There is a lot to be said for the reduction of chemicals and hazardous substances in production and products. After all, health and safety matters to us, animals, and our planet. However, the natural alternatives we used in the past are not always something we would like to return to, particularly when it comes to leather tanning. Think of some of the most unpleasant smells you have encountered - some of them are likely part of old-fashioned leather tanning. Let’s get into the smelly stuff!

Tannery odours: An odoriferous business

Leather tanneries were, in the past, located on the outskirts of towns, preferably on the far end so the wind would not carry the smell. Animal hides were originally processed using urine and dog faeces. The urine would help clean the leather, whereas the faeces contain enzymes that helped prepare the collagen in the hide. As this was often done in the open air, the smell of urine, faeces and decaying wastes were carried by the wind. Not particularly enjoyable, but a regular part of city life in the olden days. Up until the Victorian age, it was perfectly normal for the inhabitants of cities to collect dog faeces and urine for the exact purpose of leather making, therefore, the streets were relatively clean.

Modern processing has removed the need for these odorous ingredients (and therefore that of picking up after one’s loyal pet). Even vegetable tanning, a method that also used the old technologies, has modernised. However, modern use of chemicals and other agents bring with them a new challenge: to manage the processes and chemistry to minimise any odour.

Smelly hides, bad leather

Smells arising from tanneries usually result from mistakes in the process, but issues with the hides can also cause unpleasant smells. Hides and skins that haven’t been stored properly and have allowed to start decompose (rot) can be a source of foul odours.  Rotting is not beneficial to the leather quality either, as it creates a substandard product, both reasons why modern tanneries have strict transport and storage requirements. Degrading of protein and fat in the hides is, therefore, to be avoided, resulting in European tanneries keeping their hides fresh and healthy by using them directly or freezing.

Odour-less facilities today

Modern tanning methods may have gaseous emissions. More often than not, the chemistry of tanning is carefully controlled to prevent any emissions, but where they do arise, the gaseous emissions are controlled using modern technology, such as filtration systems, which capture any emissions and prevent them from going anywhere. The University of Northampton has a tannery on campus where extensive air filtration and closed processes prevent any smell leaving the facility. A lot of the opinions relating to leather production smells are merely prejudiced, as research has shown only the beamhouse processes (liming, bating, degreasing) are likely to produce most of the highly undesirable odours (Akyüz, 2009).

A lot of the opinions relating to leather production smells are merely prejudiced

Air emissions and leather production

The leather industry does, however, produce gasses that are controlled.

Tannery emissions in gaseous form are summarized as follows:

  • Processing emissions due to the use of chemicals: H2S, VOCs and NH3
  • Emissions from thermal systems in the tannery: CO2, CO, NO2
  • Wastewater emissions: H2S and NH3
  • Finished leather emissions: VOCs – solvents, CHO (formaldehyde)

While chemical management is part of every-day operation of a modern tannery, some chemistries require particular attention, such as those with potential to create NO2, SO2, and H2S. Careful management is important, as these gaseous emissions inside the tannery can affect worker health when no proper are in place.

The European Leather Industry reports significant reductions in VOCs in leather production. Greener chemistry and effective (waste)water management have contributed to a reduction of pollutants in the exhausts from tanneries. Tanners have also invested in circular processes where process heat is reused for thermal processes (or green energy, lowering CO2 and NO2 Production.

We all know the ‘new leather’ smell, a combination of certain tannins and oils used to condition the leather to make it soft and supple. These are regarded as VOCs, and as a result of the trend to reduce VOCs the

“’new leather’ smell from finished leather is vastly reduced due to water-based chemical solutions, degassing processes, and more thorough cleaning.

  • Hydrogen sulphide and oxides of sulfur (SOx) form after sulfur is used in the beamhouse process to prepare the hides. More precisely, these gases emerge when the chemical treatment treats the fresh hides in liming, bating, and pickling. Closed drums often cause the chemicals to be emitted in the wastewater. Tanners try to prevent H2S, but it can form when wastewater mixes or wrong chemical combinations are applied (due to automated processes this is ever less likely).
  • Ammonia similarly forms from deliming in the beamhouse process. It can be used in leather tanning, but most likely develops in the effluent and solid waste heat generation process. Careful chemical management of floats and processes help to reduce this to a minimum.

Controlling air emissions and managing wastewater
Most of the European tanneries now use air filtration systems throughout the leather production process. Many regions of the EU have statutory controls that restrict the levels of odour (specifically H2S, SOx and NOx) in and around industrial facilities. Innovations in the processes can yield vast reductions of VOCs (up to 40%), and wastewater filtration can purify the effluent by 99% according to a case study by the European Commission (2018). Air filtration, respiratory hazard control, leak detection, repair programmes (LDARP) and upgraded abatement technologies, also contribute to limiting tannery air pollution. Innovative solutions will allow the future of the leather industry to be odour-free.

[1] Formaldehyde has virtually disappeared from leather emissions due to strict legislation.

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