By A. Clare
Approximately 6.5 million tons of wet-salted hides/skins are processed worldwide every year. For every ton of raw material, approximately 600 kg of solid waste is generated in the form of trimmings, fleshings, chrome shavings, and buffing dust. Unsuitable treatment or disposal of solid waste causes environmental pollution problems associated with its organic and inorganic matter, chromium, and suspended solids. Solid tannery waste can be classified as:
Tannery hair waste
Untanned solid waste
Chrome-containing solid waste
Tannery sludge
Finished Leather waste
Sustainable leather processing requires tanneries to reduce the amount of waste produced and increase the return on potential by-products. Solid waste leather by-products are often protein-rich and, therefore, they can be recovered and reused in alternative applications. Over the last several years, advancements in technologies have allowed recovery of leather wastes to produce useful products.
Utilisation of Tannery Hair Waste:
Hair is rich in keratin, a family of highly structured insoluble proteins with high sulfur content and unique physiochemical properties. Hair proteins, produced during the unhairing process, result in wastewater with high sulfide levels - the major pollutant formed during beamhouse processes. Technological developments have allowed recovery of approximately 95% of hair from tannery effluents (figure 1); however, keratin is difficult to dissolve. Therefore, researchers have focused on extracting this protein from hair waste to take advantage of its unique properties.
Figure 1 - Tannery hair waste produced during the unhairing process.
Application of Keratin in Biomedicine:
Keratins excellent biocompatibility, biodegradability and mechanical durability makes it a useful material for biomedical applications. Although the use of keratin from leather wastes in biomedicine is still in the research stage, the success of this protein from other sources in the biomedical field has been of great success. The biocompatibility and absorptivity of keratin make it an excellent polymer for application in wound healing and tissue engineering. Furthermore, the high mechanical strength and fast swelling of keratin enable hydrogel formation. Keratin hydrogels can subsequently be used as scaffolds during cell culture.
Application of Keratin as a Pollutant Adsorbent:
Keratin has many polar and ionizable functional groups and, therefore, this polymer is a promising biosorpent for the removal of metals and organic compounds. Pollutant adsorbents must be low cost and have a high surface area, high porosity, and high adsorption capacity. Keratin-derived biosorpents have shown success in the adsorption of dyes from aqueous solutions and the removal of trace metals from industrial wastewater.
Utilisation of Untanned Solid Waste:
Untanned solid waste is derived from fleshing and trimming of raw hides/skins (figure 2). Although this raw waste material is biodegradable, disposal is often difficult and, the material provides a hotspot for pathogenic bacteria. Untanned solid waste is rich in protein and fats, with one ton producing 600 kg and 230 kg of collagen and fat, respectively. High-value collagen and fat can be extracted from raw material by enzymatic, acid or alkali hydrolysis, before being applied to other applications.
Figure 2 - Leather trimming solid waste.
Utilising Collagen for Biodegradable Packing:
Gelatin is a protein derived from collagen by hydrolysis. Gelatin has high water solubility and can be added to composite films to improve mechanical properties. Gelatin and polyvinyl alcohol can be used to make composite films with properties like polyethene, such as excellent strength and good saleability. The physical and mechanical properties of composite films made with tannery-derived gelatin and starch have shown to be on par with those derived from commercially available gelatin. Gelatin films produce optimum results in mechanical strength, thermal saleability, and printability tests, required for packaging materials.
Utilising Collagen in Biomedicine and Cosmetics:
Collagen is a versatile biomaterial, due to its excellent biodegradability, biocompatibility, and low antigenicity properties. Tannery-derived collagen has demonstrated success in providing a porous membrane for burns, with good antibacterial and anti-inflammatory properties. The introduction of collagen hydrolysate into the body promotes tissue recovery. Tannery-derived collagen, therefore, has the potential to be used cosmetically in dermal fillers and anti-ageing facial products.
Utilising collagen in concrete manufacture:
After fats are extracted from the hide/skin, the remaining protein can be hydrolysed using an alkali to produce a product for use as a concrete surfactant. The resulting product contains protein hydrolysate and alkaline soap, which can be used as an additive in concrete and cement. The collagen-derived product improves mechanical strength and eases workability of building materials, as well as reducing sweating and particle segregation.
Utilising Fat in Biofuel Production:
Leather fleshing waste is rich in lipids, which can be extracted using acid hydrolysis and used to produce biofuels as an alternative for diesel engines. The fat is transesterified into biodiesel using an alcohol such as methanol. Biodiesels, derived from tannery waste, produce lower emissions (oxides of nitrogen and carbon monoxide) and contribute less to global warming, compared to diesel fuel. Furthermore, the performance of biofuels matches diesel and, no energy modification is required for their application. As the fat from hides/skins is a waste product, it is a low-cost and renewable feedstock for biofuel production.
Utilisation of Chromium-containing Solid Waste:
Chrome-containing solid waste includes chrome shavings, splitting (figure 3) and trimmings. These wastes contain 1-3% trivalent chromium and up to 90% collagen. Approximately 25% of solid tannery wastes contain chromium and, the impact this waste has on the environment is a lively debate. Trivalent chromium results in chromium VI in some circumstances which must be proactively managed. Between 1968-1988, lots of research focused on developing methods to recover chrome from tannery wastes and allow indirect utilisation of the remaining by-product. Chromium recovery is achieved by thermal treatment, alkaline hydrolysis, ionic liquid extraction, biochemical or oxidation methods.
Figure 3 - Chromium-containing solid leather splitting waste.
Utilising Collagen Hydrolysate as a Biofertilizer Additive:
Collagen hydrolysate, a nitrogen-rich protein, can be used as an additive to promote plant growth. The dilution of the protein can be applied to plants when mixed with potassium overbasic naphthenates, a type of aliphatic acid which can penetrate plant cell membranes. Collagen hydrolysate solutions, derived from chromium waste, increase tomato and aubergine production by 10-15% when applied in an appropriate dose. Furthermore, soybeans treated with a similar solution yielded higher seeds and were as productive as those treated with commercial fertilizer.
Utilising Collagen Hydrolysate as an Animal Feed:
Collagen adds value to animals feed and, therefore, this protein is in high demand for the feed industry. Gelatin is often used as an additive in feed to extend shelf life and able transportation. Furthermore, gelatin is an excellent source of energy and is important for the healthy growth of animals. Studies have shown gelatin derived from leather industry waste can replace nearly 75% of a normal protein mean for poultry, with no significant influence on organism growth or meat characteristics.
Utilisation of Tannery Sludge:
Tannery sludge is produced during the treatment of tannery wastewater and consists of predominantly sulfide, chromium, lime, dissolved hair, and protein. The high concentration of sulfide and chromium classifies tannery waste as hazardous and, therefore, the waste cannot be disposed of by landfill or incineration. The development of effective recycling methods is required to prevent pollution from tannery sludge and utilise available elements.
Anaerobic Digestion of Tannery Sludge for Biogas Generation:
Utilising tannery sludge for biogas generation has demonstrated success in semi-pilot scale studies. A nutrient solution can be added to the bioreactor to provide favourable conditions for microorganism growth. The addition of lipase can increase biogas generation by up to 15% and reduce the anaerobic digestion period by 30%, relative to the control. Research results suggest a midsize tannery could reduce electric- and thermal energy consumption by 6.8% and 1.6%, respectively. Tannery sludge has the potential to be used as a renewable energy source whilst reducing costs associated with disposing of tannery waste.
Utilising Tannery Sludge for Building Materials:
Tannery sludge, containing immobilized chromium, can be used to produce building materials, such as bricks, ceramsite and ceramic. Tannery sludge-derived bricks are of excellent quality and bear good mechanical and physical properties, however, there is potential for chrome leaching throughout their life. Alternatively, ceramsite derived from tannery waste with low chromium leaching can be produced. The ceramsite has high strength, good thermal insulation and is shock resistance. Tannery sludge can also be mixed with clay to provide a material with similar mechanical properties to ceramic bricks. Furthermore, the finished ceramic product immobilized chromium, as demonstrated by leaching tests.
Conclusion:
Unsuitable treatment or disposal of leather industry solid waste can cause severe environmental damage. Technological innovations have allowed recovery of solid waste to provide materials for alternative industries and implement a promising waste management strategy. Furthermore, valorisation of wastes offers commercial benefits, as value is added to products and costs associated with waste disposal are reduced. Future research requires the continuous improvement and development of novel reusage technologies to full take advantage of leather waste resources.
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