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by W. B. Hart, Pantech, Inc.
Cotton Bleaching and Dyeing Wastes — A Specific Solution and a General Prescription COTTON bleaching and dyeing wastes are a problem in treatment and disposal. Not only their composition, but the irregularity of occurrence, make the handling of wastes from bleacheries, and the dye houses which usually are associated with them, one of the most difficult in all the waste treatment problems of industry. Usually such plants are not large; often they are not grouped as to any particular locality; and they are seasonal as to activity. Much of their output is material very subject to style and style changes in both design and coloration. The plant must be sufficiently flexible to meet these changes, and if dilution water is not available in ample supply, or is irregular in flow quantity, the problem is magnified many times, for the condition of lowest availability of dilution in the receiving stream must be the basic consideration to meet requirements of the state in which the plant is located. Characteristics of the Wastes
The wastes to be corrected usually are water solutions of caustic soda, sodium carbonates, soaps or detergents, sodium silicate, peroxide, sodium sulfate, wetting agents, and compounds designed for special purposes of treatment. Sulfuric acid is used also, but not in great quantity. The characteristics of the wastes that require correction prior to discharge to the stream are high alkalinity (pH), suspended matter, color, and biochemical oxygen demand (B.O.D.). The source of the wastes thus far considered is the so-called bleachery section of the processing. The wastes from the dyeing operations require reduction of color, biochemical oxygen demand, and suspended matter, and adjustment of hydrogen ion concentration. Treatment of these wastes will be considered separately.
Treatment of Bleachery Wastes
A plant in the eastern part of the United States was studied in order to devise procedures by which its wastes could be treated to correct those characteristics which create pollution in the stream into which the wastes are being discharged. The stream is seasonal in flow, and at its highest flow rate does not provide a dilution that could be considered sufficient for even moderate treatment procedures. As a result, a rather complete treatment is required. In the design of this treatment the effect of the highly variable waste composition became very evident. The wastes are discharged from a number of different machines, each with a specific purpose and using a different composition of solution through which the textile must pass. In some cases the washings of the material, following the actual treatment wash, can be discharged without treatment, for it usually is free of color, is of satisfactory pH, and contains little or no suspended matter. The B.O.D. may be somewhat high, but not so high that it will preclude disposal with minor correction. Treatment wastes from cleaning and bleaching of cotton goods show : Operation Quencher Singe wash Chemick squeeze Post-Chemick wash Sour squeeze White wash Color Pale yellow Gray Pale yellow Pale yellow Ignored Practically colorless
pH 4 . 0 to 5.0 5.0 to 6.0 11.5 to 12.5 6 . 5 to 10.0 2 . 0 to 3.5 6.5 to 7.5 B.O.D., P.P.M. 4000 to 4500 800 to 900 900 to 1100 20 to 30 Ignored 20 to 30
On occasion, foreign goods are treated in the bleachery, largely for cleaning or for color reduction or removal. Usually this operation is conducted in a sodium peroxide bath followed by washing. Wastes from
this treatment are colored ; this color must be removed before they can be merged with those just mentioned for reduction of pH, suspended matter, and B.O.D. These wastes will be discussed later. The first step in the correction of the bleachery wastes is the reduction of the pH to a level that will not interfere with later biological treatment. This reduction requires the introduction of an acidic compound to neutralize to the pH level desired. There often will be a choice of how this is done. One procedure is of higher capital cost and the other of somewhat higher operation cost but considerably lower capital cost. Stack gases, available from the power house, must usually be cleaned before they can be introduced into the exhauster which serves to collect them and to deliver them to the point of application. Cleaning is accomplished by a water scrubber tower followed by a water spray cooling tower from which the gas enters the exhauster. The discharge pressure of the exhauster drives the gas into a distribution line placed along, the bottom of a reaction trough. The line is perforated for release of the gas at a fairly uniform rate and progressive sections are reduced in diameter to maintain pressure throughout its length. The down-stream end is capped. Control of the pH is maintained by a meter which changes the speed of the exhauster up or down to maintain the desired pH at the outlet end of the trough. The flow from the reaction trough is then cleared of suspended matter by a flotation-type separator and moves to a two-stage recirculating biofilter for reduction of B.O.D. Take-off from the biofilter, on the basis of laboratory data, will meet the strictest state requirements. This procedure involves a considerable capital outlay, and, not VOL. 49, NO. 3
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infrequently, a considerable maintenance cost. T h e r e is some advantage, however, in the use of boiler house stack gases when there has been neighborhood complaint about these gases. T h e y also undergo cleanup. T h e purchase and installation cost of the scrubber, cooler, a n d piping, as well as the cost a n d installation and operation of the exhauster, can be avoided by the use of a n inexpensive acid for neutralization of the excess caustic to r e d u c e the p H . I n this installation a storage tank for the acid, a p u m p for its transfer, and a small-diameter pipeline for delivery to the reaction trough, will be the only e q u i p m e n t necessary. T h e p H control system must be retained to take care of the acid delivery rate. From this point on the systems are identical. Neutralization with acid provides no help in control of complaints arising from the boiler house stack gases. M e n t i o n was m a d e earlier of a sodium peroxide kier b a t h for cleaning and decolorizing foreign cotton goods. This process usually is carried out in a kier from which a somew h a t highly colored, highly alkaline waste is discharged. This waste m a y be decolorized, usually by a chlorine bleaching t r e a t m e n t ; then it can be merged with the other wastes from the bleachery operations and introduced into the t r e a t m e n t accorded the other bleachery wastes. Decolorization of the peroxide kier wastes requires a ratio of from 300 to 400 parts of chlorine to 1,000,000 pounds of waste water. T h e a m o u n t varies with the different kinds of dyes used on the material being processed a n d with the extent of soiling the goods have experienced. Presentday processing seems capable of meeting all requirements. Dirt and Grease Must Be Removed Prior to Bleaching One. of the outstanding needs of all bleachery processing is that the material being treated be carefully cleaned a n d freed of all oils or greases. T h u s one of the earliest steps in the bleachery t r e a t m e n t is the elimination of these interfering substances. T h e t r e a t m e n t for this removal is carried out in a steamheated kier with a solution containing caustic soda, sodium carbonate, 82 A
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a n d various detergents. T h e operation requires several hours at almost boiling t e m p e r a t u r e . Problems in wastes from this operation are high alkalinity a n d susp e n d e d matter, usually a deep red to reddish brown color, a n d high B . O . D . I n the course of the kiering operation a true color often is formed a n d remains in the waste after all suspended m a t t e r has been removed. This condition can be overcome in disposal by dilution, if a m p l e water is available, or by t r e a t m e n t with chlorine for decolorization. The latter procedure can be expensive, and the cost of a sewer to discharge to an a m p l e supply of dilution water soon can be recovered by not having to purchase chlorine and apply it. O n e successful procedure involves a chemical t r e a t m e n t of the waste with the cheapest acid available to produce a p H of 6.0 to 6.5. T h e waste is air blown for about one hour, the reaction then is changed to the alkaline side, a n d aeration continued for another hour. T h e waste t h e n is passed t h r o u g h a lift type of clarifier for removal of suspended matter. This can be retained in a s u m p until sufficiently drained of water, then mixed with oil a n d b u r n e d in a small rotary kiln. T h e clarified waste water, a t this point practically free of color, still will require B . O . D . reduction. This condition can usually be corrected by flowing the waste over a saw tooth side of a distributing trough to a cleaned and prepared area of ground measuring about 10 to 12 sq. feet per p o u n d of B . O . D . It is i m p o r t a n t t h a t the area for the flow t r e a t m e n t be prepared for not too great slope, which would cause too m u c h channeling. This reduces the bacteria action time, giving an insufficient B . O . D . reduction. Further, since there seems to be no great need to cover the area, heavy rains m a y cause channeling on too great a slope and there will be insufficient time for oxidation. T h e lower end of the treating area again is a trough extending the full width of the bed. It should be built so the back is high to turn the flow to the end most convenient for disposal to the receiving stream. Some Caustic Recovery Is Possible W h e r e the plant is equipped for mercerizing, the recovery of caustic
is entirely possible and profitable t h r o u g h dialysis of the waste. I n some cases, where there is not a great a m o u n t of waste caustic from mercerizing, there is little need to concentrate the waste to m o r e t h a n the 1 to 3 % it contains. But when there is a large a m o u n t of the waste to be disposed of, it can be concentrated to perhaps 50 to 6 0 % caustic strength a n d charged to a dialyzer. Theoretically about 5 0 % of the sodium hydroxide is recoverable in clean solution for re-use. This can be an i m p o r t a n t saving not only in the outlay for caustic, but even more so in the cost of treating the waste for disposal. T h e plant engineer readily can determine w h a t other wastes can be included in the caustic recovery operation because of high content of this material.
Wastes from Dyeing Operations It is not unusual for a cotton goods bleachery to be equipped for dyeing the materials it prepares. Some dyes, particularly sulfur colors, give a waste carrying a rather high concentration of suspended dye m a t e rial. O t h e r dyes produce wastes carrying true color. Wastes of this nature can be corrected by flocculation, followed by biofiltration for reduction of the B.O.D. I n t e r m e d i a t e between the flocculation treatment a n d the biofiltration, a lift-type air clarifier will be very helpful in removing the suspended matter, which then can be p o n d e d a n d burned. Bleachery plant wastes present one of the most complicated problems in all waste t r e a t m e n t experience. T h e treatments required must meet the d e m a n d s for which they are designed, itself a problem, and at the same time must be sufficiently flexible to take care of constantly changing waste characteristics. T h e r e is comparatively little information available for use in devising t r e a t m e n t for cotton bleaching and dyeing plants. Further, good treatment requires such extensive e q u i p m e n t that only plants of comparatively large size can afford the cost. T h e r e is great opportunity for research on these wastes, a n d development of processes t h a t will be more a d a p t a b l e to wastes which vary in composition, and can be provided at lower cost.
INDUSTRIAL A N D ENGINEERING CHEMISTRY For further information, circle number 83 A on Readers' Service Card, page 135 A
