WO1982000817A1 - Wastewater treatment process - Google Patents

Abstract

Wastewater treatment process in which organic impurities such as oil, organic brighteners, and chlorinated solvents are removed from the upper surface of wastewater from an industrial process and metal values are precipitated and withdrawn from the wastewater. The remaining clear liquid containing dissolved salts is used to generate working steam in a boiler for an industrial purpose. The wastewater is heated in the boiler to produce an aqueous phase having an increased concentration of impurities and a steam component which is used for the industrial purpose and condensed to produce water which is recycled for use in the industrial process and/or boiler. The accumulated impurity component is removed from the boiler periodically or continuously.

Description

WASTEWATERTREATMENTPROCESS

Background of the Invention

There has been a growing concern on the part of naturalists, industrialists, federal, state, and local legislative bodies, and the general population about controlling industrial pollution of the environment. Pollution of air and surface waters by direct emission of wastes from industrial plants into the air or into lakes and streams has been of concern for many years. More recently, pollution of underground waters as well as of air and surface waters as a result of storing solid and liquid wastes at land disposal sites has been recognized as a serious problem. Federal and state statutes, municipal ordinances, and regulations issued by agencies such as the EPA increasing the stringency of pollution standards are almost continuously being enacted and promulgated. While the ideal standard from the standpoint of protection of the environment is zero discharge of pollutants, for many areas of technology, it is widely believed at present that this standard is not practical, and standards have been accordingly established at levels which are considered feasible.

The present methods of treating industrial wastewater effluents include ion exchange, reverse osmosis, evaporation, filtration, and chemical destruction of contaminants. The use of these processes, which are discussed in more detail below, has been unsuccessful in economically reaching the ultimate goal of zero discharge of liquid effluent.

In ion exchange processes the effluent is passed through a bed of solid ion exchange resins. A reversible chemical reaction takes place between the ion exchange resins and the effluent by means of which the harmful ions contained in the effluent are interchanged with non-polluting ions from the ion exchange resins. The purified effluent can then be discharged or returned to the process which generates the effluent . In time, the ion exchange resins become contaminated and must be decontaminated and regenerated by backwashing. In the process of backwashing the ion exchange resins, wastewater is generated which is more highly contaminated than the original wastewater and which must be disposed of by some method. Also, the method is expensive and the ion exchange resins have to be replaced periodically.

Reverse osmosis is effective in some cases but is limited in the types of applications to which it may be applied because calcium salts deposit in the semi-permeable membranes and most industrial processes include a lime treatment which introduces additional calcium to further foul the membranes. Moreover, chromic acid and high pH cyanide baths attack and destroy the membranes.

In evaporation processes the effluent passes through one or more evaporator units which concentrate it for further handling. An example of one such evaporation process is disclosed in the United States Patent No. 3,973,987 to Hewitt et al. Evaporation processes have the disadvantages that the evaporator units are relatively expensive and use considerable amounts of energy. As energy becomes less abundant and more expensive, it will be even more difficult to justify using this method than it is now. Moreover, if the effluent contains cyanide at relatively high concentrations and sufficiently high hydrogen ion concentrations, carry-over of cyanide into the "purified" water is a problem.

Chemical destruction methods are perhaps the most common, and lend themselves to both continuous and batch type operations and can be used on small or large volumes of effluent. Most toxic contaminants are reduced to an acceptable level but some, such as cadmium, cause problems whereby the present and anticipated pollution control standards cannot be met. Substantially zero discharge can be accomplished for a short period of time by chemical destruct methods by recirculating the treated effluent. However, soluable salts build up in the treated effluent, and consequently the treated effluent can be recycled only relatively few times. At some point, it is necessary to dump the recirculated effluent in which soluble contaminants have built up to a high concentration.

Filtration methods have long been used to separate contaminants from industrial wastewater, but dissolved contaminants must first be precipitated to remove them. Use of chemical precipitants can introduce additional contaminants into the "purified" water. Consequently, this method is not only limited to precipitable contaminants, but can also be slow, costly, and/or partially self-defeating.

Summary of the Invention

It is accordingly one object of this invention to provide a process and system for economically removing impurities from industrial wastewater.

It is another object to provide an economically feasible process for substantially zero discharge of contaminants in liquid effluent.

It is another object to minimize the energy cost of removing impurities from industrial wastewater.

It is another object to provide a process and system for recycling effluent within an industrial process for extended periods of time.

It is another object to provide a process for removing organic impurities from wastewater.

It is still another object to provide a process and system for separating organic compounds from heavy metal salts and for recovering heavy metal salts in a form suitable for recycle to the metal users or producers.

It is yet another object to provide a process and system for removing impurities from industrial wastewater which results in minimizing the use of make-up water for the industrial process. It is another object to provide a process and system for economically achieving zero discharge of liquid effluent in treating industrial wastewater using equipment which is not only readily available commercially, but, to a large extent, will already be present in systems which are operating.

In accordance with this invention, there is provided a process for purifying wastewater from an industrial process comprising introducing said wastewater into a steam boiler, heating said wastewater within said boiler to produce a steam component and a liquid component concentrated in impurities, removing steam from said boiler and using it for an industrial purpose such as for heat or a source of energy to provide mechanical motion, and removing from the boiler at least a portion of the liquid component containing a high concentration of dissolved salts and possibly precipitated solids. The condensed steam is recycled to the boiler or for use in the industrial process.

In the preferred method of carrying out this invention suspended solids, and soluble and/or insoluble organics such as oil are removed from the wastewater before it is introduced into the boiler.

The use of a steam boiler to concentrate impurities and concomitantly produce steam for use as an energy source meets the objects set forth above. Unexpectedly, industrial waste water can be used as a feedwater to a steam boiler safely and without harm to the boiler. This flies in the face of conventional wisdom on feedwater quality requirements for a boiler, which teaches that boiler feedwater should be as pure as possible to prevent corrosion and scale formation in the boiler. Scale formation not only reduces the rate of heat transfer and increases the amount of fuel required, but it may cause hot spots resulting in burnout of the heat transfer surface.

The process described herein is very adaptable to existing equipment in most industrial operations, being usable with existing steam boilers in the plant. Very little extra energy is required by the boiler to produce steam from the contaminated industrial wastewater as compared to the usually very pure boiler feedwater. Thus, the process and system are relatively economical and energy efficient, and adds only a few percent to the cost of energy consumed in an industrial plant. Significant economies also result from recycling the treated wastewater to the industrial process which produces the wastewater. Thus, in most cases, the treated water is sufficiently pure to use as all or apart of the make-up water required for the industrial process. In addition, valuable chemicals may be recovered from the aqueous phase containing high concentrations of salts formed in the steam generating unit. The concentrated aqueous phase formed in the steam generating unit which may contain precipitates may be further concentrated by driving off nontoxic constituents so that there will only be a small amount of toxic constituents remaining which can be easily and safely disposed of. Metal salts in the form of substantially pure crystals can be obtained by evaporating water from the concentrated aqueous phase removed from the boiler.

Brief Description of the Drawings

For a better understanding of the invention, reference is made to the following description of a preferred embodiment thereof, taken in conjunction with the figures of the accompanying drawing in which:

Fig. 1 is a schematic diagram of a system in accordance with this invention and which illustrates a process also in accordance therewith;

Fig. 2 is a sectional view of a fire-tube boiler usable with a system in accordance with the present invention;

Fig. 3 is a schematic diagram showing modifications to the system shown in Fig. 1.

Cross-Reference to Related Applications

This application is a continuation-in-part of application Serial No. 65,816 filed August 13, 1979 which was a continuation-in-part of application Serial No. 907,190 filed May 18, 1978.

Detailed Description of the Invention

In carrying out this invention, liquid wastewater from an industrial process is introduced into a steam boiler to produce steam and a liquid component enriched in impurities. The feed to the boiler may be any industrial waste and this invention is not restricted to but is-particularly useful for those industrial wastes containing a high concentration of heavy metal salts. The invention will accordingly be described in detail with particular reference to metal processing, including metal surface finishing, metal plating, pickling and similar processes wherein the wastewater contains high concentrations of heavy metal salts.

Materials which may be present in the wastewater include but are not limited to salts of one or more of the elements including aluminum, cobalt, copper, nickel, cadmium, zinc, chromium, gold, silver, antimony, lead, rhodium, iridium, palladium, molybdenium, iron, tin, arsenic, barium, boron, calcium, lithium, magnesium, manganese, mercury, potassium, sodium and titanium. The anions which may be present include but are not limited to fluoride, chloride, sulfate, nitrate, cyanate and cyanide. Carbon powder may also be present.

The concentration of impurities in the feedwater may be at any level; however, for most efficient operation of an industrial plant, it will range from a level where the impurity concentration is almost too high to be useful in the industrial process to its saturation point under ambient conditions. While a feed containing solids could be introduced into the boiler, in the preferred operating mode solids are separated from the liquid, such as by settling, and the clarified liquid is fed to the boiler. Such a clarified liquid will normally be saturated with at least the component forming the solids at ambient temperatures, e.g. about 10 to about 30ºC. Typically, the wastewater fed to the boiler will contain from about 200 ppm by weight to about 10,000 ppm by weight, and usually about 2,000 ppm by weight of dissolved salts, but amounts of dissolved salts as high as 30,000 ppm and higher may be present. In the preferred method of carrying out this invention the feed to the boiler is wastewater which at least in part has been subjected to repeated precipitation of heavy metals with suitable precipitating agents. Lime and sodium hydroxide are suitable precipitating agents and the selection of one or the other, or even another agent will depend on the specific conditions under which the process is carried out. The use of lime has the advantage that the concentration of dissolved salts will increase at a relatively slow rate in view of the low solubility of calcium salts, especially the carbonate. Lime has the disadvantage that it maintains the concentration of calcium at a level at which some scale may form in the boiler.

On the other hand, the use of sodium hydroxide as a precipitating agent permits the invention to be carried out virtually free of scale-forming materials. However, in view of the high solubility of sodium compounds the concentration of dissolved salts increases more rapidly than when lime is used. The boiler can readily keep the concentration of the sodium salts at an acceptable level; however, during periods in which the boiler may not be needed to produce energy, as in the summertime when the boiler is not used to produce steam for heat, lime may be the chemical of choice, at least during the off-period for the boiler.

Many wastewaters, especially those from metal plating processes, contain water-miscible and/or water-immiscible organic compounds and/or materials such as acetone, rust- preventive oils, organic brighteners, such as benzaldehyde, chlorinated solvents such as perchloroethylene and trichloro- ethylene, and organic-metallic compounds. If these organic impurities are introduced into the boiler they steam distill and appear in the condensate, and thus to some extent contaminate the purified water. In the preferred method of carrying out this invention the organic impurities are removed from the wastewater before it is introduced into the boiler. In the preferred method of removing organic impurities from wastewater before it is introduced into the boiler the wastewater is contacted with a water-immisicible organic compound in which the organic impurities are soluble, thereby stripping the organic impurities from the water. Oil is the preferred organic stripping agent since it may already be in the system, for example as a result of its use as a rust-preventing agent, and advantage can be taken of its water-immisci bility, density, and miscibility with organic impurities. Surprisingly, organics soluble in water, such as brighteners as exemplified by benzaldehyde, as well as the water-immiscible ones are removed by the oil. In the preferred method of removing organic impurities wastewater containing the organic impurities is passed through a layer of oil floating on a body of wastewater as is discussed in detail below in the discussion of Fig. 3.

This method of removing organic compounds from wastewater .may be used with purification methods other than those involving use of a boiler to separate inorganic impurities, such as reverse osmosis, and is not limited to the use of a boiler to concentrate impurities.

In order to reduce corrosion in the boiler the feed should be adjusted to a pH so that the pH of the wastewater in the boiler is in the range of 8 to 10. Suitable compounds to control pH level and to minimize scale development are discussed in more detail below.

The boiler may be operated at any pressure for which the boiler was designed and the operating pressure will usually be determined by the nature of the steam usage. For instance, if steam is used for heating the boiler may operate below about 15 psig, while if steam is used to supply mechanical energy the pressure may be as high as that for which the boiler- was designed.

It has unexpectedly been discovered that the presence of cyanide in a low concentration within the boiler has a beneficial effect in helping prevent corrosion. While the mechanism is not known, it is believed that the cyanide acts as a scavenger for oxidizing agents which may be present. Consequently, in the preferred method of carrying out this process a low concentration of cyanide is maintained within the boiler. In order to keep the cyanide from distilling over with the steam, it is necessary to keep the pH of the wastewater in the alkaline range. At a pH of 6 all cyanide will distill over. At a higher pH the amount distilling over will depend upon the concentration of the cyanide. For example, at a cyanide concentration of 200 ppm by weight some cyanide will distill at a pH of 7.5, while none will distill at a pH greater than 8.0. At a concentration of 2000 ppm some cyanide will distill over at a pH of 8 while none will distill at a pH of 9. The cyanide concentration is preferably kept below 2000 ppm, since in higher concentrations cyanide may attack portions of the boiler as, for example, welds. The most preferred cyanide concentration is within the range of from about 1 ppm to about 200 ppm.

Many plating wastes contain cyanide, and it may be necessary to treat the wastewater to prevent the cyanide from building up within the boiler to too high a level. Cyanide may be maintained at a proper level within the boiler by adding calcium hypochlorite to oxidize it to carbon dioxide and nitrogen. However, it has unexpectedly been discovered that oxidation of cyanide with H-0 results in a significant reduction of solids being carried over with the steam. For example, when cyanide was destroyed with calcium hypochlorite, solids were present in the condensed steam at a level of about 40 ppm by weight. When the oxidizing agent was H₂O ₂ the concentration of solids in the condensed steam was reduced to about 4 ppm by weight. The concentration of solids in the condensate in a system having a steam trap was reduced even further, to.about 1 ppm by using stainless steel or CPVC pipe after the steam trap. The use of oxidizers in a boiler containing steam is believed to be contrary to usual boiler operating practice.

In the preferred method of carrying out this process chromate is present in a concentration of at least about 5 ppm by weight and preferably in a concentration of at least about 10 ppm. Concentrations of about 10 ppm to about 2000 ppm are most typical, but the concentration may range to 5000 ppm or higher. Chromate helps reduce corrosion of metal within the boiler, and it may be added if it is not already present in the wastewater fed to the boiler.

It has unexpectedly been found that for use in treating wastewater from a metal plating process where the heavy metals comprise cadmium, copper, nickel, tin, zinc, chromium and iron, the amount of scale formed within the boiler is negligible and dissolved solids can accumulate to a level greater than about 40 percent by weight at the temperatures within the boiler. The absence of a significant amount of materials having an inverted solubility curve, such as calcium and magnesium salts, may explain the low incidence of scale formation. In the preferred method of carrying out this invention the concentration of Ca⁺⁺ is maintained at a level below about 200 ppm by weight, more preferably below about 100 ppm by weight, and in the most preferred method Ca⁺⁺ is maintained below about 10 ppm. The total concentration of material having an inverted solubility curve is preferably kept below about 300 ppm and most preferably below about 20 ppm by weight.

The concentration of metal salts in the boiler, dissolved plus precipitated, may suitably range from a level slightly higher than that of the wastewater feed to a level of about 55 percent by weight. Inasmuch as zero liquid effluent from the industrial plant is the ultimate goal, and a necessary adjunct to that goal is the production of the impurities in the form of solids, in the preferred method of carrying out this invention the salts are concentrated in the steam boiler to as high a level as possible consonant with safe and efficient operation of the boiler. The preferred concentration of solids, dissolved and precipitated, is in the range of about 5 percent to about 30 percent by weight and the most preferred concentration is from about 10 percent to about 20 percent by weight. The concentrated solids are removed as by blowdown when the desired concentration is reached, and the removal may be either batch or continuous.

In this process of treating wastewater and recycling the the substantially pure water produced in accordance with this invention, the industrial process requires little or no make-up water from other sources. This feature has the advantages that not only is the cost of make-up water drastically reduced, but calcium does not accumulate in the system as it would if continuously added in make-up water. In order to keep calcium and magnesium concentrations low, make-up water is preferably softened or ft may consist of rainwater which has been captured and added.

The invention may be embodied in various forms, and the embodiments shown in the drawings are only provided to illustrate the invention.

Referring to Fig. 1 of the drawing, an industrial process which produces contaminated wastewater effluent is generally designated as 10. The industrial process may include just about any industrial process which produces contaminated wastewater effluent, but the process according to the present invention will be described with particular reference to metal processing, including metal surface finishing, metal plating, pickling and the like. It is to be understood that the present invention has application to a wide range of other industrial processes providing an effluent with a relatively high concentration of impurities and the reference to metal processing is merely by way of example.

The wastewater effluent from the industrial process 10 preferably either does not contain extremely large amounts of corrosive chemicals or it contains corrosion-resisting chemicals. The wastewater froπua chrome plating process, wherein the effluent contains chromium ions which aid in protecting boiler tubes from corrosion, is an example of effluent containing corrosion-protecting chemicals.

The wastewater effluent from process 10 is conducted by means of conduit 12 through valve 13 directly into a steam boiler 14. Steam boiler 14 may be of any conventional construction and may include, for example, fire-tube, water-tube or package-type boilers. In steam boiler 14, the wastewater effluent is heated to produce a steam component thereby concentrating the impurities in the boiler in the aqueous phase. Although the impurities are concentrated to a level exceeding their solubility at ambient temperature, they may either remain in solution at the temperature within the boiler, or they may precipitate.

Standard boiler compounds may be introduced into the wastewater effluent before it enters boiler 14 to inhibit or minimize the build-up of scale and to reduce corrosion in the boiler. Alternatively, the boiler compounds can be added to the boiler. Where the boiler compounds are added directly to boiler 14, they are introduced through entry duct 16 by means of pump 18 since these boilers are pressurized vessels. In using these compounds it is desirable to adjust the pH of the wastewater within the boiler to be within a range of about 8 to 10.

Suitable boiler compounds are well known to those skilled in the art. The choice and appropriate amount of a proper boiler compound or compounds may easily be determined by mere routine experimentation, taking into consideration the type of wastewater effluent. Suitable boiler compounds include, for example, sodium phosphate, soda ash, ammonia, volatile amines, such as morpholine and cyclohexylamine, chelating agents, such as EDTA, and polyacrylamides of the type made according to United States Patent 3,463,730 to Booth et al.

While the heat transfer surfaces within the boiler are not harmed by the wastewater, it has unexpectedly been found that the conventional level controls made of brass may corrode and it is preferred to use level controls made of stainless steel.

The concentrated impurities, which may contain precipitates, will accumulate in the boiler 14 and may form a sludge which can be removed by blowdown through conduit 20 and conventional blowdown valve 21. A combination of sludge and scale may accumulate and can be removed by blowdown valve 20 and/or scraper devices.

From boiler 14, the steam component is conducted through conduit 22 as working steam used for any industrial purpose as indicated at 24, such as heating a plant or heat exchanger or for driving turbines. As the steam is used for the indus trial purpose it condenses forming relatively pure water which is conveyed through the conduit 26 to a condensate return tank 28.

The condensed water can be selectrively pumped from condensate return tank 28 directly to boiler 14 by pump 35 through conduit 31 and valve 33 when there is insufficient untreated or pretreated wastewater effluent entering the boiler. Preferably, valve 33 is controlled by a standard water level sensing means in the water tank.

Alternatively, the water may be conducted from condensate return tank 28 through conduit 30 to a storage tank 32. The water from storage tank 32 is conducted through conduit 34, valve 36, pump 38 and check valve 43 back to the original industrial process to be used therein. Where pump 38 is necessary, an accumulator device 45 should be used to compensate for any surge in line pressure resulting from the starting of the pump and otherwise help to maintain uniformity of pressure. The accumulator device may be any standard device incorporating a piston, diaphragm, or bellows. Pump 38 may be unnecessary where gravity feed may transfer water from storage tank 32 to industrial process 10. Another variation would be to let the condensate go directly to the industrial process 10 or to the process from the condensate return tank 28.

Many industrial processes yield wastewater effluents containing insoluble materials. In this case, a process according to the present invention can include some pretreat ment of the wastewater effluent. One such process is described with reference again to Fig. 1 of the drawing.

Wastewater effluent containing dissolved ions and solids produced by industrial process 10 is conducted through conduits 40, 42 and 44 and valves 46 and 48 into settling tanks 50 and 52. Of course, depending upon the system, any number of settling tanks may be used.

For the purpose of illustration, assume that the waste water effluent contains 1,000 ppm suspended and dissolved solids. Suitable flocculents or precipitating agents, such as lime, are added to the wastewater effluent in settling tanks 50 and 52. Lime is useful as an agent to remove calcium or magnesium present as bicarbonates forming insoluble carbonates as illustrated b the e uation:

However, the concentration of calcium is preferably minimized as by using another agent such as sodium hydroxide instead of lime.

The tanks 50 and 52 are preferably used alternately, that is, one tank is filled then the other, so that the process is a batch type process. A continuous system can also be used if desired. After a period of time, the effluent separates into two components, a relatively clear component 54 and 56 containing only dissolved solids, such as sodium and potassium chlorides, nitrates, sulfates, etc. in a concentration of about 1,000 ppm, and a sludge or precipitated component 58 and 60, having a concentration of solids of about 2-5%.

In many instances, the wastewater effluent may be recycled back to the industrial process for use therein after the suspended solids are removed. Thus, the component 54 and 56 containing the dissolved solids is removed through conduits 62 and 64 and through valves 66 and 68 from settling tanks 50 and 52, respectively. Conduits 62 and 64 are connected to tanks 50 and 52, respectively, at a point above the anticipated level of sludge 58 and 60 so that only components 54 and 56 are removed. The component containing the dissolved solids is then conveyed through conduit 70 to storage tank 72.

The level of liquid in tank 72 can be raised and the concentration of dissolved solids therein diluted by adding water from condensate return tank 28. Water is selectively conveyed from tank 28 to tank 72 through conduit 37, valve 39 and pump 41. The pump and valve can be controlled by level sensing devices and concentration sensing devices known to those skilled in the art. The liquid containing the dissolved solids in tank 72 is recycled through conduit 74, valve 78, pump 82, check valve 87 and conduit 86 back to industrial process 10. Of course, where gravity feed is available, pump 82 is unnecessary. Where the pump is used, accumulator device 89 is also used for maintaining uniformity of pressure. The recycling of the wastewater effluent component containing only dissolved salts aids in greatly reducing the amount of water necessary from primary sources, such as the municipal water system, thus conserving water, a valuable natural resource. In addition, many of the dissolved chemicals contained in the component containing the dissolved salts are beneficial for the industrial purpose. Thus, there may be a two-way cost savings. Typically, the component containing the dissolved salts may be recycled for a long period of time, such as, for example, one year. The recycled component will eventually contain too large a concentration of dissolved salts to be useful in the industrial process. At that time, it is introduced into boiler 14 through conduit 76, valve 80, pump 84 and conduit 88. Boiler compounds are not necessary, but in the preferred method of carrying out this invention they are added to the pretreated, recycled component before it is introduced into the boiler. The boiler produces steam for an industrial use during which the steam is condensed and the resulting water is recycled to industrial process 10 and/or boiler 14 as set forth hereinabove.

Preferably a portion of the component containing the dissolved salts in tank 72 is continuously recycled to the industrial process whiie a smaller portion is being conveyed continuously to the boiler. In this manner, the industrial process receives a recycled component containing dissolved salts and a substantially pure component which has gone through the steam and condensation cycle as set forth hereinbefore. Processes that require high quality water can receive condensate continuously and this method can eliminate ion exchange units.

Sludge 58 and 60 in tanks 50 and 52 may be pumped through conduits 90, 92, 99 and through valves 94 and 96 by a pump 98 to a concentrator tank 100. The sludge 58 and 60 from settling tanks 50 and 52 may typically have a concentration of about 2-5% solids. The sludge is transferred to concentrator tank 100 and after standing overnight produces a relatively clear component 102 containing dissolved salts and a concentrated sludge component 104 which may over a period of time build to 15% solids content. Component 102 is recycled to tank 50 through conduit 108 and valve 109 for recycling to industrial process 10 and/or to be conveyed to boiler 14 as described hereinbefore. When concentrated sludge 104 becomes too concentrated or builds up to a predetermined level in tank 100, it is discharged through conduit 106 and valve 107.

Concentrated sludge 104 and any sludge or scale formed in boiler 14 may be concentrated further by any suitable process. the more concentrated sludge and scale is reduced to a very small volume and may be readily discarded, or recycled to metal processors.

The energy in the boiler stack gases may be used to concentrate sludge by heat exchange between the hot gases which are the combustion products of the boiler fuel and the sludge, and a process wherein the sludge from the boiler is introduced into the boiler stack and water is removed from the sludge by evaporation is highly energy efficient. The water content can thus readily be reduced to less than about 2 percent by weight.

In practicing this invention it has been found that the nature of the industrial wastewater fed to the boiler 14 may cause a foaming problem in the boiler and the liquid would then. have a tendency to surge up and down therein. As a result, the boiler 14 may shut down and/or discharge the wastewater instead of steam from the boiler. It is thus an important aspect of this invention to provide headroom when necessary at the top of the boiler to accommodate the foam and to relieve the problem of discharging water instead of steam. For most boiler s it is believed that there should be provided at least about one foot of space between the top of the wastewater and the top of the boiler 14. If necessary a conventional float switch can be provided at the desired water level in the boiler 14 and can operate a valve at the inlet to the boiler to prevent overloading the boiler with wastewater.

With reference to Fig. 2, boiler 10a represents a standard fire-tube boiler for use with this invention. In this boiler heat travels from hot combustion gases within the tubes through the tube walls to water within the boiler's water tank. The direction of temperature drop across the tube wall is from the combustion gases to the wastewater. The transfer of heat is represented by the equation

Q = μA ΔT where

Q is the amount of heat transferred per unit time,

A is the area of the surface through which the heat is transferred, μ is the overall heat transfer coefficient, and ΔT is the difference in temperature between the fluid being heated and the hot combustion gases.

The ΔT for steam boilers is high relative to that of evaporators where the heat required for evaporation usually is supplied by condensing steam, and consequently the heat transfer area and thus the size of the boiler can be much smaller than that of an evaporator having an equivalent capacity for converting water to steam. An additional disadvantage of an evaporator which increases the cost of a system using an evaporator is that it needs a source of energy, which in most cases is a steam boiler.

Boiler 10a includes outer side walls 12a and 14a, outer bottom wall 16a and outer top wall 18a which may be integral or contiguous with water tank top wall 20a. In addition to top wall 20a, tank 12a comprises bottom wall 22a and side walls 24a and 26a. No novelty is claimed in the precise construction of the boiler or the water tank. The drawing is merely representative of standard fire-tube boilers in which the present invention is operable.

Water is pumped into tank 12a through conduit 12 and valve 13 which may be controlled by a standard water level detector associated with tank 12a. Conduit 12 is also provided with check valve 11a. When the boiler is used to purify industrial wastewater, conduit 12 is connected to a source of industrial wastewater such as industrial process 10 shown in Fig. 1. The wastewater may be conveyed directly to boiler 10a or may be pretreated in accordance with the process previously described or any other desired process. The water is introduced into the tank to a level 27a just above the uppermost row of boiler tubes 50a so as to allow space in the tank for steam 29a. Steam produced by the boiler exits through conduit 22 and its flow is controlled by any conventional valve, not shown. The boiler may include any conventional blowdown valve and conduit, not shown, and any conventional valved inlet port, not shown, for the addition of standard boiler compounds to minimize scale build-up and corrosion.

Burner 28a may be any suitable, conventional burner of the type used in boilers, such as a gas burner, oil burner, coal burner or a combination thereof. Heat from burner 28a travels through chamber 30a between the outer boiler walls and the water tank walls. The heat is then routed by baffle 32a through fire tubes 34a, 36a and 38a into a chamber 42a. Chamber 42a is defined by boiler outer wall 14a, tank wall 26a and baffles 40a and 44a. From there, the heat progresses through fire tubes 46a, 48a and 50a into chamber 54a bounded by boiler outer wall 12a, tank wall 24a and curved baffle 52a. In its path through the boiler, hot gases transfer their heat through the fire tubes to the water and then are exhausted through flue 56a.

Attached to tank 19a is scale receptacle 58a for receiving any scale scraped from tank 19a. Suitable conventional gasket material or sealing means may be used to prevent water from leaking out of tank 19a or scale receptacle 58a.

According to a preferred embodiment which is particularly helpful when the wastewater contains water-immiscible organic liquid contaminants, especially those having a higher specific gravity than the wastewater, with or without undissolved metal salts, the wastewater is vigorously agitated, prior to passage of the wastewater through the oil layer, such as by injecting bubbles of a gas (including gas mixtures), for example oxygen, nitrogen, carbon dioxide and preferably air. For example the liquid may be agitated in a first body of liquid and be passed through the oil layer in a second, separate body of liquid. The operations of agitating liquid in the first body of liquid and transferring liquid from the first to the second body of liquid may each occur continuously or noncontinuously, and during certain periods these operations may occur simultaneously, alternately or in other time relationships. Preferably the liquid which is passed through the oil layer is withdrawn from an upper portion or the surface of the first body of liquid in a first vessel and is transferred from that vessel to a second vessel which contains the second body of liquid and in which the oil layer constitutes at least the upper portion of the second body of liquid. Moreover, it is desirable that the first body of liquid exhibit a gradient with respect to the mass of precipitated metal salts per unit volume of liquid which is positive with increased depth of liquid. Thus, there are more suspended and/or settled metal salts solids in a lower portion of the first body or vessel as compared to its upper portion. The desired gradient may be produced in any convenient way, such as for example by providing less vigorous agitation in the upper portion of the first body or vessel and/ or by discontinuing the agitation operation during at least a portion of the time when the transferring operation is being conducted. According to a particularly preferred embodiment the liquid is transferred only after a period of reduced or no agitation sufficient to cause appreciable or substantial settling of suspended solids, and this is preferably but not necessarily combined with withdrawing liquid from only the surface of the first body.

Regardless of the point of withdrawal and sequence of agitating and tranferring, the transferred liquid is caused to flow into and through at least a portion of the thickness of the oil layer while sufficiently restricting agitation of the oil layer for maintaining it substantially intact. Assuming the oil layer has a lower specific gravity than any other liquid which may be present in the second body, which is usually the case, organics from the first body of liquid (even those which may be heavier than water) become dispersed or dissolved in the oil layer of the second body of liquid, while the aqueous portion of the first body forms or passes into a lower aqueous layer in the second body. One convenient and preferred technique for effecting transfer is to withdraw liquid from the last-mentioned aqueous layer and to propel such withdrawn liquid into contact with an upper portion or the surface of the first body of liquid in the direction of a dam or weir over which the liquid at the surface of the first body is thus caused to flow. The liquid overflow may then be passed downwardly, preferably along a downward-directed surface upon which it flows, to the oil layer.

A system of recycling wastewater from a metal plating process showing portions of the system of Fig. 1 with modifications is given in Fig. 3. In this system, the wastewater from the industrial process is added through line 40 and valves 46 or 48 to either tank 50 or tank 52 where it is treated as by adding a precipitating agent. The resulting mixture is agitated by air introduced from an air source (not shown) through line 124 or line 125.

As in Fig. 1, tanks 50 and 52 are used alternately, i.e., when one tank is full and the wastewater therein is ready to be treated the other tank is empty and ready to receive wastewater from the industrial process. Oil and other organic compounds often found in wastewater, such as chlorinated solvents and brighteners, are removed from the wastewater by passing the wastewater through oil layer 130 within recirculation tank 136. This is accomplished by adding enough liquid to tanks 50 or 52 either from line 40 or from recirculation tank 136 through lines 128 or 129 to cause wastewater to overflow into recirculation tank 136.

The oil layer 130 is effective in removing from wastewater organic compounds soluble in the oil including watersoluble organics such as brighteners as well as water-immiscible compounds, and is effective in removing emulsified particles which would be difficult to separate from water by differences in specific gravity.

If an oil layer does not form within one or two cycles after starting the process, enough oil should be added to form layer 130 from about 1/4 inch to about 3 inches thick. The oil layer is preferably maintained at a thickness of from about 1 to about 2 inches. While thicknesses greater than these may be used, there appears to be no advantage to thicker layers. The term "oil" refers to lighter petroleum fractions ordinarily used for rust preventive purposes or for lubrication such as oils designated as SAE No. 30.

By use of the air agitation system the water-immiscible organics which are heavier than water, such as chlorinated solvents as exemplified by perchloroethylene and trichloroethylene, are prevented from accumulating in the sludge by being dispersed throughout tanks 50 and 52 and thus overflow into recirculation tank 136. The sludge for recycle to metal processors is thus relatively free of organic compounds. In using the air agitation system to disperse the heavy organics throughout tanks 50 and 52 the agitation is preferably intermittent to permit solids to settle while wastewater overflows into tank 136.

Operation of the air agitation system and recirculation pump keeps the concentration of total organic carbon in the condensed steam at a relatively low level. If heavy organics, such as the chlorinated solvents, are not present, recirculation alone will keep the concentration of total organic carbon in the steam condensate at a negligibly low level.

The presence of a heavy organic impurity, such as trichloroethylene and perchloroethylene, in the wastewater may require controlling the composition of oil layer 130 to maintain a density less than that of the wastewater. The density may be reduced, if necessary, by adding additional oil to the layer, either with or without a step of removing a portion of the material from layer 130. The specific gravity of the oil layer is preferably maintained below about 0.9.

The clear, oil-free water 135 from the recirculation tank 136 is moved by pump 126 or 127 through conduits 128 or 129 into tank 50 or 52 through spray heads (not shown) . This recirculating water serves to provide water to tank 50 or 52 to float the oil and other organics into the recirculation tank, or to flush sludge 58 or 60 from tanks 50 or 52 when they are being emptied. Clear liquid from tanks 50 or 52 is pumped into storage tank 72 through line 70.

The treated wastewater from tank 72 may be recycled as is shown in Fig. 1.

The process according to the present invention provides for substantially zero contaminated wastewater effluent discharge. The wastewater effluent is treated in accordance with the present invention and need not ever leave the system. The only contaminants which leave the system are in the form of oil (organics), highly concentrated sludge and/or scale which are easier to dispose of than large amounts of dilute liquid effluents, certain of which can be dried thus further concentrating them and placing them in a form suitable for processing by metal manufacturers.

The process will now be illustrated using the following specific, nonlimiting examples:

Example 1

Approximately 500 gallons of recycled wastewater effluent were obtained from the wash and rinse baths of an electroplating process. The recycled wastewater effluent, which had been used in the baths for about one year, contained heavy metals, such as cadmium, copper, nickel, tin, zinc and iron. In addition, it contained cyanide, hexavalent chrome, oil, alkaline cleaner and various acids.

The cyanide was destroyed by normal chlorination. The hexavalent chrome was partially reduced by a hydrosulfite and the oil removed continuously with an oil separator. The heavy metals were precipitated with excess lime and polyamine flocculents. After this treatment, a sludge component and a clear component remained. The pH of the clear component was adjusted to approximately 8 and it was pumped to a reservoir for use in the electroplating process as needed. The clear component was recycled once or twice each week and after about a year the water became unusable due to a build-up of dissolved solids and interference with the plating operation. The dissolved solids, in a concentration of about 8,500 mg/1, seemed to consist mostly of sodium sulfate, sodium chloride and sodium nitrate. Other cations, such as potassium, calcium, magnesium and ammonia were present, but no efforts were made to determine exact amounts. Organic materials, such as wetting agents, were also present.

The recycled component containing the large concentration of dissolved solids was then introduced into a small laboratory boiler for testing to see if the boiler would separate the contaminants from the steam and not damage the boiler.

The steam produced by the boiler at about 15 p.s.i.g. was condensed and the water condensate was relatively clean. It contained some ammonia and iron and had a pH of 8.8. The sludge produced in the boiler was soft and oozed out of a control valve (corresponding to a typical blowdown valve) and the experiment progressed. The boiler contained an average of about 4 gallons of the component containing the high concentration of dissolved solids as the 500 gallons of wastewater were passed through the boiler. When the boiler was disassembled, some hard scale was found and removed.

Example 2 A 55 gallon drum of chrome waste was obtained from another plant that processed copper and copper alloys. The chrome was reduced to the trivalent state and the sludge represented about 50% of the solution by volume. The pH of the solution was adjusted to 8, the solution was agitated and allowed to stand for about 20 minutes. The sludge was still about 50% by volume and remained so after leaving the sludge stand overnights Solids by weight of the sludge were about 5%.

The waste was then introduced into the laboratory boiler. There was concern that the sludge would be difficult to concentrate in the boiler because of its voluminous nature. However, this did not prove to be the case. The green sludge did not show up in the boiler sightglass or enter the steam port. Difficulties, however, were encountered in other aspects. Even though the pH was adjusted and maintained at about 8, the steam had a pH of 2.4 and badly corroded the boiler steam lines. Excess sodium sulfite was present, yielding corrosive sulfur dioxide and sulfurous acid. Hexavalent chrome was added to remove the excess sodium sulfite and the experiment repeated. When approximately 5 mg/1 of hexavalent chrome was maintained in the boiler, no further sulfur dioxide was carried over with the steam and the pH of the condensate was about 8. There appeared to be no further corrosion of the boiler system.

Example 3 Another experiment was carried out with the same chrome wastewater as used in Example 2. The conditions in the boiler were 5-10 ppm of hexavalent chrome and the pH ranged from about 8 to about 10. Morpholine was added to the boiler to adjust the pH of the steam so that as the steam entered the condensate tank, the pH was between 7.5 and 8.5. The steam pressure was about 15 p. s. i.g.

The chrome sludge did not interfere with the normal boiler conditions. The condensate showed the presence of morpholine and a pH of about 8. No noticeable corrosion could be detected in the boiler or in the steam lines. The concentrated trivalent chrome was removed from the boiler through the control valve at about 60 percent dissolved salts and solids. No hard scale formed on the inside of the boiler.

A portion of the impurities removed from the boiler were further concentrated by placing the sludge on a cloth which was placed on a steam table. More water was driven off and the solids were concentrated to about 97 percent by weight. The sludge was dark green in color and hard. It was crumbly and easily separated from the cloth.

Another portion of the 60 percent solids removed from the boiler was placed in the exhaust stack for the boiler, where water was driven off and the solids concentrated to about 98 percent. Example 4 Sludge from the boiler formed as in Example 3 and consisting of 60 percent solids were pumped from the boiler to a stainless steel conveyor designed to carry the sludge into the exhaust stack of the boiler. The exhaust gases, which were at a temperature from about 350°F to 450°F, further concentrated the solids.

Example 5

The following example was conducted in the laboratory as part of an economic study. It is believed that the economic savings set forth below would be achieved.

Conditions at another plant were observed and samples of wastewater effluent taken. Particular emphasis was placed on the economics of this plant which illustrated the energy savings associated according to the present invention. Approximately 750,000 pounds of steam were generated daily in the winter for heating and processing in the plant. In the summer, about 200,000 pounds of steam were used daily. The water discharge varied between 100,000 and 140,000 gallons per day throughout the year. About 75 percent of the water was used in the electroplating department. Even though the water was cleaned by chemical treatment before being discharged to a stream, it was not considered clean enough for recycling to the plating department. Samples of the chemically treated wastewater effluent were run through the laboratory boiler and the water condensed from the steam produced by the boiler provided to be of high quality and satisfactory for the plating operation.

If the wastewater effluent from the plating operation only were run through a separate evaporator, the additional cost for energy would be in excess of $2,000 per day, which would double the budget cost. However, if the wastewater effluent were introduced into the existing plant boiler, the energy costs would be only slightly increased. For illustration, on a cold winter day, 765,000 pounds of steam were generated and 583,440 pounds (78,000 gallons) of steam were used in the plating area. By passing all of the wastewater through the existing boiler in accordance with the present invention, there would be more than enough water for daily usage. A further advantage would be that the condensate would be warm (75-100°F), which would facilitate rinsing in the plating operation.

Since the steam had to be generated anyway, the only additional costs would be heating the condensate return water more than previously done. It was estimated that 10 percent additional energy would be required for this purpose, but this would be offset by less blowdown, so that the net energy loss would be only 4 to 5 percent.

In the summer, not enough steam would be generated to process the water according to the present invention each cycle. A determination would have to be made as to which process in the plant was most critical and would require the pure high quality condensate produced in accordance with the present invention. The rest of the operation would use recycled water from normal chemical destruct methods. The net result would be a substantially closed loop system and substantially no water would ever leave the system in liquid form containing pollutants, except in the concentrated sludge from the boiler.

Example 6 Several 55 gallon drums of water effluent were collected from a plant before waste treatment procedures were carried out on the waste effluent. The cost of chemicals at this plant was very high for reducing hexavalent chromium and precipitating heavy metals. The only pretreatment before passing the waste through the boiler was to adjust the pH to 9 and add polyamines to prevent the scale formed from sticking to the boiler plate. Hexavalent chromium was maintained in the boiler. The water which was condensed from the steam produced by the boiler was of excellent quality, but the scale did adhere somewhat and mechanical scraping was necessary. Example 7

Trichloroethylene was introduced into the recycled water in the system of Fig. 3 to determine whether it would appear in the condensate if. it were introduced into the boiler and to determine whether it would be removed by passing the recycled water through an oil layer. Trichloroethylene was selected since it has been found in ground waters and is considered to be a carcinogen.

When trichloroethylene was a component of the wastewater introduced into the boiler, the condensed steam was blue in color. The reason for the color is not known, but it made for easy visual detection of the presence of trichloroethylene. The air agitation system and the recirculation pumps were then turned on for about 30 minutes forcing the water over the overflow through a 3 inch thick layer of oil. The wastewater was permitted to settle overnight and clear water was drawn into the boiler. The condensate was clear indicating the absence of significant amounts of trichloroethylene.

Example 8 The experiment of Example 7 was repeated without air agitation. The condensed steam from the boiler was blue, indicating the presence of trichloroethylene in Ole oυndetrsate.

Example 9

Drawing oils and organic plating brighteners, but without any chlorinated solvents, were introduced into the recycled water introduced into the boiler. No blue color appeared in the condensate; however, analysis showed a significant concentration of total organic carbon in the condensate.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. For example, the industrial wastes may be from sources other than metal processes, as for example chemical processes, biological processes, mining industries, or pharmaceutical industries. The pressure at which the boiler is operated is determined by its capability and the use to which the steam is put. Pressures as high as 150 p. s . i may be desirable for processing waste from the pharmaceutical industry or for biological wastes to ensure destruction of all viruses and thermophiles. Accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

Claims

What is Claimed is:

1. A process for substantially zero discharge of polluting impurities in wastewater comprising the steps of introducing said wastewater having a first concentration including dissolved impurities into a boiler having a heat transfer surface in contact with said wastewater, raising the temperature of said surface to produce a steam component and a component having a second concentration of impurities, said second concentration being greater than said first concentration, and removing said steam component from said boiler.

2. The process of claim 1 wherein said steam removed from said boiler is used for an industrial purpose resulting in formation of condensate, and at least a portion of said condensate is recycled for use in said industrial process.

3. The process of claim 2 wherein apportion of the condensate is recycled to said steam boiler.

4. The process of claim 1 wherein the wastewater fed into said boiler contains from about 200 to about 5,000 ppm by weight of impurities.

5. The process of claim 1 wherein the liquid component within said boiler contains from about 5 percent to about 30 percent by weight impurities.

6. The process of claim 1 wherr-in the liquid component within said boiler contains from about 10 percent to about 20 percent by weight impurities.

7. The process of claim 1 wherein the impurities in the wastewater comprise at least one salt of alkali, alkaline earth, and heavy metals.

8. The process of claim 7 wherein the heavy metals are selected from the group consisting of cadmium, copper, nickel, tin, zinc, chromium, iron and aluminum.

9. The process of claim 31 wherein the concentration of cyanide within the boiler is less than about 2,000 ppm.

10. The process of claim 31 wherein the concentration of cyanide within the boiler is from about 1 to about 200 ppm.

11. The process of claim 1 wherein compounds having an inverted solubility curve comprise no more than about 300 ppm by weight of impurities in the wastewater.

12. The process of claim 1 wherein calcium ions in the wastewater comprise no more than about 200 ppm by weight.

13. The process of claim 27 wherein a portion of said partially purified aqueous waste component is recycled to said industrial process prior to processing in said boiler.

14. The process of claim 38 wherein said boiler has an exhaust stack for evacuating hot gases generated from raising the temperature of said heat transfer surface, further comprising the steps of introducing said removed component to said stack and concentrating said removed component using the heat from said hot gases.

15. The process of claim 38 wherein said removed component is retained within said stack for a period of time sufficient to reduce the water content to less than about 2 percent by weight.

16. The process of claim 15 wherein at least one of the impurities is a metal salt which is crystallized in substantially pure form upon evaporation of the water in said liquid component.

17. The process of claim 15 wherein the impurities are metal salts.

18. The process of claim 1 wherein the pH of the wastewater is adjusted to a value within the range of about 8 to about 10.

19. The process of claim 1 wherein the wastewater contains at least one organic impurity and the process includes a step of removing said organic impurity from said wastewater before introducing said wastewater into said boiler.

20. The process of claim 19 wherein said impurity is removed from said wastewater by passing said wastewater through a layer of a water-immiscible solvent for said impurity.

21. The process of claim 20 wherein said layer of a water- immiscible solvent comprises an oil.

22. The process of claim 20 wherein said impurity is selected from the group consisting of oils, organic plating brighteners, and chlorinated solvents.

23. The process of claim 19 wherein at least a portion of said organic impurities are water-soluble organic compounds.

24. The process of claim 19 wherein at least a portion of said organic impurities are substantially water-immiscible.

25. The process of claim 19 wherein at least a portion of said organic impurities are substantially water-immiscible and have a specific gravity greater than that of water.

26. The process of claim 20 wherein the step of passing said wastewater through a water-immiscible organic solvent is carried out by passing said wastewater through a layer of said solvent disposed on the surface of an aqueous body.

27. A process for substantially zero discharge of polluting impurities in wastewater effluent as in claim 1 further comprising the steps of precipitating a portion of said impurities separating the resulting precipitated impurities from the resulting aqueous phase forming a partially purified aqueous waste, and then processing at least a portion of the partially purified aqueous waste in said boiler.

28. The process of claim 27 wherein said impurities also include undissolved impurities.

29. The process of claim 31 wherein the cyanide is oxidized by an oxidizing agent selected from the group consisting of hydrogen peroxide, sodium peroxide and ozone.

30. A process for substantially zero discharge of polluting impurities in wastewater effluent comprising the steps of adding an oxidizing agent to said wastewater and heating said wastewater in a boiler to produce a steam component.

31. The process of claim 30 wherein said wastewater includes cyanide.

32. The process of claim 31 wherein the preferred oxidizing agent is hydrogen peroxide.

33. The process of claim 31 further comprising the step of adjusting the pH of said wastewater to prevent distillation of said cyanide during said heating of said wastewater.

34. The process of claim 33 wherein said wastewater is adjusted to a pH greater than 9 for a concentration of said cyanide of 2000 ppm or less.

35. The process of claim 33 wherein said wastewater is adjusted to a pH of greater than 8 for a concentration of said cyanide of 200 ppm or less.

36. The process of claim 26 further comprising the step of agitating said wastewater prior to its passage through said solvent.

37. The process of claim 36 wherein said agitation is accomplished by injecting gas bubbles into said wastewater.

38. The process of claim 1, further comprising the step of removing at least a portion of said component having a second concentration from said boiler.

39. A process for substantially zero discharge of polluting impurities in wastewater effluent from an industrial process comprising the steps of separating said effluent into components including a first component containing dissolved solids, recycling a portion of said first component for use in said industrial process, and processing a portion of said first component in said boiler as in claim 1.

40. The process of claim 39, wherein said recycling step is performed for a predetermined period of time prior to any processing in said boiler.

41. The process of claim 40, wherein said predetermined period of time is that time wherein the recycled component can no longer be used beneficially in said industrial process.

42. The process of claim 39, wherein said recycling step and said step of processing in said boiler is proceeding continuously, further comprising the steps of condensing said steam removed from said boiler to form a condensate and recycling said condensate for use in said industrial process.

43. The process of claim 42 further comprising the step of combining said condensate with said first component prior to said use in said industrial process.

44. A process for substantially zero discharge of polluting impurities in wastewater from metal processing operations wherein said wastewater contains high concentrations of heavy metal salts, comprising the steps of introducing said wastewater having a first concentration including dissolved impurities into a boiler having heat transfer surfaces in contact with said wastewater, raising the temperature of said surface to produce a steam component and a component having a second concentration, said second concentration being greater than said first concentration, and discharging said steam component from said boiler.

45. The process as in claim 44, wherein said wastewater further includes an organic impurity selected from the group consisting of oils, organic brighteners and chlorinated solvents, further comprising the step of passing said organic impurity through an organic solvent for said impurity.

46. The process as in claim 44, further comprising the steps of separating said effluent into components including a first component containing dissolved salts and a second component and recycling said first component to said metal processing operations for a predetermined time period prior to introducing said wastewater into said boiler.

47. The process as in claim 46, wherein said wastewater further includes cyanide further comprising the step of adding an oxidizing agent to said wastewater prior to introducing said wastewater into said boiler.

48. The process as in claim 47 wherein said oxidizing agent is hydrogen peroxide.

49. The process as in claim 44, wherein said wastewater from said metal processing operations consists essentially of plating wastes.