Environ. Sci. Technol. 2004, 38, 871-879
Natural Leathers from Natural Materials: Progressing toward a New Arena in Leather Processing SUBRAMANI SARAVANABHAVAN,† PALANISAMY THANIKAIVELAN,‡ J O N N A L A G A D D A R A G H A V A R A O , * ,† BALACHANDRAN UNNI NAIR,† AND THIRUMALACHARI RAMASAMI† Chemical Laboratory and Centre for Leather Apparels & Accessories Development, Central Leather Research Institute, Adyar, Chennai 600 020, India
Globally, the leather industry is currently undergoing radical transformation due to pollution and discharge legislations. Thus, the leather industry is pressurized to look for cleaner options for processing the raw hides and skins. Conventional methods of pre-tanning, tanning and post-tanning processes are known to contribute more than 98% of the total pollution load from the leather processing. The conventional method of the tanning process involves the “do-undo” principle. Furthermore, the conventional methods employed in leather processing subject the skin/ hide to a wide variation in pH (2.8-13.0). This results in the emission of huge amounts of pollution loads such as BOD, COD, TDS, TS, sulfates, chlorides and chromium. In the approach illustrated here, the hair and flesh removal as well as fiber opening have been achieved using biocatalysts at pH 8.0, pickle-free natural tanning employing vegetable tannins, and post-tanning using environmentally friendly chemicals. Hence, this process involves dehairing, fiber opening, and pickle-free natural tanning followed by ecofriendly post-tanning. It has been found that the extent of hair removal and opening up of fiber bundles is comparable to that of conventionally processed leathers. This has been substantiated through scanning electron microscopic analysis and softness measurements. Performance of the leathers is shown to be on par with conventionally chrome-tanned leathers through physical and hand evaluation. The process also exhibits zero metal (chromium) discharge and significant reduction in BOD, COD, TDS, and TS loads by 83, 69, 96, and 96%, respectively. Furthermore, the developed process seems to be economically viable.
Introduction Leather-making involves conversion of a putrefiable skin or hide into nonputrescible material. The conventional mode of the tanning process involves “do-undo” operations like curing (dehydration)-soaking (rehydration), liming (swelling)-deliming (deswelling), or pickling (acidification)depickling (basification) (1). It subjects the skins or hides to wide variations in pH. Such changes in pH demand the use * Corresponding author tel: +91 44 24411630; fax: +91 44 2491 1589; e-mail:
[email protected]. † Chemical Laboratory. ‡ Centre for Leather Apparels & Accessories Development. 10.1021/es034554o CCC: $27.50 Published on Web 12/24/2003
2004 American Chemical Society
of acids and alkalis, which lead to the generation of salts (2). Among the various tanning systems, more than 90% of the leathers tanned globally contain chromium. The present commercial chrome tanning method gives rise to only about 50-70% chromium uptake (3). This poor uptake results in material wastage on one hand and ecological imbalances on the other. The international specification for the discharge of chromium-bearing streams is less than 2 ppm (4). Even a high-exhaust chrome tanning system does not provide such low discharge. Various reports on the toxicity of chromium(III) are emerging (5, 6). Discussion exists regarding the possible conversion of chromium(III) to chromium(VI) under the influence of oxidizing environment (7). Also, the disposal of chrome-containing solid wastes and sludges is posing a major challenge (8). The post-tanning processes contribute to TDS, COD, and heavy metal pollution as analyzed by Simoncini and Sammarco (9). Apart from this, a great deal of solid waste including lime sludge from tannery and chrome sludge from effluent treatment plant is being generated. This happens to be a major stumbling block for many of the tanners around the world due to the stringent environmental concerns. Attempts have been made to replace or minimize the use of toxic chemicals by revamping the individual processing steps in order to reduce the pollution. Several lime- and sulfide-free dehairing methods have been developed during the past century (10-12). To reduce soil nitrification, few ammonia-free deliming methods have been developed (13). Salt-free pickling using naphthalene sulfonic acid is wellknown (14). Several chrome management methods have been developed and reviewed by Rao et al. (15). These improvements are certain to a unit operation. Hence, there is a need for the development of integrated process technologies by revamping the sequence of operation. Very few attempts have been made to revamp the entire leather processing technology. Thanikaivelan et al. (16) have attempted to make leather in a narrower pH range from 4 to 8.0. They have also developed a biomediated three-step tanning process involving dehairing at pH 8.0 without employing lime, enzyme-based fiber opening, and pickleless chrome tanning at pH 8.0 (17). Saravanabhavan et al. (18) have successfully developed a three-step tanning method, which involves dehairing without employing lime and sodium sulfide, enzyme-based fiber opening, and chrome tanning at pH 8.0. All these methods involve the use of chromium salts and subsequent discharge into effluent as well as solid waste. However, no attempt has been made to produce natural leather with near zero pollution. In general, the vegetable-tanned leather is often referred as natural leather (19). Vegetable tannins are plant polyphenols with molecular mass from 500 to 3000 Da, capable of converting a putrefiable skin or hide into nonputrescible material. Among the various alternatives for chromium, vegetable tannins are considered to be ecofriendly because of its natural origin. Furthermore, it has been demonstrated that they have been used for medicinal applications (20, 21). Hence, an attempt has been made to produce natural leather with near zero pollution loads. The process sequence involves enzyme-based dehairing, biocatalytic fiber opening, pickle-free natural tanning, and post-tanning using ecofriendly chemicals. The physical characteristics, hand evaluation, and SEM analysis of both experiment and control leathers have been carried out. Composite liquors from both the processes have been analyzed for BOD, COD, TDS, TS, and chromium. Comprehensive input-output audits for both VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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processes as well as technoeconomic viability have been carried out.
Experimental Methods Materials. Wet salted goatskins were chosen as raw material. Since this work involves the study of extent of hair removal, opening up of fiber bundles, and distribution of tanning agent, more compact goatskins of larger area (4-6 ft2) were chosen. All chemicals used for leather processing were of commercial grade. Biodart (a dehairing enzyme based on alkaline bacterial protease, active at pH 7.5-11.0 and temperature 25-40 °C) and R-amylase (activity 3000 units/ g) were obtained from Southern Petrochemical Industries Corporation (SPIC) Limited, Chennai, India. The chemicals used for the analysis of leather and spent liquors were of analytical grade. Tanning Process. Ten wet salted goatskins were taken and soaked conventionally. The wet weight after soaking was noted for each skin and termed as soaked weight. Five skins were taken for control, and five skins were used for the experiment. The conventional and experimental process flow sheet is given in Figure 1 (for detailed process description, please see Supporting Information). Input-Output Analysis. A comprehensive input-output audit for the raw materials, water, chemicals, and other reaction products was carried out for the conventional and experimental tanning processes excluding soaking and posttanning processes. The amount of dry sludge was estimated as per the procedure described earlier (23). The mass balance for the acid and base was not calculated since they remain in the effluent in the form of reaction products. Similarly, auditing for enzymes was also not carried out as it is neither absorbed nor fixed with the skin matrix. The amount of chromium in the wastewater from chrome tanning was analyzed as per the standard procedure (24), and the mass balance was calculated based on the volume of chrome liquor collected. The amount of vegetable tannins in the wastewater was estimated as per the standard procedure (25). Shrinkage Temperature of Tanned Leathers. Tanned samples from experimental and control processes were subjected to shrinkage temperature measurements using a Theis shrinkage tester (26). A glycerol-water (3:1 ratio) medium was employed for chrome-tanned control sample. In the case of vegetable-tanned experimental leather sample, the shrinkage temperature was measured in water. Differential Scanning Calorimetric Analysis of Crust Leathers. Crust leather samples from experimental and control processes were cut from the official sampling position (27) and conditioned at room temperature and humidity (uncontrolled). Samples were subjected to thermal denaturation (melting) using a Seiko Instruments Inc. DSC 5200 series at a heating rate of 5 °C/min. The DSC instrument was previously calibrated for temperature and heat flow. Moisture content of the samples was measured following a standard procedure (28). Scanning Electron Microscopic Analysis. Samples from experimental and control leathers were cut from the official sampling position (27) after tanning and crusting processes. Samples from crust leathers were directly cut into specimens with uniform thickness without any pretreatment. Tanned samples were first washed in water. Subsequently, the samples were gradually dehydrated using a standard procedure (29). Samples were then cut into specimens with uniform thickness. All specimens were then coated with gold using an Edwards E306 sputter coater. A Leica Cambridge Streoscan 440 scanning electron microscope was used for the analysis. The micrographs for the grain surface and cross section were obtained by operating the SEM at an accelerating voltage of 20 kV with different magnification levels. 872
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Objective Assessment of Softness through Compressibility Measurements. Softness of leathers can be numerically measured on the basis of their compressibility (30). Circular leather pieces (2 cm2 area) from experimental and control crust leathers were obtained as per the IUP method (27) and conditioned at 80 ( 4 °F and 65 ( 2% RH (relative humidity) for a period of 48 h. The samples were spread uniformly over the solid base of the C&R (compressibility and resilience) tester. The initial load acting on the grain surface was 100 g. The thickness at this load was measured 60 s after the load was applied. Subsequent loads were added, and the change in thickness was recorded 1 min after the addition of each load. The logarithm of change in leather thickness (Y axis) was plotted against the logarithm of load (X axis). Physical Testing and Hand Evaluation of Leathers. Samples for various physical tests from experimental and control crust leathers were obtained as per the IUP method (27). Specimens were conditioned at 80 ( 4 °F and 65 ( 2% RH for a period of 48 h. Physical properties such as tensile strength, percent elongation at break, tear strength, and grain crack strength were examined as per the standard procedures (31-33). Experimental and control crust leathers were assessed for softness, fullness, grain smoothness, grain tightness (break) and general appearance by hand and visual examination. The leathers were rated on a scale of 0-10 points for each functional property by experienced tanners, where higher points indicate better property. Analysis of Composite Waste Liquor. Composite liquors from control and experimental leather processing were collected from all unit operations except soaking up to tanning as well as post-tanning and analyzed for BOD, COD, TDS, and TS (dried at 103-105 °C for 1 h) as per the standard procedures (25). Emission loads were calculated by multiplying concentration (mg/L) with volume of effluent (L) per metric ton (t) of raw skins processed.
Results and Discussion Input-Output Audit for Conventional and Experimental Leather Processes. The input-output audit in leather manufacture is to assess the effectiveness of the developed process as against the existing process. The input-output of the raw materials, chemicals, and water has been monitored for the control and experimental processes. The observed values have been calculated for processing 1 t of raw goat skins and are given in Table 1. The reduction in the weight of the soaked skins after liming is primarily due to the removal of hair, epidermis, soluble proteins, and water and is more or less similar for both control and experimental pelts. In the control process, the amount of dry material produced is 152 kg; of which hair of the goatskins alone contribute to nearly 94%. This is in accordance with the reported value (34). In both experimental and control processes, the hair obtained is in undamaged form, which can be further used for several applications. The amount of dry sludge produced from control process is about 9 kg, whereas the experimental process does not produce any dry sludge. Major portions of the chemicals were carried away with the skins to the reliming process. In reliming/bio-based fiber opening, the water absorbed by the skin in the case of control is nearly 39% of applied water. Whereas, in the experimental process, the water absorbed by the skin is about 43% of applied water, which suggests that the enzymatic fiber opening is capable of replacing lime. In reliming, the amount of dry sludge formed by the control process is 116 kg, and there is no dry sludge in the case of experimental process. It is intriguing to note that there is an excess of materials present in the sludge, since the reliming process employs only 100 kg of lime. This is primarily due to the lime carried over from liming and further usage of lime during reliming process. Hence, the application of negligible amount of bioproduct not only
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FIGURE 1. Conventional and experimental process flow sheet. 9
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TABLE 1. Input-Output Audit for Control and Experimental Tanning for Processing 1 t of Raw Goatskinsa control process liming/enzyme based dehairing
reliming/bio product for opening up
washing deliming
washing pickling
tanning
washing
chemicals/raw material
input (kg)
soaked skins water lime Na2S enzyme (SPIC) dry hair dry sludge dehaired skins water lime lime in suspension formc bioproduct dry sludge fleshed skins water fleshed skins water NH4Cl alkali bate delimed skins water delimed skins water salt H2SO4 pickled/bioproduct based opened up skins water basic chromium sulfate sodium sulfate from BCSe wattle GS powder (Acacia mollisima) sodium formate sodium bicarbonate chrome tanned skins water
1000 100 100 30
740 2,000 100
output (kg) 740
143 9 1025 1,280 11.6 66
750 1500 750 750 7.5 3.75 750 1500 750 600 75 9.75 750 375 60
116 750 1400 ne 735 rpd ne ne 1470 ne 300 rp rp ne 675 21.6 12.6
7.5 7.5 750 1500
rp rp ne 1470
experimental input (kg)
output (kg)
1000 100
712
10
neb 148
712 710
1015 410
2.48
ne
720 1440
720 1300
720 720
ne 400
144
14.4
a Weight of skins before soaking. b ne, not estimated. c Nearly 30-35% lime is present in the effluent as suspended form (23). product. e Assuming that BCS contains 33% of sodium sulfate.
avoids the use of a huge quantity of lime but also circumvents the problem of solid waste disposal. The conventional process sequence requires a deliming process for the pelts treated with lime. Since lime increases the pH to 12-13, it induces osmotic swelling. This hydrostatic pressure opens up the fiber bundles and removes the unwanted proteins. Deliming process employs ammonium chloride (acidic salt) and neutralizes the alkalinity. This leads to the formation of ammonia and calcium chloride and nitrogenous salts to some extent. At the completion of deliming, the pelts are treated with bating enzymes for the removal of noncollageneous proteins. Hence, deliming and bating operations leads to nearly 11.25 kg of chemical usage and subsequent discharge in the effluent as seen in Table 1. In the case of experimental pelts, deliming and bating processes were not carried out since both dehairing and fiber opening processes are carried out at pH 7.5-8.0 and the purpose of bating is already met during the bio-based fiber opening process. The conventional process sequence requires a pickling step for the subsequent chrome tanning process. This leads to the discharge of nearly 85 kg of solids into the effluent in one stage or the other. Experimental tanning does not require sodium chloride and sulfuric acid. Conventional chrome tanning process discharges nearly 22 kg of the dose of the tanning agent for processing 1 t of goatskins. Sodium sulfates present in the BCS are also discharged into the effluent, which account for nearly 13 kg for processing 1 t of raw goatskins. Furthermore, the conventional chrome tanning requires nearly 15 kg of mild alkali salts (sodium formate and sodium bicarbonate) for the basification (complexing chromium with the protein 874
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d
rp, reaction
carboxyl groups) process. These mild alkalies are primarily used to neutralize the acidity. Hence, the pH increases from 2.8 to 3.8, leading to the fixation of chromium with the hide protein. Hence, the conventional chrome tanning-basification processes lead to the discharge of nearly 50 kg of solids for processing 1 t of raw goatskins as against nearly 15 kg in the case of experimental tanning process. The total amount of chemicals and bioproducts consumed in the control and experimental tanning processes is 401 and 157 kg, respectively. This means that the experimental processing reduces the total material consumption by 61% as compared to control leather processing. However, it is essential to mention that all the materials consumed in the experimental process are of biological origin and more or less degradable. Of the total chemicals and bioproduct used, only the tanning agent is fixed with the skin matrix. The remaining chemicals are discharged as either liquid or solid. Thus, the control and experimental process release 223 and 17 kg of materials, respectively (into the effluent), which means a 92% reduction is achievable. A major portion of the chemicals is present in sludge in the case of conventional leather processing. The dry sludge formed in the case of control process is about 125 kg. However, the sludge formation is completely avoided by employing a biobased pre-tanning operation. Scanning Electron Microscopic Analysis. The experimental process employs enzymes for both dehairing and fiber opening processes. It is imperative to investigate the grain structure since it is expected that the use of enzymes could result in grain damage. Furthermore, the pickle-free vegetable tanning process is carried out at pH 7.5-8.0 against
FIGURE 2. Scanning electron micrographs of control and experimental samples showing the grain surface after tanning: (a) control (×250); (b) experimental (×250).
FIGURE 3. Scanning electron micrographs of control and experimental crust leather samples showing the cross section: (a) control (×1000); (b) experimental (×1000). the conventional pH levels of 4.5-5.0. The scanning electron micrographs of tanned samples from control and experimental tanning processes showing the grain surface at a magnification of ×250 are given in Figure 2a,b, respectively. It is seen that the grain structure of the sample from experimental tanning process is clean without any damage. The hair pores are clearly visible without any surface deposition of vegetable tannins. This is comparable to that of control leather sample. The scanning electron micrographs of crust samples at relatively higher magnification (×500) confirm the above observation (see Figure 2c,d in Supporting Information). Scanning electron micrographs of crust leather samples from control and experimental processes showing the cross section at a magnification of ×1000 are given in Figure 3a,b, respectively. Experimental sample exhibits a dense fiber structure. Whereas, the control sample shows the relaxed fiber structure. This is primarily due to the filling nature of vegetable tanning system. The fiber bundles of both samples exhibit a very fine splitting of fibers. This means that the biocatalytic fiber opening provides comparable fiber opening to that of control processing. Softness Measurements. Experimental process employs enzyme for dehairing and fiber opening. Furthermore, vegetable tannins have been employed for tanning the derived skin matrix. Vegetable tannins are known to produce hard leathers and generally employed for producing heavyduty leathers. Hence, it is important to evaluate the extent of softness on the final leather. The logarithm of change in leather thickness upon compression was plotted against logarithm of load for the control and experimental samples, which are shown in Figure 4a,b, respectively. The plots show
a linear fit. The corresponding equation of the line was obtained. The negative slope angles were calculated (30), and the values are 4.22 and 8.64° for control and experimental leathers. Higher negative angles imply more softness in the leather. It is apparent that the experimental leather exhibits higher negative slope angle (compressibility index, CI) as compared to the control leather. This shows that experimental leather sample exhibits better softness than that of control leather sample. This is due to the softening of grain as well as fiber structure upon two-stage action of enzyme. Performance of the Leathers. Hydrothermal stability of the tanned leathers was measured in order to ascertain the efficacy of the tanning systems. The conventional chrometanned leather exhibits shrinkage temperature above 120 °C. Whereas, the skin matrix treated with vegetable tannins and previously with biocatalysts exhibits a shrinkage temperature of 84 °C. Although the value seems to be lower than the chrome-tanned leather, it is comparable to that of conventional vegetable-tanned leather (35). However, it has been shown that the dry heat shrinkage temperature of the native collagen is 221 °C at 0% moisture content (36). Hence, in principle, a skin matrix tanned using a tanning agent, capable of providing adequate shrinkage temperature as well as desired properties, would suffice for its application in dry condition (36, 37). To substantiate the above argument, differential scanning calorimetric analysis was performed for the control and experimental crust leathers samples. Thermograms of both the samples are given in Figure 5. It is interesting to note that the vegetable-tanned crust leather exhibits a melting (shrinkage) temperature of 115 °C, whereas the chrome-tanned crust leather shows only 105 °C. Corresponding moisture content of the samples were 21 and VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Plot of log of load vs log of change in thickness for control and experimental leather samples: (a) control leather sample; (b) experimental leather sample.
FIGURE 5. Differential scanning calorimetric thermograms for conventional (C) and experimental (E) crust leathers scanned at 5 °C/min. 16%, respectively. These values seem to be higher than the expected values (∼10%). This could be due to the evaporation of other volatile substances such as fatty matter, organic compounds, etc. at 102 ( 2 °C (28). The difference seen in the endothermic heat flow as well as melting temperature between the two samples could be due to the differences in 876
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packing density, heat capacity, and thermal conductivity of tannins and other substances of control and experimental samples. Hence, these results provide a convincing proof for the use of enzyme-treated vegetable-tanned natural leather for a range of applications on par with chrome-tanned leathers. The strength properties such as tensile, tear, and grain crack strength values were obtained by standard physical testing methods and are presented in Table 2. It is seen that both control and experimental leathers exhibit comparable tensile, tear, grain crack, and bursting strength values to that of BIS (Bureau of Indian Standards) norms (38). Control and experimental crust leathers were evaluated for various organoleptic properties by hand evaluation. The average of the rating for the five leathers corresponding to each experiment was calculated for each functional property and is given in Figure 6. Higher numbers indicate better property. The experimental leathers exhibit better fullness as compared to control leathers due to a high filling of vegetable tannins. Other properties such as softness, grain tightness, and smoothness are also higher when compared to conventionally processed leathers. In general, the appearance of experimental leathers is slightly better than the control leathers. Environmental Benefits. The composite liquors have been collected from control and experimental leather processes excluding soaking up to tanning as well as posttanning. BOD, COD, TDS, and TS are four parameters that have been chosen for analyzing the impact of the composite liquor on the environment. Table 3 provides the amount of water employed and discharged for each process for processing 1 kg of raw goatskins. It is apparent that the experimental leather process enjoys a reduction in total water consumption and discharge by 29 and 32%, respectively, as compared to the control leather process. It is also evident that the experimental leather processing method enjoys a reduction in water consumption by 64% and discharge by 71% as compared to the conventional leather processing assuming that the green skins (unsalted) are processed till tanning. It has been estimated that, by the year 2025, 1.8 billion people will live in countries or regions with absolute water scarcity (39). In this context, the achievement of reduction of water consumption to about 3 L for preserving 1 kg of green skins permanently (tanning) is remarkable. A direct correlation of the observed BOD, COD, TDS, and TS values with the environment may not give proper consequences. Hence, the values have been converted into emission loads. The calculated emission loads up to tanning are given in Table 4. The BOD, TDS, and TS values for experimental leather processing are lower than that of the control leather processing, despite the reduced water usage in experimental leather processing. This is because of the tailored process design as well as use of biological materials as against an empirical multistep conventional process. It is seen that the COD value for experimental processes is higher than the conventional chromium-based processes. This is primarily due to two reasons, first, vegetable tannins present in the composite liquor, which are in principle biodegradable material (40), although hardly and, second, reduced water usage. The reduction in emission loads of BOD, COD, TDS, and TS till tanning for the experimental process are 83, 69, 96, and 96%, respectively, as compared to the conventional leather processing. The reduction in TDS and TS loads are primarily due to the judicious choice of tailored pretanning and tanning processes that avoids not only the use of acids, alkalies, and neutral salts but also the formation of neutral salts due to irrational pH alterations. The BOD, COD, TDS, and TS values and the calculated emission loads up to post-tanning are given in Table 5. The reduction in BOD, COD, TDS, and TS loads are 67, 60, 95, and 95%, respectively, as compared to the control process.
TABLE 2. Physical Testing Data of Control (C) and Experimental (E) Leathers grain crack strength (av valueb)
bursting strength (av valueb)
sample
tensile strength (kg/cm2) av valuea
% elongation at break av valuea
tear strength (kg/cm) av valuea
load (kg)
distension (mm)
load (kg)
distension (mm)
C E BIS norms
268 ( 5 271 ( 6 250
65 ( 2 68 ( 4 60-70
35 ( 5 48 ( 2 30
28 ( 1 30 ( 2 20
10 ( 0.3 9 ( 0.4 7
29 ( 1 32 ( 2
11 ( 0.3 10 ( 0.6
a
Average of mean of along and across backbone values.
b
Average of load and distention values.
FIGURE 6. Organoleptic properties of crust leathers obtained from conventional (C) and experimental (E) tanning.
TABLE 3. Comparison of Water Requirement and Discharge for Control (C) and Experimental (E) Leather Processing of 1 kg of Raw Skinsa control unit processes soaking liming/enzyme-based dehairing reliming/bio-based opening up washing deliming and bating washing pickling chrome tanning vegetable tanning washing post-tanning total a
experimental
input (L)
output (L)
input (L)
output (L)
9.0 0.1
8.0
9.0 0.1
8.0
2.0
1.28
0.71
0.417
1.5 0.75 1.5 0.6 0.375
1.40 0.73 1.47 0.30 0.675
1.44
1.30
0.72
0.40
2.7 14.67
2.4 12.51
1.5 3.24 20.56
1.47 3.04 18.36
Weight of skins before soaking.
Similar to the observation made in the analysis of composite liquors up to tanning, the experimental process results in greater reductions in TDS and TS loads. However, it seems that the reduction in BOD and COD are slightly lower as compared to the earlier observation. This is despite the judicious selection of high-performance post-tanning auxiliaries. These chemicals are high exhaustive in terms of chemical properties and contribute to low COD. Nevertheless, the unit specific contribution of COD from conventional posttanning process is 14 kg/t of raw skins, whereas experimental post-tanning process contributes only 8 kg/t of raw skins. This demonstrates that the chosen post-tanning chemicals are capable of reducing COD by 43%. In the case of TDS, the unit specific contribution from the conventional post-tanning process is 153 kg/t of raw skins, whereas the experimental post-tanning process is 11 kg/t of raw skins. This signifies
that the chosen post-tanning chemicals are capable of reducing TDS by an amazing 93%. These reductions, without compromising the quality of the leather, are significant in the context of greener environment. Another important pollutant from the tannery wastewater is chromium(III). The chromium uptake is found to be 64% for control leather process. The chromium concentration in the spent chrome liquor from control tanning process is about 5500 ppm. Thus, it seems that the concentration of chromium in the composite liquor is about 250 ppm. The international specification for the discharge of chromium-bearing streams is less than 2 ppm (4). Apart from this, a great amount of solid wastes such as shavings, trimmings, buffing dust, and chrome sludges is also generated that needs careful attention for disposal and utilization. However, the experimental leather process enjoys from zero chromium emission in the composite liquor as well as harmless solid wastes. Techno-Economic Viability. Development of any new process requires commercial viability and cost-effectiveness. The experimental process designed in this work primarily involves the use of biological materials in order to achieve zero discharge as well as better quality leather. The amount of water employed for both control and experimental leather processing is given in Table 3. It is apparent that the experimental leather processing enjoys a reduction in water consumption by 29% as compared to the control leather processing. This reduction in water consumption lowers the hydraulic load to the effluent treatment plant (ETP) by 32%, thereby reducing the operating cost of ETP. The consumption of energy for the control and experimental processing is given in Table 6. The reduction in energy consumption is 20% as compared to the control leather processing. It is evident that there is a significant reduction in the consumption of water and energy. This would lead to considerable reduction in the cost of leather processing. The total chemical costs for processing 1 t of goatskins through conventional and experimental process schemes are given in Table 7. It could be seen that the experimental process exhibits higher VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Composite Liquor Analysis Up to Tanning for Control (C) and Experimental (E) Leather Processinga
process
BOD (ppm)
COD (ppm)
TDS (ppm)
TS (ppm)
vol of effluent (L/t of raw skinsb)
C E
2 184 ( 12 1 350 ( 14
3 549 ( 24 3 963 ( 26
23 890 ( 28 3,586 ( 32
26,348 ( 30 3 815 ( 24
7 325 2 110
a
Composite liquors were collected up to tanning excluding soaking.
b
emission load (kg/t of raw skinsb processed) BOD COD TDS TS 16 2.8
26 8
175 7.5
193 8.0
Weight of skins before soaking.
TABLE 5. Composite Liquor Analysis Up to Post-Tanning for Control (C) and Experimental (E) Leather Processinga
process
BOD (ppm)
COD (ppm)
TDS (ppm)
TS (ppm)
vol of effluent (L/t of raw skinsb)
C E
1 447 ( 16 1 129 ( 14
3 859 ( 22 3 765 ( 24
31 644 ( 22 4 168 ( 34
36 951 ( 45 4 474 ( 43
10 365 4 510
a
Composite liquors were collected up to post-tanning excluding soaking.
TABLE 6. Power Consumption for Control (C) and Experimental (E) Leather Processinga hours consuming power unit operations
C
liming/enzyme-based dehairing reliming/bioproduct-based opening up washing deliming and bating washing pickling tanning washing total total power consumption (kWh) cost (US$)
1.0 0.16 2.0 0.16 1.91 4.91 0.16 10.3 309 26
a
E 0.5 2.5 0.16
5.0 8.16 244.8 21
At 1 h running ) 30 kWh; 1 kWh ) Rs 4.20; 1 US$ ) Rs 49.00.
TABLE 7. Cost Estimates of the Conventional and Experimental Tanning Processes (US$/t of raw skins) chemicals/bioproducts
control
lime sodium sulfide biodart (SPIC) R-amylase (SPIC) ammonium chloride alkai bate sodium chloride sulfuric acid basic chromium sulfate sodium formate sodium bicarbonate wattle GS powder (Acacia mollisima) syntan fatliquor total
24.48 16.4
experimental
24.48 10.11 1.29 5.49 3.05 3.13 36.72 3.13 2.13 57.3 24.4 177.52
9
15 5
40 16
328 18
383 20
Weight of skins before soaking.
processing 1 t of raw goatskins. Another major advantage from the experimental process is that it involves enzymatic unhairing and fiber opening, which will increase the area of the final leather to about 2% (41). This would lead to a saving of US$ 90/t of goatskins processed. Furthermore, the reduction in discharge of effluent, BOD, COD, TDS, and TS loads would provide additional benefit in effluent treatment costs (4). Apart from this, the disposal of lime-bearing sludge and chromium-containing solid waste generated through control-based process causes both ecological and economic concerns. Considering the process followed in developed countries, where about 5% lime for liming and 8% sodium chloride for pickling are employed, the application of the present experimental process leads to the reduction in chemicals usage by 50% and TDS load by 90%. Similarly, the formation of 40 kg of dry sludge can also be avoided. In this work, an attempt has made to process the skins using natural materials at semi-technical level. However, it is important to produce various kinds of leathers at commercial level. Hence, a technical level study to optimize some of the control measures such as duration of fiber opening process and selection of post-tanning chemicals for producing various kinds of leather needs to be done. The combination of bioproducts with natural tanning material leads to an achievable eco-option to the conventional intricate leather processing that can be followed in both developed as well as developing countries due to its potential to produce natural leather with near zero discharge and negotiable leather qualities.
Acknowledgments Authors wish to thank Dr. R. Rajaram for physical testing measurements.
Supporting Information Available 161.6 47.7 42.8 286.69
chemical cost as compared to the control process. This is because the control system is based on the chrome tanning procedure. Hence, there will be a significant reduction in chemical cost if one were to compare it with conventional vegetable tanning procedure. The total chemical cost of the conventional vegetable tanning process is about US$ 290, assuming a partial pickling and 20% wattle GS powder (Acacia mollisima) and conventional post-tanning chemicals. Hence, the possible reduction in chemical cost is about US$ 4, for 878
b
emission load (kg/t of raw skinsb processed) BOD COD TDS TS
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Details of the process description, screening, and selection of post-tanning chemicals and scanning electron micrographs of the crust leather samples from conventional and experimental process. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review June 4, 2003. Revised manuscript received November 16, 2003. Accepted November 20, 2003. ES034554O
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