GB1591273A - Method for treating waste water containing cyanide ion - Google Patents

Description

(54) METHOD FOR TREATING WASTE WATER CONTAINING CYANIDE ION (71) We, NIIGATA ENGINEERING CO., LTD., of No. 4-1 Kasumigaseki, 1-chome, Chiyoda-ku, Tokyo, Japan, a Japanese Company, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for removing cyanide ions from a cyanide ioncontaining waste water. More specifically, it relates to a method for removing cyanide ions from waste waters containing the cyanide ions in a low concentration, such as water treated by biological oxidation using activated sludge.

As is well known, gas liquor discharged in the process of coke manufacture contains large quantities of noxious substances such as phenols, thiocyanate compounds, ammonia, hydrogen sulfide and cyanide compounds. For example, the content of the cyanide compounds in the gas liquor is about 15 to 100 ppm. The gas liquor has previously been treated by a biological oxidizing method. But the treated gas liquor still has a cyanide content greater than that permitted for discharge water, and the problem of disposal of such gas liquor has yet to be solved.

Cyanide ion-containing waste water discharged from a metal plating plant is usually a wash water having a cyanide concentration of 20 to 100 mg/litre (as CN). Many of the CN ions form complex ions with heavy metals such as Zn and Cd. On the other hand, the concentrated waste liquid discharged from a spent plating bath has a CN concentration of 10,000 to 50,000 mg/litre.

Cyanide ion-containing waste water from plants for heat-treating iron and steel is discharged from the cementating and washing process, and usually contains 100 to 500 mg/litre of cyanide ions (as CN), and at times, as much as 1.500 to 2,000 mg/litre of cyanide ions (as CN). In this process, most of the cyanide compounds undergo oxidative decomposition, and are present as complex ions (e.g. iron cyanide ions).

Other waste waters containing cyanide ions are, for example, waste water from metal refining plants which has a concentration of 100 to 500 mg/litre (as CN), and waste water from petroleum refineries which has a concentration of 5 to 30 mg/litre (as CN).

Known methods for removing cyanide ions include an alkali chlorination method and a Prussian blue method. The alkali chlorination method, however, has the disadvantage that the cost of chemicals used is high, and complex cyano salts cannot be removed. The Prussian blue method can remove cyanide ions at relatively low cost, but the conventional procedures falling within this method (methods I to III described below) have their own problems.

(I) Method for removing cyanide ions by a ferric compound: This method involves reacting a ferric ion (Fe3+) with a ferrocyanide ion ([Fe(CN)6]4~) to form insoluble Berlin blue. If the cyanide ion concentration in the waste water is high, the ferric salt must be added in large quantities in order to obtain a satisfactory removal of the cyanide ion. Thus, sludge is formed in large quantities, and the cost of operation increases.

(II) Method for removing cyanide ions by a ferrous compound This method comprises reacting a ferrous ion (Fe +) with a ferricyanide ion ([Fe(CN)6]3-) to form insoluble Turnbull's blue, and reacting [FE(CN)6]4-, which is formed by the reaction of a small amount of a free cyanide ion present in the waste water with Fe2+, and [Fe(CN)6]4- present in the waste water with Fe + to form a precipitate of ferrous ferrocyanide thereby to remove the cyanide ions. This method has a great effect of removing the cyanide ions, but Fe2+ remains in the supernatant liquid (treated water) obtained by coagulation and separation. When the treated water is left to stand, Fe2+ is oxidized to form a precipitate of ferric hydroxide [Fe(OH)3j. In order to remove Fe2+ in the treated water, the reaction must be carried out at a high pH. Addition of alkali in an attempt to maintain the pH high results in the dissolving of the precipitate of insoluble Turnbull's blue, and the efficiency of removing the cyanide ions is reduced. Furthermore, the floc of ferrous hydroxide [Fe(OH) 2] formed by this method is difficult to coagulate and separate since it has poor coagulability.

(III) Method for removing cyanide ions by a ferrous compound and a ferric compound: The method comprises adding a ferrous compound to waste water to form a precipitate of insoluble Turnbull's blue and ferrous ferrocyanide, then adding a ferric compound to form a precipitate of insoluble Berlin blue, finally adding an alkali to form Fe(OH)3 and Fe(OH)2, and coagulating and separating the resulting precipitates together. According to this method, too, Fe + remains in the treated water at a low pH as in the method (II). Hence, on standing, a precipitate forms in the treated water. If the treatment is carried out at a high pH in order to avoid this disadvantage, the amount of alkali added increases and the cost of operation accordingly becomes high. At the same time, the precipitates again dissolve. This causes difficulty especially when the cyanide ion concentration of the waste water is high, and the amount of ferrous compound added increases.

It is an object of this invention to provide a method for removing cyanide ions from a cyanide ion-containing waste water effectively at low cost.

Another object of the invention is to provide a method for removing cyanide ions from a cyanide ion-containing waste water, in which redissolving of precipitates in a final treating step is avoided, which requires only a small amount of alkali for pH adjustment, which yields precipitates having good coagulability, and which does not permit the formation of precipitates from the treated water after the end of the entire process.

According to the present invention, there is provided a method for treating waste water containing cyanide ions, which comprises (1) adding a ferrous salt to waste water to convert cyanide ions therein to a precipitate of ferrous ferrocyanide and insoluble Turnbull's blue, (2) adding an alkali to the waste water to convert ferrous ion remaining in the waste water to Fe(OH)2, and (3) adding a ferric salt to the waste water to precipitate [Fe(CN)6]4-, which is formed by addition of the alkali to the waste water, as insoluble Berlin blue.

The accompanying drawing is a flowsheet showing one embodiment of the method of the present invention.

The cyanides, as used in this invention, denote CN-, [Fe(CN)6]4-, [Fe(CN)6]3-, and cyanide complex ions of, for example, Cd, Zn, or Cu. Generally, they are contained in waste waters as M(CN) l or 2 where M is H, K, Na, Cu, Zn or Cd, for example, XCN where X is F, Cl, I or Br, RCN where R is an alkyl group containing 1 to 17 carbon atoms or an aryl group containing 6 to 10 carbon atoms and being optionally substituted by alkyl, alkali metal salts (Na, K, Ca, etc), of [Fe(CN)6]4-, and [Fe(CN)6] 3-, and cyanide complex ions of Cu, Cd or Zn for example.

The cyanide ion-containing waste waters include, for example, gas liquor discharged from the step of quenching coke oven gas in the coke manufacturing industry or coke oven gas manufacturing industry, a product resulting from the biological oxidizing treatment of the gas liquor, and the waste waters discharged from the metal plating plants, iron and steel heat-treating plants, metal refining plants and petroleum refineries which are exemplified hereinabove. The treatment in accordance with this invention is possible whatever materials may be contained in the waste waters.

Waste waters having any cyanide ion concentrations can be treated by the method of this invention. It is however especially effective for the treatment of waste waters having a cyanide ion concentration of less than 1,000 ppm, such as general industrial waste waters.

In step (1) of the method of this invention, a predetermined amount of a ferrous compound is added according to the concentration of the cyanide ions in the waste water, and the mixture is optionally stirred. This causes the reaction of the cyanide ions in the waste water with the ferrous compound to form a precipitate of ferrous ferrocyanide Fe2[Fe(CN)6] and insoluble Turnbull's blue.

One example of an ion equation showing the formation of Turnbull's blue is shown as (a) below.

4Fe2+ + K+ + 3[Fe(CN)6]3 KFe"[Fe""Fe"(CN) 6]s -(a) When the cyanide ion in the waste water is present as a cyano complex of Cu, Cd or Zn, for example, the pH of the waste water is pre-adjusted to 3 or less, nd the complex ion is dissociated to CN-, after which the treatment with the ferrous salt mentioned above is performed.

Any water-soluble ferrous salts can be used in step (1). The salts include, for example FeCl2, FeCl2.4H2O, FeSO4.nH2O, where(n= 0, 1, 4, 5, 7), and (NH4)2SO4FeSO4.6H2O.

Generally, however, ferrous chloride or ferrous sulfate is used. A suitable amount of the ferrous salt to be used is such that the amount of Fe" is within the range of X' calculated from the following equations.

y = 0.213x- 3.8 x' = x + 25 (mg/liter) wherein y is the concentration (mg/liter) of the cyanide ions in the influent waste water measured by the method shown in Table 1, and x is the iron content (mg/liter) and the amount of ferric salt added is such that the weight ratio of Fe"' to Fe" becomes 0.5-3.

In step (2), Fe++ remaining in the waste water is converted to Fe(OH)2 which is removed as Fe(OH)3 by air oxidation. Examples of the alkali used in this step are NaOH, KOH, CaCOs, and Ca(OH)2. CaCO3 and Ca(OH)2 are not preferred to the others because they form scales. The pH of the waste water after the addition of the alkali is adjusted to 7.5 to 9.5, preferably to 8 to 9. As a result, Fe++ remaining in the waste water is converted to Fe(OH)3 and the insoluble Turnbull's blue precipitate is decomposed to Fe(OH)2, Fe(OH)3 and [Fe(CN) 6] 4-.

The decomposition of the insoluble Turnbull's blue is performed, for example, as shown by equation (b) below.

KFe"[Fe"'Fe"(CN) 6] 3 + 11 OH oK+ + Fe(OH)2 + 3Fe(OH)3 + 3[Fe(CN)6j4- (b) Step (2) can also be performed after separating and removing the precipitate formed in step (1). If this procedure is followed one additional step is required, but the amount of alkali used can be reduced. In this case, any remaining precipitate is completely removed in step (2) and subsequent steps. Hence, in the precipitate removing step, it is not altogether necessary to remove the precipitate completely.

In step (3), a ferric compound is added in a predetermined amount to the waste water, and the mixture is stirred. As a result, [Fe(CN)6]4- dissolved by the addition of alkali reacts with Fe3+ to form insoluble Berlin blue as shown by equations (c) and (d).

Fe3+ + Fe(CN)6j4- ' [Fe"'Fe"(CN)6j (c) Fe3+ + Fe'11Fe(CN)6]- Fe"'Fe"(CN)6j3. . 3 . .(d) (insoluble Berlin blue) In step (3), any water-soluble ferric salts can be used. The salts include, for example, FeCl3, FeCl3.6H2O, Fe(NO)3)3.9H2O, Fe2(SO4)3.nH2O(n = 0, 3, 6, 7, 7.5, 9, 10, 12), KFe(SO4)2.12H2O, NH4Fe(SO4)2.12H20. Generally, ferric chloride or ferric sulfate is used. The amount of the ferric salt is generally such that the Fe"'/Fe" weight ratio becomes 0.5 - 3, preferably 1 - 2.

Generally, each of the steps (1) to (3) is carried out at 15 to 25"C.

The resulting precipitates of Berlin blue, Turnbull's blue, Fe(OH)3 and Fe(OH)2 are coagulated together and separated. The precipitates (sludge) separated are suitably treated, and the supernatant liquid (treated water) left after the separation of the precipitates is subjected to another treatment and then discharged into watercourses.

Since according to the present invention, a ferrous compound is first added to a waste water containing cyanide ions, then an alkali is added, and finally a ferric compound is added, it is possible to remove the cyanide ions and the residual Fe2+ without fail. With higher concentrations of cyanide ions in the waste water, the amount of the ferrous compound to be added increases. Consequently, the amount of the alkali for removing the residual Fe2+ in the waste water naturally increases, and therefore, the redissolving of the insoluble Turnbull's blue formed by the addition of the ferrous compound increases. In the method of this invention, the ferric compound is added in the last step, and therefore, the cyanide compounds dissolved by the addition of much alkali are surely removed as Berlin blue. The present invention therefore achieves an efficient removal of the cyanide ions and Fe2+. The cyanide ions can be removed surely to below the water pollution standard value, and Fe2+ does not remain. Accordingly, even when the treated water left after the separation of precipitates is allowed to stand, no further precipitation occurs, and the purification degree of the waste water is excellent. The amount of the ferric compound used in this invention can be drastically reduced as compared with the case of the cyanide ion removing method using the ferric compound alone. Moreover, the amount of the alkali used for pH adjustment may be small. Hence, the cost of chemicals is low, and the treatment of waste waters can be performed at low cost. Furthermore, as stated hereinabove, even when the ferrous compound having a good removing effect is used, Fe2+ does not remain in the treated water, In addition, since the ferric compound is used in the final step, the coagulability of the precipitates is good, and the precipitates can be easily sedimented and separated.

Waste water treatment is therefore performed with simplicity.

One conventional method for treating gas liquor comprises diluting the gas liquor to 2 to 4 times with industrial water (water obtained by a treatment of sewage with an activated sludge, subterranean water or water from a river) or sea water, and subjecting the diluted gas liquor to an activated sludge treatment and a post-treatment (coagulation and sedimentation of cyanide ion and an adsorption treatment using granular activated carbon) [see, for example, W.G. Gousins and A.B. Mindler, J. Water Pollution Federation, Vol. 44, No. 4, 607 (1972); Paul D. Kostenoader and John W. Flecksteiner,J. Water Pollution Federation, Vo. 41, No. 2, 199 (1969l. According to this type of treatment, the treating efficiency in an activated sludge treatment device fluctuates greatly, and a stable treating performance with a high efficiency cannot be obtained. Accordingly, such a method cannot meet rigorous legislative environmental pollution standards. Moreover, the method which involves the dilution of gas liquor with industrial water is actually difficult to employ since industrial water is scarce nowadays. It has been desired, therefore, to establish a method for treating gas liquor stably at a high efficiency without the need for a dilution step.

The method of this invention is especially suitable for the treatment of gas liquor described hereinabove. For example, it is suitable for removing cyanide ions of low concentration (generally below 30 ppm) in a waste water in conjunction with a treatment process using microorganisms, i.e. biological activated sludge solids, activated carbon, or both. An example of the combination of the treatment of gas liquor and the method of the present invention comprises: (A) a pretreatment step of treating gas liquor discharged from the step of quenching coke oven gas and containing ammonia, phenols, thiocyanide compounds, cyanide compounds, suspended solids and oils to reduce the content of ammonia to about 1,000 ppm or less (B) a biological treatment step of treating the gas liquor treated in step (A) with microorganisms, or (B') a step of treating the gas liquor with activated carbon, C) a biological treatment step of treating the gas liquor treated in (B) or (B') in an aeration tank including mixed liquor suspended solids (a liquor containing suspended sol ids) consisting of powdered activated carbon and activated sludge, (D) the cyanide ion removing step in accordance with this invention (E) if desired, a step of treating the treated gas liquor with powdered activated carbon, and (EF) if desired, regeneration of activated carbon.

The gas discharged from a coke oven in a coke producing plant and a coke oven gas producing plant is quenched by a primary cooler to condense steam, tarry substances and ammonia, etc., in the gas. They are discharged as a condensed liquid. The condensed liquid is separated into crude tar and gas liquor in a tar decanter. The gas liquor contains large quantities of noxious and impure substances as shown in Table 1.

Table 1 Analyzing Method 1 pH 8.5-9.5 JIS K0102-1974 8 2. CODMn 2,500 - 7,500 ppm JIS K0102-1974 14 3.CODS 3,300 - 9,500 ppm ASTM D 1252-1974 4. BOD5 1,500 - 4,000 ppm JIS K0102-1974 16 4. BOD5 5. Phenols 700 - 1,700 ppm JIS K0102-1974 20.1 & 20.2 6. Thiocyanate Nitric acid decomposi compounds 150 - 800 ppm tion method (as SCN) 7. Cyanide JIS K0102-1974 29.1.2 compounds 15 - 100 ppm (as CN) 8. NH3 3,000 - 4,000 ppm JIS K0102-1974 17.1.3 (as N) 9. Suspended solids 50 - 100 ppm JIS K0102-1974 10.2.1 10. Oils 100 - 200 ppm JIS K0102-1974 18.2 In Table 1, CODMn is the chemical oxygen demand of polluting substances in gas liquor which is measured using potassium permanganate; CODcr is the chemical oxygen demand of the polluting substances which is measured using potassium dichromate; and BODs is the biological oxygen demand of the polluting substances in gas liquor for a period of 5 days at 20"C. Nitric acid decomposition method is shown below.

The pH of an aqueous liquid containing thiocyanate compounds is adjusted to 1 - 2 with sulfuric acid. Cyanide ions in the liquid are removed from the thus obtained liquor by passing through gas or by distillation. Then thiocyanate compounds in the liquid are decomposed with HNO3 to cyanide ions and the amount of the cyanide ions is determined by, for example, the method prescribed in JIS K0102-1974 29.1.2. (This method is described in, for example, H. Weisg. "Mikrochim Acta" 1956, P.1225).

Phenols present include phenol, o, m- and p-cresols, 3,5-xylenol, cx- and p-naphthols, oxine, catechol, pyrogallol, metal salts (e.g., Na, K, Ca, Ba or Al salts) of these phenols, phenol-carboxylic acids such as salicylic acid and m-, and p-hydroxy benzoic acids and the esters and ethers of mono-, di-, and tri-hydric phenols.

Thiocyanate compounds present include thiocyanic acid (including isothiocyanic acid), ammonium salts of these acids, metal salts (e.g., Na or Fe salts) of these acids, and phenyl thiocyanate.

Cyanide compounds present include M(CN)l or 2 (where M is H, K, Na, Cu, Zn or Cd), XCN (where X is F, Cl, I, Br), RCN (where R is alkyl or aryl), and cyano complexes containing Ni, Fe, Cr, Mn, Cu, Hg, Cd, etc. [e.g., as disclosed in Encyclopedia Chimica, Vol. 7, p.727, 19th Edition, published on September 10, 1976 by Kyoritsu Shuppan, Tokyo] .

The suspended solids present are insoluble inorganic or organic compounds such as carbon in the coal, corrosion products of the equipment (for example, Fe2O3), naphthalene, and sulfur. Oils present include coal tar, pyridine, etc.

Step (A) of the method achieves the removal of ammonia which is a major inhibiting factor in the decomposition or oxidation reaction of the polluting substances by microorganisms. In steps (B) and (B'), phenols, suspended solids and oils are removed to reduce the BOD and COD ascribable mainly to the phenols. In step (C), phenols, thiocyanate compounds, suspended solids and oils are removed to reduce the COD and BOD further. In step (D), the cyanide compounds are removed.

In step (A), the reduction of the ammonia, i.e. a stripping of the ammonia is performed generally by blowing air or steam into the gas liquor. The amount of ammonia removed in this step should preferably be as large as possible, but preferably a part of the ammonia is left for use as a nitrogen source in the microbiological treatments. The optimum amount of ammonia to be left for this purpose is 50 to 200 ppm. In this case, stripping should be performed after adjusting the pH of the gas liquor to 10 to 11 by adding an alkali such as sodium hydroxide (usually in the form of a concentrated aqueous solution of the alkali).

Generally, however, it is sufficient for the ammonia to be removed such that the residual amount of ammonia becomes 800 to 1,000 ppm. At this level, there is no need to add the alkali. KOH, CaCO3, and Ca(OH)2 can also be used as the alkali. But the use of CaCO3 or Ca(OH)2 is less preferred because a scale may be formed.

The ammonia stripping is generally performed at atmospheric pressure. Further, the efficiency of the stripping is better at high temperatures. Generally, the temperature used for ammonia stripping is 60 to 1000C, preferably 90 to 1000F.

The pH of the gas liquor left after removal of the ammonia is adjusted to pH values suitable for microbiological treatment i.e., a pH 5 to 8, preferably 5.5 to 7.5. This pH adjustment is carried out generally by adding, e.g., sulfuric acid of a concentration of 10 to 80% by weight. Hydrochloric acid, nitric acid, and phosphoric acid can also be used for neutralization, but sulfuric acid is most preferred.

The gas liquor whose pH has been adjusted is then subjected to a first biological treatment step (B) using microorganisms, i.e. a biological activated sludge, e.g., as disclosed in, for example, Japanese Patent Application (OPI) No. 5949/69. This step can be performed using a suspension process such as an activated sluge method, or a fixed bed method such as a rotating disc method, a contact oxidation method (or a submerged filter method or a tube method in which microorganisms are grown on the surface of cylindrical or plate-shaped supporting media in an aeration tank). However, since the concentration of organic matter of the influent gas liquor is high, it is preferred to employ a contact oxidation method using an aeration tank filled with a synthetic resin filling material which is resistant to variations in load.

The filling material for the aeration tank used in the contact oxidation method may be a non-woven sheet having a three-dimensional network structure with irregularly interlaced fibers which is obtained by curling synthetic fibers such as those of nylon, polyvinylidene chloride or polyvinyl chloride by, for example, heat treatment, arranging the curled fibers in the form of a web or a mat, and bonding these fibers into a sheet form using a bonding agent or using a melt-adhesion of the fibers themselves by heat treatment, or a foamed resin sheet formed by foaming a synthetic resin such as polyurethane or polystyrene.

Generally, such filling materials having a thickness of 20 to 40 mm are aligned in parallel to one another at intervals of 20 to 50 mm, and a large number of aerobic, facultative and anaerobic microorganisms are held and grown on the surfaces of the filling materials and in spaces in the interior thereof.

The pH inside the aeration tank in the first biological treatment step is adjusted usually to 6.0 to 7.5. Depending on the character of the gas liquor, the pH of the gas liquor can be adjusted to the most suitable value experimentally. The pH adjustment may be performed prior to the introduction of the gas liquor into the aeration tank. Alternatively, the pH of the gas liquor may be roughly adjusted before introduction into the tank, and microadjusted in the tank by automatic control.

In the suspension process, the activated sludge concentration in the aeration tank is usually 2000 to 5000 ppm, preferably 3000 to 4000 ppm. The temperature is usually 20 to 40"C, preferably 25 to 350C. Air is introduced into the tank so that the amount of dissolved oxygen becomes usually 1 to 5 ppm, preferably 3 to 4 ppm.

In the contact oxidation method in the fixed bed system, the treatment is performed at a volume load of 4 to 8 kg CODMn/m3 day, preferably 5 to 7 kg CODMn/m3 day and a surface load of 200 to 400 g CODmn/m2 day, preferably 250 to 400 g CODMn/m2 day. The temperature and the pH may be the same as those used in the suspension process. The amount of dissolved oxygen is usually 2 to 7 ppm, preferably 4 to 6 ppm.

In step (B), phenols and other polluting materials are removed due to the action of microorganisms. Preferably, the polluting substances are removed in this step to an extent that the CODMn is reduced by at least 30%, so as to reduce the loads in the subsequent treatment with activated carbon and activated sludge and to perform the treatment in a stable manner and with good efficiency. However, removal of the polluting substances to reduce the CODMn by more than about 90% is not preferred. If the amounts of the polluting substances to be removed are small and the BOD and CODMn are high, the amount of sludge generated in the second biological treatment step increases, and this is undesirable from the standpoint of equipment cost, treating efficiency, and treating effect. If, on the other hand, the rate of removal of the polluting substances is large and the reduction of the CODMn is more than about 90% in the first biological treatment step the amount of sludge generated in the second biological treatment step is small. In such an event, the rate of addition of activated carbon in the second biological treatment step is restricted depending upon the absolute amount of excess sludge (activated sludge/activated carbon mixture weight ratio). As a result it is not preferred in the method of this invention to effect such a high removal of pollutants and reduction of CODMn in the first biological treatment step. In order to maintain a fixed rate of addition of activated carbon and keep the concentration of activated carbon in the aeration tank at a fixed value, the amount of excess sludge generated should be within a certain fixed range. When the amount of excess sludge is too large, a large amount of activated sludge must be discharged from the system in order to maintain the concentration of activated sludge in the aeration tank at the desired value. As a result, the amount of activated carbon in the excess sludge is withdrawn in an amount larger than the desired amount, and in order to maintain the concentration of activated carbon in the aeration tank at the desired value, the addition of more activated carbon becomes necessary. If, on the other hand, the amount of excess sludge generated is too small, only a small amount of activated sludge can be withdrawn in order to maintain the activated sludge concentration in the aeration tank constant. If a fixed rate of addition of activated carbon is maintained in such a situation, the amount of activated carbon in the aeration tank increases. Furthermore, in order to maintain the amount of activated carbon at the fixed value in the aeration tank, the rate of addition of activated carbon should be reduced. A decrease in the rate of addition of activated carbon deteriorates the properties of the liquor being treated.

Excess sludge from step (C) is treated with a device for regenerating activated carbon by, for example, the wet air oxidation method [Step (EF)] (This method is described in, for example, U.S. Patent 3,442,798). The sludge is oxidized and burned, and the activated carbon is supplied in an amount corresponding to the loss during regeneration (which is usually 4 to 7%).

In order to add a predetermined amount of activated carbon in step (C) while maintaining the composition of the activated carbon/activated sludge mixture weight ratio constant, the degree of decrease of CODMn in step (B) is preferably 30 to 90%, more preferably 50 to 80%.

The residence time of the liquor being treated in the treating tank is generally 10 to 15 hours when the amount of sludge returned is 100% by volume based on the volume of the starting liquor.

Step (B') may be carried out instead of step (B). It comprises treating the gas liquor, from which ammonia has been removed, with powdery activated carbon. The activated carbon used has a particle size of usually 150 to 400 mesh, preferably 200 to 250 mesh. It is added in an amount of usually 3,000 to 10,000 ppm, preferably 5,000 to 8,000 ppm, and the mixture is stirred for about 0.5 to 2 hours to remove the phenols, suspended solids and oils in the gas liquor and to reduce CODMn by 20 to 80%, preferably by 30 to 70%. This can reduce the load in the subsequent biological treatment step.

The gas liquor treated in step (B) or (B') is subjected to a solid-liquid separation, and the supernatant liquid is then subjected to step (C). In step (C), phenols and thiocyanate compounds etc. in the gas liquor are removed by the decomposition, oxidation or decomposition-oxidation action of the microorganisms in the activated sludge and the adsorption action of the activated carbon. Hence, the BOD and COD are reduced. In this step, activated sludge and activated carbon make up the mixed liquor suspended solids in the aeration tank.

The activated carbon used in this invention usually has a particle size of 150 to 400 mesh, preferably 200 to 250 mesh. Activated carbon having too small particle size is difficult to separate in the solid-liquid separating procedure, and an activated carbon having too large particle size has poor adsorbability and it is difficult to achieve good circulation within the tank.

The concentration of activated sludge in the aeration tank is usually 2500 to 5000 mg/liter, preferably 3000 to 4000 mg/liter. The activated carbon concentration is usually 10,000 to 50,000 mg/liter, preferably 20,000 to 40,000 mg/liter. The ratio by weight of the activated sludge to the activated carbon is usually 1:2 to 1:20, preferably 1:5 to 1:14. If the amount of activated carbon is less than 10,000 mg/liter, the amounts of the polluting substances, the decomposition products and the oxidation products to be adsorbed tend to decrease. If the amount of activated carbon is larger than 50,000 mg/liter, it is difficult to separate the activated carbon with good efficiency in the solid-liquid separating operation.

The treatment in this step is carried out usually at 20 to 400C, preferably 25 to 350C. The amount of air fed into the tank is adjusted such that the amount of dissolved oxygen in the tank is usually 2 to 6 ppm, preferably 3 to 4 ppm. The pH inside in the aeration tank is usually adjusted to 6 to 7.5 automatic control. The pH can be adjusted to the most suitable range experimentally depending on the character of the gas liquor. The pH of the liquor can be adjusted with an inorganic acid which is described hereinabove, such as sulfuric acid. In order to keep the weight ratio of the activated sludge and activated carbon constant, the activated carbon is added to the aeration tank in an amount of 500 to 2,000 mg/liter based on the liquor introduced. Regenerated activated carbon can be used for this purpose. The loss (which is about 4 to 7 /7s) of the activated carbon at the time of regeneration is replenished with fresh activated carbon. The residence time of the liquor in the aeration tank is usually 8 to 15 hours.

By mixing activated @ sludge and activated carbon in the step described above, an anaerobic zone is formed around the activated carbon, and an aerobic zone, on outside of the anaerobic zone. Substances adsorbed on the activated carbon are decomposed by anaerobic microorganisms, and oxidized by aerobic microorganisms. Since the polluting substances are adsorbed on the activated carbon, the load of sludge and qualitiative and quantitiative shock loads (resistance to variation in load) can be reduced, and the treating efficiency can be stabilized. Furthermore, the reactions within the system are promoted because the biological metabolites in the treating system are adsorbed.

The activated sludge-activated carbon mixture is separated and removed from the gas liquor treated in step (C), and the residue is subjected to step (D).

The precipitate formed in step (D) is coagulated and separated. The supernatant liquid, if desired, is treated with powdered activated carbon in step (E). Step (E) can be performed in quite the same way as in step (B'). The supernatant liquid obtained by solid-liquid separation after step (D) or (E) is discharged into watercourses after, if desired, having been filtered through a bed of sand, for example.

According to the method described above main ingredients which hinder biochemical reactions in step (C) are decreased or removed in step (B) or (B'), and the efficiency of the biological oxidation reaction in step (C) is increased. Furthermore, since the biological treatment step is performed using activated carbon and activated sludge at a pH of 6 to 7.5, thiocyanate compounds, phenols and other polluting materials can be surely removed by a synergistic action of the biological oxidation by activated sludge and the adsorption by activated carbon, and BOD, COD can be reduced. After the biological treatment step, the coagulating treatment using iron salts in accordance with this invention and the powdered activated carbon treatment (optionally) are carried out. Hence, the remaining polluting materials and impurities such as cyanide compounds, color ingredients and residual suspended solids can be surely removed, and CODMn can be reduced. By the effective combination of the aforesaid pre-treatment, biological treatment and the post-treatment of the invention, the gas liquor whose stable treatment has been regarded as difficult can be treated consistently and completely to afford treated water of good quality. Furthermore, the effective combination of the pretreatment, the biological treatment and the posttreatment make it possible to treat the gas liquor easily and surely without diluting it with industrial water, sea water, waste water (domestic waste water and other process waste waters) or mixtures of these. Hence, the method is very effective for treatment of gas liquor.

Referring now to the drawing, the process comprises step (A) which can be considered a pretreatment step (an ammonia stripping step al, and a neutralization step a2), a biological treatment step (B) (treatment with microorganism) or step (B') (treatment with activated carbon), a biological treatment step (C) (treatment with a mixture of activated sludge and activated carbon), a post-treatment step (D) (a coagulating-sedimentation step dl, and a final filtration step d2, and an activated carbon regenerating step (EF).

First, gas liquor is introduced via line 16 into an ammonia stripper 1, and simultaneously, air or steam is fed via line 17 into the gas liquor 16 in the stripper 1, to remove ammonia via line 18 in the liquor. (ammonia stripping step A-al).

Then, the gas liquor from which ammonia has been removed (in a predetermined amount) is introduced into a pH adjusting tank 2, and the pH of the gas liquor is adjusted to 5 to 8, e.g., with sulfuric acid. (neutralization step A-a2).

Then the gas liquor is introduced into a first biological treatment aeration tank 3. Air via line 19 is introduced into the aeration tank 3. Due to the decomposition and oxidation action of microorganisms, the polluting substances in the gas liquor are partly removed. The pH of the liquor within the aeration tank 3 is adjusted using an automatic pH controller 29.

When the above treating device is used with a fixed bed-type biological treating method, although exhausted sludge from tank 3 may be returned to the tank it is usually not necessary. The gas liquor from which the polluting substances have been partly removed in the aeration tank 3 is then introduced into a settling tank 4 and the liquor is subjected to a solid-liquid separating procedure. The supernatant liquid is introduced into an aeration tank 6. The pH of the liquid in the tank 6 is adjusted using an automatic pH controller 30.

The sludge is sent to an activated carbon reservoir tank 14 to be described hereinafter through a thickener 5, and the sludge is treated in an activated carbon regenerating device 15 simultaneously with regeneration of the activated carbon. (first biological treatment step (B) and regeneration step (EF).

The supernatant liquid introduced into the aeration tank 6 from the settling tank 4 is mixed in the aeration tank 6 with activated sludge (including return sludge via line 20) and activated carbon (regenerated activated carbon via line 21 plus replenishing activated carbon via line 22). Air via line 23 is introduced into the aeration tank 6. Due to the biological oxidation action of the microorganisms in the activated sludge and the adsorption action of the activated carbon, phenols and thiocyanate compounds, for example, in the gas liquor are removed, and the BOD and the COD are reduced. The regenerated activated carbon is, in this embodiment, activated carbon regenerated in the regenerating step (EF). The gas liquor treated in the aeration tank 6 is introduced into a settling tank 7 where the activated sludge/activated carbon mixture is separated by sedimentation. The supernatant liquid is introduced into a coagulation tank 9. A part of the sludge (the activated sludge/activated carbon mixture) is returned to the aeration tank 6 as return sludge via line 20. The remainder of the sludge is sent to a thickener 8 as excess sludge via line 24. (Biological treatment step (C)).

A predetermined amount of a ferrous compound such as ferrous chloride is added via line 25 to the supernatant liquid introduced into the coagulation tank 9, from the settling tank 7 and, optionally the mixture is stirred. Then, an alkali, such as sodium hydroxide or potassium hydroxide is added via line 26 to adjust the pH of the mixture. Then, a predetermined amount of a ferric compound such as ferric chloride is added via line 27. The mixture is stirred to remove the cyanide compounds from the gas liquor, and the COD is reduced. The treated liquor is subjected to a solid-liquid separating procedure in a settling tank 10. The supernatant liquid is introduced into a sand filtering device 13, and the sedimented sludge is supplied to a thickener 11.

The sedimented sludge is introduced into a sludge treating device 12' through the thickener 11, and separately treated. (coagulating sedimentation step D-dl). The supernatant liquid introduced into the sand filtering device 13 is completely filtered, and released as treated liquor via line 28. (final filtering step D-d2).

The sludge in the thickener 5 in the first biological treatment step (B) and the sludge (the activated sludge/activated carbon mixture) in the thicker 8 in the second biological treatment step (C) are each transferred to the activated carbon reservoir 14 in the activated carbon regenerating step (EF), and are mixed in the activated carbon reservoir tank 14. The mixture is introduced into an equipment 15 for regenerating activated carbon using a wet air oxidation method. The used activated carbon is reactivated and regenerated, and the excess sludge is burned there. In the activated carbon regenerating step (EF), the regeneration of powdered activated carbon and the treatment of excess sludge are performed simultaneously. The activated carbon regenerated in the regenerating equipment 15 is returned to the aeration tank 6. In this way, the activated carbon is recycled, and the cost of treatment can be reduced (regeneration step (EF).

When step (B') is employed instead of step (B) and the treatment with activated carbon in step (E) is also performed, the activated carbon used in step (E) may be used directly in step (B') to utilize its remaining adsorbability effectively. The activated carbon used in step (B') is generally concentrated and then sent to a regenerating device where it is treated together with the activated carbon used in step (C). The regenerated activated carbon is recycled to step (C) and step (E). When the treatment of gas liquor with powdered activated carbon in stage (E) is not performed, the regenerated activated carbon is recycled to step (C). Thus, the cost of treatment can be reduced when step (B') is employed, steps (A), (C), and (D) to be combined with it are the same as in the case of employing step (B).

The following Examples and Comparative Examples specifically illustrate the present invention.

EXAMPLE I FeCl2 (100 ppm) was added to gas liquor containing 5.6 ppm of a cyanide ion (CN-) which had been subjected to a biological oxidizing treatment. The mixture was rapidly stirred at 150 rpm for 2 minutes by a jar tester (a stirring apparatus whose speed of rotation is freely controllable). The pH of the mixture was adjusted to 8.4 with sodium hydroxide, and it was rapidly stirred for 2 minutes. Furthermore, 150 ppm of Fecal2 was added, and the mixture was rapidly stirred for 2 minutes. Finally, the mixture was slowly stirred at a speed of 30 rpm for 10 minutes, and coagulated and separated. After the coagulation and separation, the supernatant liquid (treated water) had a pH of 6.5. Analysis showed that it contained 0.7 ppm of CN-. Thus, the cyanide ion could be surely removed. The treated water did not form a precipitate on standing for a long period of time.

COMPARATIVE EXAMPLE I FeCl2 (100 ppm) was added to the same waste water as in Example 1, and after rapid stirring for 2 minutes, 150 ppm of FeCl3 was added. The mixture was rapidly stirred for 2 minutes. The pH of the mixture was adjusted to 6.5 with sodium hydroxide, and the mixture was rapidly stirred for 2 minutes. It was finally stirred slowly for 10 minutes, and then coagulated and separated. Analysis showed that the treated water contained 0.7 ppm of CN- but on standing, a precipitate of Fe(OH)3 was formed from the treated water.

When the procedure was repeated under the same conditions as set forth above except that the amount of sodium hydroxide was increased to adjust the pH of the mixture to 7.4, no precipitate was formed from the treated water but the CN- concentration of the treated water was 1.1 ppm.

The amount of sodium hydroxide required to increase the pH to 8.4 in Example 1 was the same as that of sodium hydroxide required to raise the pH to 6.5 in Comparative Example 1.

A comparison of Example 1 (the method of the invention) with Comparative Example 1 shows that Example 1 required a smaller amount of alkali than Comparative Example 1, and in Example 1, the cyanide ions can be surely removed and the method can fully cope with an increase in the amount of Fe Cl2 that is required with an increase in the cyanide ion content in the influent waste water in coagulating sedimentation.

COMPARATIVE EXAMPLE 2 To the same waste water as used in Example 1 was added Fecal3 in an amount of 500 ppm, 1,000 ppm and 1,500 ppm, respectively. The mixture was rapidly stirred for 2 minutes, and adjusted to pH 7 with sodium hydroxide. It was again rapidly stirred for 2 minutes, finally slowly stirred for 10 minutes, and coagulated and separated. Each of the treated waters was analyzed for cyanide ions, and the results are shown in Table 2.

Table 2 CN- concentration of the Amount of Fe Cl3 added treated water (ppm) (ppm) 500 2.1 1,000 1.5 1,500 1.3 As can be seen from Table 2, the method of Comparative Example 2 requires a large amount of Fecal3 in order to remove the cyanide ions without fail. When the amount of Fecal3 increases, the amount of sludge formed increases accordingly and leads to a high cost of operation.

COMPARATIVE EXAMPLE 3 Fecal2 (250 ppm) was added to the same waste water as used in Example 1, and the mixture was rapidly stirred for 2 minutes. Then, its pH was adjusted to 7 with sodium hydroxide, and the mixture was stirred rapidly for 2 minutes and then slowly stirred for 10 minutes. It was then coagulated and separated. But since the coagulability of the Flock was poor, the coagulation and separation could not be effected well. Hence, 1 ppm of a polyacrylamide coagulant was added, but complete coagulation and separation were neither possible.

The treated water left after the separation of the flock was filtered through a No. 5 filter paper (JIS T-3801 standards). The filtrate was found to have a CN- concentration of 0.8 ppm, but on standing, a brown precipitate of Fe(OH)3 formed in the filtrate.

It was ascertained that the pH at which no precipitate formed from the filtrate on standing was 9.2. When the waste water was treated at this pH, it had a CN- concentration of 1.8 ppm.

Thus, in Comparative Example 3, the ratio of removal of CN- was good, but coagulation and separation were difficult. Moreover, Fe2+ remained, and an attempt to perform treatment at such a high pH as to prevent the remaining of Fe2+ resulted in the dissolving of CN-.

EXAMPLE 2 Fecal2 (200 ppm) was added in the same way as in Example 1 to gas liquor containing 15 ppm of cyanide ions which had been subjected to a biological oxidizing treatment. The pH of the mixture was adjusted to 8.4 with sodium hydroxide, and then 150 ppm of FeCl3 was added. The treated water had a CN- concentration of 0.6 ppm. No precipitate was formed from the treated water. The pH of the treated water was 7.3 and the water had a residual iron content of 0.4 ppm. Thus, good treatment of the gas liquor could be performed.

EXAMPLE 3 A gas liquor as shown in Table 4 below was treated under the conditions shown in Table 4 below using the process, that is process (A) - (B) -(C) - (D) as shown in the flowsheet shown in the Figure. The biological treatment steps were performed continuously, and the pre-treatment and post-treatment steps were performed batchwise.

The biological treatment steps were performed as described below. The specifications of the test unti used is shown in Table 3.

Table 3 Specifications of the Testing Unit Biological Step Biological Step (B) (C) Fixed bed type Treatment with biological activated sludge treatment and activated carbon Raw Waste Storage Tank 200 liters Aeration Tank 2.5 liters (2 tanks) 10 liters Settling Tank 5 liters 10 liters Treated Liquor Storage Tank 50 liters 50 liters Pump for Supplying Starting Liquor 0-30 ml/min 0-30 ml/min Pump for Returning Sludge - 0-30 ml/min Air Pump 15 N* liters/min 15 N* liters/min Air Flow Meter 0-5 N liter/ 0-10 N liters/ min (2 air flow min meters) Surface Area of Filling Material 0.11 m2 Material of Filling Material Polyvinylidene chloride non-woven sheet * N means as measured at OOC and at atmospheric pressure.

The gas liquor which had been subjected to the pretreatment step (ammonia stripping and neutralization with sodium hydroxide) was fed into the aeration tank of the first biological treatment step at a flow rate of 12 liters/day. Air was introduced into the aeration tank at a flow rate of 2 to 4 N* liters/min. The amount of dissolved oxygen was maintained at 3 to 6 ppm, and the pH of the liquor in the aeration tank was maintained at 6 to 7 using an automatic pH adjuster; While the gas liquor was present in the aeration tanks (two tanks each with a capacity of 2.5 liters) for 10 hours, the gas liquor underment the decomposition and oxidation actions of aerobic, facultative, and anaerobic microorganisms held and grown on the surface of the filling material and the interior spaces inside the filling material. As a result, the CODMn was reduced by about 60%.

Air was fed into the aeration tank of the second biological treatment step at a flow rate of 2 to 3 N * liters/min, and the amount of dissolved oxygen was maintained at 2 to 4 ppm. The pH of the liquor in the aeration tank was maintained at 6 to 7 using an automatic pH adjuster. The concentrations of the activated sludge and activated carbon in the aeration tank were 4,100 and 39,000 ppm, respectively. The weight ratio of the activated sludge to the activated carbon was 1:9.5. The rate of addition of regenerated activated carbon was 1,520 ppm and fresh activated carbon was 80 ppm based on the raw waste. While the gas liquor was present in the aeration tanks for 12 hours, the gas liquor was purified and clarified by the synergistic combination of the biological oxidation action by the activated sludge and the physical adsorbing action by the activated carbon. The amount of return sludge was 100% by volume based on the amount of the influent liquor fed.

In the coagulating and sedimenting step, 200 ppm of ferrous chloride was added to the treated liquor of biological treatment step (C), and the mixture was rapidly stirred at 150 rpm for 2 minutes, and then sodium hydroxide was added to adjust the pH of the mixture to 8.5. Then, 100 ppm of ferric chloride was added, and the mixture was rapidly stirred at 15 rpm for 2 minutes, and then slowly stirred at 30 rpm for 10 minutes. The liquor was then filtered with a filter paper (NO 5-C by JIS standards).

EXAMPLE 4 A gas liquor as shown in Table 4 below was treated using the same procedures as in Example 3 under the conditions shown in Table 4 below.

In this Example, the gas liquor was immediately subjected to an activated sludge treatment after the pretreatment step, and then subjected to the post-treatment step.

The results obtained in Example 3 and Example 4 are also shown in Table 4 below.

Table 4 Example 3 Example 4 Ammonia Ammonia stripping stripping Pretreatment and pH and pH Step adjustment adjustment Raw Waste Gas pH 9.5 9.5 Liquor BOD5 (ppm) 2960 2830 CODMn (ppm) 4500 4300 SCN Compounds (ppm) 680 655 CN Compounds (ppm) 30 25 Phenols (ppm) 1020 980 NH3 (ppm) 3100 3300 Biological Treatment Step Influent Gas pH 6.1 6.5 Liquor BOD5 (ppm) 1850 1700 CODMn (ppm) 3000 3000 SCN Compounds (ppm) 670 650 CN Compounds (ppm) 20 20 Phenols (ppm) 622 500 NH3 (ppm) 810 450 Table 4 (continued) Example 3 Example 4 Fixed bed type bio First Biological logical Treatment Step treatment Conditions COD Volume Load (kg/m3.D) 7.2 COD Surface Area Load (g/m2.D) 327 Aeration Time (hours) 10 Effluent Gas pH 6.7 Liquor BODs (ppm) 300 CODMn (ppm) 1270 SCN Compounds (ppm) 580 CN Compounds (ppm) 18 Phenols (ppm) 10 Treatment with Acti vated sludge Second Biological and activated Activated Treatment Step carbon sludge Conditions COD Volume Load (kg/m3.D) 1.27 1.0 COD-SS Load (kg/.D) 0.31 0.25 Sludge Concentration (ppm) 4100 3950 Activated Carbon Con centration (ppm) 39000 Amount of Activated 1600 Carbon Added (ppm) (regenera ted: 1520 flesh: 80) Aeration Time (hours) 12 36 Table 4 (continued) Example 3 Example 4 Treatment with acti vated sludge Second Biological and acti- Activated Treatment Step vated carbon sludge Effluent Gas BODs (ppm) 3.8 30 Liquor CODMn (ppm) 20 350 SCN Compounds (ppm) 0.01 40 (trace) CN Compounds (ppm) 15 18 Phenols (ppm) 0.01 5 (trace) Floccula tion and precipi tation, and Post-Treatment filtering Step through sand Effluent BOD5 (ppm) less than 2 20 Liquor (trace) CODMn (ppm) 12 115 SCN Compounds (ppm) less than 40 0.1 (trace) CN Compounds (ppm) 0.6 0.8 Phenols (ppm) less than 0.5 0.01 (trace) Total Aeration Time in Biological Treatment 22 36 Steps (hours) Ratio of Sludge Returned: 100% by volume based on the raw waste gas liquor volume.

COD Volume Load: COD of the influent liquor per day per cubic meter in the aeration tank.

COD Surface Area Load: COD of the influent liquor per day per square meter of the filling material in the aeration tank.

COD-SS Load: COD of the influent liquor per day per kilogram of suspended solids in the aeration tank.

EXAMPLE 5 In Example 3, the gas liquor was treated in the same way except using ferrous sulfate and ferric sulfate instead of the ferrous chloride and ferric chloride. The treating conditions and the results obtained are shown in Table 5 below together with the results obtained in the case of using iron chlorides in Example 3.

Table 5 Coagulating-Sedimenting Step Iron Chlorides Iron Sulfates Conditions Ferrous Salt (ppm) 200 300 As Fe (ppm) 88 84 pH (using NaOH) 8.5 8.4 Ferric Salt (ppm) 100 150 As Fe (ppm) 34 55 Influent CODMn (ppm) 20 22 Liquor BOD5 (ppm) 3.8 4.7 (effluent liquor from SCN Compounds (ppm) less than 0.01 less than 0.01 the second (trace) (trace) biological CN Compounds (ppm) 15 13 treatment step) Phenols (ppm) less than 0.01 less than 0.01 (trace) (trace) Effluent CODMn (ppm) 12 14 Liquor from BODs (ppm) less than 2 less than 2 the Coagu- (trace) (trace) lating-Sedi menting SCN Compounds (ppm) less than 0.01 less than 0.01 Step (trace) (trace) CN Compounds (ppm) 0.6 0.7 Phenols (ppm) less than 0.01 less than 0.01 (trace) (trace) All values in Table 5 were measured in the same manner as in Table 1.

EXAMPLES 6-13 Gas liquor shown in Table 6 was treated under the conditions shown in Table 6. Only the biological treatment was performed continuously, and the pretreatment and the posttreatment were carried out batchwise.

The biological treatment step was performed in the following manner.

The gas liquor which had been pretreated was treated using a flow-type activated sludge treating device composed of an aeration tank having a capacity of 10 liters and a settling tank having a capacity of 10 liters connected to each other. The concentrations of activated sludge and activated carbon in the aeration tank were 3780 and 16540 ppm, respectively.

The mixing weight ratio of the former to the latter was 1:4.4.

The gas liquor which had been subjected to ammonia stripping and a pretreatment of adsoprtion with powdered activated carbon was adjusted to pH 6.5 with sulfuric acid, and then fed into the aeration tank at a flow rate of 10 liters/day. Air was fed into the aeration tank at a flow rate of 4 - 5 N liters/min to maintain the amount of dissolved oxygen at 2 to 4 ppm. The gas liquor was caused to reside for 12 hours in the aeration tank, and during this time, was purified by a synergistic action of the biological oxidative decomposition by activated sludge and adsorption by activated carbon. It was then flowed into the subsequent settling tank, and subjected to solid-liquid separation. The supernatant liquid was flowed into a treated water tank, and a part of it was sampled for analysis. On the other hand, a part of the spearated sludge was withdrawn as an excess sludge from the bottom of the settling tank. The remainder was returned to the aeration tank. The amount of the return sludge was 100% by volume based on the amount of the influent gas liquor.

In the pretreatment step of adsorption with powdered activated carbon, the activated carbon discharged from the post-treatment step of adsorption with powdered activated carbon was used, and added in an amount of 4,000 ppm to the gas liquor, followed by stirring the mixture for 60 minutes.

In the post-treatment step (D), FeCl2 (the amount in each Example is shown in Table 7) was added to the gas liquor, and the mixture was rapidly stirred at 150 rpm for 2 minutes by a jar tester. Then, sodium hydroxide was added to adjust the pH to 8.5. FeCl3 (the amount in each Example is shown in Table 7) was addcd, and the mixture was stirred rapidly at 150 rpm for 2 minutes. Finally, the mixture was slowly stirred at 30 rpm for 10 minutes, followed by coagulation and separation.

In step (E) (adsorbing treatment with powdered activated carbon), powdered activated carbon which was used and has been regenerated by wet air oxidation method was used.

4000 ppm of the activated carbon was added to the gas liquor and the liquor was stirred for 60 minutes.

COMPARATIVE EXAMPLES 4-11 Gas liquor shown in Table 6 was treated in the same manner as in Examples 6-13 except the post-treatment step (D). In the post-treatment step (D), 500 ppm of Fecal3 (172 ppm as Fe"') was added to the gas liquor, and the mixture was rapidly stirred at 150 rpm for 2 minutes, and then slowly stirred at 30 rpm for 10 minutes.

The results obtained in Examples 6-13 and Comparative Examples 4-11 are shown in Table 6.

Table 6 Treating Conditions and Example Comparative Example Comparative Items of Analysis 6 Example 4 7 Example 5 Pretreatment Step Ammonia Stripping Step - yes Powdered Activated Carbon Adsorption Step Type of Biological Treatment Step AS AS Dilution Ratio in the Biological Treatment 1 1 Type of Diluting Water Law Waste Gas Liquor pH 9.4 9.5 BOD, ppm 1,480 1,840 CODMn, ppm 2,500 2,800 SCN Compound, ppm 650 650 CN Compound, ppm 6.2 10.5 Phenols, ppm 500 700 NH3, ppm 3,300 3,500 Influent Water in the Biological Treatment pH 9.4 8.5 BOD, ppm 1,480 1,480 CODMn, ppm 2,500 2,500 SCN Compound, ppm 650 650 CN Compound, ppm 6.2 6.1 Phenols, ppm 500 500 NH3, ppm 3,300 450 Effluent Water from the Biological Treatment BOD, ppm 300 50 CODMn, ppm 800 350 SCN Compound, ppm 450 70 CN Compound, ppm 4.8 3 Phenols, ppm 50 5 Discharge Water from the Post-Treatment BOD, ppm 200 200 30 30 CODMn, ppm 260 260 80 80 SCN Compound, p

Table 6 (continued) Treating Conditions and Example Comparative Example Comparative Items Analysis 8 Example 6 9 Example 7 Pretreatment Step Ammonia Stripping Step yes yes Powdered Activated Carbon Adsorption Step yes yes Type of Biological Treatment Step AS AS + AC Dilution Ratio in the Biological Treatment 1 1 Type of Diluting Water Low Waste Gas Liquor pH 9.5 BOD, ppm 2,100 CODMn, ppm 3,400 SCN Compound, ppm 700 CN Compound, ppm 7.5 Phenols, ppm 900 NH3, ppm 4,000 Influent Water in the Biological Treatment pH 6.5 BOD, ppm 710 CODMn, ppm 1,500 SCN Compound, ppm 700 CN Compound, ppm 5.3 Phenols, ppm 270 NH3, ppm 910 Effluent Water from the Biological Treatment BOD, ppm 15 4.3 CODMn, ppm 150 40 SCN Compound, ppm 30 0.01 CN Compound, ppm 2.7 2.1 Phenols, ppm 3 0.01 Discharge Water from the Post-Treatment BOD, ppm 11 11 3.1 3.1 CODMn, ppm 60 60 10 10.0 SCN Compound, ppm 20 20 0.1 0.1 CN Compound, 0.4 1.2 less than 0.8 ppm 0.3 Phenols, 0.1 0.1 less than 0.01 ppm 0.01 Treatment Conditions in the Aeration Tank COD Volume Load, kg-COD/m3 day 1.50 1.47 COD-SS Load, kg-COD/kg-SS day 0.370 0.390 Concentration of Activated Sludge, ppm 4,050 3,780 Concentration of Activated Carbon, ppm - 16,540 Table 6 (continued) Treating Conditions and Example Comparative Example Comparative Items of Analysis 10 Example 8 11 Example 9 Pretreatment Step Ammonia Stripping Step yes yes Powdered Activated Carbon Adsorption Step Type of Biological Treatment Step AS AS + AC Dilution Ratio in the Biological Treatment 23 23 Type of Diluting Water Industrial Water Industrial Water Law Waste Gas Liquor pH 9.2 BOD, ppm 2,530 COD Mn, ppm 3,450 SCN Compound, ppm 650 CN Compound, ppm 25 Phenols, ppm 750 NH3, ppm 3,500 In fluent Water in the Biological Treatment pH 7.2 BOD, ppm 992 CODMo, ppm 1,521 SCN Compound, ppm 310 CN Compound, ppm 8.2 Phenols, ppm 360 NH3, ppm 300 Effluent Water from the Biological Treatment BOD, ppm 8.5 5.9 CODMn, ppm 250 40 SCN Compound, ppm 10 0 CN Compound, ppm 5.2 3.4 Phenols, ppm 0.5 0.01 Discharge Water from the Post-Treatment BOD, ppm 5.7 5.7 4.5 4.5 CODMn, ppm 80 80 15 15 SCN Compound, ppm 7 7 0.01 0.01 CN Compound, ppm 0.7 2.8 0.5 0.9 Phenols, ppm 0.2 0.2 0.01 0.01 Treatment Conditions in the Aeration Tank COD Volume Load, kg-COD/m3 day 1.51 1.50 COD-SS Load, kg-COD/kg-SS day 0.398 0.403 Concentration of Activated Sludge, 3,790 ppm 3,790 3,720 Concentration of Activated Carbon, ppm - 16,500 Table 6 (continued) Treating Conditions and Example Comparative Example Comparative Items of Analysis 12 Example 10 13 Example 11 Pretreatment Step Ammonia Stripping Step Powdered Activated Carbon Adsorption Step Type of Biological Treatment Step AS AS + AC Dilution Ratio in the Biological Treatment 1.82 1.89 Type of Diluting Water Industrial Water Industrial Water Law Waste Gas Liquor pH 9.2 BOD, ppm 2,100 CODMn, ppm 3,000 SCN Compound, ppm 650 CN Compound, ppm 28 Phenols, ppm 750 NH3, ppm 3,500 Effluent Water in the Biological Treatment pH 7.4 7.4 BOD, ppm 1,210 1,150 CODMn, ppm 1,650 1,580 SCN Compound, ppm 360 340 CN Compound, ppm 15.3 14.8 Phenols, ppm 410 400 NH3, ppm 1,925 1,850 Effluent Water from the Biological Treatment BOD, ppm 48 25 CODMn, ppm 310 250 SCN Compound, ppm 180 100 CN Compound, ppm 13.2 10.5 Phenols, ppm 20 8 Discharge Water from the Post-Treatment BOD, ppm 20 20 13 13 CODMn, ppm 95 95 75 75 SCN Compound, ppm 80 80 60 60 CN Compound, ppm 0.8 8.5 0.6 7.1 Phenols, ppm 2.0 2.0 0.5 0.5 Treatment Conditions in the Aeration Tank COD Volume Load, kg-COD/m3 day 1.42 1.48 COD-SS Load, kg-COD/kg-SS day 0.356 0.361 Concentration of Activated Sludge, ppm 3,975 4,100 Concentration of Activated Carbon, ppm - 16,300 Table 7 Ferrous Salt (ppm) Ferric Salt (ppm) Example No. Fecal2 as Fem Fecal3 as Fe 6 100 44 150 50 7 75 33 150 50 8 75 33 150 50 9 100 44 150 50 10 100 44 150 50 11 75 33 150 50 12 200 88 300 100 13 200 88 300 100 Attention is drawn to the Specification and claim of our British Patent Application No.

8012998 (Serial No. 1591274) which is divided from the present Application and which claims a method for treating gas liquor discharged from the step of quenching coke over gas.

WHAT WE CLAIM IS: 1. A method for treating waste water containing cyanide ions, which comprises (1) adding a ferrous salt to waste water to convert cyanide ions therein to a precipitate of ferrous ferrocyanide and insoluble Turnbull's blue, (2) adding an alkali to the waste water to convert ferrous ion remaining in the waste water to Fe(OH)2, and (3) adding a ferric salt to the waste water to precipitate [Fe(CN)6]4-, which is formed by addition of the alkali to the waste water, as insoluble Berlin blue.

2. A method as claimed in Claim 1, wherein the ferrous salt is ferrous chloride or ferrous sulfate.

3. A method as claimed in Claim 1 or 2, wherein the ferric salt is ferric chloride or ferric sulfate.

4. A method as claimed in any preceding Claim wherein the amount of the ferrous salt added is such that the amount of Fe" is within the range of x' calculated from the following equations, y = 0.213x- 3.8 x' = x + 25 (mg/litre) wherein y is the concentration (mg/litre) of the cyanide ion in the influent water, and x is the iron content (mg/litre) in the ferrous salt, and the amount of the ferric salt added is such that the weight ratio of Fe"'/Fe" becomes 0.5-3.

5. A method as claimed in any preceding Claim, wherein an alkali is added to the waste water to adjust its pH to 7.5-9.5.

6. A method as claimed in any preceding Claim, wherein the treatment of step (2) is carried out after the precipitate formed in step (1) has been separated and removed.

7. A method as claimed in any one of Claims 1 to 6, wherein the cyanide ion is derived from a cyano complex ion of Cu, Cd, or Zn which has been in the waste water.

8. A method as claimed in Claim 7, wherein the cyanide ion is derived from by dissociation of the cyano complex ion by adjusting the pH of the waste water to not more than 3.

9. A method as claimed in claim 1, substantially as hereinbefore described in of Example 1 or 2.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   Table 7 Ferrous Salt (ppm) Ferric Salt (ppm) Example No. Fecal2 as Fem Fecal3 as Fe

   6 100 44 150 50

   7 75 33 150 50

   8 75 33 150 50

   9 100 44 150 50

   10 100 44 150 50

   11 75 33 150 50

   12 200 88 300 100

   13 200 88 300 100 Attention is drawn to the Specification and claim of our British Patent Application No.

   8012998 (Serial No. 1591274) which is divided from the present Application and which claims a method for treating gas liquor discharged from the step of quenching coke over gas.

   WHAT WE CLAIM IS: 1. A method for treating waste water containing cyanide ions, which comprises (1) adding a ferrous salt to waste water to convert cyanide ions therein to a precipitate of ferrous ferrocyanide and insoluble Turnbull's blue, (2) adding an alkali to the waste water to convert ferrous ion remaining in the waste water to Fe(OH)2, and (3) adding a ferric salt to the waste water to precipitate [Fe(CN)6]4-, which is formed by addition of the alkali to the waste water, as insoluble Berlin blue.

   2. A method as claimed in Claim 1, wherein the ferrous salt is ferrous chloride or ferrous sulfate.

   3. A method as claimed in Claim 1 or 2, wherein the ferric salt is ferric chloride or ferric sulfate.

   4. A method as claimed in any preceding Claim wherein the amount of the ferrous salt added is such that the amount of Fe" is within the range of x' calculated from the following equations, y = 0.213x- 3.8 x' = x + 25 (mg/litre) wherein y is the concentration (mg/litre) of the cyanide ion in the influent water, and x is the iron content (mg/litre) in the ferrous salt, and the amount of the ferric salt added is such that the weight ratio of Fe"'/Fe" becomes 0.5-3.

   5. A method as claimed in any preceding Claim, wherein an alkali is added to the waste water to adjust its pH to 7.5-9.5.

   6. A method as claimed in any preceding Claim, wherein the treatment of step (2) is carried out after the precipitate formed in step (1) has been separated and removed.

   7. A method as claimed in any one of Claims 1 to 6, wherein the cyanide ion is derived from a cyano complex ion of Cu, Cd, or Zn which has been in the waste water.

   8. A method as claimed in Claim 7, wherein the cyanide ion is derived from by dissociation of the cyano complex ion by adjusting the pH of the waste water to not more than 3.

   9. A method as claimed in claim 1, substantially as hereinbefore described in of Example 1 or 2.