environmental controls

WASTEWATER TREATMENT SYSTEMS FOR FINISHING OPERATIONS

BY ALAN MONKEN

One of the most common growing areas of concern in organic finishing operationsis waste disposal. Where wastewater discharge into municipal sewers wasonce common place, greater and greater restrictions are being placed on anyeffluent from manufacturing operations, not only for obvious problem areassuch as plating operations, but also for water once considered innocuous, suchas spray washer rinse stage overflow. In some situations, it is possible to conservewater usage/discharge with filtration systems; it is also possible to findwaste haulers to remove contaminated water from the plant. However, both canbe costly and neither is a long-term answer to the ever-increasing regulationsgoverning disposal of industrial waste. The best solution is to pursue installationof an in-plant treatment system, putting the control and reduction ofcontaminants in any effluent directly in the hands of the manufacturer. Beforedoing so, however, it is necessary to determine what types of materials may beentering your waste stream and the methods available to treat/remove them.

SOURCES AND TYPES OF WASTE CONTAMINANTS

The type of operation at each individual finishing shop largely determines thetypes of materials that will enter the waste stream, and the type of treatment thatwill be required for the resultant waste influent. The types of operations typicallyfound include metal forming processes such as drawing, stamping, and bending,chemical treatment processes such as plating and phosphatizing, and coatingprocesses such as painting.

Metal Forming Operations

In metal fabrication there are a number of processes that may been countered.One of the most common is drawing, the process by which sheet metal stock(or other material) is formed in a press into a cup like or box like shape. Duringthis process lubricants known as drawing compounds are normally required toprevent scoring and damage from the metal-to-metal contact between the stockmetal and the die.Coolants may be required for this process due to the heat offriction produced, which can reduce die life. These lubricants and coolants arenormally oil-based compounds, either “natural” (i.e., petroleum or animal-fatderived) or synthetic. In addition, metal can be drilled, cut, forged, stamped, orcast, each of which may require additional coolants or lubricants. While thesecoolant and lubrication systems are typically closed (i.e., not directly tied to thewastewater stream), residue from these materials normally must be removedfrom the work in process.

Paint Pretreatment Operations

In metal fabrication operations the normal sequence of events in production isformation of raw metal stock into component parts, which are assembled and,most typically, painted. After the forming process it is necessary to go throughseveral pretreatment steps prior to painting, including chemical or physicaltreatment to remove rust or other surface defects (such as mill scale) resultingfrom the forming process or handling; cleaning of the parts or assembled productwith oils, greases, and other soils present due to the forming processes (such asdrawing compounds and lubricants); and conversion coatings (such as iron andzinc phosphates), which are applied to promote enhanced paint performanceand provide corrosion inhibition. A similar sequence of events is used in plasticsmanufacture, with cleaners, alkaline and acidic, used to remove shop soils andmold release agents, and conditioning agents applied to promote better paintadhesion. Because most of these processes are aqueous-based, a number of opportunitiesexist for contaminants to enter the waste stream. From the derusting orpickling operations, extremely low pH solutions, often high in iron and otherdissolved metals, require eventual disposal. Alkaline cleaner solutions containsurfactants, which are present to help remove/disperse oils and greases but canthemselves add to the organic pollutants requiring removal in waste treatment.In addition ,the more alkaline caustic-based cleaners require pH neutralizationwhen treated for disposal.These cleaners may also contain chelants, which are chemical compoundspresent to tie up metal fines and particulates in the water solution. When sent totreatment, these chelants may prevent the easy precipitation of metals. The tanksolutions of alkaline cleaners will also contain high levels of oils and greases comingfrom the drawing compounds, etc., being removed as soils.Conversion coating baths are typically at a low pH during use. Depending onwhether the process is iron or zinc phosphating, there will be a high concentrationof that particular metal when the tank is dumped; in either case, there will be alarge amount of phosphates, both soluble and insoluble (in the form of sludge).In cleaning and prepaint treatment systems one of the most important processsteps is the clear water rinse. These rinses may be continuously overflowed orrecirculated, or a combination of the two.The rinse stages will gradually becomecontaminated with the same materials as the chemicalprocess stages due to carryover and drag-out from stage to stage.The final stage of a multiple stage washer often is used to apply a rust inhibitorymaterial or other final sealing rinse material. These treatments can includechromium, zinc, and other exotic metals, which may require special treatmentfor removal.

Paint Operations

Once the formed parts are cleaned and pretreated, they are ready for painting.Sprayed liquid paint is applied in an apparatus called a spray booth,whichis typically a water system. Although these booths are closed systems, with thesludge removal taking place at the booth site, there maining water from cleanoutof the booth is often pumped directly to waste treatment for disposal. Thiswater may have a high pH (if caustic-based detackifying chemicals are in use)or high dissolved solids (if a polymer system is in use). Surfactants, misciblesolvents,and other debris may also be present. When water-based paints aresprayed, it may be more practical to continuously cycle the dispersed paint-andwatermixture directly to waste treatment. The waste treatment scheme has tobe adjusted to account for this other material.

ADDITIONAL SOURCES OF OPERATIONAL CONTAMINANTS

There are a number of other processes, which may be in use inorganic finishingoperations that will significantly impact waste treatment.Electroplating generates copious amounts of wastewater to be treated, normallyfor removal and/or destruction of materials such as chromium,cyanide,nickel, cadmium, copper, lead, and zinc. The wastewater from such systems isusually at the extremes of the pHscale depending upon which stageis being treated, thus requiringneutralization. Processes such asaluminum anodizing will also producesignificant amounts of pollutants,similar in some cases toelectroplating systems, with highamounts of hexavalent chromiumand other metals to be removedand highly acidic and alkalinewastewater to be neutralized.

WASTE TREATMENT

SYSTEMS

Waste treatment systems are putinto place to remove the variouspollutants entering the wastestream from plant operations.These systems have grown insophistication over time fromsimple settling ponds to complexosmotic filtration units. The typicalwaste treatment system consistsof a series of tanks in whichwastewater can be collected andchemically treated as necessary to remove contaminants. Depending on the rateof water flow, the system may be continuous or may involve batch treatment. (Asa rule, systems in which spray washer rinse run-off and “dumped” washer stagesare the prevalent material in the waste stream can typically be treated on a batchbasis; systems consuming large amounts of water on a continuous basis, such aselectroplating or electrocleaning lines, are often treated in a continuous system.)In the case of materials such as hexavalent chromium a dedicated tank might benecessary for segregation/treatment of a particular pollutant. The material canthen be treated, adjusted, and, quite possibly, removed from the water, whichthen moves on in the treatment system. Other tanks may simply be used forpH adjustments, such as those to neutralize highly acidic or alkaline materials.Once the wastewater is adjusted to the desirable state, it moves into the areaof solids removal. This may be done through physical filtration,such as a sandfilter system, or through gravity separation, such as would be done in a settlingpond. Commonly, however, the particles/pollutants remaining in the water atthis point either are not heavy enough to rapidly settle in a simple still pond orare not in a form to ever settle under normal means. To facilitate this process,inorganic materials such as lime or alum can be added to help flocculate thesolid pollutants, bringing them together in a mass. Organic polymers can alsobe used to coagulate the smaller particles, as can combination products madeup of polymers and inorganic salts.To further facilitate the settling of these pol- lutants, a piece of equipment known as a clarifier is used. The overall purpose ofthis type of equipment is to remove solids from water streams by gravitationalsettling in a relatively small area. In much the same way that the polymericdetackifier/flocculent programs remove paint solids,the basic principle involvescapturing lightweight dispersed solids and increasing their density/weight withthe organic polymers or inorganic materials.Clarifiers come in various designs, ranging from large rectangular pits tocircular tanks (see Fig. 1). The larger circular clarifiers are quite common incontinuous treatment type systems with daily flow rates in excess of 250,000 gal/day. For smaller systems, as are typically found in metal fabrication operations,the lamella-type clarifier is quite common (see Fig. 2).The lamella makes use ofstacked flow plates to effectively increase the settling surface area to equal thatof a much larger tank-type clarifier, resulting in a system that will separate alarge 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, wasteprocess water, and any other waterborne waste materials is cycled into thetreatment system, either continuously or in a batch process;

2. Chemical additions are made to the wastewater, including adjustments topH and reduction of metals such as hexavalent chromium;

3. Treatment chemical additions are made to the adjusted wastewater to aidprecipitation (settling) of solids; this treatment may consist of addition ofinorganic materials, such as alum or ferrous sulfate, or organic polymers, orsome combination of the two;

4. The precipitated solids are pumped from the clarifier to a secondary systemfor further dewatering; the dewatering system may be anything from a sludgeconsolidation pit to a plate and frame filter press.The waste treatment system allows suspended solid pollutants to settle out ofthe water stream for collection and, in addition, can remove dissolved, dispersed,or otherwise-distributed contaminants by treating them (typically chemically) toseparate them from the water in the waste stream. Examples of these contaminantsinclude oils and greases dispersed by surfactants and metals made solubleby chelants. Once these dispersed materials have been “destabilized,” the normalmethods of collection in the waste treatment system allow the “solids” to settleout. Other additives such as polymers are added to increase the settling rate ofthe “solids” by increasing the density/weight of the particles. The net result isthe removal of all materials infiltrating the water stream from the point of enteringthe facility to the point of leaving it. With the ever-increasing regulationsconcerning the contents of discharged water,it may often be the case that theeffluent water is of a higher overall quality than the influent. These systems canbe run effectively with a minimum of effort on the part of the organic finisherby recognizing what pollutants enter the stream within the plant and how eachimpacts the treatment program. By working with the various chemical supplierswithin the functional areas, problems of treatment for the finisher should beminimized and discharge limits in all areas easily met.