably used.

  The solid-liquid separation device 30 separates solid components such as calcium carbonate contained in the concentrated waste water supplied from the concentrated liquid tank 31 and the concentrated liquid tank 31 that stores the concentrated waste water generated by the first concentration device 20. And a centrifugal dehydrator 32. In order to facilitate solid-liquid separation, the centrifugal dehydrator 32 may be added with an aggregating agent such as aluminum sulfate, polyaluminum chloride, and ferric chloride, and a chelating agent that collects heavy metals. The solid-liquid separation device 30 may have a configuration capable of solid-liquid separation by sedimenting solids, and is not necessarily limited to one having power. The concentrated waste water from which the solid matter is separated by the centrifugal dehydrator 32 is supplied to the second concentrating device 40 together with the supernatant liquid in the concentrated liquid tank 31.

  The second concentrating device 40 is configured in the same manner as the first concentrating device 20, and a plurality of heat transfer tubes 41 through which the heating fluid passes and nozzles that spray the stored wastewater on the surface of the heat transfer tubes 41. 42 and configured to circulate the waste water through the circulation line 43 in a state where the pressure is reduced by a vacuum pump (not shown), thereby heating and evaporating the waste water on the surface of the heat transfer tube 41. . The steam generated in the second concentrator 40 is guided to the condenser 2 to become condensed water, and is stored in the condensed water tank 4.

  The crystallizer 50 includes an evaporator 51 that stores the re-concentrated wastewater supplied from the second concentrator 40, and a heater 52 that circulates and heats the stored re-concentrated wastewater. The inside of 51 is decompressed to further evaporate and concentrate the reconcentrated wastewater, thereby precipitating salt crystals contained in the reconcentrated wastewater. The steam generated in the evaporator 51 is guided to the condenser 2 to become condensed water, and is stored in the condensed water tank 4.

  The salt separator 60 includes a centrifuge 61 that separates salt crystals from the slurry of the reconcentrated wastewater supplied from the crystallizer 50, and a filtrate tank 62 that stores the filtrate from which the crystals have been separated. . The filtrate in the filtrate tank 62 other than the supernatant is supplied to the drying device 70, while the supernatant is returned to the evaporator 51.

  The drying device 70 is composed of a drum dryer, and supplies the reconcentrated waste water to the drum heated by sucking the heated steam and rotates the drum to evaporate and dry to obtain a solid matter. The drying device 70 may have another known configuration such as a disk dryer or a vacuum dryer.

  In the present embodiment, a waste heat boiler 91 that recovers heat of combustion exhaust gas discharged from the waste incinerator 90 to generate steam is provided, and the generated steam is heated by the superheater 92 and then steam. The generator 94 is driven by being introduced into the turbine 93. The combustion exhaust gas after supplying heat to the waste heat boiler 91 is discharged through a dust collector or a filter (not shown), and a part thereof is supplied as a carbon dioxide-containing gas to the diffuser pipe 11 of the pretreatment tank 10.

  On the other hand, part of the exhaust steam discharged from the steam turbine 93 is supplied to the heat transfer tubes 21 and 41 of the first concentrating device 20 and the second concentrating device 40 and used as a heating fluid. After the heat exchange with the waste water, the remaining exhaust steam from the steam turbine 93 is guided to the condenser 95 and supplied to the waste heat boiler 91 again.

  Further, from the middle of the steam turbine 93, steam having a pressure higher than that of the exhaust steam (for example, 0.5 MPaG) is extracted, and this extracted steam is supplied to the crystallizer 50 and the drying device 70. The extracted steam supplied to the crystallizer 50 is used as a heat source for the heater 52, and the extracted steam supplied to the drying device 70 is used as a heat source for evaporating and drying the reconcentrated waste water.

  Next, a method for treating wastewater using a wastewater treatment apparatus having the above configuration will be described.

  First, the wastewater to be recycled is stored in the pretreatment tank 10, and the carbon dioxide-containing gas is released from the diffuser pipe 11 and brought into contact with the wastewater.

  In this embodiment, the wastewater stored in the pretreatment tank 10 is wastewater in a cleaning factory, but is not particularly limited as long as it contains calcium ions, such as other factory wastewater or leachate in a disposal site. There may be. In addition, in this embodiment, the carbon dioxide-containing gas is used as combustion exhaust gas from the waste incinerator 91 in the cleaning factory, but combustion exhaust gas generated in other combustion equipment such as a thermal power plant can also be used. In addition, carbon dioxide generated in the factory can be effectively used to reduce emissions. However, carbon dioxide-containing gas other than combustion exhaust gas (including only carbon dioxide gas) can also be used. By performing such pretreatment, calcium ions and carbonate ions in the waste water react to produce calcium carbonate.

  During the production of calcium carbonate, the pH of the wastewater is adjusted by appropriately supplying alkali (for example, sodium hydroxide aqueous solution) into the pretreatment tank 10 while operating the stirring device 13 and monitoring the measured value of the pH meter 12. Control the value. The pH value may be any condition as long as calcium carbonate is generated, and may be, for example, 8 or more. However, if the pH value is too low, calcium carbonate crystals are hardly formed, leading to a decrease in the ratio of calcium carbonate crystals to residual calcium ions. . For this reason, in the case of general wastewater such as wastewater from a cleaning factory, the pH value is preferably greater than 9, and more preferably 9.5 or more. The upper limit of the pH value is not particularly present, but is preferably 11 or less, for example. The pH adjustment of the wastewater by supplying the alkali is continuously performed.

  In this way, most of the calcium ions in the waste water react to produce calcium carbonate, and the waste water containing the calcium carbonate is continuously supplied to the first concentrator 20.

  In the first concentrating device 20, the waste water is circulated through the circulation line 23, and is heated and evaporated by heat exchange with the heating fluid passing through the heat transfer tube 21. The heating fluid supplied to the heat transfer tube 21 uses the exhaust steam of the steam turbine 93 in the present embodiment, so that the waste heat of the combustion furnace 91 is effectively used. The fluid after heat exchange is stored in the condensate tank 95.

  The temperature of the exhaust steam of the steam turbine 93 is, for example, about 65 ° C., and the TDS (total salt amount) can be concentrated to about 10% (concentration magnification is about 5 times). In this case, since the boiling point rise is as small as about 1.0 ° C., the necessary heat transfer temperature difference can be ensured even when the first concentrating device 20 is a two-effect type, and the exhaust of the steam turbine 93 is eliminated. Steam can be used effectively as a heat source. In addition, when using things other than the exhaust steam of the steam turbine 93 as a heat source, it is also possible to make the density | concentration of TDS still higher by using a heat source with a higher energy level.

  In the wastewater supplied to the first concentrating device 20, since calcium ions remain slightly, there is a possibility that scale is generated with the evaporation and concentration of the wastewater. However, since the calcium carbonate contained in the wastewater becomes a seed crystal, the calcium ions in the wastewater can be brought into contact with the seed crystal and can be grown on the surface of the seed crystal. Adhesion can be prevented. Thus, by using the calcium carbonate generated in the pretreatment step as a seed crystal, it is possible to produce a seed crystal effect without adding a seed crystal to the first concentrator 20 separately.

  The steam generated by the first concentrator 20 becomes condensed water in the condenser 2 and is collected in the condensed water tank 4. This condensed water can be reused for applications where water is currently used (washing, cooling, watering, etc.). As described above, when the concentration ratio is set to about 5 times, about 80% of the water in the waste water can be recovered as condensed water. On the other hand, the concentrated waste water generated by the first concentrator 20 is supplied to the solid-liquid separator 30.

  In the solid-liquid separator 30, concentrated drainage is stored in the concentrate tank 31, and anything other than the supernatant is supplied to the centrifugal dehydrator 32, and heavy metals such as lead and mercury are recovered together with calcium carbonate as solids, and landfilled. Will be disposed of. In the centrifugal dehydrator 32, solids can be easily separated by adding a flocculant.

  The concentrated waste water from which the solid matter has been removed by the centrifugal dehydrator 32 is supplied to the second concentrating device 40 together with the concentrated liquid tank 31, and is again evaporated and concentrated. As for the heated fluid supplied to the heat transfer pipe 41, the exhaust steam of the steam turbine 93 is used from the viewpoint of effective use of waste heat, as in the case of the first concentrator 20.

The exhaust steam of the steam turbine supplied to the first concentrator 20 and the second concentrator 40 has a low pressure and low utility value, but the wastewater treatment method of the present embodiment also includes such low-pressure steam. It can be used effectively. The combustion facilities that serve as heat sources are not necessarily the same, and wastewater treatment may be performed using waste heat from a plurality of different combustion facilities. The heat sources of the first concentrator 20 and the second concentrator 40 are not necessarily limited to the exhaust steam of the steam turbine,
For example, the steam generated in the first concentrator 20 or the second concentrator 40 is configured to be guided to the heat transfer tubes 21 and 41 by a compression blower, or steam generated from a gas engine, gas turbine cogeneration or boiler, Hot water can also be used.

Concentrated wastewater in the second concentrator 40 is almost completely free of calcium ions, but sulfate ions, chlorine ions, sodium ions, etc. are present, so these dissolved components (NaCl, Na ₂ SO _4, etc.) ) Is not oversaturated (for example, the TDS concentration is about 30% and the concentration factor is about 3 to 4 times), thereby suppressing the generation of scale due to NaCl, Na ₂ SO _4, etc. Can be prevented.

  The steam generated by the second concentrator 40 becomes condensed water in the condenser 2 and is collected in the condensed water tank 4. The condensed water that can be collected here is about 70% of the water that could not be collected by the first concentrator 20 when the concentration factor is about 3 to 4 times.

  Thus, after separating the solid body of calcium carbonate contained in the concentrated wastewater generated by the first concentration device 20 by the solid-liquid separation device 30, the concentrated wastewater that has passed through the solid-liquid separation device 30 is separated into the second concentration device. Since evaporative concentration is again performed at 40, a large amount of water can be recovered from the wastewater while facilitating the removal of calcium contained in the wastewater.

The re-concentrated waste water generated by the second concentrating device 40 is supplied to the evaporator 51, heated by the heater 52, and evaporated and concentrated under reduced pressure. As a result, crystals of salts (mainly NaCl, Na ₂ SO ₄ ) contained in the reconcentrated waste water precipitate inside the evaporator 51. The steam generated in the evaporator 51 becomes condensed water in the condenser 2 and is collected in the condensed water tank 4. In this way, the reconcentrated waste water that has been evaporated and concentrated in the first concentrating device 20 and the second concentrating device 40 can be further evaporated and concentrated to obtain condensed water. Water can be recovered and reused. As a heating source of the heater 52, it is preferable to use high-pressure steam of 0.5 MPaG or more. For example, by using steam extracted from the middle of the steam turbine 93 as in the present embodiment, surplus steam of the steam turbine 93 is used. Can be used efficiently.

The slurry-like re-concentrated waste water in which salt crystals are deposited is supplied to the centrifuge 61, and salts such as NaCl and Na ₂ SO ₄ and trace amounts of heavy metals in the waste water are collected as solids and disposed of by landfill. Is done.

  The filtrate after salt separation is stored in the filtrate tank 62. Since the supernatant liquid in the filtrate tank 62 contains a very small amount of ions, it is possible to promote precipitation of salts by supplying this again to the evaporator 51 and mixing it with the reconcentrated waste water. The filtrate other than the supernatant liquid in the filtrate tank 62 is supplied to the drying device 70.

  The filtrate supplied to the drying apparatus 70 has almost all the ions removed, and most of the recoverable water is recovered as condensed water, and as a result, contains a high concentration of organic substances and heavy metals. The drying device 70 evaporates a little remaining water and releases it to the atmosphere, thereby obtaining organic substances and heavy metals as solids. This solid matter is disposed of by landfill or the like. As for the heating source of the drying apparatus 70, it is preferable to use high-pressure steam of 0.5 MPaG or more, similarly to the heating source of the heater 52, and use surplus steam extracted from the middle of the steam turbine 93 as in this embodiment. Is possible. The steam after heat exchange is stored in a condensate tank 95.

  As described above, according to the wastewater treatment method of the present embodiment, most of the water contained in the wastewater can be recovered and reused, and the water-saving effect (water consumption and sewerage discharge in factories) can be obtained. The flow rate reduction effect) can be increased, and the carbon dioxide emission can be indirectly reduced.

  In addition, when the factory has combustion equipment, the combustion exhaust gas generated in the combustion equipment can be effectively used as the carbon dioxide-containing gas introduced into the pretreatment tank 10, and the waste heat of the combustion exhaust gas can be used as the first and second waste gases. Since it can be effectively used as a heat source for a concentrator, a crystallizer, and a dryer, a direct carbon dioxide reduction effect and an economic effect can be obtained.

  As mentioned above, although one Embodiment of this invention was explained in full detail, the specific aspect of this invention is not limited to the said embodiment. For example, in the present embodiment, by disposing the solid-liquid separation device 30 between the first concentration device 20 and the second concentration device 40, the waste water before reconcentration by the second concentration device 40 is reduced. However, the solid-liquid separation device 30 is disposed between the second concentrating device 40 and the crystallizing device 50. The solid-liquid separation device 30 is disposed between the second concentrating device 40 and the crystallizing device 50. It is also possible to do. In either case, calcium carbonate can be separated and removed before the treatment in the crystallizer 50, and the clogging of the filter cloth in the centrifuge 61 can be prevented.

In this embodiment, the first concentrator 20 and the second concentrator 40 are used as the concentrator for evaporating and concentrating the wastewater. However, when the amount of wastewater treatment is small (for example, 250 m ³ / day). 2), only the first concentrator 20 may be used as shown in FIG. In this case, since the evaporation load of the first concentrating device 20 is not so large, a single effect type can be used, and the exhaust steam of the turbine can be used as a heat source as in the configuration of FIG. In FIG. 2, the same components as those in FIG. 1 are denoted by the same reference numerals (the same applies to the following drawings).

Moreover, when calcium is not contained in waste water and the main removal object is only a heavy metal, as shown in FIG. 3, it can also be set as the structure which is not equipped with the pretreatment tank 10 and the solid-liquid separator 30. . When the amount of wastewater treatment is large (for example, 250 m ³ / day or more), it is preferable that the first concentrating device 20 and the second concentrating device 40 be a two-effect type and a single-effect type, respectively. It is possible to use the exhaust steam of the turbine as a heat source. It is preferable to set the concentration magnification in the first concentration device 20 and the second concentration device 40 in the same manner as the configuration shown in FIG.

In the configuration shown in FIG. 3, when the amount of wastewater treatment is small (for example, less than 250 m ³ / day), only the first concentrator 20 may be used as shown in FIG. As the first concentrating device 20, a single effect type can be used as in the configuration shown in FIG.

As an example showing the effect of the wastewater treatment method using the wastewater treatment apparatus shown in FIG. 1, the results of calculating the water mass balance in the case where the amount of wastewater is 500 m ³ / day are shown in Table 1. While the weight of the waste water is 507 t / day, the amount of water that can be collected by the waste water treatment method of this embodiment is 495.2 t / day, and 97% or more of the waste water can be collected.

次に、二酸化炭素削減効果及び経済的効果の一例として、排水量が５００ｍ^３／日のケースで算出した結果を表２及び表３に示す。表２において、ＣＯ_２の排出減量は、１年３００日稼働として計算している。また、表３は、電気料金をkWhあたり7.7円で計算し、上水購入費用は480円/m³、下水放流費用は360円/m³で計算している。

Next, as an example of the carbon dioxide reduction effect and the economic effect, Table 2 and Table 3 show the results calculated in the case where the amount of drainage is 500 m ³ / day. In Table 2, the CO ₂ emission reduction is calculated as 300 days a year. Table 3 shows the calculation of electricity charges at 7.7 yen per kWh, water purchase costs at 480 yen / m ³ , and sewage discharge costs at 360 yen / m ³ .

It is a schematic block diagram of the waste water treatment apparatus used for the waste water treatment method which concerns on one Embodiment of this invention. It is a schematic block diagram of the waste water treatment apparatus used for the waste water treatment method which concerns on other embodiment of this invention. It is a schematic block diagram of the waste water treatment equipment used for the waste water treatment method which concerns on other embodiment of this invention. It is a schematic block diagram of the waste water treatment equipment used for the waste water treatment method which concerns on other embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Waste water treatment apparatus 10 Pretreatment tank 20 1st concentrator 30 Solid-liquid separator 40 Second concentrator 50 Crystallizer 60 Salt separator 70 Dryer 90 Waste incinerator 93 Steam turbine

Claims (4)

A concentration and recovery step of evaporating and concentrating wastewater containing heavy metal in a concentrator, condensing the generated steam and recovering it as condensed water ;
A precipitation step of condensing the generated steam by condensing the generated waste water generated by the concentrating device and further condensing the generated steam as condensed water by precipitating and condensing the salt contained in the concentrated waste water ;
A salt separation step of separating precipitated salt crystals with a salt separation device and taking out the crystals as a solid;
A wastewater treatment method comprising: a drying step of drying the filtrate that has passed through the salt separation device to evaporate excess water and leave a solid matter.   The wastewater treatment method according to claim 1, wherein exhaust heat from a steam turbine that uses thermal energy of combustion exhaust gas generated in a combustion facility as a drive source is used as a heat source used in the concentration recovery step. The wastewater contains calcium along with heavy metals,
Before the concentration and recovery step, the waste water is brought into contact with a carbon dioxide-containing gas, and a pretreatment step for generating calcium carbonate is provided.
The wastewater treatment method according to claim 1, further comprising a calcium separation step for separating calcium carbonate solids between the concentration recovery step and the precipitation step. As the carbon dioxide-containing gas used in the pretreatment step, utilizing the combustion exhaust gas generated in the combustion facility,
As the heat source used in the concentration recovery step, utilizing the exhaust steam of the steam turbine that uses the thermal energy of the combustion exhaust gas generated in the combustion facility as a drive source,
The wastewater treatment method according to claim 3, wherein steam extracted from the middle of the steam turbine is used as a heat source used in the precipitation step and the drying step.

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