Article pubs.acs.org/est
Remediation of Environmental Pollution by Substituting Poly(vinyl alcohol) with Biodegradable Warp Size from Wheat Gluten Lihong Chen,†,‡ Narendra Reddy,‡ and Yiqi Yang*,†,‡,§,∥ †
Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, Donghua University, Shanghai, China 200051 Department of Textiles, Merchandising & Fashion Design, §Department of Biological Systems Engineering and ∥Nebraska Center for Materials and Nanoscience, 234, HECO Building, East Campus, University of Nebraska-Lincoln, Lincoln, Nebraska 68583-0802, United States
‡
ABSTRACT: We report the development of wheat gluten as an environmentally friendly sizing agent that can replace poly(vinyl alcohol) (PVA) and make the textile industry more environmentally friendly. Wheat gluten applied onto polyester/cotton (P/C) and polyester as warp sizing agent provided sizing performance and biodegradability in activated sludge necessary to substitute poly(vinyl alcohol) (PVA). PVA is one of the most widely used sizing agents and provides excellent sizing performance to synthetic fibers and their blends but is expensive and difficult to degrade in textile wastewater treatment plants. Although considerable efforts have been made to replace PVA, it has not been possible to develop a warp sizing chemical that can match the sizing performance of PVA and at the same time be cost-effective and biodegrade in effluent treatment plants. At similar % add-on, wheat gluten provided similar cohesion to P/C but much higher abrasion resistance to polyester fabrics compared to PVA. With a biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratio of 0.7 compared to 0.01 for PVA, wheat gluten was readily degradable in activated sludge. Wheat gluten has the ability to replace PVA for textile warp sizing applications.
1. INTRODUCTION Using renewable materials and eco-friendly processing with clean technologies are essential for the textile industry to be sustainable in future.1,2 Textile processing involves considerable use of materials, labor, energy, and other resources and warp sizing/slashing is a critical and essential step in producing woven products.3 During sizing, a protective layer is imparted to enable warp yarns withstand the abrasive forces during weaving. After weaving, a process called desizing is used to remove the size on the fibers and the size is later released into the wastewater. Sizing and desizing processes are energyintensive, consume large amounts of water, and release nondegradable chemicals into the effluents that have led to environmental concerns.4 Processing a ton of textiles is estimated to consume about 80−100 m3 of water and release 115−175 kg of COD per ton of finished product.5,6 Sizing chemicals are reported to be responsible for the high COD in textile effluents and desizing process consumes large amount of water. Therefore, attempts have been made to develop new sizing chemicals and/or sizing technologies that can reduce pollution and provide good sizing properties. Traditionally, starch and starch derivatives for cotton and PVA for synthetic fibers and their blends have been used as sizing agents.7 Although starch-based sizes are inexpensive and provide good performance properties on cotton, they are unsuitable for synthetic fibers and their blends. Similarly, PVA provides excellent sizing performance but is relatively © 2013 American Chemical Society
expensive. Poor degradability in textile effluent treatment plants is another major disadvantage of using PVA as warp size. PVA has been found to be prevalent in effluent water released from textile plants. Studies have shown that only 60% of PVA was degraded under aerobic conditions after four months and 0−12% of PVA was degraded in anaerobic sludge after 77 days.8 PVA has a low BOD5/COD ratio 0.01 indicating the poor biodegradability.9 It is therefore preferable to avoid using PVA for textile warp sizing. Unfortunately, it has not been possible to develop a cost-effective alternative to PVA that can provide the sizing performance similar to PVA and also be degradable in textile wastewater treatment plants. For instance, starch has been chemically modified by grafting and other techniques to make starch suitable for sizing synthetic fibers and their blends.10 Also, chemical modifications decrease the biodegradability of starch and increase the price making modified starch unattractive as a sizing chemical compared to PVA. Ability to form films on the surface of the fibers, adequate cohesion between fibers, and resistance to abrasive forces during weaving are the primary requirements for a good sizing chemical.7 In addition, a good size should also be able to be Received: Revised: Accepted: Published: 4505
October 30, 2012 March 19, 2013 March 20, 2013 April 3, 2013 dx.doi.org/10.1021/es304429s | Environ. Sci. Technol. 2013, 47, 4505−4511
Environmental Science & Technology
Article
depending on the desired % add-on) up to 90 °C, held at that temperature for 30 min and later sized at 90 °C based on the conditions recommended by the manufacturer. Sized materials were air-dried and later heated in an oven at 105 °C to determine the dry weight. Differences in the dry weights before and after sizing were used to calculate the amount of size (% add-on) on the materials. Sized materials were conditioned at 65% relative humidity and 21 °C until a constant weight was obtained before testing 3.2. Properties of the Sizing Solution. A rheometer (Brookfield, model R/S plus) was used to determine the viscosity of the sizing solution at various conditions. A CC-25 spindle and cup and 70 g of solution was used for each measurement at a shear speed was 3500 s−1. Viscosity measurements (mPa.s) were done at 90 °C using a water bath. At least three separate measurements were done for each sample and the average and ± one standard deviation are reported. 3.3. Evaluating Sizing Performance. Properties of the sized materials were determined in terms of increase in tensile strength, abrasion resistance and cohesiveness. Tensile properties of the rovings tested on an MTS tensile tester (model Q test 10) using a gauge length of 10 cm and a crosshead speed of 50 mm/min, and were used as a measure of the cohesiveness of the size to the fibers. Yarn tensile properties were determined on an Instron tensile tester (model 4444) using a gauge length of 10 cm and a crosshead speed of 18 mm/min. At least 20 samples were tested for each condition and the experiments were repeated three times. Average and standard deviation between the three replications were calculated and reported. Fabric abrasion resistance before and after sizing was determined on a CSI universal Wear Tester using Norton “0” grain emery paper. A weight of 1 lb was used for the polyester fabrics and 1/2 lb for the P/C fabrics. The number of cycles required to create a hole in the fabric was used as an indication of the resistance to abrasion. At least six samples each from three different experiments for a total of 18 specimens were tested for each condition and the averages and ± one standard deviations for the three groups are reported. 3.4. Desizing. Desizing (removal of size) was done by treating the wheat gluten and PVA sized fabrics with a known amount of % add-on in water at different temperatures and wash/rinse times. After desizing, the fabrics were dried in an oven and weighed. Differences in the weight of the fabrics before and after desizing were used to calculate the % size removal. 3.5. Film Preparation and Properties. Films were prepared by heating 6% wheat gluten in 0.5% alkali at 90 °C for 30 min. After heating, the solution was cast onto Teflon coated glass plated and allowed to dry at 20 °C and 65% humidity for about 3 days. PVA (6% w/w) dispersed in water was heated to 90 °C, then cooled to 50 °C and also cast to form films. Films formed were tested for tensile properties on a MTS tensile tester (QTest 10) according to ASTM Standard 822e. According to the standard, film samples measuring 8 cm x 1.5 cm were cut and tested using a gauge length of 2 in. and crosshead speed of 10 mm/min. At least 20 samples from three different films were tested for tensile properties and the averages and standard deviations are reported. 3.6. Biodegradation. 3.6.1. COD and BOD. Ability of the wheat gluten and PVA used for sizing to degrade in wastewater treatment plants was studied. Parameters such as chemical oxygen demand (COD), biochemical oxygen demand (BOD5),
removed (desized) from the fabrics after weaving because incomplete desizing creates problems in dyeing and further finishing of the fabrics.11 In addition to food applications, plant proteins such as wheat gluten and soyproteins have been made into films, fibers, nanoparticles, and extrudates for various applications.12 Wheat gluten also has excellent binding properties and has been used as adhesive and also as matrix for composites. With respect to textile applications, wheat proteins (gluten and gliadin) have been made into regenerated protein fibers.13,14 It has also been demonstrated that wheat gluten could be used as a binder in textile printing paste.15 Low cost, excellent film forming properties, adhesiveness, and easy biodegradability make wheat gluten a very attractive choice as a sizing agent. However, the ability of wheat gluten to form film on the surface of fibers and the performance of the sized materials have not been studied. It is also necessary to understand the biodegradability of wheat gluten in wastewater treatment plants to be used as a biodegradable size to substitute PVA. In this research, wheat gluten has been used to size P/C and polyester rovings, yarns, and fabrics. Effect of size preparation and sizing conditions on properties of the sized materials have been compared with the same materials sized with commercially used PVA. Ability of the wheat gluten to degrade in activated sludge has also been determined in terms of changes in COD, BOD5 and total and ammonia nitrogen released.
2. MATERIALS Wheat gluten (Whet pro 80) containing 80% proteins used for the study was kindly supplied by Archer Daniels Midlands Company, Decatur, IL. P/C (65/35) rovings (70s Hank) and ring spun yarns (15s Ne) were supplied by Mount Vernon Mills, Mauldin, SC. Polyester (100%) staple fiber rovings and yarns were supplied by Shuford Yarns LLC, Hickory, NC. Fabrics P/C (65/35, type 7435) and 100% polyester (Dacron type 54) were purchased from Test Fabrics Inc., (West Pittston, PA). Reagent grade sodium hydroxide was purchased from VWR International, Bristol, CT. Two types of commercially available PVA based sizes were obtained from major sizing chemical manufacturers in the United States. PVA 1 was a fully hydrolyzed copolymer with a molecular weight (Mw) of approximately 65 kDa. Viscosity of PVA 1 was 11.6−15.4 cps at 4% solid content and 20 °C and pH of the solution was between 5 and 7. PVA 2 containing 80−85% PVA with the other ingredients being humectant, lubricant, defoamer had a hydrolysis value between 88 and 96% and a viscosity of 14−25 cps for 4% solid content at 20 °C. 3. METHODS 3.1. Sizing. Wheat gluten was used as a sizing agent after pretreating with alkali at different temperatures and time. The concentration of alkali used was varied between 0.01 to 0.1% with a soyprotein to water ratio of 1:20 and the pretreatment was carried out 90 °C for 30 min. After the pretreatment, the pH of the solution was about 9.5 and was adjusted to pH 8 by adding acetic acid to avoid damage to polyester fibers under high pH and temperature. Rovings and yarns wound on frames were immersed in the sizing solution at 90 °C for 5 min. Fabrics were also immersed in the sizing solution and later squeezed in a laboratory padder to ensure uniform size pick-up and penetration. Commercial size (PVA powder) used on the P/C and polyester materials was heated in water (1−2 wt % 4506
dx.doi.org/10.1021/es304429s | Environ. Sci. Technol. 2013, 47, 4505−4511
Environmental Science & Technology
Article
major effect on the strength or elongation of the polyester rovings whereas the strength and elongation of the P/C rovings were lower at high alkali concentrations as seen from Figure 1.
and changes in the concentration of total and ammonia nitrogen were monitored from 0 to 6 days. Activated sludge from local wastewater treatment plant was collected and acclimatized for 2 days under laboratory conditions (20 °C, 65% humidity and pH of the sludge was between 7.5 and 8). About 300 ppm of the sizing solution was added into the sludge and aerated in beakers using an oxygen pump. After the desired treatment time, the sludge with and without the sizing solutions was collected and centrifuged to remove solid materials. The supernatant obtained was used to measure the COD, BOD5 and nitrogen values. COD was measured according to U.S. Environmental Protection Agency method 800 using a high range (20−1500 mg/L) test kit (TNT 822e) supplied by Hach Company (http://www.epa.gov/glnpo/monitoring/sop/ chapter_6/LG602.pdf). BOD was determined according to American Public Health Association guidelines in the Standard Methods for the Examination of Water and Waste Water. Size samples (300 ppm) were added into the BOD bottles and incubated at room temperature (21 °C) without exposure to light for 1−5 days. BOD5 measurements were taken during initial incubation and after the COD had reached less than 100 mg/L. Water used for dilution was seeded and the total oxygen depletion of water was less than 0.2 mg/mL in five days. Levels of oxygen in the samples before and after five days of incubation were measured using a dissolved oxygen probe (Hach Company, HQ 440D multi). 3.6.2. Total and Ammonia Nitrogen. The total nitrogen in the sludge was calculated based on the alkaline persulfate oxidation digestion method and ammonia nitrogen was calculated using salicylate and hypochlorite in an alkaline phosphate buffer (http://www.epa.gov/greatlakes/lmmb/ methods/nh3lw.pdf). The buffer turns the effluent water green and the intensity of the color is proportional to the amount of ammonia in the sample. To determine the ammonia nitrogen, the sample was added with salicylic acid (50g/L), potassium sodium tartrate (50 g/L), sodium nitroferricyanide (10g/L), and sodium hypochlorite solution (3.5 g/L active chlorine). The mixture was allowed to stand for 1 h at room temperature. The samples were later measured on a spectrophotometer for absorbance at 697 nm. A calibration curve prepared with known concentrations of ammonia was used to determine the ammonia in the samples. Total nitrogen was determined by adding potassium persulfate into the sample and digesting the mixture at 120 °C for 40 min. After digestion, 1 mL HCl was added to terminate the digestion. The total nitrogen was determined by measuring the absorbance at 220 and 275 nm for the blank (distilled water) and the sample with size, respectively. The concentration of total nitrogen was calculated using calibration curves that had R2 > 0.999.
Figure 1. Tensile strength and elongation of the polyester/cotton (P/ C) and polyester rovings sized with wheat gluten pretreated with various concentrations (0.01−0.1%) of alkali. Sizing was done at 90 °C for 5 min and the % add-on on the Roving was 10 ± 1%. For each property, data points with different letters symbolize statistically significant difference.
Lower strength of the P/C rovings at high alkali concentrations should be due to the hydrolysis of proteins. Polyester rovings had 13% lower strength whereas P/C rovings had 38% lower strength when pretreated with 0.1% alkali compared to the properties when treated with 0.01% alkali. Poor affinity between hydrophilic wheat gluten and hydrophobic polyester should be the major reason for the relatively inferior performance of the wheat gluten size on polyester. In addition, wheat gluten contains about 20% starch which is a good sizing agent for cotton but not for polyester. Hence, there was a pronounced effect of wheat gluten on P/C rovings compared to polyester rovings. Wheat gluten size had lower viscosity (1.5 mPa.s) compared to 2 mPa.s for PVA which would also make the protein size to penetrate deeper into the fibers compared to PVA. Having viscosity similar to that of PVA is important because changes in viscosity are reported to adversely affect mechanical properties of the films and the sized materials.16 Wheat gluten is reported to have excellent binding ability and has been used as adhesive and also as a binder in printing paste.15 Without sizing, the Roving had almost no strength since the Roving was a loose assembly of fibers. Wheat gluten adhered to the fibers and provided cohesion leading to improved strength. Ability of wheat gluten to provide good improvement in tensile strength even when treated with low (0.01%) of NaOH meant that the sizing preparation would be simple and cost-effective. pH of the sizing solution had contrasting effects on the properties of the polyester and P/C rovings as seen from Figure 2. While the strength and elongation of the polyester rovings were higher at pH 9.5 compared to pH 8, P/C rovings had higher strength and elongation at pH 8. Differences in charge carried by the proteins at the two pHs should be responsible for the variation in the tensile behavior of the polyester and P/C Roving. Only two pHs were studied in this research because pHs lower than 8 caused precipitation of wheat gluten and the pH of the size solution after cooking was 9.5 and we did not
4. STATISTICS Data generated was analyzed using Tukey’s multiple-pair wise comparison using SAS program (SAS Institute, Raleigh, NC). A significance level of α = 0.05 was considered as statistically significant. In each figure, data points with significant differences are marked with different letters or symbols. 5. RESULTS AND DISCUSSION 5.1. Influence of Size Preparation Conditions on Properties of Sized Materials. Increasing the concentration of alkali used to hydrolyze the wheat gluten did not show any 4507
dx.doi.org/10.1021/es304429s | Environ. Sci. Technol. 2013, 47, 4505−4511
Environmental Science & Technology
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Polyester rovings sized with wheat gluten had much lower increase in strength compared to both the PVAs studied. Poor interaction between the hydrophilic wheat gluten and hydrophobic polyester should be the major reasons for the inferior strength of the polyester rovings sized with wheat gluten. In addition, gluten contains about 20% starch which is not a good size for polyester. Also, PVA formed a much stronger film (24.3 ± 3.9 MPa) with considerably high elongation (278 ± 34%) compared to 2 ± 0.3 MPa strength and 1.6 ± 0.4% elongation for wheat gluten films. Stronger film and higher elongation would provide the rovings sized with PVA higher tensile strength compared to wheat gluten. Additives, commonly present in sizing agents, including salts and plasticizers that can improve the attraction between polyester and wheat gluten and may be helpful to make wheat gluten more suitable to size polyester rovings. Based on the strength improvement in the rovings, wheat gluten is more suitable to size P/C to improve cohesion than on polyester. 5.3. Performance of the Sized Roving at Different Humidities. Tensile strength and elongation at three different humidities revealed that the P/C roving was more susceptible to changes in humidity than the polyester rovings as seen from Figure 4. Polyester rovings did not show noticeable change in
Figure 2. Tensile strength and elongation of the polyester/cotton (P/ C) and polyester rovings sized with wheat gluten at two different pHs. Sizing was done at 90 °C for 5 min and the % add-on on the Roving was 10 ± 1%. For each property, data points with different letters symbolize statistically significant difference.
add additional alkali to increase the pH of the sizing solution. The P/C Roving had higher strength than the polyester Roving due to the larger size, but lower elongation due to the inherently lower elongation of cotton compared to polyester. 5.2. Comparison of Fiber Cohesiveness between Wheat Gluten and PVA. Cohesion between fibers is necessary to provide good strength and abrasion resistance during weaving. Since rovings are a loose bundle of fibers without much strength, the strength of the rovings after sizing has been used a measure to determine the cohesiveness of the sizing agent. Figure 3 shows the comparison of the improve-
Figure 4. Influence of % relative humidity on the tensile properties of polyester and polyester/cotton (P/C) roving treated with wheat gluten and having an add-on of about 10 ± 1%. For each property, data points with different letters symbolize statistically significant difference.
strength or elongation when the humidity was changed between 55 and 75% but the P/C rovings had substantially lower strength and elongation at 55 and 75% humidity compared to the respective properties at 65% humidity. At low humidities, the fibers and the films formed on the surface of the fibers will have low moisture and the films will be relatively brittle. Therefore, the strength and elongation of the rovings decreased at 55% humidity. At a humidity of 75%, the proteins and the fibers contained high levels of moisture that probably softened the proteins and decreased the tensile strength but provided better elongation.16,17 Since polyester had low moisture absorption and was less sensitive to changes in moisture, no change in tensile properties was observed with the polyester rovings as the humidity was changed. 5.4. Resistance to Abrasion. Polyester fabrics sized with wheat gluten showed similar improvement in abrasion resistance whereas P/C fabrics had considerably poor abrasion
Figure 3. Comparison of the tensile strength of the polyester/cotton (P/C) and polyester roving at different % add-on of wheat gluten and two different PVAs. Sizing was done at 90 °C for 5 min.
ment in strength of the rovings treated with wheat gluten and two different commercially available sizing agents at various % add-ons. Overall, wheat gluten had relatively poor cohesion with polyester fibers compared to PVA. P/C rovings sized with wheat gluten had similar increase in strength (160 N) compared to P/C Roving treated with PVA2 (176 N) but much lower than that of PVA1 (226 N). In addition, a higher % add-on of 15.6% was necessary for wheat gluten sized rovings to obtain comparable improvement in tensile strength. 4508
dx.doi.org/10.1021/es304429s | Environ. Sci. Technol. 2013, 47, 4505−4511
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5.5. Desizing. Wheat gluten can be easily removed from the fabrics even at low temperatures as seen from Table 1. Easy removal of size from the fabrics (desizeability) is necessary to avoid problems during subsequent processing such as dyeing and finishing and removal of size at low temperatures and using low amounts of water would save energy and reduce effluents and pollution.18 More than 90% of the size was removed when the polyester fabrics were treated at 90 °C for 5 min using one rinse with water to fabric ratio of 5:1. At the mildest conditions (20 °C, 5 min and one rinse), the wheat gluten treated fabrics had 86% removal compared to 62% for PVA. P/C fabrics had relatively lower size removal at all the desizing conditions compared to polyester fabrics indicating that wheat gluten had better affinity to cotton. When desized at room temperature, the P/C fabrics with both wheat gluten and PVA had relatively low size removal of 76 and 82%, respectively. It should be noted that after one rinse at 90 °C using 5:1 ratio of water to fabric, wheat gluten was more easily removed than PVA for both polyester and P/C fabrics. However, increasing the rinse times or the liquor ratio resulted in better removal of PVA from the fabrics. Overall, the ability to remove more than 97% of the size using normal desizing conditions means that there is no need to use additional water, chemicals or enzymes for desizing the fabrics sized with wheat gluten leading to lower environmental pollution and savings in sizing costs. 5.6. Biodegradation in Activated Sludge. Figures 6 and 7 depict the degradation behavior of the wheat gluten in activated sludge. At a concentration of 300 ppm, the PVA size had a COD value of 585 mg/L and BOD5 of 6 mg/L. Corresponding values for wheat gluten were 398 and 279 mg/ L. Lower COD and higher BOD5 for wheat gluten suggested that the proteins were more easily degradable than PVA. Treating the size in activated sludge continually decreased the COD values for wheat gluten but the COD values for PVA did not show any considerable decrease except after 1 day of treatment (28% decrease) in the sludge. After 3 days of treatment, the COD values of the wheat gluten size decreased to 87 mg/L, lower than the permissible level (100 mg/L) in wastewater released into the environment. As for BOD5, the values were 18 mg/L and 4 mg/L after 3 days of treatment for wheat gluten and PVA, respectively. There was almost no change in the BOD5 for PVA whereas wheat gluten had only about 6% of the initial BOD5 value after 3 days of treatment suggesting that most of the wheat gluten was already degraded. Since the degradation of proteins releases nitrogen that could
resistance compared to the respective fabrics treated with PVA as seen from Figure 5. P/C fabrics sized with wheat gluten
Figure 5. Abrasion resistance of polyester and P/C fabrics treated with wheat gluten and PVA at different % add-ons. For each property, data points with different letters symbolize statistically significant difference.
showed a 50% increase in abrasion resistance at an add-on of 5.5% compared to the untreated fabrics. However, PVA treated fabrics had a substantially higher increase (113%) even at low add-on (2.8%). Polyester fabrics sized with wheat gluten showed considerably better improvement in abrasion resistance than P/C fabrics. As seen from Figure 5, wheat gluten provided substantially higher abrasion resistance (274 cycles) to the polyester fabrics compared to 186 cycles for PVA sized fabrics even at low % add-on (2.7%). Abrasion resistance is mostly related to the film forming ability of the proteins and the size of the film on the surface of the fibers. Since wheat gluten contained starch, was hydrophilic and had better affinity to cotton than polyester, wheat gluten was able to penetrate into P/C fabrics and therefore formed a thinner film on the surface. Repulsion between polyester and wheat gluten would result in more size on the surface of the polyester fibers leading to a thicker film and consequently higher abrasion resistance. Although the strength increase provided by wheat gluten was low on polyester rovings compared to PVA, the excellent improvement in abrasion resistance which is the primary criteria for a good size, means that wheat gluten is useful for sizing polyester yarns.
Table 1. Comparison of the % Size Removed from Wheat Gluten and PVA Treated Polyester and P/C Fabrics at Various Desizing Conditionsa desizing conditions
% size removed
water to fabric ratio type of fabric polyester
poly cotton
a
temp, °C
time, min
washing
rinsing
rinse times
90
5
20
5
5:1 5:1 10:1 10:1
5:1 5:1 5:1 5:1
1 2 1 1
90 97 98 86
± ± ± ±
3 2 1 3
90
5
20
5
5:1 5:1 10:1 10:1
5:1 5:1 5:1 5:1
1 2 1 1
87 91 91 76
± ± ± ±
4 7 3 1
wheat gluten
PVA 77 96 99 62
± ± ± ±
5 4 1 3
77 ± 4 100 100 82 ± 4
Add-on on the fabrics was approximately 5%. 4509
dx.doi.org/10.1021/es304429s | Environ. Sci. Technol. 2013, 47, 4505−4511
Environmental Science & Technology
Article
compared to commercially available PVA based sizes. Although higher add-ons are required for the wheat gluten size, wheat gluten sells at about $0.50−0.80 per lb compared to $1.70 per lb of PVA in the United States market. In addition, wheat gluten size was easily removable (>94% removal) (desized) than PVA even at low temperatures and using low amounts of water. Therefore, wheat gluten could be a cost-effective replacement to PVA. Simple preparation, good sizing properties, easy removal during desizing and complete degradation in effluent treatment plants makes wheat gluten an ideal candidate to replace PVA for textile warp sizing. Wheat gluten as size would help to lower sizing costs, reduce dependence on PVA, and also make the textile processing more environmentally friendly.
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Figure 6. Changes in the COD and BOD5 values for wheat gluten and PVA in activated sludge from 0 to 3 days. COD was measured until the concentration decreased below 100 mg/L and BOD5 was measured at 0 day and at day 3 when the COD concentration was
