A waste water concentrator which is usually used in a nuclear power station, for instance, is constituted by an evaporator 1 and a condenser 4 as shown in FIG. 12. To the evaporator 1, a heater 14 heating waste water by using station-service heating steam 15 is connected via a circulating pump 13. Steam generated through evaporation in the evaporator 1 is sucked by an eductor 5 and brought to a condenser 4 to be turned into condensed water. The waste water is introduced into a concentrator through a valve 7 on the supplying tank side. Concentrated waste water containing solid-state wastes, which is obtained through concentration in the evaporator 1, on the other hand, is introduced into a subsequent process through a take-out valve 8. Although a vapor-liquid separator is provided in the upper part of the evaporator 1, the rate of removal of mist is low and therefore the mist is often transferred onto the condenser side.
In recent years, a method has been contrived wherein concentration itself is conducted by a membrane and thereby a high decontamination factor is obtained. For instance, a method has been proposed, as described in the Official Gazette on Japanese Patent Laid-Open No. 164195/1986, wherein, a hydrophobic polymer porous membrane allowing steam to permeate but not water to do is employed, nuclear power station waste water at a prescribed temperature is made to contact directly this porous membrane on one side thereof, steam is generated from this water and made to permeate onto the other side of said porous membrane, the steam permeating therethrough is cooled down to be condensed, and thus the waste water is concentrated, while deionized water is collected efficiently. This method is not accompanied by the problem of the transfer of mist onto the condensed water side, which occurs when the evaporator and the condenser are employed.
Out of the above-described prior arts, the method using the combination of the evaporator and the condenser has an inferior efficiency in removing metal ions and inorganic substances existing in the mist, and consequently the decontamination factor (hereinafter mentioned as DF) is improved thereby only to about 10.sup.3. Such an efficiency of removal DF as low as about 10.sup.3 is too low in terms of the prevention of radioactive contamination in the disposal of waste water containing radioactive substances, in particular, and therefore it requires additional introduction of a demineralizer treating condensed water in the posterior stage of a concentrator.
As for the method of concentration using the porous membrane, it has such a problem as stated below owing to the direct contact of waste water with the porous membrane therein. With regard to the waste water which is the object of waste disposal, waste water of high electroconductivity coming out of a power station (mainly waste water called floor drain) contains a variety of metal ions and soap waste, and in a membrane disposal of a contact type, there is a large possibility of a membrane being clogged therewith. Although DF is raised somewhat in the membrane disposal of the contact type, accordingly, it can reasonably be assumed that the reliability of the machinery and the plant dependent on the life of the membrane lowers. Disclosure of Invention
An object of the present invention is to furnish a waste water concentrator and a concentrating method which enable the attainment of high DF and are operable with high reliability for a long time.
A second object of the present invention, in addition to the above object, is to furnish a waste water concentrator and a concentrating method which facilitate the adjustment of a quantity of disposal.
A third object of the present invention, in addition to the above objects, is to simplify a waste water disposal plant.
A fourth object of the present invention, in addition to the above objects, is to simplify a disposal plant of waste water of high electroconductivity in a nuclear power station.
A fifth object of the present invention is to furnish a method of concentrating waste water, such as washing waste water, containing a surfactant, by using a hydrophobic porous membrane which allows steam to permeate but not water to do.
The present invention enables the attainment of high DF and high reliability for a long time by a construction wherein a porous membrane which has a property allowing gas such as steam to permeate but not allowing a liquid such as water to do is employed and, on the occasion of concentration of waste water by means of this porous membrane, the waste water is made, as a vapor flow, to contact the porous membrane to be concentrated. By turning the waste water into the vapor flow, in other words, most of various metal ions, a surfactant, etc. existing in the waste water are separated from the vapor flow (DF is about 10.sup.3). Mist or metal ions contained in the mist or a very minute quantity of inorganic substance scattered therein, which are contained in the vapor flow, are removed from the steam permeating the porous membrane (DF is about 10.sup.6) by using the porous membrane having the property allowing a gas to permeate but not allowing a liquid to do, as a mist separator. Accordingly, the employment of the porous membrane in a mode of non-contact with waste water makes it possible to avoid the problem of clogging of the membrane involved in the prior art, enabling the prolongation of the life of the membrane and thus enabling the effective and long stable utilization of a DF function which the porous membrane has.
Moreover, the present invention makes it easy to control a pressure difference between the front and rear sides of the porous membrane and consequently to adjust the quantity of disposal by refluxing the vapor flow not permeating the porous membrane to a part wherein it is generated. In other words, by refluxing the vapor flow not permeating the porous membrane onto the side of the part wherein it is generated, a pressure acting on the membrane on the side of the generation of the vapor flow is kept constant, and therefore a differential pressure across the porous membrane can be adjusted by regulating a pressure on the permeation side of the membrane. The adjustment of the quantity of disposal is further facilitated by providing a means to make variable the pressure difference across the porous membrane. Concretely, the pressure difference across the porous membrane may be adjusted, for instance, by an eductor provided on the permeation side of the porous membrane. The quantity of disposal (efficiency) can be increased easily by reducing the pressure on the permeation side of the membrane by the suction by means of the eductor.
Furthermore, the present invention simplifies the waste water disposal plant to a large extent by using the above-mentioned waste water concentrator for constructing the waste water disposal plant and by providing this plant with a tank accommodating the steam permeating the porous membrane of the concentrator as condensed water and a tank accommodating a concentrated liquid. When the waste water disposal plant is constructed with the above-mentioned waste water concentrator used as the central device, in other words, the steam permeating the concentrator can be obtained with high DF, and therefore the steam permeating the membrane can be reused as condensed water without providing any special device such as a demineralizer in the rear stage of the concentrator. Besides, the steam can also be introduced as condensed water into a tank provided for outside the system releasing. Because of these effects, the disposal plant can be simplified to a large extent.
In addition, the present invention is aimed at simplifying the waste water disposal plant of a nuclear power station by adapting the aforesaid waste water concentrator to the concentration of waste water of high electroconductivity of the nuclear power station and by introducing the steam permeating the porous membrane as condensed water into a condensate storage tank without interposing the demineralizer. When the aforesaid waste water concentrator is used for the plant for disposal of high-conductivity waste water of the nuclear power station, the condensed water of the steam permeating the concentrator can be obtained with high DF, and therefore the demineralizer provided conventionally in the rear stage of the concentrator can be dispensed with. Moreover, since DF can be raised to so high a degree as to enable the reuse of the substance which can not be reused and so is stored separately according to prior art, the waste water disposal plant can be simplified to a large extent by introducing the same into a condensate storage tank wherein waste water of low conductivity subjected to disposal is stored.
Besides, the present invention enables the disposal of waste water containing a surfactant by the use of the hydrophobic porous rembrane, by a construction wherein the waste water is subjected to evaporation as the first stage in the concentration of the waste water containing the surfactant such as washing waste water, so as to concentrate the component of the surfactant in the waste water, and the waste water wherefrom the component of the surfactant is removed is further concentrated by means of the hydrophobic porous membrane. When the waste water containing the surfactant such as the washing waste water is subjected to the evaporation conducted as the first stage, in other words, the surfactant is left in the waste water since it hardly becomes mist. When the substance wherefrom the component of the surfactant is separated by the evaporation is made to contact the hydrophobic porous membrane, accordingly, steam and water can be separated from each other in a stable manner since the hydrophobicity of the porous membrane is not damaged by the surfactant, and thus the disposal of the waste water containing the surfactant is enabled. Although the waste water wherefrom the surfactant is separated might be made to contact, in the state of liquid, the hydrophobic porous membrane as well for disposal, in this case, the method wherein the waste water is put in the state of liquid and then concentrated by the hydrophobic porous membrane is not very preferable in terms of thermal efficiency and others, and therefore it is preferable that the waste water is introduced as the vapor flow into the hydrophobic porous membrane so as to be concentrated thereby.
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