Techniques for Pollution Prevention in Textile Wet Frocessing

8/7/2019 Techniques for Pollution Prevention in Textile Wet Frocessing

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TECHNIQUES FOR POLLUTION PREVENTION IN TEXTILE WET FROCESSINGCPERATIONSDavid B. Williams, Environmental EngineerN.C. Office of Waste Reduction

Textile wet processing operations are coming under increasedscrutiny from environmental regulators because of the complexwastewaters and air emissions they generate. This new regulatoryeffort comes at a time when textile companies are already facedwith the need to reduce costs to respond to increasingcompetition. Pollution prevention offers an opportunity fortextile companies to reduce pollutant releases and save money; atthe same time, it helps them become more efficient. This paperdiscusses some of the opportunities for using pollutionprevention to reduce waste and save money.Contrary to conventional waste management which only results inincreased cost, pollution prevention can help a company reduceits costs through reduced raw material usage, reduced wasterequiring treatment and disposal, reduced water and energyrequirements, and improved product quality. Applicable pollutionprevention techniques include purchasing and inventory control;water and chemical conservation; improved operation, maintenance,and housekeeping; production process modification; recovery andrecycle/reuse; and source separation. The following sectionsdiscuss ways to apply some of these pollution preventiontechniques to reduce pollutant loadings in wastewater; conservewater and energy; and reduce air emissions, hazardous wastegeneration, and worker exposure to toxic chemicals. Case studiesare presented to demonstrate that these techniques andtechnologies have been implemented effectively by othercompanies. The techniques are discussed in a low-tech to high-tech order. It,isimportant to keep in mind that low-tech low-cost approaches are often the most effective for preventingpollution.

Purchasins and inventory controlHundreds of different chemicals and raw materials used by dyeingand finishing operations. Consumption of these materials rangesfrom 5 lbs. per year to several tons per week. Processperformance can be affected dramatically by changes in theconstituents of these raw materials, particularly when tracecontaminants exist in the chemicals or raw materials. Trackingraw material quality and managing each of these raw materialsrequires significant effort. The effort is rewarded, however,with fewer off-quality products and reworks. Chemical screeningand quality control are two ways that have proven effective forreducing waste management headaches and costs.


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Chemical screeninq. Many companies have established a committeeto review chemicals used and to review new chemicals before theyare approved for use to ensure that the chemicals do not containingrsdients that may cause water or air pollution problems,increased risk to workers, or increased hazardous waste.Criteria for evaluating chemicals may include:aaaaaaaa

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worker safety effectsheavy metal contentbiodegradabilityaquatic toxicity potentialinclusion on list of EPA priority pollutantsSARA listed compounds (particularly 33/50 chemicals)hazardous waste characteristicsVOC and hazardous air pollutant contentcost and performanceozone-depleting chemical content

Increasingly, substitutes are becoming available to replaceharmful components with less toxic and less regulatedalternatives. Chemical substitution is discussed further in alater section of this paper.Ensurinq auality of raw material shiDments. Variation in thechemical and physical properties of raw materials can have adramatic effect on the quality of the dyeing and finishingprocesses.is prior to use rather than after time, energy, and otherresources have been invested in using the material. Simplephysical tests (pH, viscosity, density, color, etc.) conducted onraw materials and compared to test results from a standard forthe material can reveal changes in the raw material in time forappropriate process adjustment or return of the material to thesupplier.Frequently raw fiber and chemicals contain significantundesirable impurities which may cause pollution problems. Testsof raw materials and chemicals for contaminants may help pinpointa source of wastewater pollutants that the wet processor may nothave anticipated.concentration from 75 to 100 mg/kg, and sodium sulfate oftencontains significant levels of zinc. The extent of tracecontaminants in raw materials, especially natural minerals andfibers, is highly dependent on the source of the material.example, cotton produced in certain southwestern areas of theU.S. has been shown to have elevated levels of arsenic.

The best time to detect changes in the raw materials

Cotton fibers may contain metals ranging in

For

Inventory manaqement. To minimize the quantity of surpluschemicals required to be managed as hazardous waste, it isimportant to order chemicals in reasonable quantities and usefirst-in-first-outinventory management principles to ensure thatthe shelf life of chemicals is not exceeded. It is alsoimportant to store ingredients under proper conditions.instance, bagged ingreaients and fiber drums should not be storedFor


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on a wet dyehouse floor. When materials are spilledaccidentally, they should be swept or vacuumed up in a dry formwhen possible rather than simply being washed down the drain.

Water and chemical conservationWater and chemical conservation offers one of the greatest wastereduction and cost reduction opportunities for wet processors.Water conservation. Frequently, mills can reduce water.use by 30percent or more with no decrease in product quality.*sources of unnecessary water waste include:

Common

0 hoses and faucets left running,0 leaking fittings, valves, and pumps,0 excessive water use in washing and rinsing operations0 excessive water use in clean-up procedures, and0 cooling water left running when machinery is not operatingBekause much of the energy required for wet processing operationsis used for heating process water, reducing water consumption andreusing water lowers energy costs substantially.Water reuse. Of all wet processing operations, washing andpreparation operations typically consume the greatest amount ofwater. These operations also offer the greatest opportunity forwater conservation. Continuous preparation operations can employcountercurrent washing to allow water to be used several timesbefore it must be discharged. Countercurrent washing is used inmany mills for desizing, scouring, mercerizing, bleaching,dyeing, and printhouse soaper ranges.3Rinsewater can be used as make up for subsequent batch orcontinuous processes.scouring range can be used to make up the.scouring bath, or insome cases, as make up for desizing. Mercerizing or bleachwashwater can be used for either scouring or desizing operations.

As an example rinse water in a continuous

As color and chloride limits for wastewater discharges becomeprevalent, the incentive for reusing dyebaths will increase asboth of these dyehouse wastewater components are difficult andexpensive to remove. Dyebath reuse may reduce the need for colorand chloride removal. Water costs and wastewater management costswill also be reduced with dyebath reuse. Because many of thedyes and chemical specialties are not completely exhausted, theresidual chemicals can be accounted for as the baths arezeconstituted, thereby lowering chemical costs. Energy savingsshould be realized as well because the recycled dyebath generallystarts at a higher temperature. Bleachbaths and caustic can berecovered and reused as


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Clean-up procedures. It is not unusual to see a dyehouse ordrugroom worker turn on a faucet or hose and leave the waterrunning for several minutes while he attends to some other need.Meanhhile large volumes of water are wasted. Automatic shutoffvalves and nozzles can help reduce the likelihood of hoses beingleft running. In-line flow restrictors and flow control valvesare also useful for reducing water consumption for clean-up. Ifscoops and handling utensils are dedicated to individualingredients, the need to clean each utensil after each use can beeliminated.

CASE STUDY - WATER CONSERVATION & REUSEAMITAL S P I N N I N G CORPORATION - N e w B e r n , N . C .A m i t al S p i n n i n g Co rp. i n s t i t u t e d a w a t e r c o n s e r v a t i o n andr e u s e p ro gr am . N o n co n ta c t c o o l i n g w a t e r i s r e u s e d f o r dyel i q u o r p r e p a r a t i o n , and s p e n t d y e b a t h s a r e r e u s e d wheneverp o s s i b l e . B e c a u s e the c o o l i n g w a t e r i s r m e r (SOOF) t hant a p w a t e r , s t e a m r e q u i r e m e n t s f o r h e a t i the d y e b a t h a r e;educed, and t i m e8 t o 1 0 m i n u te s p e r hs a l l o w s c h em i ca l st h a t r e m a i n i n the sed a s w e l l .Chemical cos t s fo p e r b a t c h lowert h a n f o r n e w b a t c h e s . W a t er u r e d u c ed f r o m3 2 0 , 0 0 0 t o 9 2 , 0 0 0 g a l l o n s roduc t ion h a si n c r e a s e d f r o m 12Es t i m a t e d co December 1 9 9 1t o t a l l e d $21s a v i n g s .

b a t h i s reduced by

o f y a r n p e r d ay .for w a t e r s a v i n g s and $ 5 2 1 , 1 0 0 f o r e n e r g y

Chemical conservation. Substantial quantities of chemicalspecialties and dyes are wasted in wet processing mills.Chemicals are either used in excessive amounts, usedunnecessarily, or spilled. These wastes cause potentialpollution problems and a drain on profits.to grow over time as new ingredients are added to counteract anundesirable effect caused by other ingredients.Dyebath recipes tend

An example of an ingredient that is often used unnecessarily orin excessive quantities is defoamer. The most judicious approachis to seek out the cause of the foaming problem and correct itrather than to add something else to counteract the effect.Foaming indicates that either too many other chemicals are beingused or that some of the chemicals in use foam excessively.Often adjusting temperature, increasing batch time, or optimizingpH can eliminate the need for specialty chemicals such asretarders or leveling agents. Optimized physical conditionsinstead of additional chemical ingredients improves the dyersability to repeat shades, thereby lowering the re-dye percentage

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and reducing dye use. Wastewater contaminants are reduced aswell since retarders and levelers are not typically exhausted i nnormal dyeing operations.One recent study found that in some cases up to half the totalwastewater pollutant loading comes from spillage, clean-up, andother nonprocess sources.6 Measuring, weighing, and handlingutensils should be large enough that ingredients do not r u n overor spill out during transport.

Chemical substitutionAs discussed, it is a good idea to review all chemicals used,particularly those used in large quantities, to identify thosethat may contribute to wastewater problems or other problems. Inmany situations, alternative chemicals are available f o rchemicals that contribute to particular environmental concerns.Process modification may sometimes be necessary to accommodatesubstitute chemical. Situations where chemical substitution isoften employed are discussed below.Metal containins dyes. Many dyes and pigments, particularlyblues and greens, contain metals such as copper and nickel aspart of the dye molecule. In many situations these metal-containing dyes can be replaced with dyes that do not containmetals or that contain lower metal content. An example issubstituting metal-free vat dyes for metal-containing direct orfiber reactive dyes for dyeing cellulosic materials. Typicallyfor direct dyes, only 85-95% of the metals exhaust into thefiber; the remainder is left in the dyebath and is dumped.Dyeing conditions should be optimized to maximize exhaustionwhere metalized dyes are used.Surfactants. Surfactants are contained in nearly every chemicalspecialty. Surfactants vary in their toxicity to aquatic lifedepending on their structure. The ultimate toxicity ofsurfactants is affected by the degree to which they biodegradeduring wastewater treatment processes. Relatively toxicsurfactants that biodegrade easily are actually less harmful toaquatic life than less toxic surfactants that do not biodegradewell. For example, ethoxylated octyl- or nonylphenol (AP)surfactants are less toxic than linear alcohol ethoxylates (LAE),but they are only 25% degradable, whereas LAEs are nearly 100%degradeable.6 The result is that LAE exhibits far less pass-through toxicity than AP. Therefore, LAE is commonly substitutedfor AP to help reduce effluent toxicity. Unfortunately, morebiodegradable surfactants generally have a higher biologicaloxygen demand (BOD),so much consideration must go intosubstitute surfactants to ensure that one problem does notreplace a different one.Phosphates. Effluent phosphorus limitations have forced manycompanies to evaluate strategies reduce phosphorus. A number of


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common processing chemicals contain high levels of phosphorusincluding buffers, builders for scouring, water conditioners, andsurfactants.for Almost every situation where phosphates are used. As was thecase with surfactants, many of the non-phosphate alternativechemicals may have a higher BOD or may be more toxic than thosethey replaced. Material safety data sheets do not alwaysindicate the presence of phosphates since they are not generallyconsidered hazardous materials.chemical suppliers to obtain information about phosphoruscontent.

Alternatives which do not contain phosphates exist

It may be necessary to contact

CAL SUBSTITUTION

Production Process ModificationOften, large reductions in water, chemical, and energy usage andpollutant loadings can be made through modifying the preparationand dyeing processes to make them more efficient.waste reduction through process modification is presented in thefollowing sections.

Examples of

Low liquor ratio dveins eauiDment. The amount of water requiredfor dyeing a pound of fabric, the liquor ratio, differs greatlywith the type of dyeing equipment used. Table 1 shows typicalliquor ratios for various types of dyeing equipment. Low liquorratio equipment not only reduces the amount of water used fordyeing, but it also reduces energy required for heating the waterand the amount of electrolyte and other chemicals required.Reducing electrolyte usage will become more important in theyears to come as effluent chloride limits become common. Manytypes of low-liquor ratio dyeing equipment have a relatively poorwashing efficiency. For this reason, a dyeing machine with a1O:l liquor ratio will not necessarily use half as much water asa machine with a 20:l ratio.


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Tab l e 1: Liquor ratio of dyeing equipment

~ Dyeing Water Consumption Typical liquorMachine gal./lb. of fabric ratioContinuousBeckJetJigBeamPackagePaddleStock

2 02 824122 02 23 52 0

1:l17:l12:15:11O:l1O:l40:l12 :1

Skein 30 17:l

For low liquor ratio equipment it is important to consider theeffects of water conservation on the ultimate concentration ofpollutants in the effluent stream. Many municipalities haveallowed dischargers to switch their permitted discharge limits toa pounds-per-day basis rather than a concentration basis.Companies with such limits are, therefore, not penalized forconserving water.

Process Control. A great deal of water and chemical conservationcan be accomplished by more closely controlling water andchemical inputs. Water levels in dyeing equipment should bevaried depending upon the size of the load. Reducing the filllevel also reduces the quantity of chemicals required to achievethe desired concentration.Pad batch dyeing. Pad batch dyeing eliminates the need for saltand chemical specialties from dyebaths, and it helps reduceenergy requirements since it is a cold process. Pad batch isapplicable for dyeing cellulosic fibers with fiber reactive dyes.Pad batch dyeing requires only about 2 gallons of water per poundof fabric compared to approximately 20 gallons per pound with anatmospheric beck. It is particularly appropriate for dyeing darkcolors such as forest green that require large quantities ofel-ectrolyte when dyed with direct dyes. Shade repeatability,reduced labor requirements, and high production speeds are otheradvantages of the pad batch method.2


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CASE S TUDY - PROCESS CONTROLCRAFTSMEN FABRICS INDUSTRIES - Concord, N . C .

Craftsmen Fabr ics Ind us t r ie s modi f i ed i t s dye ing machineproces s con t ro l l e r s t o a ll ow the equ ipmen t t o be f i l l e d t ovary ing l e v e l s depending upon load si z e . Chemical usage wasvaried accord ingly. Because l e s s water needed t o be heated ,l e s s energy was re qu ire d. Depending upon th e type o f dyeingmachine used t o ta l co st red uc tio n ranged from 1 t o 4 c e n t sper pound of fa b r i c processed. Total annual cost savings arees timated to be $ 1 7 2 , 0 0 0 . 8

Vacuum or mechanical extraction. If excess solution is extractedfrom goods prior to rinsing and washing operations, less carry-over or drag-out of concentrated dyeing solution into subsequentprocesses occurs. The result is that the number of rinsing stepsrepired can often be reduced. One manufacturer was able toreduce the number of wash boxes from eight to three simply byinstalling vacuum extraction equipment on the wash boxes.Heat Recovery. Tremendous opportunities exist for recoveringenergy from textile wastewater. Wastewater dischargetemperatures often exceed 130F compared to raw watertemperatures of approximately 5 5 O . Many companies have reclaimedthis excess heat from wastewater using heat exchangers and usingit to preheat incoming water. One company is saving more than$1,000 per day by recovering waste heat.g

SummaryPollution prevention offers textile manufacturers an opportunityto reduce water pollutant loadings and VOC and HAP emissions andsave money at the same time. Many companies may be able to comeinto full compliance with all environmental regulations withoutinstalling expensive pollution control equipment. Pollutioncontrol equipment can only pay for itself through avoidedenvironmental fines and other intangible costs such as improvedenvironmental quality and improved public relations. The casestudies presented here show companies which have reduced waterpollutants and air emissions with less than one year paybackthrough reduced water, chemical, and energy use. These companiesalso realized the intangible savings mentioned above.Pollution prevention has been proven by countless companies to bethe most cost-effective approach to environmental protection.The North Carolina Pollution Prevention Program is available tohelp companies identify opportunities for pollution prevention.Companies interested in receiving free non-regulatory technicalassistance should call ( 9 1 9 ) 571-4100. .


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REFERENCES

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Smith, Brent. "Chemical Screening and Inventory Control."American Dyestuff Reporter. June 1987. pp. 24/26.Smith, Brent. A Workbook for Pollution Prevention by SourceReduction in Textile Wet Processing. North CarolinaPollution Prevention Program. Raleigh, N.C. 1988."Water Conservation for Textile Mills." North CarolinaPollution Prevention Program. Raleigh, N.C."Dyebath and Bleach Bath Reconstitution for Textile Mills."North Carolina Pollution Prevention Program. Raleigh, N.C.1992 Governor's Awards for Excellence in Waste Manaqement.Presented April 22, 1993. Raleigh, N.C.Smith, Brent. Identification and Reduction of Toxic. Pollutants in Textile Mill Effluents. North CarolinaPollution Prevention Program. Raleigh, N.C. 1989.Accomplishments of North Carolina Industries: CaseSummaries. North Carolina Pollution Prevention Program.Raleigh, N.C. 1987.Bishop, Clifton. Craftsmen Fabrics Industries, Inc.Personal Communication. 1994.Smith, Brent. Identification and Reduction of PollutionSources in Textile Wet Processinq. North Carolina PollutionPrevention Program. Raleigh, N.C. 1986.

Techniques for Pollution Prevention in Textile Wet Frocessing

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