GB1571847A

GB1571847A - Waste water treatment

Info

Publication number: GB1571847A
Application number: GB1199577A
Authority: GB
Original assignee: Systems Eng & Manufacturing
Current assignee: Systems Eng & Manufacturing
Priority date: 1974-01-02
Filing date: 1977-03-22
Publication date: 1980-07-23
Also published as: DE2713684A1

Description

(54) WASTE WATER TREATMENT (71) We, SYSTEMS ENGINEERING & MANUFACTURING CORPORATION trading as BAKER BROS. of Campanelli Parkway, Stoughton, Massachusetts 02072, United States of America, a Corporation organised under the laws of the State of Delaware, United States of America, 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: The present invention is concerned with a method for removing heavy metal contamination from waste water.

In many industrial processes, waste water is obtained with high concentrations of heavy metal ions and other contaminant materials, such as various anions and cations, including fluorides, cyanides and hexavalent chromium. Because of the high concentration of heavy metals and other contaminant materials, the water cannot be recycled in many industrial procedures and also cannot be discharged to environmental outlets because of the polluting effect of such heavy metal ions and/or other contaminant materials. In the metal finishing industry in the U.S.A., the Environmental Protection Agency has set certain tentative discharge limits for maximum amounts of heavy metal ions which can remain in water discharged to the environment. The industry has attempted to meet the pollution problem by several long known and by other recently developed methods. In one method which is often used, waste water containing heavy metal ions and other contaminant materials resulting from metal finishing rinses is treated with an alkaline or acidic material to adjust the pH and to form insoluble hydroxides or other precipitates of the metals, the insoluble hydroxides or other precipitates then being separated from the waste water. A problem here is that many of the heavy metal ions and other contaminant materials are soluble to some extent, even in their hydroxide or precipitated form, so that waste water treated in this manner often still has undesirably high levels of certain metal ions. In a more recent development, ion exchange methods have been employed to remove heavy metal ions and other contaminant materials.

However, such methods are often expensive and complex. Other problems arise with the use of reverse osmosis techniques, high concentrations and high pressures being part of reverse osmosis operational problems. Reverse osmosis is limited to approximately 2000 parts per million (ppm) concentration and the efficiency of the process is inversely proportional to the concentration.

It was believed in the art that heavy metal hydroxides tend to form a gelatinous precipitate and thus block filter membranes by penetration into filter pores or by building a continuous gelatinous precipitate on top of a membrane face area. Thus the art often removes and avoids use of heavy metal hydroxides in filtering procedures in order to avoid the known clogging problems.

It is an object of the present invention to provide a method of treating waste water in order efficiently to remove materials therefrom, permitting recovery of such materials and/or reuse or discharge of the treated water.

Thus, according to the present invention, there is provided a method of removing heavy metal in the form of ions and small particle size material and contaminants from waste water feed containing no more than about 4000 ppm of said heavy metal ions and contaminants, said method comprising combining said heavy metal ions and small particle size material with a combining agent at a pH of from 7.1 to 14 to form bound material having a particle size of at least 10 Angstroms, said combining agent being a hydroxide of an element selected from iron, aluminium, tin, copper, zinc, cadmium, nickel, cobalt, silicon, lead, barium, calcium, manganese and chromium said combining agent being present in a molar ratio of from 1:1 and above with respect to moles of heavy metal ions and the small particle size material but in an amount permitting pumping of said waste water, and subsequently filtering the waste water to separate the bound material from the waste water.

The wastewater is preferably filtered through an ultrafiltration skinned membrane at a flux of at least 30 gfd under a pressure of at least 9 psig.

As stated above, the combining step is carried out at a pH of from 7.1 to 14, a pH of 8 to 12 being preferred.

In one embodiment of the present invention, wherein the pH is basic, the wastewater, after filtration, contains an amount of heavy metal ions and small particle size material below the water solubility limits of the heavy metal ions and small particle size material, the small particle size material consists essentially of heavy metal from wastewater feeds containing no more than about 4000 ppm of the heavy metal and the ultrafiltration pressure is from 9 to 150 psig.

In another embodiment of the present invention, wherein the small particle size material consists essentially of contaminant heavy metal resulting from a metal finishing procedure, the method further comprises carrying out the combining in a reacting tank and filtering the wastewater in an ultrafiltration unit, obtaining a concentrate from the ultrafiltration unit and recycling this concentrate to the reacting tank and obtaining the filtrate from the filtration unit and recycling the filtrate to the metal finishing procedure and passing wastewater from this procedure to the reacting tank, all the steps being carried out continuously in a closed loop.

The heavy metal ions and small particle size material can be originally present in the wastewater in an amount of more than 1 ppm, the small particle size material consisting essentially of contaminate heavy metal resulting from a metal finishing procedure.

According to another embodiment of the present invention, the method can additionally comprise carrying out the combining in a reacting tank and filtering the wastewater in an ultrafiltration unit, obtaining a concentrate from the unit, obtaining a filtrate from the filtration unit and recycling the filtrate to a metal finishing operation and passing wastewater from this operation to the reacting tank.

The wastewater feed can be concentrated so that the combining agent is combined with heavy metal ions in the wastewater feed when the concentration of the heavy metal ions is above 2000 ppm.

According to the present invention, a reaction tank can be used to contain the combining agent and wastewater feed which form a reaction mixture, the reaction mixture being passed to an ultrafiltration skinned membrane with the filtrate collected and the concentrate recycled continuously to the reaction tank in a continuous method.

According to yet another embodiment of the present invention, the combining is carried out in a first tank to form an aqueous reaction mixture, the reaction mixture is transferred to a second tank, the wastewater is filtered in an ultrafiltration unit and concentrate obtained from the ultrafiltration unit is recycled to a second tank, filtrate being continuously obtained from the filtration unit.

Yet again. according to the present invention, in a method of removing heavy metal ions from an aqueous solution obtained from metal finishing and plating operations, the steps comprise combining a hydroxide of at least one of the elements given in claim 1 with the metal ions of said solution at a pH of from 8 to 12 with said element of the hydroxide being present in said solution in a concentration of at least 1700 ppm and the hydroxide being present in said solution in a ratio of at least 1:1 with respect to the heavy metal ions, to form bound material, and filtering the combined solution through an ultrafiltration skinned membrane at a flux at least as high as 30 gf2d under pressure at least as high as 9 psig to separate bound material from the solution.

The wastewater treated by the process according to the present invention is preferably wastewater from a commercial metal finishing operation, such as metal plating and the combining agent is one of the above-mentioned elements in its oxide and/or hydroxide form and preferably an oxide or hydroxide or iron and/or aluminium.

The combining agent is preferably used in a concentration of at least 1700 parts per million in the water to be filtered and more preferably of at least 2000 ppm.

The filtering step is preferably carried out through an ultrafiltration anisotropic or skinned membrane which permits high filtering rates with small membrane area and relatively low pressure.

The method can be carried out in a batch operation or in a continuous operation which continuously produces filtered, usable water and permits the drawing off of the concentrate resulting from the filtration to an area where it can be treated for discharge to the environment or permits the recovery of the metal ions.

The method of the present invention can be carried out in an economical manner and permits reuse or discharge of the treated wastewater. The heavy metals and other contaminant materials removed can also be concentrated so that recovery or disposal of these metals is possible.

The present invention comprises using a combining agent to increase the size of the materials to be removed so that various filtration methods can be practically employed to recover or discharge cleansed water. The combining agent method, when used with filtration devices and preferably ultrafiltration, has significant advantages in permitting high fluxes with extremely good separation of pollutants, such as heavy metal ions, to reduce such pollutants to concentrations well below concentrations which could ever be obtained by precipitating materials previously used. In fact, the pollutants are reduced to levels well below the water solubility limits of the materials removed. When a continuous system is used, large amounts of water can be re-used in a closed continuous cycle. When such closed continuous cycle is used, a conventional polishing step can be incorporated in the cycle. The disposal volume of wastewater is reduced to an absolute minimum. The concentration of the metal ions and/or other contaminant material remaining after filtration is raised sufficiently to recover or destroy the materials economically. In the case of hexavalent chromium, a one-step mechanical process combines the hexavalent chromium directly with the combining agent used according to the present invention. This avoids the need to reduce chromium from Cr6 to Cr3 as is customarily done in a separate step in previously used methods.

Surprisingly, we have now found that, by the use of a high concentration of the hydroxide combining agent, ultrafiltration can be used to achieve a product water flow through the filter which is higher than that achieved where a low concentration of hydroxide is used. It is thought that the excess of hydroxide permits the formation of a substantially crystalline precipitate rather than a more gelatinous one which often causes filter clogging. A steady product water flow can be obtained over long time periods when using a large excess of combining agent in accordance with the present invention.

The advantages and features of the present invention will be better understood from the following description, with reference to the accompanying drawing, which illustrates a preferred closed loop wastewater retreatment method of the present invention.

In the metal finishing and plating industry, rinse waters are used during processing which waters often cannot be re-used unless treated to remove contaminating heavy metal ions and certain other contaminant or pollutant ions which are picked up by the water in the procedure. Common heavy metal ions and other anions and cations picked up in such wastewater include the following, tentatively permissible discharge limits for the concentrations of such ions which may be discharged to the environment being listed below: Effluent parameter mg/litre phosphorus 0.6 aluminium 0.2 antimony 0.3 arsenic 0.05 barium 1.0 beryllium 0.3 boron 0.3 cadmium 0.1 chromium Cr +6 0.05 Cr +3 0.2 Cr Total 0.25 cobalt 0.3 copper 0.2 iron 0.5 lead 0.05 manganese 1.0 nickel 1.0 silver 0.05 selenium 0.3 tin 0.3 zinc 0.5 gold 0.3 ammonia 1.0 mercury 0.2 cyanide Dest. by Cm2 1) 0.03 Refract.2' 0.5 Total 0.53 fluoride 2.0 1) cyanide determined by destructive distillation with chlorine 2) cyanide determined by refractive index measurement.

The term "heavy metal ions" used herein includes all of the metals listed above in their pure ionic form and/or in bound form, as in chromates. The term "contaminants" used herein includes all the non-metallic materials listed above. In all cases, the heavy metal ions and contaminants referred to herein include all unwanted materials in wastewater which can be removed by the combining agents of the present invention.

Separated rinse waters can be obtained from a plating tank rinse and treated in accordance with the process of this invention. For example, in gold plating, the rinse water used can be separated and treated in accordance with the method of the present invention to enable removal of gold ions from the wastewater. In this case, the concentrate remaining has a high concentration of gold ions without a large variety of other contaminant materials, thus making the separation of the gold ions from the concentrate readily achievable by conventional methods. Alternatively, rinse water from a variety of metal finishing steps can be combined to produce a wastewater having various combinations of many of the above heavy metal ions and other contaminants. In this case, the clear filtrate obtained can be re-used and it is often most practical to dispose of the heavy metal ions and other contaminants in the concentrate, using conventional methods.

In most cases, the non-metallic ions and materials present in rinse wastewaters do not in any way affect the method of the present invention. In fact, in some cases, non-metallic ions, such as fluorides, chromates and cyanides, can be removed, together with the heavy metal ions, during the process of the present invention. In some cases, these non-metallic ions or other particles do not prevent re-use of the wastewater if the heavy metal ions are first removed. In some cases, the non-metallic contaminants, if present, can combine with the combining agent to form complexes, enabling removal thereof in that form. For example, ammonia can combine with preferred combining agents of the present invention and is thus removed during the filtration step in many cases.

In the method of the present invention, the heavy metal ions and other contaminants to be removed, which may be any one or more of the above-mentioned materials, are treated with a combining agent to increase the particle size of the contaminants, which are normally of small particle size, to a size large enough to permit filtration of the unwanted contaminants from the waste-water. The particle size is increased to a minimum of 10 Angstroms and normally the particle size is increased to somewhere in the range of from 10 to 1,000 Angstoms. Particle size is determined by the fact that the material so increased in size will not pass through a filter having pore sizes below 10 Angstroms or below any specific particle size to which the contaminants are increased.

The particle size of the contaminant materials to be removed from the wastewater is increased by using a combining agent or combination of combining agents, which may be added to the wastewater. The combining agent is preferably in its hydroxide form, although it may be in its oxide form, which has a large zeta potential with high absorption or adsorption properties. Preferred combining agents are formed from metal salts and preferably from iron and aluminium salts. It is believed that these inorganic salts, when added to the wastewater adjusted to a basic pH, form oxides and most probably hydroxides, which act as the combining agents. In some cases, if the heavy metal ions are unionised with non-metals, such as cyanide or fluoride, the cyanide and fluoride can be removed by filtration, together with the heavy metal ions. Other inorganic oxides and hydroxides which create agglomerates large enough to be filtered and which have high absorption and/or adsorption properties to heavy metal ions and other contaminants of wastewater, due to electrical potential on their surface, zeta potential, hydrogen bonds, van der Waals bonds and other forces, can also be used and include oxides and hydroxides of tin, copper, zinc, cadmium, nickel, cobalt, silicon, lead, barium, calcium, manganese and chromium.

When hydroxides or oxides are used as the combining agent, while they can be added directly to the wastewater to be treated, it is preferred to form them in the wastewater as by the addition of inexpensive salts. For example, chlorides, fluorides, sulphates and nitrates of one or more of the above elements can be added to the wastewater. Sulphates, such as ferrous sulphate, ferric sulphate and aluminium sulphate, are preferred due to their ready availability and low cost.

The pH of the wastewater is adjusted to a range of from 7.1 to pH 14 and preferably of from 8 to pH 12, to allow formation of the hydroxides. It is preferred to make the pH adjustment to a level as close to neutral as is permissible for the efficient separation of the contaminants.

When high pH values are used, the filtered wastewater may have to be treated with acidic materials to lower the pH for discharge to the environment, without causing basic contamination. In general, the higher is the pH, the better is the separation of unwanted contaminants, although excelllent separation and filtration can be obtained at pH values which are not above 9. The pH value can be adjusted in the wastewater treated by the addition of inorganic hydroxides as is known in the art. For example, sodium hydroxide, potassium hydroxide and calcium hydroxide can be used, calcium hydroxide being preferred, due to its ready availability and low cost.

It is preferred to employ a large excess of the combining agents since it itself can be easily filtered and a large excess will assure the binding of the maximum amount of heavy metal ions and other contaminants present in the wastewater. Therefore, molar ratios of at least 1:1 and preferably in the range of from 1:1 to 10:1 moles of combining agent to moles of metal salt or other contaminant to be removed are used. Thus, when inorganic salts are used to form the combining agents in the wastwater, they are added in molar ratios at least 1:1 and preferably higher. In most cases, molar ratios of 1:1 are sufficient to permit the removal of the heavy metal ions and contaminants, molar ratios above 10:1 being unnecessary, although no detrimental effects are encountered by higher molar ratios.

In addition to using the molar ratios indicated above, it is necessary to have a minimum amount of metal hydroxide present in order to ensure efficient ultrafiltration without blocking the filter. Crystalline precipitates are preferred to avoid rapid clogging of the filter membrane, as often occurs in a short time when gelatinous precipitates are used. Therefore, at least 1700 ppm and preferably at least 2000 ppm of the combining agent in the aqueous solution to be filtered is used. There is no upper limit to the concentration of the combining agent used except that the solution or suspension must remain capable of being pumped in ordinary commercial equipment.

The combining step can be carried out over a wide range of temperatures with a wide range of time periods being possible. In most cases, combining and binding of the heavy metal ions and other contaminants occur substantially instantaneously at ambient temperature, although the temperature range can be varied greatly, for example from 5 to 95"C., as may be encountered in various commercial operations. The time required for the combining step can vary from a few seconds in certain continuous recycling operations to several hours in batch operations, although periods of from 30 seconds to 15 minutes are normally sufficient.

After the combining step with the resulting formation of large particle size materials, which can be soluble and/or insoluble, carried in the wastewater, filtration is used to complete the separation and remove the heavy metal ions from the wastewater. For example filter paper can be used to remove insoluble, large particle size materials formed, while reverse osmosis can also be used to remove soluble or insoluble large particle size materials. However, according to the preferred embodiment of the present invention, it is preferred to use ultrafiltration for the separation step.

Ultrafiltration is a process in which particles of small size are retained by a filter medium, while solvent, together with certain accompanying low molecular weight solutes, are allowed to pass through. In a preferred embodiment, ultrafiltration using an anisotropic or skinned plastic membrane is found to give the best results. Such ultrafiltration can be carried out at low pressures of from 9 to 1000 psig or higher and preferably 10 to 150 psig. High flux rates are obtained even at low pressures, flux rates of from 30 to 1,000 gfd or higher (gallons, foot2 of membrane per day) preferably being used. As is known, higher pressures result in higher flux rates and high flux rates permit the use of a smaller membrane surface area. Because of the low pressures and high flux rates obtained with anisotropic or skinned membranes, practical separation of the bound heavy metal ions and other contaminants can be obtained in commercial production. The porosity of the skinned membrane is designed to prevent passage of the enlarged particle size heavy metal ions and other contaminants in their bound form. In ultrafiltration, the filtration process is carried out on the top surface of the skin, which reduces clogging of the filter and thus allowing operation of the filter over long periods of time.

The Figure in the accompanying drawing is a schematic illustration of a system 10 for the continuous recycling of cleansed wastewater, using the method of the present invention in a metal plating procedure. The system comprises a rinse tank 12, which may contain one or more of the metal ions of other contaminants picked up by water used for rinsing work pieces.

For example, fresh water going into the rinse tank may be used to wash a plated object so that contaminants are picked up by the clean water, which is converted to wastewater having a high concentration of unwanted heavy metal ions and/or other contaminants. Appropriate conduiting and pump or gravity feed means connect the rinse tank to a reacting tank 13, which is, in turn, connected via pump means 15 to an ultrafiltration unit 14. The concentrate from the ultrafiltration unit can be removed through conduit 23 to a reacting tank 17, where conventional procedures can be used to remove the contaminants from the concentrate, enabling reclamation of the highly concentrated materials present. Alternatively, conduiting 18 can be used to pass the concentrate through line 19 back to the reacting tank 13.

Conduiting 22 enables reagents or make-up water to be added if water is removed to tank 17.

Conduit 20 receives the cleansed water filtrate, which can be returned for re-use in the rinse tank 12 via conduit 21 or can be discharged to the environment, if desired.

In the described system 10, continuous recycling can be carried out as follows. Wastewater is obtained from rinse tank 12 and passed to the reaction tank at, for example, a rate of 10 gpm (gallons per minute). The reaction tank has a reservoir volume of approximately 800 gallons. The wastewater contains high concentrations, such as 100 ppm, of contaminant heavy metal ions. The reagent used to form the combining agent, together with a pH adjustment means, for example ferrous sulphate and calcium hydroxide, are added to the reacting tank, the ferrous sulphate being in large excess at an initial concentration in tank 13 of 5% and remaining at at least a molar ratio of 1:1 to the metal ions and other contaminants present. The pH is adjusted to 9 and monitored, calcium hydroxide being added, when necessary, to maintain the pH at 9. The reacting tank preferably contains a mixing device for ensuring thorough mixing of the reactants with the wastewater, although the continuous flow is often sufficient to ensure uniform mixing. The contaminants and heavy metal ions react with the combining agent in the reacting tank and may be passed through pump 15 to the ultrafiltration unit 14 at a rate of 700 gpm pressurised to about 50 psig. Cleansed water passes through the skinned membrane of the ultrafiltation unit at a rate of 10 gpm and is returned to the rinse tank 12. The concentrate is returned to the reacting tank 13 at a rate of 690 gpm through lines 18 and 19, permitting re-use of the reagent. If, for example, the rinsewater contains 100 ppm of heavy metals, after 660 hours of continuous operation, 400,000 gallons of water will have been treated. At that time, the strength of the reject concentrate flowing through line 18 will be 5 % and it can be removed through line 23 and replaced by fresh water and reagent. The treatment of 400,000 gallons of rinsewater generates only 700 gallons of concentrate. If a separated rinsewater containing only one or few contaminants has been used, it is relatively simple to obtain and reclaim the contaminant materials. Alternately, conventional methods can be used to treat the reject concentrate in order to render it harmless and suitable for disposal.

Alternatively, the system 10 can be operated continuously by adding make up water, calcium hydroxide and reagent, such as ferrous sulphate, while continuously removing concentrate through conduit 23 during continuous operation of the system. Thus, the operation can be continuous without the need to stop the system when a high concentration of contaminants is reached in tank 13. Since removal through line 23 is only in small amounts, such small amounts of concentrate can be treated for disposal or recovery over a prolonged time period.

The following Examples 1 to 5 and 8 to 10 describe experiments and Examples 6,7 and 11 to 14 are given for the purpose of illustrating the present invention: Example 1 200 mil of 0.01 molar (2980 ppm) aqueous sodium bichromate is mixed with 200 ml. of water in a reaction tank so that the chromate concentration is 1510 ppm. The pH is increased by 9 by adding calcium hydroxide. The solution is filtered through Whatman number 2 filter paper and analysed. The filtrate is found to contain 1510 ppm bichromate. Thus, no metal was removed.

In a modification of this procedure, 200 ml. of 0.01 molar sodium bichromate solution is mixed with 200 ml of aqueous ferrous sulphate solution and the pH adjusted to 9 with calcium hydroxide to form a combining agent, which is believed to be ferrous hydroxide. The ferrous sulphate is added in an amount sufficient to make the total solution 0.0025 molar to ferrous sulphate. The solution is allowed to react at ambient temperature (25 C) for 5 minutes and then passed through Whatman number 2 filter paper. Analysis indicates that the chromium (Cr) in the filtrate has a value of 490 ppm, equivalent to a bichromate value of 1510 ppm.

The above steps are repeated, except that the final concentration of ferrous sulphate reagent in the solution is increased, the following results being obtained at the molar levels indicated: 0.025 M reagent - 55 ppm Cr < 170 ppm bichromate 0.037 M reagent - 0.075 ppm Cr < 0.23 ppm bichromate o.05 M reagent - 0.003 ppm Cr < 0.009 ppm bichromate Thus, at reagent to contaminant molar ratios of 5:1 and above, significant removal of contaminants occurs.

Example 2 0.01 molar aluminium sulphate in water is treated at ambient temperature. The pH is adjusted to 9 with sodium hydroxide and ferrous sulphate is added.

When the ferrous sulphate is added in a molar ratio of 1:5 contaminant aluminium sulphate to ferrous sulphate, on filtration through a Whatman number 2 filter paper, the filtrate water contains more than 0.2 ppm of aluminium. When ferrous sulphate is added in a ratio of 1:10, the concentration of aluminium in the filtrate water is 0.018 ppm.

Example 3 In this Example, reactions are carried out at ambient temperature for periods of 5 minutes, with stirring to mix the materials used uniformly. 100 ml of 0.01 molar aqueous zinc chloride solution is mixed with calcium hydroxide to raise the pH to 11. 100 ml of aqueous ferrous sulphate solution are then admixed therewith in a molar strength of 0.005 and the aqueous solution is filtered through Whatman number 2 filter paper and analysed. The filtrate is found to contain 0.27 ppm of zinc.

When this Example is repeated but omitting the ferrous sulphate or using higher molar concentrations thereof, the following results are obtained: Concentration of Zn in the filtrate no reagent 0.8 ppm 0.01 molar reagent 0.37 ppm 0.05 molar reagent 0.12 ppm 0.1 molar r Example 4 In a series of tests, all at ambient temperature, a 50 ml of aqueous chromium sulphate solution containing 1000 ppm chromium is admixed with varying amounts of aqueous ferrous sulphate solution, water is added to maintain a similar volume for all tests and the resultant solution is rendered alkaline with aqueous sodium hydroxide solution. In each test, the solution is then filtered through Whatman number 2 filter paper and the chromium concentration determined. The following results are obtained: ml. ferrous sulphate ml. water chromium concentration in 10,000 ppm added added filtrate (ppm) at pH 8 9 11 35 15 0.03 0.03 > 0.5 32.5 17.5 0.13 1.8 3.2 30 20 0.29 2.8 5 25 25 4 38 5.2 Example 5 Example 4 is repeated, except that 50 ml aqueous aluminium sulphate solution with a concentration of 1000 ppm is used instead of chromium sulphate, the results in the filtrate being indicated below: ml. ferrous sulphate ml water aluminium concentration 10,000 ppm added added in filtrate (ppm) at pH 9 11 35 15 0.005 0 32.5 17.5 0.022 0.029 30 20 0.018 0.069 25 25 0.035 0.075 0 50 70.2 Example 6 In this Example, use is made of the continuous system 10 illustrated in the accompanying drawing. The ultrafiltration unit used is an Abcor commercial unit HFA 300 (FEG tubes) manufactured by Abcor Corporation of Cambridge, Massachusetts, U.S.A. The unit contains 4 ultrafiltration tubular membranes having a diameter of 1 inch and a length of 5 feet. The feed to the reaction tank contains the heavy metal ion contaminants listed below. The pH is adjusted via conduits 22 and 19 to a pH of 9 by addition of sodium hydroxide and ferrous sulphate is added in large excess (about 25.00 ppm). The solution is passed through the ultrafiltration tubes at a pressure of 50 psi and the pure water is collected and analysed, the results obtained being given below: Analysis of the "Pure Water" Feed in ppm product "pure water" in ppm aluminium 350 0.02 iron 1000 0.01 copper 1500 0.025 zinc 30 0.08 chromium 70 0.01 In this Example, the flow rate of pure water from line 20 is 700 ml. per minute, which is equivalent to 35 ml. per minute per foot of ultrafiltration tube length. The pure water filtrate is drawn off and analysed directly and the concentrate is passed directly to tank 17. Thus, the system was not operated on a continuous recycling basis, only a single pass of the wastewater being made through the ultrafiltration unit.

Example 7 In this Example, using the system 10 of the accompanying drawing, line 21 is connected to a series of metal plating baths where the returning pure water is used as rinsewater. A manifold system collects the rinsewater from each of the baths and brings it to tank 12 to make the system a closed loop. In this Example, the Abcor unit of Example 6 is again used as the ultrafiltration unit and flow rates are established at 10 gallons per hour from tank 12 to tank 13 and 60 gallons per minute at a pressure of 50 psi from tank 13 to the ultrafiltration unit. 10 gallons per hour of the filtrate are collected in line 20 and continuously passed through line 21 to the metal plating rinse baths. 3590 gallons per hour of concentrate are passed through lines 18 and 19 back to the reaction tank 13. Tank 13 is a 250 gallon tank containing 200 gallons of the starting rinsewater. At the start of the system, calcium hydroxide and ferrous sulphate are added through line 19 in an amount to bring the volume of wastewater in tank 13 to a pH of 9 and a value of 8400 ppm ferrous sulphate.

The system is operated continuously at ambient temperature for a period of 8 hours, after which the system Is shut down and bled through line 23 to obtain a concentrate in tank 17 having a concentration of 3% contaminants, as opposed to a starting total concentration of metal salts of 0.5%. When the system is run for 16 hours before shutdoWn, the concentration of the concentrate bled off in tank 17 is 6%.

The table below indicates the makeup of the starting rinsewaters combined with the ferrous sulphate reagent fed into the tank 13 and the concentration of contaminants in the filtrate water taken off from the ultrafiltration unit at line 20: Tank solution: Na2Cr207 275 ppm NiSO4 275 ppm Al2(SO4)3 100 ppm CuSO4 275 ppm ZnCl2 275 ppm FeSO4 8400 ppm Analysis of output: pH = 8.7 Na2Cr207 0.002 ppm 85 gallons NiSO4 0.03 ppm Al2(SO4)3 0.01 ppm CuSO4 0.04 ppm ZnCl2 0.06 ppm FeSO4 0.07 ppm Example 8 In a series of tests, 100 ml. of aqueous ferrous sulphate solution are admixed with 100 ml of aqueous sodium bichromate (concentration of 2 ppm) solution. Concentrated sodium hydroxide is added to adjust the pH to 9. The combined solution is then filtered through Whatman number 2 filter paper. The analysis of the filtrate obtained at the ppm of the particular ferrous sulphate solution used in each batch is indicated below: 200 ppm FeSO4 - 0.002 ppm Na2Cr2O7 20 ppm FeSO4 - 0.36 ppmNa2Cr2O7 10 ppm FeSO4 - 1 ppm Na2Cr2O7 1 ppm FeSO4 - 1 ppm Na2Cr2O7 Example 9 Example 8 is repeated using 100 ml. of aqueous copper sulphate solution (concentration of 2 ppm) in place of the sodium bichromate of Example 8. Similar results were obtained in that the copper is effectively removed at concentrations above 10 ppm concentration of the ferrous sulphate used.

Example IO 100 ml of 0.1 M aqueous manganese sulphate solution are admixed with 100 ml. of 0.01 M aqueous sodium bichromate solution at ambient temperature. 2 ml of sodium hydroxide are added to adjust the pH to 9. The total solution is filtered through a Whatman number 2 filter paper. The filtrate, on analysis, is found to contain 0.008 ppm sodium bichromate.

Example 11 In this Example, system 10 of the accompanying drawing is used. However, the rinse tank is connected by a single pipe to the reacting tank 13, lines 18 and 19 carry the concentrate back to the reacting tank 13 and clear filtered water is drawn off through line 20 for discharge to the environment. In tank 12, there is placed a 15 gallon sample of a rinse water which contains the following heavy metal ions: Fe 40 ppm Ni 27 ppm Zn 0.6 ppm Cr 108 ppm 110 grams of ferrous sulphate are added. After 20 minutes, the pH is adjusted to 9.2, using 56 ml of 50%sodium hydroxide. The mixture is then transferred to reacting tank 13 and pumped through the filtration unit 14 at a pressure of 10 to 15 psi. The analysis of the effluent is as follows: Fe 0.035 ppm Zn 0.04 ppm Cr 0.01 ppm Ni negligible The product water flow through the ultrafiltration membrane is 1000 ml per minute per 0.8 ft.2 of membrane area, equal to 500 gf2d. When the water level in the tank 13 is low, an additional 15 gallons of rinse sample, containing 12.2 ppm aluminium, is added. The product water in line 20 is found to contain 0.04 ppm aluminium.

Example 12 The system described in Example 11 is used. 90 gallons of combined rinse water containing; Fe 1300 ppm Ni 27 ppm Cu 300 ppm Al 300ppm Cr 300 ppm Zn 38 ppm are added to the rinse tank.

The rinse water pH is adjusted to 9.3 by the addition of 50% sodium hydroxide solution.

This solution is transferred at a rate of 1000 ml. per minute to a 25 gallon recirculating tank 13 which contains 15 gallons of an aqueous solution containing 2000 ppm of iron in the form of iron hydroxide, with a pH of 8.7 to 9.2. The solution is recirculated through a 3 foot long, 1 inch diameter ultrafiltration membrane tube in the filtration unit 14 at a rate of 23 gpm. The product water flow of 1000 ml. per minute is collected. After most of the 90 gallons has passed through the filter, with recirculation through lines 18 and 19, the amount of sludge in the recirculating tank is 10 gallons and the product water is analysed with the following results: Fe 0.02 ppm Ni negligible Cu 0.06 ppm Al 0.018ppm Cr 0.008ppm Zn 0.05 ppm It should be noted that, in this case, while the initial rinse waters or wastewaters contained less than 3000 ppm of contaminants and heavy ions to be removed, the reacting tank has a build up of levels well above that amount as water is removed and the concentrate recirculated to the reacting tank 13 through lines 18 and 19.

Example 13 In a recirculating tank containing 100 ppm ferrous ions and 60 ppm zinc ions in aqueous solution, sodium hydroxide is added to adjust the pH to 9. The solution is pumped at a rate of 1000 ml. per minute at 13 psi through filtration unit 14. It is found that the flow rate drops to 280 ml./minute after 1400 minutes.

Using the same system, except that the concentration of the ferrous ions is adjusted to 2000 ppm, with the product water flow started at 1260 ml./minute, the flow is 1400 ml. per minute after 1300 minutes. In both cases, the analysis of the product water issuing from line 20 is: 0.02 ppm Fe 0.06 ppm Zn The correlation between original ferrous ion concentration and product water flow is given below under the conditions of the above Example, in a series of different ferrous ion concentrations: Ferrous ion Product water flow ml/min concentration after 24 hours of passage through ultrafilter 100 ppm 280 1000 ppm 350 1250 ppm 400 1500 ppm 500 1750 ppm 670 2000 ppm 1400 It should be noted that below 1500 ppm of iron combining agent, product water flow drops of substantially in a 24 hour period to a level not acceptable for large scale commercial operations. On the other hand, at 2000 ppm and above, after 24 hours, product water flow is at an economically efficient and practical level. The limiting feature seems to be above 1700 ppm of combining agent to obtain a product water flow per minute of at least 600 ml/minute after 24 hours of use.

Example 14 Using the system described in Example 11, 2000 ppm of chromium in the form of chromium hydroxide is present in the reacting tank 13. A pressure of 15 psi is used to create a water flow from the tank 13 to the filtration unit 14 at a rate of 120 ml. per minute. When the concentration of the chromium is raised to 5000 ppm in the reacting tank 13 because of the return of the concentrate through lines 18 and 19, the flow through the filter is found to be 880 ml. per minute, which leveled off after 72 hours of operation to 600 ml. per minute. The product water contains no measurable amount of chromium ions. When contaminant iron ions are added to the tank 13 at a concentration of 200 ppm, the product water at line 20 is found to contain 0.04 ppm iron after a single pass through the ultrafiltration membrane.

While specific Examples of the present invention have been described above, such Examples are not to be considered as limiting the present invention. Thus, other filtration units can be used. In fact, other filtration methods, including reverse osmosis, may have application in certain embodiments, so long as the particle size to which the contaminant ions are raised is sufficiently large to prevent passage through the filter. However, problems such as clogging and low flux rates can be encountered, depending upon the particle size involved and the particular filtration system used. Ultrafiltration through skinned membranes gives significantly improved efficiency when rapid filtering times and continuous systems are desired.

While metal finishing, such as plating and etching wastewaters have been discussed, other wastewaters or any contaminated water can be treated in accordance with the method of the present invention. For example, metal contaminants in polluted waterways can be treated by the method of the present invention.

The combining agents formed may be insoluble or soluble in the wastewater being treated.

Similarly, the product formed by reaction with the heavy metal ions and other contaminants to be removed may be soluble and/or insoluble. It is an important feature of the present invention that the combining agents used can act to remove heavy metal ions and other contaminants present in concentrations below the maximum solubility of such materials.

Thus, the present invention enables residual metal concentrations in wastewater to be reduced to below the solubility of corresponding metal hydroxides in water at a corresponding pH.

When inorganic salts are used as reagents, they can be added directly to the wastewater in solid form but are preferably added in the form of aqueous solutions. The reagents can be added before, after or simultaneously with pH adjustment of the wastewater. In some cases, no pH adjustment is necessary, for example when the wastewater to be treated has the preferred pH value.

Ferrous sulphate is highly effective as the additive reagent for forming the combining agents for removing metal ions from wastewaters containing one or more of the contaminants listed below at the pH values given: aluminium sulphate (pH 9) zinc chloride (pH 11) copper sulphate (pH 9) chrome sulphate (pH 9) sodium bichromate (pH 9) nickel sulphate (pH 9) Ferric sulphate is highly effective as the additive reagent for forming the combining agents for removing metal ions from wastewaters containing one or more of the contaminents listed below at the pH values given: nickel sulphate (pH 9) nickel chloride (pH 9) zinc chloride (pH 11) Aluminium sulphate is effective as the additive reagent for forming the combining agents for removing metal ions from wastewaters containing one or more of the contaminants listed below at the pH values given: nickel sulphate (pH 9) nickel chloride (pH 9) chromium sulphate (pH 9) Manganese sulphate is effective as the additive reagent for forming the combining agent for removing bichromate from wastewaters.

However, all of the reactants and combining agents of the present invention, when used in amounts above the minimum molar ratio of 1:1 can act to remove one or more heavy metal ions and/or other contaminants, for example those mentioned hereinbefore, to obtain concentrations of the heavy metal ions and/or other contaminants in the filtrate at levels which can be at or below the concentration levels mentioned hereinbefore as tentative discharge limits. In most cases, the concentration of the heavy metal ions or other contaminants to be removed from water prior to treatment by the method of the present invention is 1 ppm or higher. At concentrations in wastewater of below 1 ppm of contaminant, it is sometimes necessary to increase the molar ratio above the 1:1 value in order to ensure highly efficient removal. Removal can be effective in highly concentrated wastewaters up to and beyond the solubility limits of the metal ions or other contaminants to be removed. Wastewater feeds from most conventional metal finishing and plating operations contain no more than about 4000 ppm and often less than 2000 ppm of such ions and contaminants. Of course, after operation of the system of the present invention as in Example 7 or in other cases where the concentrate from the ultrafilter is recycled to the reacting tank, the concentration of the heavy metal ions and contaminants in the reacting tank just prior to ultrafiltration is extremely high and can easily rise well above 4000 ppm over a normal time period of operating the system. In some cases, the metal ions to be removed are present in the wastewater and dissolved therein, although, in other cases, for example where the pH is basic, the metal ions may be partially in solution and partially in suspension. The combining agents used according to the present invention permit the removal of metal ions which may be in both suspended and dissolved form. Partial or complete removal can be made, depending upon the particular conditions used, for example reagents, molar ratios, pH, filtration and the like. In some cases, the filtered water from line 20 is polished as by passage through a deioniser to remove alkaline salts before re-use.

WHAT WE CLAIM IS: 1. A method of removing heavy metal in the form of ions and small particle size material and contaminants from wastewater feed containing no more than about 4000 ppm of said heavy metal ions and contaminants, said method comprising combining said heavy metal ions and small particle size material with a combining agent at a pH of from 7.1 to 14 to form bound material having a particle size of at least 10 Angstroms, said combining agent being a hydroxide of an element selected from iron, aluminium, tin, copper, zinc, cadmium, nickel, cobalt, silicon, lead, barium, calcium, manganese and chromium and said combining agent being present in a molar ratio of from 1:1 and above with respect to moles of heavy metal ions and the small particle size material but in an amount permitting pumping of said wastewater, and subsequently filtering the wastewater to separate the bound material from the wastewater.

2. A method according to claim 1, wherein the combining agent is present in an amount of at least 1700 ppm in the wastewater.

3. A method according to claim 1 or 2, wherein the wastewater is filtered through an ultrafiltration skinned membrane at a flux of at least 30 gfd under a pressure of at least 9 psig.

4. A method according to any of the preceding claims, wherein the pH is basic, the wastewater, after filtration, contains an amount of heavy metal ions and small particle size material below the water solubility limits of the heavy metal ions and small particle size material, the small particle size material consists essentially of heavy metal from wastewater feeds containing no more than about 4000 ppm of the heavy metal, and the ultrafiltration pressure is from 9 to 150 psig.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. for removing metal ions from wastewaters containing one or more of the contaminents listed below at the pH values given: nickel sulphate (pH 9) nickel chloride (pH 9) zinc chloride (pH 11) Aluminium sulphate is effective as the additive reagent for forming the combining agents for removing metal ions from wastewaters containing one or more of the contaminants listed below at the pH values given: nickel sulphate (pH 9) nickel chloride (pH 9) chromium sulphate (pH 9) Manganese sulphate is effective as the additive reagent for forming the combining agent for removing bichromate from wastewaters. However, all of the reactants and combining agents of the present invention, when used in amounts above the minimum molar ratio of 1:1 can act to remove one or more heavy metal ions and/or other contaminants, for example those mentioned hereinbefore, to obtain concentrations of the heavy metal ions and/or other contaminants in the filtrate at levels which can be at or below the concentration levels mentioned hereinbefore as tentative discharge limits. In most cases, the concentration of the heavy metal ions or other contaminants to be removed from water prior to treatment by the method of the present invention is 1 ppm or higher. At concentrations in wastewater of below 1 ppm of contaminant, it is sometimes necessary to increase the molar ratio above the 1:1 value in order to ensure highly efficient removal. Removal can be effective in highly concentrated wastewaters up to and beyond the solubility limits of the metal ions or other contaminants to be removed. Wastewater feeds from most conventional metal finishing and plating operations contain no more than about 4000 ppm and often less than 2000 ppm of such ions and contaminants. Of course, after operation of the system of the present invention as in Example 7 or in other cases where the concentrate from the ultrafilter is recycled to the reacting tank, the concentration of the heavy metal ions and contaminants in the reacting tank just prior to ultrafiltration is extremely high and can easily rise well above 4000 ppm over a normal time period of operating the system. In some cases, the metal ions to be removed are present in the wastewater and dissolved therein, although, in other cases, for example where the pH is basic, the metal ions may be partially in solution and partially in suspension. The combining agents used according to the present invention permit the removal of metal ions which may be in both suspended and dissolved form. Partial or complete removal can be made, depending upon the particular conditions used, for example reagents, molar ratios, pH, filtration and the like. In some cases, the filtered water from line 20 is polished as by passage through a deioniser to remove alkaline salts before re-use. WHAT WE CLAIM IS:

1. A method of removing heavy metal in the form of ions and small particle size material and contaminants from wastewater feed containing no more than about 4000 ppm of said heavy metal ions and contaminants, said method comprising combining said heavy metal ions and small particle size material with a combining agent at a pH of from 7.1 to 14 to form bound material having a particle size of at least 10 Angstroms, said combining agent being a hydroxide of an element selected from iron, aluminium, tin, copper, zinc, cadmium, nickel, cobalt, silicon, lead, barium, calcium, manganese and chromium and said combining agent being present in a molar ratio of from 1:1 and above with respect to moles of heavy metal ions and the small particle size material but in an amount permitting pumping of said wastewater, and subsequently filtering the wastewater to separate the bound material from the wastewater.

5. A method according to any of the preceding claims, wherein the pH is from 8 to 12.

6. A method according to any of the preceding claims, wherein the small particle size material consists essentially of contaminant heavy metal resulting from a metal finishing procedure and further comprising carrying out the combining in a reacting tank and filtering the wastewater in an ultrafiltration unit, obtaining a concentrate from the ultrafiltration unit and recycling this concentrate to the reacting tank and obtaining the filtrate from the filtration unit and recycling the filtrate to the metal finishing procedure and passing wastewater from this procedure to the reacting tank, all the steps being carried out continuously in a closed loop.

7. A method according to any of the preceding claims, wherein the heavy metal ions and small particle size material comprise a mixture of at least two different heavy metals.

8. A method according to claim 7, wherein the heavy metal ions and small particle size material are originally present in the wastewater in an amount of more than 1 ppm, the small particle size material consisting essentially of contaminate heavy metal resulting from a metal finishing procedure.

9. A method according to any of the preceding claims, wherein the combining agent is formed by adding a large excess of ferrous sulphate to the wastewater.

10. A method according to any of claims 1 to 5 which further comprises carrying out the combining in a reacting tank and filtering the wastewater in an ultrafiltration unit, obtaining a concentrate from the unit, obtaining a filtrate from the filtration unit and recycling the filtrate to a metal finishing operation and passing wastewater from this operation to the reacting tank.

11. A method according to any of the preceding claims, wherein the combining agent is iron hydroxide or aluminium hydroxide.

12. A method according to any of the preceding claims, wherein the wastewater feed is concentrated so that the combining agent is combined with heavy metal ions in the wastewater feed when the concentration of the heavy metal ions is above 2000 ppm.

13. A method according to any of claims 2 to 12, wherein the combining agent is present in an amount of at least 2000 ppm in the wastewater.

14. A method according to any of claims 1 to 5, wherein a reaction tank is used to contain the combining agent and wastewater feed which form a reaction mixture, the reaction mixture being passed to an ultrafiltration skinned membrane with the filtrate collected and the concentrate recycled continuously to the reaction tank in a continuous method.

15. A method according to any of claims 1 to 5, wherein the combining is carried out in a first tank to form an aqueous reaction mixture, the reaction mixture is transferred to a second tank, the wastewater is filtered in an ultrafiltration unit and concentrate obtained from the ultrafiltration unit is recycled to a second tank, filtrate being continuously obtained from the filtration unit.

16. A method according to any of the preceding claims, wherein the combining agent is formed by adding ions of at least one of the elements given in claim 1.

17. In a method of removing heavy metal ions from an aqueous solution obtained from metal finishing and plating operations, the steps comprising combining a hydroxide of at least one of the elements given in claim 1 with the metal ions of said solution at a pH of from 8 to 12 with said element of the hydroxide being present in said solution in a concentration of at least 1700 ppm and the hydroxide being present in said solution in a ratio of at least 1:1 with respect to the heavy metal ions, to form bound material, and filtering the combined solution through an ultrafiltration skinned membrane at a flux at least as high as 30 gf2d under pressure at least as high as 9 psig to separate bound material from the solution.

18. A method according to claim 17, wherein the hydroxide is formed by adding a large excess of ferrous sulphate to the aqueous solution.

19. A method according to claim 1 for treating waste water, substantially as hereinbefore described and exemplified.