A New Development For (Textile Mill) Wastewater Treatment. Prepared for Publication in The American Dyestuff Reporter June 1988

Transcription

1 A New Development For (Textile Mill) Wastewater Treatment Prepared for Publication in The American Dyestuff Reporter June 1988 By Timothy R. Demmin, Ph.D., and Kevin D. Uhrich Andco Environmental Processes, Inc. Amherst, New York

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3 Introduction The textile industry, from its beginnings, has been hampered by the large volumes of water required for the preparation and dyeing of cloth. More recently, water consumption and waste generation have become considerable concerns for textile manufacturers and finishers. Textile industry wastewater is characterized primarily by measurements of BOD, COD, color, heavy metals, total dissolved and suspended solids. BOD, or biochemical oxygen demand, is the measure of the oxygen consuming capabilities of organic matter. Water with high BOD indicates the presence of decomposing organic matter and subsequent high bacterial counts that degrade its quality and potential uses. COD, or chemical oxygen demand, measures the potential overall oxygen requirements of the wastewater sample, including oxidizable components not determined in the BOD analysis. Color is defined as either true or apparent color. True color is the color of water from which all turbidity has been removed. Apparent color includes any color that is due to suspended solids in the water sample. Color and turbidity both cause an aesthetic and real hazard to the environment. The aesthetic value considers the discoloration of recreational streams and waterways. The real hazards caused by color and solids in waste are dye toxicity and the ability of the coloring agents to interfere with the transmission of light through the water, thus hindering photosynthesis in aquatic plants. Heavy metals, typically chromium and copper, are very hazardous to human and aquatic life at relatively low concentrations. Heavy metals are introduced into the wastewater of textile manufacturing through the use of premetalized dyes and heavy metal afterwashes, which are used to increase the light fastness of the finished product. Total dissolved solids, or TDS, characterize the general purity of water and is often largely due to soluble ions such as sodium, chloride, and sulfate. Obviously, high TDS is detrimental to fresh water aquatic life. There are many technologies currently available for treating wastewater from the textile industry. Included are: 1) biological treatment, 2) chemical precipitation, 4) ultrafiltration, 3) carbon adsorption, and 5) oxidation with ozone. These approaches, along with their associated pros and cons, are outlined in Table 1. The main drawback with these technologies is that they generally lack the broad scope treatment efficiency required to reduce all the diverse pollutants present in textile wastewater. However, when one approach does look promising then the capital costs, or operating costs often become prohibitive when applied to the large scale water needs common to this industry. Electrochemical technology developed in the 1970's by Andco Environmental Processes, Inc. of Arnherst, New York, has shown that many of the major chemical components in textile industry wastewater can be effectively and economically removed. The scope of this new electrochemical treatment is a significant advance to the existing technologies. Electrochemical Wastewater Treatment The electrochemical technology was developed over fifteen years ago for removing hexavalent chrome and other heavy metals from wastewater at very high flowrates. The process uses sacrificial iron electrodes to produce ferrous hydroxide in solution which chemically reacts with hexavalent chrome and coprecipitates other heavy metals, e.g. Cu, Co, and Ni, to produce a disposable hydroxide sludge. The process very simply involves passing the aqueous waste between carbon steel plates across which a DC current is applied. These sacrificial iron electrodes generate Fe(+2) and OH- according to Faraday's Law (see Equation 1). As the insoluble Fe(OH)* forms and precipitates from solution soluble and insoluble pollutants are removed from the effluent (see Equations 2 and 3).

4 The original application of the electrochemical technology in the textile industry was for the removal of heavy metals. However, it was soon discovered that besides heavy metal removal the system was very efficient at color, BOD, and COD reduction. A number of experiments on synthetic wastes and actual textile mill wastewater have been conducted and the results are presented here. It will be shown that the benefits of the electrochemical system include: 1. Reduction of BOD and COD from wastewater in the range of 50-70% with retention times of less than 10 minutes. 2. Removal of color constituents produced from a wide variety of dyes and pigments including soluble and insoluble dyes. 3. Coagulation of the total suspended solids present in the wastewater. 4. Heavy metal removal down to ug/l ranges. 5. Unlimited flow capability. BOD/COD Reduction The level of BOD and COD is a critically important factor in evaluating the extent of "organic" pollution in textile wastewater. Substantial sewer surcharges are often imposed when the local POTW limits are exceeded. Depending on the type of fibers, dyes and additives, various textile operations will routinely generate water having levels of BOD in the range mg/l and COD in the range mg/l. Discharge permits may typically restrict these parameters to 250 mg/l and 500 mg/l respectively. In numerous preliminary electrochemical tests at Andco on typical textile wastewater samples, BOD levels were generally reduced by 30-55% and COD values were lowered by 50-70%. Results are contained in Tables II and III. The BOD and COD reductions presumably result from adsorption of the organic constituents onto the precipitating iron hydroxide matrix. The resulting sludge, which in many cases should be delistable as a non-hazardous material, may then be disposed in a sanitary landfill.

5 Color Reduction The presence of color in textile effluent is obviously visual pollution. More importantly, many dyes biodegrade slowly and are potentially poisonous to plant and animal life. Guidelines for color assessment in textile wastewater are nebulous at this. time primarily because no economical technology exists to treat the flowrates involved. Situations are usually described as a "color problem" which the local POTW or on-site biological system cannot adequately handle. The application of Andco's electrochemical technology to color removal has involved tests on pure dye samples as well as actual textile effluent. Table IV shows dramatic results of color removal from water containing various dye classes. These results indicate that electrochemical color removal is very broad in scope and efficient in many cases. It is significant that in actual wastewaters containing mixtures of color components, the extent of color removal is usually "across the board", i.e. the color removal occurs over the entire visible spectrum. As an example; Figure 1 represents the electrochemical color removal from a cotton textile manufacturer's effluent containing a mixture of fiber reactive dyes. Figure 2 is another example of a reactive dye mixture, an intensive shade of red from an exhausted cotton dye bath, that is successfully treated using 300 mg/l of electrochemically generated iron. Figure 3 shows how a mixture of blue dye components containing copper phthalocyanine, as well as other dyes, undergoes effective color removal employing only 100 mg/l of iron. All of this data in Table IV and Figures 1,2, and 3 are indicative of the wide ranging capabilities of the electrochemical approach to color removal. The spectra in Figures 1,2, and 3 were obtained on a COLOR GRAPH double beam spectrophotometer generously provided by the Milton Roy Company/Analytical Products Division of Woburn, Massachusetts. In addition to removing the various types of synthetic dyes from wastewater, the electrochemical technology can also effectively treat natural coloring agents, e.g. the dark brown coloring associated with cola beverage production (see the results of Sample 3 in Table IV) and lignins associated with the bleaching operations in the paper and pulp industry. These dark brown-black species degrade the environment aesthetically and can severely damage the quantity and quality of fish and plant life. Preliminary "color removal" tests on pulp bleaching wastewater look promising and further studies are planned. With regard to suspended solids removal, the iron hydroxide matrix generated electrochemically is especially suited to virtually eliminating this component from effluent water. Regardless of influent levels most treated samples contain residual TSS in the range mg/l after a standard clarification. Heavy Metal Removal As mentioned earlier, Andco's patented electrochemical process was originally developed for heavy metal removal and it was for this direct application that the first textile effluent was specifically tested. This rinsewater was comprised of premetalized dyes containing Cu, Cr, and Co used in the manufacture of automotive upholstery. The very low heavy metal discharge limits were easily achieved by the electrochemical process and the results are contained in Table V. Table VI and Figure IV represent the metal removal data for a single sample of actual textile wastewater treated with increasing levels of electrochemically generated iron. This data represents the typical shape of the removal functions seen with most contaminants. Irregularities in the Cu and Co data are due to increased analytical error close to the detection limits of these metals.

6 Theoretical Interpretation The mechanism for removing varied contaminants from wastewater is not fully understood. It is believed to be primarily an adsorption process with the various organic and inorganic contaminants being adsorbed onto the iron hydroxide matrix that is formed in the electrochemical cell. Assuming equilibrium is attained during treatment, data from adsorption phenomena often follow the Freundlich isotherm equation. The Freundlich isotherm equation can be mathematically represented by q e = K F Ce 1 /n where K F and n are constants qe = weight of contaminant adsorbed per weight of adsorbant and Ce = the final equilibrium concentration of contaminant In the case of our tests q represents the reduction of contaminant per ppm of iron added and Ce represents the final contaminant level. This equation will generate a straight line if the two variables are plotted on logarithmic scales. This was done for the cobalt data listed in Table VI and the results are presented in Figure 5. A linear regression was performed on the logarithmic data and a correlation coefficient of 0.98 was attained. Although this quick check does not prove that all the contaminants are removed through an adsorption process, it does support this hypothesis for heavy metal removal. To conclusively prove that true adsorption is occurring, extensive controlled laboratory tests must be performed. Water Reuse Thus far, data has been presented to show how electrochemically generated ferrous hydroxide removes various pollutants from textile effluent. The treated water is then suitable for discharge to surface waters or to a POTW for further purification. However, textile finishers use vast quantities of water often at a price which contributes substantially to the overall cost of production. Thus, many producers are interested in the " water reuse capability in any treatment scheme since this would represent an attractive payback on their investment. Water quality is critical to all textile dyeing operations and each process facility has its own stringent requirements. Experiments on reusing electrochemically treated effluent on a case by case basis are being conducted at several major textile mills. Summary Electrochemically generated ferrous hydroxide removes a broad spectrum of components in textile processing wastewater, including BOD, COD, color, suspended solids, and heavy metals. Depending upon the type of effluent the efficiency in pollutant removal is often greater than 90%.

7 Table 1: Textile Wastewater Treatment Current Technologies Description Area of Efficiency Disadvantages Biological Treatment Activated sludge, extended aeration, aerated lagoons, land treatment Efficient BOD reduction Long residence times, may require nutrients, very large aeration tanks, lagoons, land areas, many toxic compounds not removed, variable color removal Chemical Precipitation Addition - precipitation with ph adjustment Heavy metals, suspended solids, BOD(?), COD(?) Color removal varies with dye class and dyeing process, little information on on BOD, COD removal, chemicals handling can be a problem 3) Activated carbon Passing water through a bed of carbon (as a pretreatment to further treatment) BOD,COD, color Expensive capital investment, long residence times,low adsorption capacity, frequent and expensive regeneration 4) Ultrafiltration Permeation of water under pressure through specialized polymeric membranes BOD, COD, color Membranes foul easily, heavy metals not removed, frequent cleaning or replacement of membranes 5) Ozone Ozone, generated by electrical discharge, is used to oxidize organics BOD, COD, color Very expensive capital investment, heavy metals and solids require separate treatment Table II: Electrochemical Treatment BOD 5 Reduction Source Description Influent Electrochemical B0D 5 (mg/l) Iron(mg/L) Effluent BOD 5 (mg/l) 96 Reduction Nylon dye mixture containing Cu Cotton dye mixture of vat, disperse, sulfur, and reactive dyes Nylon carpet Mill # Nylon carpet Mill # Nylon carpet Mill #

8 Table III: Electrochemical Treatment COD Reduction Source Description Influent COD(mg/L) Electrochemical Iron(mg/L) Effluent COD(mg/L) 96 Reduction Nylon carpet Mill #I Nylon carpet Mill #2 Nylon carpet Mill #3 Nylon carpet Mill #4 Nylon carpet Mill #5 Nylon dye mixture containing Cu Cotton dye mixture of vat, disperse, sulfur, and reactive dyes Table IV: Electrochemical Treatment Color Removal Source Description Influent Color: Absorbance (nm), Pt-Co Units or mg/l Electrochemical Iron (mg/l) Resulting Effluent Color (same units) 96 Reduction in Color 1. Nylon dye mixture containing heavy metals (400 nm) Cotton dye mixture of vat, disperse, sulfur, and reactive, dyes (400 nm) (465 nm) (525 nm) (600 nm) I Beverage product natural coloring agents 7412 Pt-Co Units (465 nm) 7412 " Brilliant blue, fiber reactive dye (1:l000 dilution) (400 nm) Brilliant red, fiber reactive dye (1:l000 dilution) 6.00 (525 nm) Acid red (532 nm) Basic red mg/l mg/l 90

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