DESCRIPTION METHOD FOR PURIFICATION TREATMENT OF PROCESS WATER Technical Field The present invention is related to a method for purification treatment of byproduct water to obtain water that can be used for various uses, by treating byproduct water which is produced during- the production of a liquid hydrocarbon mixture from carbon monoxide gas and hydrogen gas based on Fischer-Tropsch reaction, etc. Background Art Fischer-Tropsch reaction (synthesis) is known as a method for synthesizing a liquid hydrocarbon mixture from a synthesis gas comprising carbon monoxide and hydrogen based on a catalytic reaction, and it is also abbreviated as "FT" method. According to FT method, for example, a solid material such as coal, woody fuel, biomass, and carbon-containing waste or a gas material such as natural gas is partially combusted or gasified using steam to prepare synthesis gas (CO, H 2 ) described above, and the synthesis gas is transformed into a liquid hydrocarbon using an iron catalyst or a cobalt catalyst based on FT method. Conventionally, as basically cheap petroleum oil has been supplied in a massive amount, the use of FT method has not been so popular. However, liquid fuel obtained from FT method, for example diesel fuel or jet fuel, is low in sulfur content and 1 has little effect on environment, thus the method is now newly evaluated. In addition, to use natural gas of rich reserve as an alternative for petroleum oil, it is required to convert natural gas into a liquefied hydrocarbon based on FT method. In addition, by using a liquefied hydrocarbon derived from natural gas as raw materials, a product that is equivalent to those derived from petroleum oil can be also produced. That is, with conversion into a liquefied hydrocarbon, the use of natural gas which is present in relatively rich reserve in nature can be diversified. In addition, by liquefying natural gas based on FT method, it can be stored and transported as a liquid at room temperature without having the natural gas in a liquid state by cooling down to extremely low temperature, as it has been treated according to a conventional method. Methane gas, which is included in methane hydrate and currently draws big attention, can be also liquefied based on FT method. Meanwhile, although a demand for water is higher than ever before due to an increase in world population increases, industrialization of a third world, increased production of bioethanol and the like, water resources are geographically localized due to global warming. As a result, water deficiency becomes a more threatening problem. To solve this problem, technological development such as quality improvement of treated water obtained from drained water and enhancement of recovery ratio of water, etc. is now strongly required. 2 Herein, a chemical reaction based on FT method can be described as follows. (2n+1) H 2 +nCO -> CnH 2 n+ 2 +nH 2 0 Thus, according to FT method, water is produced as byproduct water together with hydrocarbons from hydrogen gas and carbon monoxide gas (i.e., Fischer-Tropsch byproduct water) In terms of amount, it is produced more than liquefied hydrocarbons. A mixture comprising this Fischer-Tropsch byproduct water and hydrocarbons is separated into gas (gas phase hydrocarbons), oil (liquid phase hydrocarbons) and byproduct water by using a three-phase separator or a coalescer, etc. Thus separated liquefied hydrocarbons are used as an alternative for petroleum oil. In the remaining byproduct water, unremoved hydrocarbons in a floating state, hydrocarbons dissolved in water, and metals derived from a catalyst, etc. are included as impurities. [Definition of the terms] In the technical disclosure of the present invention, the hydrocarbons that are included in untreated byproduct water as described in the above are defined according to four separate categories. Based on such categories, more detailed descriptions are given in the following. The term "hydrocarbon-based organic substance" indicates every organic substance that is included in untreated byproduct water. The term "hydrocarbon" indicates an organic substance 3 that remains in a floating state (i.e., oil) as not being completely removed by a separator, i.e., a mixture containing aliphatic/aromatic/alicyclic hydrocarbons and slightly-water soluble and oxygen-containing hydrocarbons. Examples thereof include hexane, benzene, phenol, benzaldehyde and the like. The term "non-acidic oxygen-containing hydrocarbon" indicates a hydrocarbon which is water soluble and contains oxygen that is not acidic. Examples thereof include methanol, ethanol, acetone, formaldehyde and the like. The term "acidic oxygen-containing hydrocarbon" indicates carboxylic acids which are water soluble and acidic. Examples thereof include formic acid, acetic acid, propionic acid and the like. Meanwhile, there are two types of Fischer-Tropsch reaction, i.e., the reaction is carried out at low temperature or at high temperature. For a low temperature reaction, a cobalt catalyst or an iron catalyst can be used. For a high temperature reaction, an iron catalyst is used. Due to these separate types of Fischer-Tropsch reaction having different methods, the components that are included in byproduct water are also different from one another. Byproduct water described in the above contaminates an environment when it is disposed as it is, and considering that an effective use of byproduct water is now required, it is water-treated and then released as waste water or reused as industrial water, etc. As a water treatment (i.e., purification) of byproduct 4 water, for example, several methods have been suggested including a method comprising multi-steps of distillation, microfiltration, ultrafiltration, membrane separation based on a reverse osmosis membrane, etc. (for example, see Patent Document 1 and Patent Document 2). According to these methods, distillation is carried out as a primary treatment of a multi-step treatment. As a result, not only the "hydrocarbons" but also most of "non-acidic oxygen-containing hydrocarbons" can be removed. In addition, extremely small amount of the above described two organic substances and "acidic oxygen-containing hydrocarbons" are removed by membrane separation. In addition, there is a method that, by using synthesis gas comprising carbon monoxide and hydrogen as described above, dimethyl ether (DME) is synthesized and used as diesel fuel, etc. Byproduct water is also generated from the DME synthesis method using synthesis gas, and it needs to be treated in the same way as the above described byproduct water. [Patent Document 1] Japanese PCT National Publication No. 2006-514579 [Patent Document 2] Japanese PCT National Publication No. 2006-534469 Disclosure of the invention [Technical problems to be solved by the invention] Meanwhile, when water treatment is carried out based on microfiltration membrane separation, ultrafiltration membrane 5 separation, semi-permeable membrane separation and the like, it is mostly carried out in a cross flow manner. Thus, permeate water which permeates the membrane and contains no impurities, and concentrated water which does not permeate the membrane and contains concentrated amount of the impurities are obtained. In this case, when recovery ratio is for example 70% for membrane separation, i.e., for the case wherein treated water obtained after distillation is separated by a single or multi-step membrane separation treatment, when final permeate water is 70%, there generates 30% of concentrated water. It is obvious that such 30% of concentrated water cannot be used as industrial water, etc., as it is. Even when it is drained, it is difficult to say that it is fully purified. As such, a separate water treatment of this concentrated water is required. The present invention, which is devised in view of the above circumstances, is to provide a purification treatment method which can be used for an efficient treatment of concentrated water when a membrane separation is used. [Means to solve the problems] In order to achieve the above described purpose of the invention, the method for purification treatment of byproduct water as described in Claim 1 is related to a method for purification treatment of byproduct water which is generated during the synthesis of a liquefied hydrocarbon mixture from carbon monoxide gas and hydrogen gas, characterized in that it comprises steps of: 6 carrying out a distillation treatment of the byproduct water to give primary treated water, separating the primary treated water into purified water and concentrated water by using a semi-permeable membrane, having the concentrated water as secondary treated water, carrying out a biological treatment for at least part of the secondary treated water and also having the treated water resulting from solid-liquid separation of the biological treatment as tertiary treated water, and conveying at least part of the tertiary treated water to the primary treated water to carry out again the separation by a semi-permeable membrane. According to the invention described in Claim 1, after removing most of the "hydrocarbon-based organic substance" included in byproduct water via distillation, a semi-permeable membrane separation treatment is carried out to obtain permeate water (i.e., purified water) which can be used as industrial water, irrigation water, drinking water, and the like. In addition, by subjecting concentrated water which is generated from a semi-permeable membrane separation treatment after distillation to a biological treatment, concentration of the "hydrocarbon-based organic substance" can be lowered. Meanwhile, according to a biological treatment, bacterial cells are always present under any condition and the amount of floating materials is high in treated water. As described in the above, membrane separation such as microfiltration or ultrafiltration can be used for solid-liquid separation to 7 remove most of the floating materials. However, there is a possibility that some fine floating materials may leak. In addition, dissolved salts or organic substances that are resistant to a biological degradation cannot be removed. Thus, it is necessary to have tertiary treated water derived from concentrated water again subjected to the semi-permeable membrane separation. According to the present invention, concentrated water (i.e., brine), that is generated during a semi-permeable membrane separation treatment, is subjected to MBR, and at least part of it is again subjected to a semi-permeable membrane separation treatment (i.e., conveying process) . As a result, quality of treated water that is released from entire system is improved. Further, by having this conveying process, fine floating materials or dissolved salts can be removed. Meanwhile, only 20% or so of "non-acidic oxygen-containing hydrocarbon" such as methanol and the like is removed by a semi-permeable membrane treatment, and therefore the "non-acidic oxygen-containing hydrocarbon" that cannot be completely removed by distillation is hardly processed with a semi-permeable membrane treatment and just permeates the membrane. However, according to the technology of the present invention, water treated by a membrane bioreactor (MBR), which can process almost 100% of methanol, is conveyed for merge. As a result, due to a dilution effect, methanol concentration in water treated by a semi-permeable membrane treatment can be lowered. In addition, since organic substances that are basically difficult to be degraded by a 8 biological treatment are repeatedly subjected to the biological treatment and microorganisms capable of degrading such organic substance can flourish, slowly the degradation is promoted. Further, primary treated water is at high temperature and comprises a great deal of "acidic oxygen-containing hydrocarbons" as described above, thus its pH is low. To carry out efficiently a semi-permeable membrane separation as a secondary treatment, temperature should be lowered to near room temperature and pH needs to be increased. Meanwhile, since tertiary treated water is water obtained after a biological treatment, in general it is at room temperature and has near neutralpH. For such reasons, by conveying the tertiary treated water to the primary treated water, amount of energy or an alkali addition that is required for temperature reduction or neutralization of pH can be saved. According to Fischer-Tropsch reaction, inorganic components derieved from a carrier can be included in drain water, and sometime scale components are included in the inorganic components. With a semi-permeable membrane separation as a secondary treatment, primary treated water as supply water is gradually concentrated. That is, when concentration is carried out to the level that is higher than the acceptable dissolution concentration of a scale component, the scale component will not be dissolved and further membrane separation may become difficult. In other words, concentration of the scale component is a limiting factor for recovery ratio of permeate water (purified water) compared to 9 supply water (primary treated water) in a semi-permeable membrane separation treatment. Meanwhile, as tertiary treated water is water obtained after a biological treatment, the scale component is adsorbed to sludge during a biological treatment. Thus the concentration of the scale component is lower in the treated water (i.e., tertiary treated water) compared to a supply water for a biological treatment (i.e., secondary treated water). Thus, by conveying the tertiary treated water to the primary treated water, concentration of the scale component can be lowered and also recovery ratio of a semi-permeable membrane separation (i.e., secondary treatment) can be improved. For such reasons, according to the present invention, tertiary treated water that is separated based on solid-liquid separation during a biological treatment is conveyed to the primary treated water, mixed together and subjected again to the separation using a semi-permeable membrane. As a result, most of the tertiary treated water becomes useful permeate water and some becomes again concentrated water. However, such concentrated water is subjected to a repeated biological treatment as described above, thus the concentrated water that is generated during membrane separation can be efficiently purified. Moreover, with a semi-permeable membrane separation treatment, dissolved salts or small organic substances having low molecular weight can be removed so that separated purified water can be also used as drinking water. 10 In this case, although the tertiary treated water is conveyed to get mixed with the primary treated water and subjected to a semi-permeable membrane separation treatment as described in the above, some of it is also subjected to another semi-permeable membrane separation treatment using an equipment that is separate from the one used for the semi-permeable membrane separation treatment described above. As a result, not only the above described concentrated water can be purified to the level that is required for industrial water, irrigation water, drinking water and the like but also load applied on a semi-permeable membrane during a semi-permeable membrane separation treatment that is carried out as a secondary treatment can be lowered. Further, by appropriately selecting a semi-permeable membrane suitable for various water quality requirements, operation cost can be reduced and energy can be saved. Although it is necessary to process concentrated water which is generated by said semi-permeable membrane separation treatment, it can be returned to byproduct water, thus an operation process can be simplified. The method for purification treatment of byproduct water as described in Claim 2 is characterized in that, regarding the invention described in Claim 1, solid-liquid separation is carried out based on membrane separation for the above mentioned biological treatment. According to the invention described in Claim 2, treated water which includes a great amount of suspended solid such as 11 bacterial cells, etc. resulting from a biological treatment is subjected to solid-liquid separation using a separating membrane. Thus, compared to conventional solid-liquid separation based on natural precipitation, treatment time can be shortened, a small-sized treatment equipment can be used, and equipment cost can be reduced. Further, as suspended solid are removed by a separating membrane, filtration property and separation property of a semi-permeable membrane separation treatment (i.e., secondary treatment) can be improved. The method for purification treatment of byproduct water described in Claim 3 is characterized in that, regarding the method of Claim 1 or Claim 2, part of tertiary treated water obtained after the biological treatment but not conveyed to the primary treated water is again separated into purified water and concentrated water by using a semi-permeable membrane that is different from the above described semi-permeable membrane followed by separation into purified water and concentrated water by using any of these semi-permeable membranes, and then at least part of the resulting concentrated water is conveyed to byproduct water before any purification. The method for purification treatment of byproduct water described in Claim 4 is characterized in that, regarding the invention described in any of Claims 1 to 3, after treating the tertiary treated water with activated carbon treatment and/or carrying out ultrafiltration treatment, the tertiary treated water is again separated into purified water and concentrated water by using a semi-permeable membrane that is different from 12 the above described semi-permeable membrane. According to the invention described in Claim 4, before carrying out a treatment using a semi-permeable membrane, residual impurities are removed by an activated carbon treatment or ultrafiltration and/or microfiltration having mesh size larger than the semi-permeable membrane so that load on a semi-permeable membrane is reduced and lifetime of the membrane is extended, etc. As a result, overall cost can be reduced. The method for purification treatment of byproduct water as described in Claim 5 is characterized in that, regarding the invention described in any of Claims 1 to 4, as the above described semi-permeable membrane and/or a semi-permeable membrane that is different from the above described semi-permeable membrane, a low fouling reverse osmosis membrane is used. According to the invention described in Claim 5, since a semi-permeable membrane is a low fouling reverse osmosis membrane, deterioration in performance of a semi-permeable membrane due to fouling can be prevented. When chemical fouling (i.e., chemical contamination) wherein organic substances (hydrocarbons) dissolved in byproduct water are adsorbed on a membrane surface or bio fouling (i.e., biological contamination) wherein microorganisms that flourish as having dissolved organic substances as their nutrient source are adsorbed on a membrane surface occurs, there is a problem that water permeating 13 property and membrane separation property of a semi-permeable membrane are deteriorated. On the other hand, by using a low fouling reverse osmosis membrane, such deterioration in properties due to fouling can be inhibited. Effect of the invention According to the present invention, byproduct water that is produced during the production of a liquid hydrocarbon mixture from synthesis gas based on a conventional method such as FT method, etc., can be cleaned and purified with low cost. Brief description of the drawings Fig. 1 is a process flow chart showing each step of the method for purification treatment of byproduct water, that is related to an embodiment of the present invention, is shown. Fig. 2 is a schematic drawing which shows constitution of a fluid separation element using a semi-filtration membrane in flat membrane shape, as used in the present invention. Detailed Description of the Invention Herein below, in view of the drawings, embodiments of the present invention will be explained. According to the present invention, after separating hydrocarbons and byproduct water from each other wherein said hydrocarbons and byproduct water are produced from catalytic reaction of synthesis gas based on Fischer-Tropsch reaction, etc., the separated byproduct water is cleaned and purified to 14 the level satisfying any one of water quality that has no significant effect on environment when the separated byproduct water is drained as it is, water quality that is safe to be used as industrial water or irrigation water, or water quality that is safe to be used as drinking water. The method for purification treatment of byproduct water of the present example relates to, as it is shown in the process flow chart of Fig. 1, by carrying out a distillation treatment of byproduct water which is separated from the reaction product obtained by the production of a liquid hydrocarbon mixture using synthesis gas (1: primary treatment), primary treated water is obtained. Subsequently, by carrying out a semi-permeable membrane separation treatment to the primary treated water in a cross-flow manner (2: secondary treatment), purified water (permeate water) which can be used as industrial water, irrigation water or drinking water, and concentrated water as secondary treated water are obtained. Further, by carrying out an aerobic treatment and/or anaerobic treatment as a biological treatment to the secondary treated water (3: tertiary treatment) and also by carrying out a solid-liquid separation of bacterial cells, tertiary treated water is obtained. Subsequently, by subjecting the tertiary treated water to an activated carbon treatment and/or a membrane separation treatment for removing residual impurities by ultrafiltration (4; fourth treatment), fourth treated water is obtained. Meanwhile, when not so high-level quality is required for treated water, the activated carbon treatment 15 and/or the membrane separation treatment by ultrafiltration can be omitted. Further, part of the fourth treated water is conveyed to the primary treated water, and a semi-permeable membrane separation 2 as the above mentioned secondary treatment is carried out. Further, it is also possible that all of the fourth treated water is conveyed to the primary treated water. Further, when the fourth treatment is omitted, whole or part of the tertiary treated water is conveyed to the primary treated water. Further, for the portion of the fourth treated water that has not been conveyed, a semi-permeable membrane separation is carried out in cross flow manner by using an equipment which is separate from the one used for the above described secondary treatment (5: fifth treatment), and purified water is obtained. This purified water and the purified water obtained from secondary treatment water can be drained into river, ocean, etc. However, it can be preferably used as industrial water, irrigation water, drinking water, etc. Further, it is preferable that concentrated water obtained from the fifth treatment is, for example, conveyed to byproduct water and with all together a primary treatment is carried out. Further, the treatments described in the above can be a batch treatment for each step or a continuous treatment process. Further, when each step is performed as a continuous treatment, the entire process of the method for purification treatment can be carried out in a continuous treatment process. Further, COD removal ratio is similar to the removal ratio 16 of "hydrocarbon-based organic substance." In the present example, COD removal ratio is used as removal ratio of "hydrocarbon-based organic substance." When distillation treatment (1) is carried out, for example, a distillation tower (i.e., rectification tower) that is well known in petroleum chemical industries can be used. For example, when a continuous distillation is performed, byproduct water that has been vaporized by hot steam is introduced to a middle level of a distillation tower, and then distillate including a great amount of "hydrocarbons" and "non-acidic oxygen-containing hydrocarbons" can be obtained as a volatile component that is obtainable at a top region. From a bottom region, water from which these "hydrocarbons" or "non-acidic oxygen-containing hydrocarbons" are removed can be obtained. This water becomes primary treated water. Further, distillate which include a great amount of "non-acidic oxygen-containing hydrocarbons" is for example incinerated, same as the conventional method. Distillation treatment (1) has an advantage of providing high separation ratio of lower alcohols (i.e., "non-acidic oxygen-containing hydrocarbons") that are difficult to be separated from water by a semi-permeable membrane separation treatment. Next, according to semi-permeable membrane separation treatment (2), "acidic oxygen-containing hydrocarbons" or dissolved salts remaining in primary treated water can be removed, and resulting purified water can be used as industrial 17 water, irrigation water, drinking water and the like as described above. In addition, according to semi-permeable membrane separation treatment (2), not only bacteria but also viruses can be filtered off, so it can be used as drinking water. Still further, according to semi-permeable membrane separation treatment (2), dissolved salts (e.g., metal ions, etc.) can be also removed to the level that is required for drinking water. Thus, water level that is required for high quality water can be obtained. A semi-permeable membrane indicates a membrane which only allows permeation of ions or molecules having certain size or molecular weight. Examples thereof include a nanofiltration membrane or a reverse osmosis membrane. It is required for a semi-permeable membrane to have a property which can reduce concentration of solutes in filtrate water to the level that is required for recycled water. A nanofiltration membrane is defined as a filtration membrane having operation pressure of 1.5MPa or less, cut-off molecular weight of 200 to 1,000 and sodium ion blockage of 90% or less. A membrane which has a smaller cut-off molecular weight and higher blockage ratio is called a reverse osmosis membrane. When concentration of a solute or a suspended material is low, it is preferable to use a nanofiltration membrane which requires low operation pressure. On the other hand, when concentration of a solute or suspended material is high, it is preferable to use a reverse osmosis membrane. In addition, when occurrence of deterioration in water 18 permeability or removal property by chemical fouling (i.e., chemical contamination) wherein dissolved organic substances are adsorbed on a membrane surface or bio fouling (i.e., biological contamination) wherein microorganisms that flourish as having dissolved organic substances as their nutrient source are adsorbed on a membrane surface is worried, it is preferable to use a low-fouling membrane which is resistant to such fouling. For example, reduction ratio of water permeability is determined as follows. When membrane filtration is carried out at 25 0 C for one hour by using sodium chloride solution (pH 6.5, 1,500mg/L) with operation pressure of 1.0MPa and the resulting water permeability is taken as former-permeability (Fl), and then after adding a non-ionic surfactant (polyoxyethylene(10)octylphenyl ether) to the test solution to have concentration of 100mg/L and water permeability one hour after the addition is obtained as latter-permeability (F2), the reduction ratio of water permeability is defined by the following formula; Reduction ratio of water permeability = 1-(F2/Fl). Such ratio is 0.35 or less, or preferably 0.20 or less for the present invention. By using a membrane with such property, almost no adsorption of organic substances occurs on membrane surface and deterioration in water permeability is negligible, and therefore permeate water can be stably obtained. Examples of a method for producing a low fouling membrane include, a method of coating a polymer on a polyamide membrane 19 surface to inhibit flux deterioration caused by fouling (see, WO 97/34686 and Japanese Patent Application Laid-Open (JP-A) No. 2000-176263), a method of carrying out a surface treatment with a compound which can react with surface-remaining acid chlorides or amino groups (see, JP-A No. 2002-224546 and JP-A No. 2004-243198), a method of irradiating electronic beam, UV light, radioactive ray, etc. to membrane surface or modifying a surface by graft polymerization (see, JP-A No. 2007-014833), a method of reducing surface area available for adsorption by smoothing out the surface (see, Eric M. Vrijenhoek, Seungkwan Hong, Menachem Elimelech, "Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes, " Journal of Membrane Science 188 (2001) 115-128) and the like. As an example of a low fouling reverse osmosis membrane, semi-permeable membranes such as TML20 series (manufactured by Toray Industries, Inc.), LF10 series (manufactured by Nitto Denko Corporation), LFC series and ESNA-LF series (manufactured by Hydranautic), BW30-FR series (manufactured by Dow Chemical Company), Seasoft HL series (manufactured by OSMONICS) and the like can be mentioned. When supply water is filtered using a semi-permeable membrane, operation pressure that is higher than permeation pressure between supply water side and permeate water side is required. A pressurizing pump to obtain such pressure is not specifically limited as long as it can provide pressure that can pressurize filtering water. 20 A nanofiltration membrane or a reverse osmosis membrane can be in a shape of a hollow fiber membrane or a flat membrane, and both can be used for the present invention. In addition, to improve a handling property, a fluid separation element in which a hollow fiber membrane or a flat membrane is contained in a casing body can be used. When a semi-permeable flat membrane is used as a nanofiltration membrane or a reverse osmosis membrane, a fluid separation element preferably has a structure as shown in Fig. 2, for example, wherein a semi-permeable membrane (10) and a membrane unit including a permeate water side flow channel material (12) such as tricot and the like, and a supply side flow channel material (11) such as a plastic net and the like are wrapped around a tubular-shaped perforated center pipe (17), and the resulting structure is placed in a casing body. It is also preferable that, by connecting multiple fluid separation elements in a serial or parallel manner, a separation membrane module is obtained and used. With respect to the fluid separation element, supply water (13) is supplied into the unit from an end of one side. Then, during the period the water reaches the end of the other side, permeate water (15) which permeated a semi-permeable membrane (10) flows through the center piper (17) and is drained from the center pipe at the end of the other side. Meanwhile, supply water (13) which did not permeate through the semi-permeable membrane (10) is drained as concentrated water at the end of the other side. As a material for a semi-permeable membrane (10), polymer 21 materials such as a cellulose acetate-based polymers, or polyamides, and the like can be used. In addition, with respect to a membrane structure, it can be any one of an asymmetric membrane which has a dense layer on at least one side of the membrane and fine pores of which diameter becomes gradually larger towards the inside or the other side of the membrane, or a complex membrane which has a very thin separative functional layer for separation on a dense layer of an asymmetric membrane wherein the separative functional layer is made of a different material. For the above described support, various commercially available filter materials such as "Millipore VSWP" (trade name; manufactured by Millipore Corporation), "Ultra Filter UK10" (trade name; manufactured by Toyo Roshi Kaisha, Ltd.) and the like can be used. In general, it can be produced according to a method described in "Office of Saline Water Research and Development Progress Report" (No.359 (1968) ) . As a basic material, a homopolymer such as polysulfone, polyamine, polyester, acetic acid cellulose, nitric acid cellulose or poly vinylchloride, etc. or a blend thereof is used. Preferably, polysulfone having high chemical, mechanical and thermal stability is used. For example, on top of a densely-woven polyester fabric or non-woven fabric, dimethylformamide (DMF) solution of polysulfone is cast with even thickness and the fabric is wet-solidified in an aqueous solution including sodium dodecylsulfate (0.5 wt%) and DMF (2 wt%). As a result, a 22 microporous support membrane having fine pores with diameter of several tens of nanometers or less present on most of the membrane surface can be obtained. As a material for a microporous support membrane, polyamide or polyester is also preferably used in addition to polysulfone. In addition, operational conditions of a semi-permeable membrane (e.g., filtration flow rate, recovery ratio, etc.) can be appropriately determined depending on type of a nanofiltration membrane or a reverse osmosis membrane to be used, quality of water to be treated, and quality requirement for permeate water, etc. However, filtration flow rate is preferably determined while keeping in mind that membrane fouling should be inhibited as much as possible. Regarding recovery ratio, which is ratio of permeate water compared to water to be treated, higher value is preferred. However, it has to be cautioned that higher ratio may yield poor quality of permeate water. In addition, if recovery ratio is too high, incompletely dissolved solutes can be precipitated on a membrane surface so that a dent may be formed on the membrane or a flow channel may be blocked. As such, recovery ratio has to be set within the range wherein no precipitation occurs. Of course, when a scale inhibitor is added, precipitation can be inhibited to a certain level and therefore higher recovery ratio can be set. In addition, when recovery ratio is extremely high, such ratio can be maintained by reducing flow amount of water to be treated. However, reducing flow amount of water to be treated to an extremely low level may cause an accumulation on 23 a membrane surface (i.e., stronger concentration polarization causes deterioration in performance), thus the flow amount of water to be treated should be set within a recommended range. As such, for the purpose of maintaining the flow amount of water to be treated within an appropriate range, a nanofiltration membrane or a reverse osmosis membrane is used in multi-steps and as a result recovery ratio can be improved. In this case, according to the semi-permeable membrane separation treatment (2) that is performed in a cross flow manner, concentrated water is generated. The biological treatment (3) is performed on this concentrated water as secondary treated water using an apparatus for a biological treatment. During the biological treatment (3), an aerobic treatment and/or an anaerobic treatment is carried out as a pre-step. A biological treatment is to treat organic substances included in water with bioorganisms (in particular microorganisms). That is, as a substrate for bioorganisms (microorganisms), the organic substances are treated by being ingested by them. There are two main types of a biological treatment, i.e., an aerobic treatment and an anaerobic treatment. For an aerobic treatment, any well known method can be used. In the present case, considering the fact that hydrocarbon concentration is already reduced by a primary treatment and less amount of excess sludge is favorable, a biological membrane method in which microorganisms are maintained on a carrier can be preferably used. In this regard, 24 an activated sludge method as a general method can be also used. In addition, an anaerobic treatment such as methane fermentation, etc. can be also preferably used. When concentration of "hydrocarbon-based organic substances" is high in byproduct water, aeration energy can be reduced, thus it is particularly preferable. For a biological treatment, a various combination is possible based on concentration of a component in water to be treated, required water quality of treated water after solid-liquid separation, etc. For example, when the concentration of organic substances (COD, BOD, TOC and the like) is high in concentrated water (for example 2,000mg COD/L or more), carrying out an anaerobic treatment followed by an aerobic treatment is favorable in terms of saving energy or reducing cost. Further, since sludge made of bacterial cells of microorganisms, etc. are generated by the biological treatment (3), a solid-liquid separation treatment has to be carried out after the treatment. Any kind of a solid-liquid separation based on a general coagulation and precipitation method can be used without a problem. Still, in the present examples, it is preferable to carry out the solid-liquid separation not with a precipitation method but with a membrane separation method. In addition, when a membrane separation unit comprising a separation membrane is immersed and placed in a treatment bath, it is preferable that water to be filtered by membrane which surrounding the membrane separation unit is set in a flow motion 25 by using air or othermeans, etc. In addition, when the membrane separation unit is installed outside, water to be filtered by membrane in the treatment bath is supplied to a membrane separation unit installed outside the treatment bath in a cross flow manner and treated water which did not go through the membrane separation unit is again conveyed to the treatment bath. Regarding the structure of a separation membrane that is used for membrane filtration method, a porous membrane or a complex membrane in which a functional layer is complexed to a porous membrane and the like can be mentioned, but it is not specifically limited thereto. Polyfluorovinylidene porous membrane or polytetrafluroethylene porous membrane is particularly preferred as such membrane, since they have high resistance to chemicals. In addition, a complex membrane in which cross-linked type silicone, polybutadiene, polyacrylonitirle butadiene, or rubber type polymers such as ethylenepropylene rubber or neoprene rubber is complexed as a functional layer applied on the porous membrane can be mentioned. Further, regarding a shape of a separation membrane, there are a flat membrane, a rotatable flat membrane, a hollow fiber membrane, a tubular membrane and the like, but it is not specifically limited thereto. A diameter of a membrane pore in a separation membrane is preferably a pore diameter which is useful for separating activated sludge into a solid component and a dissolved component by a solid-liquid separation. A 26 microfiltration membrane or an ultrafiltration membrane can be mentioned as an example. When the diameter of the membrane pore is large, membrane water permeability is improved but it tends to have a high possibility that solid components are contained in membrane filtered water. On the other hand, when the diameter of the membrane pore is small, there is a less possibility that solid components are contained in membrane filtered water, but membrane water permeability tends to deteriorated. Specifically, the diameter of the membrane pore is preferably in the range of 0.01 to 0.5pm, more preferably, 0.05 to 0.2pm. Thus, since a precipitation bath, etc. for separation of sludge is not required and no sludge are drained, high concentration of sludge or saving a working space, etc. can be expected. In addition, even for a biological fixation method, excess sludge are present as a floating material. Those can be also easily separated by using a membrane. As described in the above, by using a membrane separation method for solid-liquid separation, efficiency for space utilization and cost reduction can be achieved. In addition, since solid components can be removed by a separation membrane, when tertiary treated water is treated using a semi-permeable membrane, filtration property and separation property of the semi-permeable membrane can be improved. For the biological treatment (3), a membrane separation activated sludge method (MBR) can be preferably used. That is, 27 as the biological treatment (3) an aerobic treatment is performed, and as a solid-liquid separation treatment, the membrane separation activated sludge method using a microfiltration membrane and/or an ultrafiltration membrane can be used. In one prior art document (i.e., Patent Document 2) a treatment comprising "distillation", MBR, and semi-permeable membrane (RO) in said order is described as a flow for GTL byproduct water treatment. When this flow is compared to the flow of the present invention, it is found that, although the amount of treated water by RO is the same, the load of water amount on MBR is smaller in the present invention since the amount of treated water by MBR is reduced as much as a recovery amount by RO (60% or more). In addition, although it is believed that load of organic substances on MBR is almost the same, since concentration of organic substances contained in MBR original water is high in the present invention, energy efficiency of a treatment (aeration) is improved. Regarding the membrane separation activated sludge method, an immersion-type membrane separation activated sludge method in which the separation membrane is immersed in a treatment bath, and a circulation-type membrane separation activated sludge method in which a membrane separation apparatus accommodating a separation membrane is installed outside the treatment bath, sludge contained in the treatment bath are supplied to a membrane separation apparatus to carry out membrane filtration while using liquid flow of the supply 28 flow and surface of the separation membrane is washed, and sludge that did not separated by the membrane separation are conveyed back to the treatment bath can be preferably used. In particular, an immersion-type membrane separation activated sludge method which can lower energy consumption by using aeration both for a biological treatment and cleaning of membrane surface is preferred. As a method for obtaining membrane permeate water by carrying out membrane filtration, there is a method in which a suction pump is used for secondary step of a membrane filtration or a method in which water head difference is utilized, etc. Concentration of activated sludge that are in contact with a separation membrane is preferably in the range of 2,000mg/L to 20,000mg/L. In addition, it is preferable that an air diffuser is installed in a lower region of a separation membrane, oxygen-containing gas (air, etc.) is supplied from an aeration apparatus (e.g., blower, etc.) that is installed so as to be continuously connected with an air diffuser, and membrane filtration is carried out while peeling off membrane-adhered activated sludge components from membrane surface. Residence time of water to be treated in a biological treatment bath is generally 1 to 72 hours. However, depending on properties of water to be treated and conditions for a biological treatment, the best time period can be selected. In addition, with installment of an apparatus for adding a coagulant, a coagulant can be added to water to be treated which contains activated sludge that are accumulated in a biological 29 treatment bath. Flux for membrane filtration (flow amount of membrane filtration per unit area of membrane surface) is preferably 0.1 to 1.5m/d. Tertiary treated water that is obtained after the biological treatment (3) is then subjected to an activated carbon treatment and/or an ultrafiltration separation treatment (4). Activated carbon treatment is a treatment by which secondary treated water is brought into contact with activated carbon to have impurities included in the secondary treated water (i.e., biological metabolites, etc.) get adsorbed on activated carbon and to remove them from the secondary treated water. In the present invention, type of activated carbon is not specifically limited. It can be either granular activated carbon or powdered activated carbon. In addition, raw materials of activated carbon can be any thing that is generally used, including palm shell, coal, cokes and the like. These raw materials are carbonated and activated to prepare activated carbon. Method for activation is not specifically limited. For example, activated carbon including activated carbon that is produced by using active gas such as steam, oxygen, carbon dioxide, etc. according to the method described in the literature ("Activated carbon Industries", The Heavy & Chemical Industries News Agency (1974), p.
23 to p.37), or chemically activated carbon using phosphoric acid, zinc chloride, etc. can be used. According to an activated carbon treatment, residual impurities such as organic substances and the like can be 30 removed by adsorption. Further, an ultrafiltration membrane treatment is membrane filtration for a biological treatment in which an ultrafiltration membrane is used. In this case, regarding a shape of a separation membrane, there are a flat membrane, a rotatable flat membrane, a hollow fiber membrane, a tubular membrane and the like and it is not specifically limited thereto. Depending on quality of original water and treatment condition, etc. it can be appropriately selected. Regarding an apparatus for ultrafiltration membrane, it can be any of an internal or an external pressure type. When viscosity of original water is high or a great amount of suspension materials are comprised, an external pressure type is preferable considering that it is relatively free of clogging. In addition, regarding a type of membrane filtration, it can be any of bulk filtration type module or a cross flow filtration type module. Meanwhile, the cross flow type is resistant to fouling but it is characterized in that it has high energy consumption. For a general water treatment, less energy consumption is considered important so that a bulk filtration type module is more commonly used. In addition, it can be any of pressure type module or an immersion type module. In this regard, a pressure type module is characterized in that it can be operated under high flux and membrane area can be decreased, while an immersion type module is characterized in that it does not require any pressure-resistant vessel so that it can be performed with low cost. 31 Regarding a hollow fiber membrane that can be used for a membrane module, any kind of a porous hollow fiber membrane can be used without specifically limited. However, organic materials such as polyfluorovinylidene (PVDF) or polyacrylonitrile are preferred, in terms of having high membrane strength and high resistance to chemicals, or high hydrophilicity and high resistance to contamination, respectively, along with inorganic materials such as ceramic, etc. Diameter of a pore present on surface of a separation membrane can be appropriately selected within the range of 0.001Lpm to O.l m. In addition, when a hollow fiber membrane is used as a separation membrane, outer and inner diameters of a hollow fiber membrane is not specifically limited. Still, caution is required since flow resistance becomes high when it is too thin and filling ratio of a membrane can be decreased when it is too thick. Still further, considering that a hollow fiber membrane has a high vibrational property and an excellent washing property, it is preferably in the range of 250pm to 2, 000m. With an ultrafiltration membrane separation treatment, although salts having a small molecular weight cannot be removed, an organic substance having a large molecular weight can be removed. These are carried out as a pre-treatment for a semi-permeable membrane separation treatment (2, 5) which is performed in the next. By reducing the amount of organic substances, etc. remaining in tertiary treated water that is subjected to a semi-permeable membrane separation treatment (2, 32 5), load applied on the semi-permeable membrane can be reduced, and as a result life time of the semi-permeable membrane can be extended and process cost can be saved. In addition, when an ultrafiltration membrane separation treatment is performed in a cross flow manner, concentrated water can be conveyed to the tertiary treatment (i.e., biological treatment (3)) or the primary treatment (i.e., distillation treatment (1)). In addition, fourth treated water that is obtained after the activated carbon treatment and/or the ultrafiltration membrane separation treatment (4) is conveyed to and mixed with the primary treated water and then subjected to a semi-permeable membrane separation treatment (2), which is a secondary treatment. As a result, dissolved salts, fine floating materials derived from bacterial cells, and some amount of the "hydrocarbon-based organic substances", that cannot be removed by a biological treatment as tertiary treatment, can be removed by carrying out again filtration through a semi-permeable membrane. Alternatively, it is also possible to omit fourth treatment and tertiary treated water is conveyed to secondary treatment side. According to the present example, in order to avoid excess load on the semi-permeable membrane with conveying of tertiary treated water, part of tertiary treated water, for example about 50%, is conveyed to primary treatment water side and the remaining 50% is subjected to a semi-permeable membrane separation treatment (5) as a fifth treatment by using an 33 equipment for semi-permeable membrane separation that is separate from the one used for the above described secondary treatment. Alternatively, tertiary treated water can be subjected to a semi-permeable membrane separation treatment (2, 5) without fourth treatment. A semi-permeable membrane separation (6) is performed basically in the same manner as a semi-permeable membrane separation treatment (2) . However, since concentration of hydrocarbon-based organic substances in fourth treated water (tertiary treated water) is already decreased by tertiary treatment compared to that of primary treated water, with smaller scale equipment compared to the equipment for a semi-permeable membrane separation treatment of secondary treatment, a semi-permeable membrane separation treatment as fifth treatment can be carried out. In this case, concentrated water is also generated from a semi-permeable membrane separation treatment (5) as a fifth treatment. Such concentrated water is conveyed to and mixed with byproduct water, secondary treated water, etc. For instance, in the present example, it is conveyed to byproduct water. In addition, by again conveying tertiary treated water that is biologically treated after a semi-permeable membrane separation as secondary treatment to primary treated water side for semi-permeable membrane separation, purified water can be produced with efficiency. EXAMPLES 34 (Example 1) Byproduct water that is produced by FT method was purified according to the method described below. Specifically, byproduct water is subjected to distillation (i.e., distillation treatment (1)) followed by a semi-permeable membrane separation treatment (2) . Thus purified water was used as water such as industrial water, etc. and concentrated water as secondary treated water was subjected to a biological treatment (3) . In addition, tertiary treated water which has been separated into solid and liquid based on membrane separation of the biological treatment was admixed with the primary treated water. Distillation was carried out at 100*C under atmospheric pressure. In addition, as biological treatment (3), the above described membrane separation activated sludge method (MBR)was employed. For a biological treatment of MBR, circulation type nitiriding/denitiring method was employed. As a separation membrane, a microfiltration membrane made of polyfluorovinylidene (average pore diameter 0.08pim, manufactured by Toray Industries, Inc.) was used. First, secondary treated water was introduced to an oxygen-free bath comprising activated sludge. After a denitriding treatment, activated sludge mixture was introduced to a next nitriding bath. Inside the nitriding bath, an aerobic treatment was carried out with aeration (i.e., degradation of organic substances and nitriding reaction), and part of the mixture was conveyed back 35 to the oxygen-free bath for circulation. In this case, circulation flow amount was four times compared to the flow amount of the primary treated water. In addition, part of the activated sludge mixture contained in the nitriding bath was introduced to a membrane separation bath. Inside the membrane separation bath, a flat membrane element equipped with the above described separation membrane was immersed and in the lower region of the flat membrane element an air diffuser was installed to perform aeration both for cleaning of membrane surface and supplying of oxygen. The activated sludge mixture solution in the membrane separation bath was conveyed to the nitriding bath in a flow amount three times higher than that of the secondary treated water. Activated sludge contained in the membrane separation bath were subjected to solid-liquid separation with application of negative pressure on permeation side of the separation membrane by using a suction pump. As a result, permeate water was obtained as tertiary treated water. In addition, recovery ratio of water was 80% for the semi-permeable membrane separation treatment (2) (remaining 20% was drained as concentrated water). As a semi-permeable membrane, low fouling reverse osmosis membrane TML20-370 (flat membrane made of polyamide, manufactured by Toray Industries, Inc.) was used. In this case, original supply water for a semi-permeable membrane treatment was introduced into the semi-permeable membrane by using a centrifugal pump to obtain permeate water and concentrated water. Results of the treatments are shown in Table 1. 36 [Table 1] FT RO RO MBR byproduct permeated brine treated water Distillation water water "Non-acidic oxygen containing mq/L hydrocarbons" 15, 000 "Acidic oxygen containing mg/L hydrocarbons" 1,000 700 30 3,200 50 "Hydrocarbons" mg/L <10 CODCr mg/L 15,000 850 50 4,000 150 As it is indicated in Table 1, the byproduct water includes "non-acidic oxygen-containing hydrocarbons" at 15, 000mg/L and "acidic oxygen-containing hydrocarbons" at 1000mg/L. Concentration of "hydrocarbons" was less than 10mg/L. In addition, oxygen demand due to potassium dichromate (i.e., CODCr) was about 15,000mg/L. In addition, after the distillation, concentration of "acidic oxygen-containing hydrocarbons" was 700mg/L and CODCr was 850mg/L. Thus, by the distillation, COD removal ratio which is an approximated value of removal ratio of "hydrocarbon-based organic substances" was 94.3%. For purified water (permeate water) of the semi-permeable membrane separation treatment (2), concentration of "acidic oxygen-containing hydrocarbons" was 30mg/L and CODCr was about 50mg/L. Further, for the concentrated water (secondary treated water) generated from the semi-permeable membrane separation treatment (2), concentration of "acidic oxygen-containing hydrocarbons" was 3,200mg/L and CODCr was 4,000mg /L. Still 37 further, for the tertiary treated water obtained from MBR treatment of the secondary treated water, concentration of "acidic oxygen-containing hydrocarbons" was 50mg/L and CODCr was 150mg /L. Meanwhile, 30% of the flow amount of the tertiary treated water obtained after the biological treatment was admixed with the primary treated water and again subjected to the semi-permeable membrane separation treatment (2). With the above described treatments, byproduct water can be transformed into water which can be satisfactorily used as industrial water or irrigation water. In addition, it is also possible to purify it to the level that is required for drinking water. In addition, compared to prior art technologies, equipment cost or operation cost can be saved as described above and also even concentrated water obtained after membrane separation can be reliably and efficiently purified. (Example 2) Except that part of the tertiary treated water which had not been conveyed to the primary treated water was again separated into purified water and concentrated water as fifth treatment by using a reverse osmosis membrane that is different from the reverse osmosis membrane of the previous step and at least part of the resulting concentrated water was conveyed to byproduct water before any purification, the purification up to tertiary treated water was carried out exactly in the same manner/means/flow as Example 1. Specifically, byproduct water was first distilled and then subjected to a semi-permeable 38 membrane separation treatment. As a result, purified water was used as various water for industrial water, etc. and concentrated water as secondary treated water was subjected to a biological treatment. Then, with membrane separation of the biological treatment, solid-liquid separation was achieved to give tertiary treated water. Organic substances that are contained in concentrated water of fifth treated water had undergone the biological treatment, and they are a component resistant to degradation. By circulating this concentrated water to the distillation treatment, it becomes possible that organic substances are more efficiently treated and degraded. A reverse osmosis membrane that is different from the reverse osmosis membrane of the previous step and used in the present example is low fouling membrane TML20 series that is manufactured by Toray Industries, Inc. Operation condition of the membrane corresponded to a typical condition in pertinent art. [Table 2] MBR Secondary RO Secondary RO treated water permeate concentrated water water Non-acidic oxygen containing mg/L hydrocarbons" _ _ ""I "Acidic oxygen containing mg/L 50 1 200 hydrocarbons" "Hydrocarbons" mg/L CODCr mg/L 150 10 600 39 [Description of the symbols] 1: Distillation 2: Semi-permeable membrane separation treatment 3: Biological treatment 4: Activated carbon treatment and/or ultrafiltration membrane separation treatment 5: Semi-permeable membrane separation treatment 10: Semi-permeable membrane 11: Supply water flow channel material 12: Permeate water flow channel material 13: Supply water 14: Concentrated water 15: Permeate water 16: End plate 17: Center pipe 40