GB2359302A

GB2359302A - Treatment of waste water with ozone

Info

Publication number: GB2359302A
Application number: GB0024708A
Authority: GB
Inventors: Keiji Hirano; Tsutomu Taira; Jun Tanaka; Shigeto Yoshida
Original assignee: Organo Corp; NEC Corp
Current assignee: Organo Corp; NEC Corp
Priority date: 1999-10-07
Filing date: 2000-10-09
Publication date: 2001-08-22
Also published as: KR20010050843A; TW487686B; GB0024708D0; KR100741351B1; JP2001104971A

Abstract

pH is adjusted optimally for each substance to be treated in waste water containing substances not amenable to conventional biological treatment because of odor generation, foaming property, toxicity to microorganisms, or the like. Once the pH is adjusted, the substance is then subjected to ozone oxidation treatment. The waste water is preferably subjected to a biological treatment after completion of each oxidation treatment. The waste water may be subjected to heating and degassing treatments prior to the oxidation treatment.

Description

2359302 METHOD OF TREATING WASTE WATER AND APPARATUS THEREFOR

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a method of treating waste water and an apparatus therefor, and more particularly, to a method and apparatus for treating waste water containing substances not readily treatable by a biological treatment due to odor generation, foaming property, toxicity, or the like. The waste water may be that from a semiconductor manufacturing process which typically contains organic substances at high concentrations.

Prior Art

Waste water containing organic substances is commonly treated by general biological treatment. However, in industrial processes such for example as semiconductor manufacturing., it is not uncommon' for a number of high molecular organic substances to be used. The waste water subsequently discharged from such a manufacturing process often contains high concentrations of surfactants, such as alkylbenzensulf onic acid (ABS) and the like, and organic solvents, containing dimethyl sulfoxide (DMSO) or phenol or the like, and these organic substances can not be treated by the conventional activated sludge process because of problems of foaming, odor generation, toxicity to microorganisms. Waste water containing substances not amenable to biological treatment, as described above, usually ends up as an industrial waste to be burnt which requires incineration facilities and specialized management which entail 1 exorbitant costs. There is therefore a great demand for technologies, which permit treating at low costs waste water containing these hitherto untreatable substances.

Several methods have been proposed for treating waste water containing these hitherto untreatable substances, but which have not yet been commercialized because of the difficulty in treating waste water containing high concentrations of difficult-to-treat substances and the need for construction of specialized facilities. Methods capable of treating waste water containing organic substances in concentrations of thousands of ppm (mg/L) include, for example, (1) decomposing organic substances by biological treatment, (2) decomposing organic substances by oxidation at high temperatures and high pressures, (3) oxidizing organic substances by addition of ozone under alkaline conditions as described, for example, in Japanese Patent Laid-Open Publication No. 10-174984, and (4) oxidizing organic substances by addition of hydrogen peroxide and irradiation with ultraviolet rays.

However, the above-described method (1) leaves much to be desired in that it is ineffective, when the organic substances contain the likes of ABS and DMSO because of their low biodegradability. In the case of the above-stated method (2), while it can remove such difficult-to-treat organic substances by oxidation, it is disadvantageous in that extensive and costly facilities are required and operating costs are also high.

In the case of the above-described method (3), while it is recognized that oxidation by addition of ozone is effective particularly for treatment of waste water containing organic substances in high- concentrations, there are problems that, while 2 the removal efficiency is high for certain organic substances, it is not for all - This became clear in the course of completing the present invention. Examining, for, example, DMSO contained particularly in high concentrations in the waste water discharged from the semiconductor manufacturing process, it was confirmed that the ozone oxidation rate under alkaline conditions rather slows down and that the treatment ef f iciency is poor. In the case of the above-described method (4), since optimization of facilities for the waste water to be treated in which the content and kind of organic substances vary is often difficult, there are many cases where there is no choice but to install excessive facilities, leading to the problem that the cost of plants and equipment increases remarkably at present.

Therefore, the treatment of waste water containing difficult-to-treat substances such as DMSO and the like in high concentrations discharged from semiconductor manufacturing facilities is contracted out for off-site disposal.

SUMMARY OF THE INVENTION

Preferred embodiments of the invention provide a method of treating waste water and an apparatus particularly suited to treatment of waste water discharged from semiconductor manufacturing facilities or the like.

In preferred embodiments of the invention,' difficult-to-treat substances can first be converted to substances amenable to biological treatment by carrying out ozonation under conditions optimum for the substances in the waste water. Such ozonation is followed by biological treatment further to decse the substances. Embodiments 3 of the present invention are realized from focusing on the effectiveness of a method of oxidation treatment of organic substances by addition of ozone and on the cost-performance advantages associated with such a method, particularly when treating highly concentrated waste 5 water.

In preferred embodiments of the invention, substances present in waste water are oxidized by ozone while the pH is adjusted optimally for substances to be treated in the waste water containing substances not suited to biological treatment because of odor generation, foaming property, toxicity to microorganism. That is, pH is adjusted optimally for substances corresponding to the types of difficult- to-treat substances present,. and oxidation treatment of the substances by addition of ozone is. carried out under such adjusted pH.condition.

For example, when the waste water contains a sulf oxide-f amily organic sul fur compound (for example, the above-described DMSO), there is a process in which pH is adjusted to a neutral or acidic condition and the compound contained in the waste water is subjected to oxidation treatment by addition of ozone under such an adjusted pH condition. When the waste water contains an organic substance having a benzene ring, most typically phenol, there is a process in which pH is adjusted to a neutral or acidic condition, and the organic substance contained in the waste water is subjected to oxidation treatment by addition of ozone under such adjusted pH condition. When the waste water contains a surfactant having a benzene ring, there is a process in which pH is adjusted to an alkaline condition, and the surf actant contained in the waste water is subjected to oxidation treatment by addition of ozone under such 4 adjusted pH condition. Furthermore, when the waste water contains ammonia or a compound having an imino group or amino group (including primary amine to tertiary amine), there is a process in which pH is adjusted to an alkaline condition, and the substance contained in the waste water is subjected to oxidation treatment by addition of ozone under such an adjusted pH condition. when the pH is adjusted -to alkaline, it is preferable to adjust it to 10.5 and above.

In preferred embodiments, each substance contained can be treated sufficiently, rapidly, and effectively because the substance is subjected to oxidation treatment by addition of ozone under the optimum pH condition as described above, even when the substance is a. hitherto difficult-to-treat substance. After the difficult-to-treat substance present in the waste water.is.

subjected to oxidation treatment in the oxidation treatment process by addition of ozone to such a degree that the substances can be easily biologically treated, the substance thus oxidized in the waste water can be further decomposed by biological treatment and finally treated. The biological treatment may be carried out under aerobic or anaerobic condition according to the substance to be treated. In a manner as described above, the substance which has been conventionally considered not amenable to biological treatment can be biologically treated, making it possible to take full advantage of both oxidation treatment by addition of ozone and biological treatment.

With the preferred embodiments, it is possible to first remove chemical compounds with a low boiling point present in the raw waste water prior to the above-described pH adjustment and oxidation treatment by addition of ozone.

Removal of the chemical compounds with a low boiling point may be carried out by, for example, heating the waste water to temperatures above the boiling point of a chemical compound in question. There are substances for which the treating efficiency becomes high when oxidation treatment by addition of ozone is carried out while maintaining the waste water at high temperatures. In such a case, ozone should be added to the waste water while the waste water is still at a temperature of, for example, 2ST or more, without cooling the waste water which has been heated above the boiling point of a target chemical compound.

With the preferred embodirrents, it is erable to degas the waste water to be treated in order to enhance the solubility of ozone. Since oxygen and the like gasses dissolved in the waste water to be treated will be decreased by degassing, ozone added will be dissolved more rapidly and effectively, thereby further improving the efficiency of the oxidation treatment.

Further, with the preferred embodinients, it is also possible to judge the termination of oxidation reaction by measuring the concentration of ozone in the discharged gas in the oxidation treatment process. The termination of the oxidation reaction is detected f rom change in the concentration of ozone in the discharged gas. That is to say, since most ozone added is consumed and the concentration of discharged ozone gas becomes near constant when almost all substances to be treated has been decomposed by oxidation, it is possible to judge the practical termination of oxidation reaction from a change in the ozone concentration of the discharged gas. And, on the basis of this judgement, the waste water to be treated may be trans f erred to the next step, a biological treatment 6 step, for example, or addition of ozone may be stopped in order to conserve the amount of ozone used.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig - 1 is a block diagram of an apparatus for treating waste water of the present invention.

Fig. 2 is a characteristic diagram obtained f rom an experiment carried out in order to confirm effectiveness of the method of treating waste water.

Fig. 3 is another characteristic diagram obtained from an experiment carried out in order to confirm effectiveness of the method of treating waste water---.

Fig. 4 is a another characteristic diagram obtained from an experiment carried out in order to confirm effectiveness of the method of treating waste water.

Fig. 5 is a another characteristic diagram obtained from an experiment carried out in order to confirm effectiveness of the method of treating waste water.

Fig. 6 is a another characteristic diagram obtained from an experiment carried out in order to confirm effectiveness of the method of treating waste water.

Fig. 7 is a another characteristic diagram obtained from an experiment carried out in order to confirm effectiveness of the method of treating waste water.

Fig. 8 is a another characteristic diagram obtained from an experiment carried out in order to confirm effectiveness of the method of treating waste water.

7 Fig. 9 is a characteristic diagram illustrating operating cost effects of the method of treating waste water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The preferred embodimentsof the present invention will be described below with reference to the appended drawings.

Fig. 1 shows an apparatus for treating waste water according, to a preferred embodiment of the present invention. Waste water 2 containing dif f icult-to-treat substances which can not be readily treated biologically because of difficulties ascribed to odor generation, foaming property, toxicity to microorganisms or the like is supplied to the waste water treating apparatus 1.. In. this embodiment of the present invention, the supplied waste water 2 is f irst heated above the boiling points of particular substances with low boiling points by a heating apparatus 4 within an apparatus 3. At this step, the substances with low boiling points are removed from the waste water 2, which is then transferred to a storage tank 5. A discharge pipe 6 for discharged gas is provided in an upper section of the storage tank 5. The discharge pipe 6 may be opened or closed as necessary.

The waste water 2 now stored in the storage tank 5 is transferred by a pump 8 via a circulation line 7 to an oxidation-treatment apparatus (reactor) 9 as a means for oxidation-treatment, and treated water from the oxidationtreatment apparatus 9 is returned to the storage tank 5. A pHadjusting apparatus 10 as a means for optimally adjusting pH for each substance to be treated is connected via a supply line to the 8 oxidation-treatment apparatus 9 in the circulation line 7, and alkali or acid for adjusting pH, such as, for example, NaOH, H2S04 or the like is added from the pH-adjusting apparatus 10 to the water to be treated as required. The oxidation-treatment apparatus 9 comprises a tower type reactor to the lower section of which is connected an ozone generator 11. The ozone generator 11 generates ozone from dry air 12, and generated ozone is added to the water to be treated in the oxidation-treatment apparatus 9. In this embodiment, ozone gas is introduced in the water to be treated from an air dif f user 9a. A discharge gas line 13 is provided at the upper section of the oxidation-treatment apparatus 9, and an apparatus,for measuring the concentration of discharged ozone gas-14 is..provided on the discharged gas line 13.

A pH-sensor 15 for. detecting the pH of the treated water from the oxidation-treatment apparatus 9 and a degasser 16 for deaerating dissolved oxygen or the like gases in order to enhance the dissolution of ozone in the oxidation-treatment apparatus 9 are provided along the circulation line 7 from the ox idat ion- treatment apparatus 9 to the storage tank 5. The degasser 16 may be, f or example, a vacuum degasser for performing degassing in a vacuum container, a membrane degasser for performing degassing via a membrane, or the like. When a membrane degasser is employed, a membrane with sufficiently high ozone resistance must be selected.

In this embodiment, hitherto difficult-to-treat substances present in the waste water are treated by oxidation in the oxidation treatment process by addition of ozone while the waste water to be treated is circulated between the storage tank 5 and the oxidation- treatment apparatus 9 via the circulation line 7. This 9 continues until the substances are decomposed into substances that can be easily treated biologically. After the termination of a given oxidation treatment, the treated water is transferred to a biological -treatment apparatus 17 and the substances thus oxidized in the treated water is subjected to biological treatment under aerobic or anaerobic conditions to further decompose the substances. The biological-treatment apparatus 17 may be a conventional activated sludge system. Switching between the circulation of the waste water to be treated via the circulation line 7 and supply of the treated water to the b io logical -treatment apparatus 17 may be carried out by opening and closing control of. electromagnetic valves 18,19.

In this embodiment, the sequence of operations of the apparatus described above may be controlled manually or automatically. A controlling apparatus 2 0 is provided for automatic control. Signals f rom the apparatus f or measuring the concentration of the discharged ozone gas 14 and the output of the pH-sensor 15 are supplied to the controlling apparatus 20, and input information 21 concerning the substances in the waste water and set values for operating conditions for respective parts of the apparatus 1 is also provided to the controlling apparatus 20, as necessary. Operating signals are output from the controlling apparatus 20 to the heating apparatus 4, pump 8, pH-adjusting apparatus 10, ozone-generator 11, and electromagnetic valves 18,19.

The method of this invention making use of such an apparatus as described above may be carried out, for example, in such a manner as described below.

A basic technical idea of the embodiments is based on the discovery that there exists an optimum pH for oxidation-treatment by addition of ozone for each substance present in waste water, and the subsequent realization that a most effective treatment can be achieved by adjusting pH optim ally for each such substance.

Hitherto untreatable substances can be decomposed into product substances which can be biologically treatedi so that the treated water can be further treated in a biological treatment apparatus.

Experiments as described below were carried out using the apparatus shown in Fig. 1 in order to conf irm. that there does indeed exist a pH condition optimum for ox idat ion- treatment corresponding to the type of the substance contained in the waste water. The waste water to be treated was assumed to be waste water discharged from a semiconductor manufacturing facility, and experiments were conducted particularly on waste water containing dimethylsulf oxide (DMSO), sodium dodecylbenzene sulfonate(DBS) as a kind of alkylbenzene sulfonic acid(ABS), and phenol to study characteristics of these substances.

A reactor 5 meters high and with a 20 liter capacity was used as the oxidation-treatment apparatus 9. ozone was generated from dry air by an ozone generator 11 and was forced into the reactor 9 from the bottom at a predetermined constant flow rate. The waste water to be treated was brought into contact with ozone as it moved downward f rom the upper portion of the reactor 9. The depth of water was set at 4 meters or above to maintain the pressure at the place where ozone gas was introduced, and pH of the waste water was detected by the pH-sensor 15 at an outlet of the reactor 9, on the basis of which control was performed.

Samples for evaluation were prepared by dissolving in pure 11 water phenol, DMSO, and DBS as a typical ABS in a set concentration. In addition, the concentration of waste water discharged from the semiconductor manufacturing process containing DMSO as a main component was adjusted using pure water to prepare samples of waste 5 water at set concentrations of DMSO.

First, the treatment characteristics and oxidized products were examined to f ind a pH optimum for the reaction of each substance present in the oxidation-treatment process. Fig. 2 shows the reaction characteristics by ozone oxidation-treatment for each single solution with respect to DBS, phenol, and DMSO. As shown in Fig. 2, the concentration of each of these substances decreased as additional ozone was added.

-In the case of the DBS solution, while the reaction ef f iciency was considerably low at a neutral pH,.91% of DBS was decomposed by 4.3-fold amount of ozone (4.3g-ozone/lg-DBS) and foaming property disappeared. It may be considered that this is because when ozone is dissolved under alkaline condition and at high pH, the amount of hydroxyl radical (. OH) produced is large and the untreatable substance can be effectively decomposed by the strong oxidizing ef f ect of OH.

on the other hand, in the case of phenol solution, 93% of the phenol was decomposed by a 1.8-fold amount of ozone (1.8gozone/lg-phenol) at any value of pH. While it is, accordingly, considered that decomposition of phenol proceeds according to a reaction mechanism different from that of DBS, the decomposition of both phenol and DBS conform to the decomposition characteristics of benzene and produced oxalic acid, which easily be biologically treated.

12 is In the case of a DMSO solution, while 95% of the DMSO was decomposed by 1. 2-fold amount of ozone (1.2g-ozone/lg-DMSO) at a neutral pH, approximately a two-fold amount of ozone (2.0gozone/lg-DMSO) was required at pH 10.5 and above. While DMSO produces as a reaction product a small amount of dimethyl sulfone (DMSO.) at a high pH region as shown in Fig. 3, this is converted to mainly methanesulfonic acid (MSA) by radical reaction with hydroxyl radical ( OH) so that little H2S04 is actually produced. In the reaction at neutral region, DMSO was converted. directly to only DMSO, by oxygen addition reaction as shown in Fig. 4. Both the MSA and the DMS02 produced are substances which can be easily biologically treated, and it is clear, as shown in Figs. 2-4, that the reaction efficiency of ozone treatment of DMSO is higher when conducted in a neutral pH region.

There aremany-cases where DMSO present in waste water discharged from a semiconductor manufacturing facility is present in high concentrations of several thousand ppm or greater. DMSO being a typical example of a hitherto dif f icult-to-treat substance which is an object to be treated by this invention, the pH dependence of ozone oxidation treatment of DMSO was examined in detail. Fig. 5 shows treatability (decomposition rate) of DMSO for a variety of pH values, from which it is evident that a higher decomposition rate of DMSO was obtained when treated under conditions f rom neutral to acidic, as compared to the results under alkaline conditions.

It should be noted that waste water from actual manufacturing facilities contains many different kinds of organic substances. The waste water from a plant evaluated in this example contained mainly much isopropyl alcohol (IPA) as a compound of low boiling 13 point. While IPA can be easily treated by general biological treatment, the decomposition rate of IPA by ozone oxidation treatment is extremely low, an d IPA interferes with ozone oxidation treatment of other organic compounds or the like. It is preferable, therefore, to first remove IPA from waste water containing much of this substance. IPA may be effectively removed through heating aeration by the apparatus for removing the compound of low boiling point 3 equipped with the heating apparatus 4, as shown in Fig.

When a compound with a low boiling point, for example, IPA which coexists with other substances in the waste water is removed by heating aeration as a pre-treatment in such a manner as described above, subsequent.ozone treatment must.be carried out while maintaining a high temperature. Thereforei the influence of water temperature on the oxidation treatment characteristics of DMSO was examined. Fig. 6 shows the treatment characteristics of DMSO for each water temperature. The improvement in the treatment. characteristics was observed at 2ST or above, and a particularly high DMSO removal rate was obtained when treatment was conducted at 40T or above. It is clear that there is no need to cool waste water heated by heating aeration as a pretreatment bef ore subjecting the waste water to ozone treatment. That is to say, it is clear that from the viewpoint of treatment efficiency, it is preferable to add ozone to the waste water heated at temperatures above the boiling point of the compound with low boiling point while maintaining a preferable temperature range corresponding to the object to be treated by subsequent ozone treatment. For example, the temperature may be maintained at 25tand above as described above.

14 Further, the influence of initial content of organic compound in the waste water on the ozone oxidation treatment was examined with respect to DMSO. As shown in Fig. 7, the treatment proceeded basically in proportion with the amount of ozone added when the waste water containing DMSO in high concentration is treated. While a slight decline in the reaction rate was observed at the region of.low concentration, the increase in treating time until DMSO has entirely disappeared was slight(treating condition: pH=4, water temperature=4 0 t. ) As described above, the higher the concentration of waste water, the more ozone is required. Accordingly, increased solubility of the ozone in the waste water to be treated is necessary for ef f ective treatment, and degassing of oxygen or the like gases dissolved in. the-water to be treated is effective for enhancement of dissolution of ozone.

is The content of the substance which is the object to be treated in the waste water decreases as ozone oxidation treatment proceeds. Accordingly, when a constant amount of ozone is injected, ozone is not used ef f ectively, and the amount of ozone in the discharged gas increases, and the increase in the amount of ozone stops at the time when the ozone 'Oxidation treatment practically finishes, or as the ozone oxidation treatment approaches the final stage, which can be detected by monitoring the concentration of ozone in the discharge gas. As is apparent from the results of experiments for DMSO-containing waste water shown in Fig. 8 (experiments at neutral or acid region), DMSO is directly oxidized to DMS02 as the ozone oxidation treatment proceeds, and the concentration of ozone in the discharged gas remains at a maximum value after the ozone oxidation treatment has practically finished. By detecting the is inflection point of the concentration of the discharged ozone gas, the termination of the ozone oxidation treat ment can be practically detected.

The information obtained from the results of the experiments as described above is incorporated into the design of the waste water treating apparatus 1 shown in Fig. 1 so as to realize the most preferable treatment possible. A sequential explanation is given below from the upstream of the line of the waste water with reference again to the organic substance-containing waste water treating apparatus 1.

First, the waste water 2 containing dif f icult-to-treat organic substance,. f or example, waste water discharged from a semiconductor manufacturing process is heated to a temperature above the boiling point of the compound with low boiling point (for example, IPA) present in the waste water 2 by the heating apparatus 4 in the apparatus 3 for removing the compound of low boiling point and the compound of low boiling point in the waste water 2 is removed by means of heating aeration. The waste water 2 f rom which the compound with low boiling point has been removed is transf erred to the storage tank 5 and stored therein. Because the compound of low boiling point has been removed, the oxidation treatment by addition of ozone described below is not disturbed by the compound of low boiling point, and, therefore, the efficiency of ozone oxidation treatment is improved. Af ter heating f or removal of the compound of low boiling point, it is not necessary to cool the waste water heated, which is transferred to the storage tank 5 keeping temperature condition intact,. Particularly in the case of waste water containing much DMSO, high treatment efficiency can be obtained with a small 16 quantity of ozone when the oxidation treatment is carried out at high temperatures as stated above.

The waste water stored in the storage tank 5 is transferred by the pump 8 to the oxidation treatment apparatus (reactor) 9.

The treated water from the oxidation treatment apparatus 9 is returned to the storage tank 5 via the circulation line 7. Accordingly, the water to be treated into which ozone is added is circulated many times. During circulation, the target substance in the water to be treated is subjected to oxidation treatment by ozone added in the oxidation treatment apparatus 9. In this embodiment, the pH of the water to be treated in the oxidation treatment apparatus 9 is detected by the pH-sensor 15, the'value for pHis.transmitted to the control apparatus 20, the'control apparatus 20 controls output in accordance with the type of substance that is the object to be treated, and, on the basis of the output, the pH-adjusting apparatus 10 is controlled.

That is to say, in the pH-adjusting apparatus 10, alkali or acid for adjusting pH such as NaOH, H2S04 or the like is added to the water to be treated to meet the demands at that time. Accordingly, while the oxidation treatment is carried out by ozone added from the ozone-generator 11 to thesubstance contained in the water to be treated in the oxidation treatment apparatus 9, it is possible to adjust pH optimally for each substance as an object to be treated by this pH adjustment, and, as a result, the oxidation treatment is carried out under the optimum pH condition. It is not objectionable that the pH-sensor 15 measures pH of the water to be treated (not treated water) in the oxidation treatment apparatus 9.

17 In the case where the waste water to be treated contains DMSO and ABS(particularly, DBS), waste water from semiconductor manufacturing often contains DMSO in extremely high concentrations (for example, 3000 ppm and above) and DBS in relatively low concentrations (for example, approximately 50 ppm.). In the case of such a waste water wherein DMSO and DBS coexist, the pH is first adjusted to the alkaline side (for example, pH 10.5 and above), and the oxidation treatment by addition of ozone is carried out under such alkaline conditions. under such conditions, DBS can be treated by ozone oxidation far more ef f ectively than in a case where pH is neutral or acidic, as stated earlier. However, as the efficiency of the ozone oxidation treatment of DMSO under alkaline conditions may not be high as shown earlier,. the oxidation treatment. of DBS is first carried out under the optimum pH condition with regard to DBS, the content of which is small. only then, after the short time required to terminate the oxidation treatment of the, small amount of DBS has elapsed, is the pH adjusted to the optimum pH condition for the oxidation treatment of DMSO. With this pH adjustment (to neutral or acidic side), the oxidation treatment of DMSO can be rapidly and ef f ectively performed using only a small amount of ozone.

While the pH adjustment to neutral or acidic side can be done by adding acid such as H2S04 or the like to the water to be treated in the pHadjusting apparatus 10, the pH spontaneously becomes acidic (via neutral) only by halting addition of NaOH or the like to terminate pH adjustment. This is because, in the case of DMSO, MSA is produced as a product of the treatment carried out under an alkaline condition, and MSA exhibits a transferring action to 18 acidity. more ozone is used under alkaline conditions because the oxidation reaction is performed via -OH radical under the alkaline condition, and the -OH radical is used to decompose all organic compounds because of non-effective consumption of the OH radical by radical chain reaction and the OH radical having no selectivity. Since,.however, the oxidation,reaction of DMSO at neutral or acid side is the reaction in which DMSO is converted directly to DMS02 without producing MSA, a small amount of ozone is effective and rapid treatment is feasible.

In such a manner as described above, pH is adjusted optimally corresponding to the substance to be treated, and the oxidation treatment by addition of ozone is performed under the optimum pH condition. There are several different.types of substances, pH of which are tobe adjusted optimally, and pH is adjusted to alkaline or neutral or acid corresponding to the organic substances present. For example, when the,waste water contains sulf oxide -f amily organic sulfur compounds, such as the above-described DMSO or the like, pH is adjusted to neutral or acid, as is the case when the waste water contains organic compounds having a benzene ring, typically phenol-f amily compounds. When the waste water contains a surf actant having a benzene ring (for example, ABS, typically the abovedescribed DBS), pH is adjusted under an alkaline condition to obtain high oxidation treatment efficiency. Also, when the waste water contains ammonia or a compound having imino or amino groups (including primary amine to tertiary amine), pH is adjusted under an alkaline condition. When the waste water contains phenol, since pH changes spontaneously to acid side similarly to the case of the above- described DMSO because oxalic acid is produced, it is possible 19 to lower pH spontaneously to acid via neutral merely by halting pH adjustment once the pH adjustment to alkaline side is finished.

Particularly when the waste water contains a plurality kinds of substances to be treated, pH is adjusted optimally so as to make it possible to perform the oxidation treatment most effectively for each substance contained to meet the demands at that time similarly to the case of the above-described waste water containing DMSO and DBS, and the pH condition thus obtained is changed and adjusted sequentially corresponding to a state inwhich the substance is contained.

Because the waste water is subjected to oxidation treatment underan optimum pH condition, targeted.oxidation treatment prepares the waste water for downstream biological treatment. The oxidation treatment can be carried out using a minimum amount of added ozone. Aa a result, the ozone treatment according to the present invention can reduce operating cost. Fig. 9 is a graph showing a comparison of operating cost between a typical conventional method and the method of this invention. When comparison is made of an example case in which 2 m' per day 20 waste water containing 10,000 mg/L of DMS is treated, the present invention can reduce operating costs by M 27.1 million per year as compared with a case wherein the waste water is treated as industrial waste to be disposed of off site, and can save V 8. 1 million per year over the ultraviolet rays/hydrogen peroxide treating method (UVM,0, treating method). The operating cost of ozone treatment is approximately 1/17 of that required for treating as industrial waste, and approximately 116 of that required for a conventional UV/H202 treatment. The higher the concentration of waste in the waste water, the greater will be the cost benef it of ozone treatment over a UV/H202 treating method.

Further, the ozone treatment has additional advantages in that it is not only liable to absorption of light (effective for oxidation) by turbidity unlike the W/H202treating method, but also not susceptible to corrosion by fluorine when treating colored turbid waste water or fluorine-containing waste water, as compared with the W/H,0, treating method..

In Fig. li while the waste water treated by oxidation with ozone in the oxidation treatment apparatus 9 is returned again to the storage tank 5 via the circulation line 7, the waste water is.deaerated by the degasser 16 provided on.the circulation line 7, and the ozone solubility in the water circulated is increased, and the ozone solubility in the oxidation treatment apparatus 9 is increased, as a result, the efficiency of the oxidation treatment by ozone can be increased and, simultaneously, the amount of ozone added can be more decreased.

Although in the apparatus shown in Fig. 1, the treated water in the oxidation treatment apparatus 9 is circulated to the storage tank 5 via the circulation line 7, the circulation line 7 can be omitted, in which case the oxidation treatment apparatus 9 may be constructed so as to make it possible to add a given amount of ozone at a given pH and take a given reaction time in the oxidation treatment apparatus 9. In such a case, it is necessary to detect 25 pH of the water to be treated in the oxidation treatment apparatus 9 by the pH- sensor 15 and supply acid or alkali into the oxidation treatment apparatus 9 by me ans of the pH-adjusting apparatus 10. In this case, the degasser 6 can not be set.

21 And, when acid or alkali is supplied directly into the oxidation treatment apparatus 9 by the pH-adjusting apparatus 10, the treated water may be returned into the oxidation treatment apparatus 9 by the circulation line 7. Further, the treated water may be returned by the circulation line 7 to any point on a pipe from the storage tank 5 to the oxidation treatment apparatus 9.

Furthermore, in the apparatus shown in Fig. 1, only one oxidation treatment apparatus 9 is provided in which pH is changed sequentially to decompose a plurality of substances. However, at least two oxidation treatment apparatuses 9 may be provided in each of which pH may be adjusted optimally for each substance as an object to be treated and the oxidation treatment.by ozone may be carried out.

Because little of the ozone added in the oxidation treatment apparatuses 9 is consumed after the termination of the treatment of the substance to be treated, this termination can be detected, as stated previously, by a change in the concentration of ozone gas in the discharged gases f rom, the oxidation treatment apparatuses 9 relative to known concentration of ozone added in the oxidation treatment apparatuses 9. Particularly, the concentration of discharged ozone gas will reach an approximately constant value. In the apparatus shown in Fig. 1, the concentration of discharged ozone gas is detected by the apparatus for measuring the concentration of discharged ozone gas 14 provided in the discharged gas line 13, and the detection results are supplied to the control apparatus 20. It is decided that an object treatment has been. terminated, that is, the oxidation treatment has been sufficiently performed to make the waste water amenable to biological treatment, 22 by the fact that the concentration of discharged ozone gas is approximately constant. Accordingly, addition of ozone can be stopped, and, as a result, the amount of ozone used can be conserved. Further, the termination of the treatment may also be detected when a change in the concentration (differential) becomes equal to zero or by detection of an inflection point where change in change (second-order differential) becomes equal to zero.

When the termination of the intended ozone oxidation treatment is detected by the apparatus for measuring the concentration of discharged ozone gas 14 as described above, electromagnetic valves 18 and 19 are switched on the basis of orders from the control apparatus 20 and the treated water is transferred to the biological treatment apparatus 17. The treated water to be transferred does not contain any s ignif icant amount of hitherto dif f icult-to-treat substances, but only oxidation products such as oxalic acid or the like which can readily be decomposed by biological treatment. The waste water can be treated by the biological treatment and then discharged or recovered for re-use. As the biological treatment apparatus 17 may be used an apparatus with specification conventionally well known. For example, an aerobic biological treatment apparatus such as an activated sludge treatment apparatus, a packed-bed reactor, or the like may be preferably used. However, when appropriate, an anaerobic treatment apparatus, such as an anaerobic packed-bed reactor or the like, may be employed.

As described above, according to the method of treating waste water and an apparatus theref or of this invention, since the ozone oxidation treatment is carried out under optimum pH condition corresponding to the type of substance contained in the waste water, 23 an intended oxidation treatment can be carried out extremely effectively in a small amount of ozone and a substance to which conventional general biological treatment can not be applied can be changed to a substance capable of being easily biologically treated without producing harmful substances as by-products. The method of treating waste water and the apparatus therefor of this invention are actually feasible from the view point of excellent treatment performance, device, and cost. Also, according to the preferred embodirrents of the invention, there can be provided a technology which is particularly suited to effective treatment of waste water discharged from a semiconductor manufacturing facility. 'As described previously, in the technology of this invention, the cost performance advantages increase when the concentration of the waste water increases. When waste.water containing DMSO in high concentration of 1 % is treated, the operating cost of ozone treatment is.approximately 1/6 that of a conventional UV/H202 treatment and only 1/17 of the cost required for treatment as industrial waste (to be disposed of off site), that is, an extremely large cost savings can be obtained. 20 The preferred embodiments can also be easily applied to colored or turbid waste water or organic waste water containing fluorine to which UV/H202 treatment can not be easily applied, that is, the type of the waste water and the range of concentration to which this invention can be applied is wide. 25 The technology of this invention is extremely effective not only in reducing environmental waste, but also in reducing costs. The principle of the technology of the present invention also has many general-purpose applications. This technology is not only 24 suited to treatment of waste water discharged from a semiconductor manufacturing facility, but also to almost all water treatment or water recycling efforts, at low cost and over a wide range.

Claims

CLAIMS:

1. A method of treating waste water containing a plurality of waste substances not amenable to biological treatment comprising the steps of:

adjusting the pH of said waste water to a suitable pH for the oxidation of one of said substances and then carrying out oxidation treatment of said one substance by addition of ozone, and adjusting the pH of said waste water which has been subjected to oxidation treatment to a suitable pH for oxidation of another of said substances and then carrying out oxidation treatment of said another substance by addition of ozone.

2. The method of claim 1, wherein said waste water is waste water containing a sulfoxide-family sulfur compound, and the pH of said waste water is adjusted to neutral or acidic side and said sulfoxide-family organic sulfur compound is subjected to oxidation treatment by addition of ozone.

3. The method of claim 1, wherein said waste water is waste water containing an organic compound having a benzene ring, and the pH of said waste water is adjusted to neutral or acidic side and said organic compound having a benzene ring is subjected to oxidation treatment by addition of ozone.

4. The method of claim 1, wherein said waste water is waste water containing a surfactant having a benzene ring, and the pH of said waste water is adjusted to alkali side and said surfactant having 26 a benzene ring is subjected to oxidation treatment by addition of ozone.

5. The method of claim 4, wherein the pH of said waste water is 5 adjusted to 10.5 or above.

6. The method of claim 1, wherein said waste water. is waste water containing ammonia or a compound having an imino group or an amino group including from primary amine to tertiary amine, and the pH of said waste water is adjusted to alkali side and said ammonia or said compound having an imino group or an amino group is subjected to oxidation treatment by addition of ozone.

7. The method of claim 6, wherein the pH of said waste water is 15. adjusted to 10.5 or above.

8. The method of claim 1, wherein said substances contained in said waste water include at least two of a sulf oxoide-f amily organic sulfur compound, an organic substance having a benzene ring, typically phenol, and ammonia or a compound having an imino group or amino group including from primary amine to tertiary amine.

9. The method of claim 1, wherein, after completion of oxidation treatment, the waste water is further subjected to biological treatment.

10. The method of claim 1, wherein said waste water is subjected to heating treatment at a f irst stage (prior to oxidation treatment) 27 to remove substances of low boiling points.

11. The method of claim 10, wherein said oxidation treatment is carried out while said waste water heated by said heating treatment still has a temperature of 25 'C or above.

12. The method of claim 11, wherein said waste water which is to be subjected to oxidation treatment is subjected to degassing treatment.

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13. The method of claim 1, wherein the concentration of ozone in the gas discharged during said oxidation treatment is measured and termination of said oxidation treatment is determined based on the measured concentration of ozone in the discharged gas.

14. An apparatus for treating waste water containing at least two substances not amenable to biological treatment, wherein the pH of said waste water is adjusted to a suitable pH for one of said substances and oxidation treatment of said one substance is carried out by addition of ozone, then the pH of said waste water which has been subjected to oxidation treatment is adjusted to a suitable pH for another of said substances and oxidation treatment of the other substance is carried out by addition of ozone.

15. The apparatus described in claim 14, wherein said apparatus for treating waste comprises; a pH-adjusting device for adding acid or alkali into said waste 28 water, and an oxidation treatment device into which said waste waster is introduced and to which ozone gas is added to be mixed with said waste water.

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16. The apparatus of claim 15, wherein, in said oxidation treatment device, when oxidation treatment of.one substance treatment is terminated at a f irst pH, oxidation treatment of the other substance is carried out at a second pH.

17. The apparatus of claim 16, further includes a circulation line for circulating the waste water treated in said oxidation treatment device to the waste water inflow side of said oxidation treatment device.

18. The apparatus of claim 17, further including a storage tank for storing the waste waster created by said oxidation treatment device, and said circulation line circulates the waste water treated in said oxidation treatment device to a side of inflow of said 20 oxidation treatment device via said storage tank.

19. The apparatus of claim 16# wherein said pH-adjusting device further includes a control device for giving signals for controlling the pH to a value adjusted according to a measured degree of progress 25 of the oxidation treatment in said oxidation treatment device.

20. The apparatus of claim 16, further including a biological treatment device for biologically treating said treated water from 29 said oxidation treatment device.

21. The apparatus of claim 16, further including a heating device for heating said waste water to remove compounds having low boiling points before the waste water is supplied to said oxidation treatment device.

22. The apparatus of claim 17, wherein said circulation line includes a degassing device for removing gases dissolved in water 10 circulated through the section.

23.. The apparatus of claim 19, further including an ozone meter for measuring concentration of ozone in gases discharged from said. oxidation treatment device, wherein said control device controls 15 the pH level based on the measured concentration of ozone.

24. A method.of treating waste water substantially as hereinbefore described with reference to the accompanying drawings.

25. An apparatus for treating waste water substantially as hereinbefore described with reference to the accompanying drawings.