Why water is so important for leather tanneries
You’ll find many tanneries around the world located near rivers. There’s a good reason for that and it’s called H2O – water. Like various other industries, water is used in many process steps and sourcing directly from rivers was historically the obvious way. Why do we need water to produce leather? Where is all that water used in the leather-making process? And more importantly, what happens to the water afterwards?
Water as the perfect solvent
When we speak about the chemical side of leather production (or chemistry in general), a solvent is a substance that dissolves a solute (a soluble substance). The result is a solution. We make a solution every day when we make a cup of tea or mug of coffee. Even the sugar (or sweetener) you use is technically a chemical that dissolves into your favorite brew.
Solvents are usually liquids, but there are gasses and solids that function as solvents. If we talk about industrial applications, water is usually excluded from the solvent label. The reason is mostly because water has no harmful chemical properties as opposed to other solvents. Water-based is often considered environmentally friendlier, but it’s still a solvent. In fact, water is the universal solvent and the reason for that is… well, chemical.
Without water no leather tanning
Each H2O molecule has a positive electrical charge (where the hydrogens sit) and a negative charge on the other end (where the oxygen sits). Due to this polar arrangement of its atomic structure, it can disrupt the bonds holding other chemicals together, so that they dissolve into their salts. It doesn’t do that with everything, but lots of salts find themselves dissolving in water. It’s a lot like magnets. Water molecules thus have a positive and negative end, which attracts. Salts are similarly made up of positive and negatives, so they are attracted to their opposites. Water has the strength to pull these salts apart and dissolving them by attracting their parts to the water molecules.
Water Function 1: solvent
In the leather making process, water has two functions: to dissolve and carry the chemicals, and to allow the leather to mix and turn in the tanning drum without scuffing or damaging it.
Water dissolves chemicals, enzymes and dyes in the processing of hides, converting them to leather and finishing them. For example, lime dissolved in the water can seep in-between the leather fibres, allowing it to open up the structure of the leather, puffing the leather up. This enables the tanning agent to penetrate the structure and fix to the collagen to make it non-putrescible. The de-liming agent then, in turn, is dissolved into the water so it can remove the lime and the leather shrinks back. Without water as carrier (solvent) in this process, tanning would be impossible.
Water Function 2: mechanical action
Water also helps in the process of mixing the hides in the drums, preventing rubbing or damage to the hides and ensuring even distribution of the above-mentioned agents. After all, we expect hides with consistent performance and a smooth appearance. (Though there is a new trend towards celebrating the natural aesthetic of leather, just as we celebrate the grain and knots in authentic wooden furniture.) This is called the mechanical action of water. New innovations look at whether this can be done with other substances; there’s even a company selling reusable plastic beads to realize this process (water is still needed to dissolve chemicals). Water fulfils various functions in the process steps, which we’ll look into next.
Water consumption in the tanning process
All data used here are rough estimates. As each tanning process is different and available methods for higher efficiency vary, these merely serve to given an indication, based on global statistics. It’s important to note that tanneries don’t just discharge water, but also purify it and often recycle it. This greatly affects the water footprint.
Beamhouse & Tanning processes ~55-70% water of total water usage
The beamhouse process is a name for the preparation stage where the hide or skin is made ready for tanning. This is, by far, the most intense process with regard to water usage.
Frequently, hides are salted or pickled to preserve them. In the soaking stage, the hides are cleaned and brought back to their original, fresh state. During this stage, residues like blood, dirt, dung, and other proteins are also removed from the hide. Using fresh hides significantly lowers the water impact here, which is what many European tanners now do.
- Liming & deliming
Hair, wool and flesh are loosened during the liming stage with mechanical removal, followed by a dehairing operation with chemical treatment. A paste of lime and/or enzymes are put onto the hide. This helps remove all unwanted elements from the hide or skin and leave only the collagen central layer. After liming, the paste needs to be removed, which is done with ammonium chloride or sulphate. Another treatment usually follows, which is bating. Here scud, short hair and grain correction takes place.
In the pickling process, the hide is made ready for tanning (bating also is important for this) as the leather needs to have the right pH values for the tanning agent to be effective. Many tanners have integrated the deliming, bating and pickling process, which saves them time, water and resources.
During the beamhouse process, between 7 and 25 m3 water per ton of hides is used (Buljan & Králl, 2019).
- Tanning Operations
The more efficient the beamhouse process is, the less needs to be done in the tanning phase. Here the hide is stabilized with tanning agents and turned into leather. The chemicals to do this need to enter the collagen structure of the dermis and penetrate the protein structure.
During the tanning operations, between 1 and 3 m3 water per ton of hides is used (Buljan & Králl, 2019).
Post-tanning process ~ 25 - 45% of total water usage
During post-tanning, the leather acquires its final properties, meaning the leather is made suitable for its final application. This is done through retanning cycles, where various chemical agents enter the hide structure.
After tanning the hide, the leather is often neutralized for post-tanning processes. Because the hide will be (depending on the tanning system) cationic, neutralization is required for effective uptake of anionic retanning agents, dyestuffs and fat liquors. Here the leather is washed, neutralized (with neutralizing agents) and washed again.
- Retanning and fixing processes
Retanning, dyeing and fat liquoring are usually done at the same time. Here again, working agents need to enter the structure of the hide and be taken out afterwards.
During post-tanning, between 4 and 8 m3 water per ton of hides is used (Buljan & Králl, 2019).
Finishing ~ 5% of total water usage
Finishing gives the leather its final coating and looks, and is often subjected to some additional treatment.
During finishing, between 0 and 1 m3 water per ton of hides is used (Buljan & Králl, 2019).
The total water impact
Though water is essential for the creation of many of the flexible materials we use, including leather, consumption and pollution are major sustainability issues. You may not be aware of it, but we use up to 250 litres of water to produce 1 kilogram of PET (polyethene terephthalate). That’s the plastic used for water bottles. The total amount of water used depends on various factors like weight, size and type. Hides used in fashion can be 27 kg per hide, where automotive uses 36 kg hides. This comes down to a water use per kilo of leather in a range of 12 l/kg to 37 l/kg (or water used per hide of 324 l/hide to 1,332 l/hide). But this excludes processes like tannery cleaning and maintenance. According to Swartz (et al, 2017), the difference in usage can be quite significant. Particularly vegetable tanning uses copious amounts of water. When we talk about pollution levels, there is also a difference if a tannery is full process or only part process, and there are many other actors to consider.
Efficiency is essential and reducing the water footprint has been a key focus point for years. In fact, water use in leather production has significantly reduced over the last 25 years. In our next article, we’ll look more closely at this topic.
- Buljan, J., Král, I. (2019) The framework for sustainable leather manufacture. UNIDO. Retrieved from: Leather Panel. [Accessed on 30 July 2020]
- Nazer, D. (2010) Saving Water and Reducing Pollution in the Unhairing-Liming Process in the Leather Tanning Industry. Published in: From Scarcity to Sustainable Water Use in the West Bank, Palestine. CRC Press, Francis & Taylor Group, United States.
- Nothing To Hide (2015) Essay Ten: Water Consumption – Reducing water use in tanneries. Retrieved from: Nothing To Hide.[Accessed on 8 July 2020]
- Olson-Sawyer, K., Madel, R. (2020) The Water Footprint of Your Plastic Bottle. Retrieved from: Foodprint. [Accessed on 8 July 2020]
- Sundar, V., Ramesh, R., Rao, PS, Saravanan, P., Sridharnath, B., Muralidharan, C. (2001) Water management in Leather Industry. Journal of Scientific & Industrial Research Vol. 30, June 2001. Retrieved from: JSIR. [Accessed on 9 July 2020]
- Swartz, C., Jackson-Moss, C., Rowswell, RA., Mpofu, AB, Welz, PJ. (2017) Water and Wastewater Management in the Tanning and Leather Finishing Industry: Natsurv 10 (2nd edition) – Report to the Water Research Commission. Retrieved from: Research Gate. [Accessed on 8 July 2020]