Alan Monken Technical Service Chemist Calgon Corporation St. Louis, Missouri

Transcription

1 - CO-MINGLING PAINT BOOTH SLUDGE WITH OTHER INDUSTRIAL WASTE Alan Monken Technical Service Chemist Calgon Corporation St. Louis, Missouri Presented at: "Finishing '93 Conference and Exposition" October 25-28, 1993 Dr. Albert B. Sabin Convention Center Cincinnati, Ohio Copyrighted by SME; Technical Paper Number to be Assigned

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3 - - CO-MINGLING PAINT BOOTH SLUDGE WITH OTHER INDUSTRIAL WASTE Industrial water-wash paint booths were originally designed for the sole purpose of providing a means of removing paint overspray from the airflow of the workplace. What was never taken into account, however, was the treatment and disposal of the paint waste generated by these systems. A variety of means of paint sludge removal have been developed, showing varying degrees of efficiency and applicability; these methods range from the labor-intensive manual dredging of sunken paint sludge to the highly-efficient continuous removal of paint solids by centrifugal equipment. Over the past several years, much emphasis has been placed upon finding ways to reduce paint booth waste. Concurrently there has been increasing pressure placed upon manufacturers to institute treatment programs for an^ waterborne waste generated in-plant. Rather than maintain two separate waste handling systems with the same facility, it would greatly improve efficiency to be able to combine paint wastes with normal plant wastes. The questions this poses are: 1. Is this possible? 2: is this practical? 3: How can this be done?

4 In fact, under many conditions it is possible to co-mingle (i.e. mix) paint booth wastes with llnormalll plant wastes for treatment through conventional waste systems. Before doing so, however, it is important to know if limitations based upon paint types exist, how to utilize paint booths and treatment systems to optimize results, and how conventional treatment systems can handle the co-mingled waste. From this information, it is ~~ possible to construct a model for treatment of paint and other waste. This paper will present both the model and a case study of its application. PAINT TYPES AND TREATMENT The type of paint being used is very important to the ultimate success or failure of combined waste treatment. There are three basic types of paint commonly encountered in metal fabrication and manufacturing: powder coating, sprayed liquid coatings, and electrocoating. In the case of powder coating, the paint, in dry form, is spray and collected immediately for reuse, requiring no waste treatment. Electrocoating operations (as well as conventional dip operations) function as self-contained immersion systems and, as they generate no oversprayed or waste paint, require no further treatment during operation. Sprayed liquid coatings, however, require some type of chemical or physical treatment to aid in both handling and removal from the painting system. When treating sprayed paint in a booth, the two areas of concern are the handling of the oversprayed material in the paint booth system and the removal of the resultant waste paint from the system. In chemical treatment of the overspray entering the booth, the handling of the paint normally is translated into terms of detackifying or llkillingll the paint spray. This detackification is especially important in the handling of solvent-borne coatings, which tend to remain tacky in water and which adhere to booth and recirculation system surfaces if left untreated. Solvent-borne liquid coatings, which have been historically the most common paints found in metal fabrication, can be detackified with a variety of different chemical means, including high alkalinity systems (the classic Itcaustic1* treatment program), alkaline salt systems, clay-based systems, and polymeric treatment systems. Polymer detackifiers have proven to be the most effective treatments to use with the high solids compliance coatings that have become prevelant in the marketplace. One efficient way of removing detackified paint solids, especially in samll sidedraft booths, is through the use of a continuous removal device such as a centrifuge. The majority of these detackifiers will also work in conjunction with polymer-based flocculents to aid in the removal of paint solids through skimming systems (commonly called consolidator units), which are more applicable to larger downdraft booths. Water-borne coatings, which are increasing rapidly in the face of stricter VOC regulations, are sprayed in the same types - - -

5 - - - of systems as solvent-bornes, but present other challenges with regards to treatment. Due to the very nature of water-borne coatings, the paints tend to be highly soluble/dispersible in water systems and, thus, do not exhibit the same level of tack as their solvent-borne counterparts. Detackification, therefore, is much less of a problem with these coatings. Because of their tendency to disperse so readily, however, solids generated by water-borne coatings are much more difficult to separate from booth water. Specialized polymers are now becoming available for dealing particularly with the separation of the dispersed solids both in the system (or in floatation systems) and in continuous removal situations. One of the differences in treatment of waterborne as opposed to solventborne coatings is that, due to the lack of need for quick detackification, the necessary polymer feedpoint may be farther lldownstreamll from the booth, at the sludge removal device, rather than at the painting site itself. Even with the best sludge removal system, however, the situation remains that paint wastes are being removed at or near the point of generation, totally separated from any other existing waste treatment plan. If it were possible to combine this waste with more conventional waste streams being generated within the plant without negatively effecting the conventional treatment operation, it would be possible both to reduce overall waste generation, reduce operating costs, and potentially eliminate multiple waste sludge generation sites. WASTE TREATMENT SYSTEMS Conventional waste treatment systems remove solids from water streams in much the same way that the polymeric detackifier/flocculent programs remove paint solids; the basic principle involves capturing lightweight dispersed solids and increasing their density/weight with organic polymers or inorganic materials. The primary difference in solids removal is that the solids are expected to settle rather than float. The Equipment most typically used for this purpose is the clarifier. These come in various designs, ranging from large retangular pits to circular tanks. (See Figure 1) One design which is gaining in popularity in metal fabrication operations is the Lamella-type clarifier. (See Figure 2) The Lamella makes use of stacked flow plates to effectively increase the settling surface area to equal that of a much larger tank-type clarifier; the overall result is to provide a system that will separate a large amount of solids while requiring a relatively small amount of floor space. The basic mode of operation followed in industrial waste treatment is: 1. The water stream containing spent detergent solutions, rinse solutions, waste process water, and any other water-borne waste materials is cycled into the treatment system, either continuously or in a batch process; 2. Chemical additions are made to the waste water, including adjustments to ph and reduction of metals such as

6 c chrome: 3. Treatment chemical additions are made to the adjusted waste water in order to aid in precipitation (settling) of solids; this treatment may consist of addition of - inorganic materials, such as alum or Ferrous Sulfate, or organic polymers, or some combination of the two: 4. The precipitated solids are pumped from the clarifier to a secondary system for further dewatering: the dewatering system can be anything from a sludge consolidation pit to a plate and frame filter press. Inlet 7 Tube Modules Weir Peripheral Effluent Flume Rectangular Clarifier In a more simplistic sense, a waste treatment system removes dissolved, Circular Clarifier dispersed, or otherwise-distributed Figure 1. contaminants bv chemically treiting them to make them separate from the water in the waste stream; examples of these contaminants would include oils and greases dispersed by surfactants and metals made soluble by chelants. The normal method of collection in a waste treatment system allows the llsolidslv to settle out by gravity; other additives, such as polymers, are added to increase the settling rate of the tlsolidstl by increasing the density/weight of the particles. Waterborne paints exist in paint booth systems in much the same way, being originally formulated to be dispersions of organic resins in water, making them ideal material to treat in a conventional waste treatment process. In contrast, solventborne paints enter the paint booth water in a insoluble state; it is up to the treatment to make the waste paint disperse (if possible) in the water without adhering to the booth itself. If the solvent-borne coating can be fully dispersed and detackified, it becomes possible for this material to also be handled/treated in a waste treatment process.

7 / A PROPOSED MODEL FOR CO-MINGLED TREATMENT Given the differences in Itnormaltt operating methods, can paint booth wastes be introduced into normal waste streams without causing upsets and, if so, why hasn't this been done in the past. As suggested earlier, using the paints and detackifying systems of the past, this type of treatment would not have been proposed; the possibility of partially-detackified sludge blocking circulation systems would have been too great with the incomplete paint llkillll normally associated with older treatment systems. One of the stipulations in comingling paint waste with other industrial waste is that the paint solids must be Figure 2: Lamella-type Clarifier completely detackified. For this reason, a co-mingling approach may not be practical with some high solids solvent-borne paints, especially high solids baking enamels that cure at only high temperature. Likewise, some solvent-borne coatings with a strong tendency to float after introduction into the paint booth, most notable polyurethanes, may not sink adequately to allow a clarifier to separate the solids. The paints that are best suited to treating in conjunction with other wastes are the water-borne coatings. The waste generated by these coatings is, in some ways, very similar to other waste water materials normally dealt with, in terms of particle size and density. The major obstacle to be overcome is the dispersion of the organic resins by surfactants, which, again, is very similar to the type of dispersion occurring in the waste from cleaning operations. In addition, the potential problems of over-dispersal which lead to collection problems of water-borne coatings in conventional systems are virtually eliminated when it is treated and/or comingled with standard waste and treated in a clarifier system. Co-mingling will work with a number of solvent-borne coatings, especially those in the low-to-mid solids range, but it is especially suited for dealing with water-bornes of every type.

8 ~ e - The only situation in which co-mingling of sludge might not be recommended would be in a case where either the paint waste or the tlnormalll waste were listed as hazardous material for disposal; this would normally be the case if the sludge contained heavy metals or other undesirable materials. Since the overall objective in most waste treatment systems is to minimize - the hazardous waste generated, any action resulting in an ~ increase in volume would obviously not be desired. - - A CASE STUDY OF WASTE CO-MINGLING Account lls1l is a steel door manufacturer doing metal fabricating, cleaning, phosphatizing and painting. They currently spray a chromate-based water-borne primer as well as a water-borne top coat into two sidedraft paint booths. The account also has a waste treatment facility handling several waste streams, including that resulting from the rinse stages of a recirculating spray washer cleaning and phosphatizing steel door parts, as well as another processing line using a chrome seal. The waste water treatment system was, therefore, already performing chrome reduction as well as general solids removal. The account was treating the paint with a polymeric detackifier and flocculent with good results, floating and manually skimming the paint sludge from the system, and disposing of the sludge separately from the other waste. Experimentation showed that the booth waste water containing the oversprayed paint and the detackification polymer could be processed through the chrome reduction process, reducing the hexavalent chrome to an acceptable level, and then treated with a flocculent which allowed it to settle in the clarifier. Further testing showed that the same water, mixed with influent water from the waste stream, was also treatable in the same fashion. The switch from in-booth treatment to treatment in the waste water system also reduced overall chemical cost, eliminating the need for the flocculent formerly used in the booth with only a slight increase in the normal waste water flocculent used. (In this case, since this flocculent was an emulsion-type polymer which was effective at very low concentrations, the savings was even greater than expected.) The booth water is now routinely dumped to the waste treatment plant on a cyclic batch basis, alternating with the dump of other plant system wastes. This has eliminated the time-consuming manual cleanout of the booth as well as the hazardous waste generated from the poorly dewatered paint solids; the co-mingled waste is processed from the clarifier to a sludge-press, achieving good dewatering and much less volume than previously. A similar process is currently under study for an automotive paint system currently spraying a combination of a waterborne colorcoat and a solventborne clearcoat. In this case, the detackifier will be added directly to the paintbooth

9 -- recirculation line following the spray area, which will allow the combination of it and the waterbased coating to aid in detackifying the solventborne paint. The water will then be pumped directly to the waste treatment system. This plan should eliminate the need for a separate sludge removal system at the booth pit site, saving $50,000 in capital costs. The co-mingling of paint booth waste water and plant influent waste water is a workable solution in many cases to the problem of waste reduction and disposal. It can reduce the amount of waste generated, eliminate waste generation points, and better utilize existing equipment. In some cases, it may even drastically cut chemical consumption. While not the answer for every system, this is a viable method worthy of consideration for many manufacturers.

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