Leather Processing: Water Pollution's Main Culprit

The leather industry is one of the most polluting industries, with leather processing causing significant water pollution. The tanning process, in particular, has been identified as a major contributor to water pollution, with 90% of the total pollution caused by the leather industry attributed to pre-tanning and tanning operations. This is due to the use of various chemicals and large volumes of water, with high levels of water pollution observed in the soaking, liming, fat liquoring, and dyeing stages. The release of hazardous chemicals, such as chromium, mineral salts, formaldehyde, coal-tar derivatives, oils, dyes, and finishes, poses a significant risk to the environment and human health. The non-biodegradability of tanned leather further exacerbates the problem, impacting agro-based activities and groundwater systems. The leather industry's water pollution has led to calls for fashion brands to reduce and phase out leather use, with vegan leather alternatives gaining popularity.

The tanning process

The high water pollution levels caused by the tanning process have been recognised, and attempts have been made to reduce liquid discharges from tanneries. Advanced Oxidation Processes (AOPs) and adsorption, for example, have been applied to increase the biodegradability of wastewater before biological treatment. However, the tanning process itself, along with pre-tanning operations, contributes about 90% of the total pollution caused by the leather industry. This highlights the need for further initiatives to address water pollution caused by the tanning process specifically.

Wet processing stages

The leather industry is one of the most polluting industries, with leather processing causing an adverse impact on the environment. The wet processing stages of leather production involve different steps that consume large volumes of water and chemicals and release effluents that pollute surface water and may harm human health.

The wet processing stages of leather production include soaking, liming, chrome tanning, rechroming, neutralization, fat liquoring, and dyeing. These stages have different effects on the environment in terms of water pollution. For example, the blue water footprint, which refers to the use of ground or surface water, is higher in the soaking, liming, and finishing stages. On the other hand, the grey water footprint, which indicates the impact of high water pollution, is higher in the soaking, liming, fat liquoring, and dyeing stages.

The soaking and liming stages, in particular, have high levels of pollutants, including heavy metals and non-biodegradable solids. The liming main bath effluent has very high levels of BOD (2500 mg/L), COD (60,000 mg/L), TDS (14,720 mg/L), and TSS (51,400 mg/L). The soaking stage, which includes presoaking, main soaking, and washing, also has high levels of pollutants, with BOD at 10 kg/ton, COD at 54 kg/ton, TDS at 66 kg/ton, and TSS at 39 kg/ton.

The tanning and rechroming stages involve acidic processing and have low pH levels, which contribute to water pollution. These stages, along with the liming stage, also produce gaseous pollutants such as ammonia, amines, aldehydes, and volatile hydrocarbons. The use of chromium in the tanning process is particularly harmful, as it is a heavy metal that is discharged into the environment, and all wastes containing chromium are considered hazardous by the EPA.

Overall, the wet processing stages of leather production have a significant impact on water pollution, with about 97% of the water footprints of tanneries attributed to these stages. The high water consumption and the use of various chemicals contribute to the high levels of pollution in the leather industry.

Solid waste management

Types of Solid Waste

The leather industry produces various types of solid waste, including animal skin trims, animal hairs, flesh wastes, buffing dust, and keratin wastes. These wastes are generated during the different stages of leather processing, such as hair-recovering, unhairing, and buffing processes.

Environmental Impact

The solid waste generated by the leather industry can have detrimental effects on the environment. For example, the non-biodegradability of tanned leather poses challenges for solid waste management systems, negatively impacting agro-based activities and groundwater systems. The presence of hazardous chemicals, such as chromium, in the solid waste can also lead to soil and groundwater contamination.

Waste Disposal Methods

Currently, the majority of waste generated by the leather industry is disposed of through landfill or incineration processes. However, these methods are not ideal due to the economic and environmental costs associated with transportation and the potential release of toxic emissions.

Reutilisation and Recycling

Effective reutilization and recycling of solid waste are essential to reducing the environmental impact of the leather industry. Some by-products, such as bovine and ovine hair, can be recovered during the tanning process and put to alternative uses. Additionally, the development of vegan leathers made from recycled plastic, pineapples, cork, mushrooms, and polyurethane offers a more sustainable alternative to traditional leather production.

Pollution Control and Treatment Technologies

To mitigate the environmental impact of solid waste, advanced treatment technologies are being explored. For example, Advanced Oxidation Processes (AOPs) and adsorption techniques have been shown to increase the biodegradability of effluents, making them more suitable for disposal or reuse. Other methods, such as electrocoagulation, have been proposed to remove carcinogens and other organic contaminants from wastewater.

Policy and Consumer Awareness

Addressing the solid waste management challenges in the leather industry requires a multi-faceted approach. It involves advocating for policy changes that hold the leather industry accountable for its environmental impact and encouraging fashion brands to reduce and phase out their leather use. Additionally, raising consumer awareness about the environmental consequences of leather production can influence purchasing decisions and promote more sustainable alternatives.

Water consumption

The leather industry's water consumption varies depending on the specific processes and stages involved. On average, the post-tanning stage requires 8.6 m3 of water per ton of leather processed. However, the water demand can be higher or lower depending on the size and location of the tannery. Large-scale tanneries may have a higher water demand, while tanneries in regions with access to wastewater treatment facilities may be able to reduce their water consumption.

The leather industry's high water consumption and pollution have significant environmental impacts. The large volumes of wastewater generated by tanneries contain high concentrations of organic and inorganic pollutants, including salts, lime sludge, sulfides, and acids. These pollutants contribute to water pollution and can have harmful effects on aquatic ecosystems and human health. The leather industry's water consumption also intersects with other environmental issues, such as land use and deforestation, as the production and transportation of leather goods require significant water resources.

To address the water consumption and pollution issues in the leather industry, several treatment technologies have been proposed and implemented. Advanced Oxidation Processes (AOPs) and adsorption techniques have been found effective in increasing the biodegradability of effluents and reducing pollution loads. Membrane Separation Processes have also been successful in treating effluents for potential reuse. Additionally, the development of vegan and synthetic leather alternatives, such as those made from recycled plastic, pineapples, cork, mushrooms, and polyurethane, offers a way to reduce the water consumption and pollution associated with traditional leather production.

Environmental impact

The leather industry has a significant environmental impact, particularly during the tanning process. The tanning stage of leather production is responsible for about 90% of the total pollution caused by the leather industry.

Leather processing requires large volumes of water and chemicals, and releases effluents into the environment that pollute surface water and may cause harm to human health. The conversion of raw hide into leather involves several mechanical and chemical operations using acids, alkalis, chromium salts, tannins, solvents, auxiliaries, surfactants, metallorganic dyes, natural or synthetic tanning agents, sulfonated oils, and salts. The tanning process stabilises the collagen or protein fibres in skins so that they stop biodegrading—otherwise, the leather would rot. The average water demand of post-tanning is 8.6 m3 per ton of leather processed, and the average chemical consumption is 360.2 kg per ton of shaved leather. The retanning and fatliquoring steps are the largest chemical consumers.

The leather industry has been associated with the pollution of over 55,000 ha of land, affecting about 5 million people directly or indirectly through the low quality of drinking water and the social environment. Tanneries release various gaseous pollutants such as ammonia, amines, aldehydes, hydrogen sulfide, and volatile hydrocarbons into the atmosphere as effluents. During the deliming, unhairing, or drying processes, ammonia is emitted, and during liming/unhairing and subsequent processes, emissions of sulfides occur. Particulate emissions contain chromium, which may occur because of the reduction of chromate or from handling basic chromic sulfate powder or even from the buffing process. The presence of carcinogens in wastewater increases the need for its treatment.

The leather industry also contributes to eutrophication, a serious ecological problem in which runoff waste creates an overgrowth of plant life in water systems, which suffocates animals by depleting oxygen levels in the water and is the leading cause of hypoxic zones, also known as “dead zones”. The leather industry is also linked to mass deforestation, causing habitat loss for millions of species, eliminating the Earth’s tree canopy, and driving climate change.

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