A catalyst for change – innovations in chemical and waste14 August 2015
It’s widely understood that the use of tanning enzymes can contribute to a better environment, but remain underused. Professor Dobre Jovanoski, a Macedonia-based international leather expert and chemical engineer, examines the environmental impact through life-cycle assessments, and explains how Novozymes’s protease and other innovations are leading the way in chemical and waste reduction.
Although less than 10% of bovine hides are soaked and unhaired using enzyme-assisted processes, results show that enzymes can actually improve product quality while reducing production costs and environmental impact. As energy is used in the production of chemicals, chemical savings in the leather industry mean a reduced contribution to climate change. And assuming that the environmental improvements are applicable worldwide, the global potential savings from using enzymes is approximately 700,000t of CO2 a year.
A life-cycle assessment (LCA) is an evaluation tool that measures the environmental impact of a process or product. Global biotechnology company Novozymes conducted an LCA, comparing an entirely chemically based approach to leather treatment with one that substituted enzymes for chemicals. The LCA calculations were based on figures from a major Chinese tannery that used chemical and enzyme-assisted methods.
The LCA demonstrated considerable environmental savings for enzyme-based processing - results that significantly outweigh the small amount of energy and CO2 emissions required for their production. The figures show that using enzymes in the soaking process led to shorter soaking times, lower tenside and soda requirements, and effecting savings in energy, CO2 and costs.
Using enzymes in the unhairing process lessened the need for sulphides, thereby lowering the sulphide content of the wastewater, as well as reducing the energy consumed and the CO2 emitted during sulphide production.
Unlike enzymatic processes, chemically based operations depend on large amounts of harsh chemicals that are produced using much greater amounts of energy. The savings in sulphide turn out to be the most important environmental benefit of enzyme use. A large quantity of coal is used to produce sulphides, and consequently a large quantity of CO2 is emitted during their production. As a considerable amount of sulphides are required for processing, the impact of chemical methods on global warming is significant. Other environmental hazards (acidification, toxicity, nutrient enrichment and smog formation) are also reduced in enzymatic processing. Overall, the environmental impact resulting from enzyme production is at least 20 times less than that incurred by the saved chemicals and energy consumption.
Furthermore, during the processing of leather, a lot of solid waste, fleshings, cuttings and shavings are produced. Instead of dumping this material, resulting in environmental problems, enzymes (alkaline proteases) can be used to recover chromium and fatty material from this waste material. Alkaline proteases are used under alkaline conditions to facilitate the separation. Depending on the nature of the waste, the protein part can be reused either in biogas production or even in animal feed stuffs. The inorganic (Cr) part can be refined for reuse in the tannery.
The global supply of bovine hide for leather production was about 8.8 million tons in 2005. Today, less than 10% of bovine hides are soaked and unhaired in enzyme-assisted processes. Assuming that the environmental improvements observed in this study are applicable worldwide, the global potential savings from using enzymes is approximately eight million gigajoules of energy and 700,000t of CO2 a year. The saved CO2 emissions would be equivalent to the annual load from 75,000 average world citizens or 170,000 cars.
One of the oldest applications of industrially produced enzymes is in the processing of hides and skins for leather. Hides and skins contain proteins and fat in between the collagen fibres, and before the hides and skins can be tanned, these substances must be partially or totally removed. The proteins can be removed by proteases and the fat can be removed by lipases, as well as by tensides and solvents. Today, proteases are used mainly for soaking, bating and enzyme-assisted unhairing. The use of lipases to dissolve and remove fat is under rapid development, and lipases are now an integrated part of leather processing in many parts of the world.
When preparing hides and skins for liming and unhairing, proper soaking of the raw stock is essential for obtaining a good quality leather. For some raw materials, notably dried stock, satisfactory rehydration may be a difficult and time-consuming process. By degrading interfibrillar protein using proteolytic enzymes, water absorption is significantly facilitated and the soaking operation can be shortened. Novozymes's soaking protease works especially well at pH of 8-9.
The conventional way to remove hair from bovine hides is to use lime and sodium sulphide in a hair-burning process. They dissolve the hair and open up the fibre structure.
Novozymes markets a protease that assists in the removal of hair. It is a unique protease because it is active at the very high pH of 12-13 found in the liming process.
Most importantly, enzyme-assisted unhairing results in a cleaner grain surface, and improved area yield and softness. The use of this specific Novozymes protease also offers tanneries a number of options. For instance, the sulphide and lime requirements can be reduced by as much as 40% while maintaining the same liming time. Alternatively, tanners can shorten the liming time by at least half without any loss of quality. Another possibility is to avoid the use of amines, which can be converted into carcinogenic compounds.
The hair-burning process is by far the most widespread. This specific Novozymes protease can also be used to advantage in the hair-saving process, which is more environmentally sound. The hair is not dissolved and can be filtered out from the liming float. In this way, it is possible to reduce COD by 50% and BOD by 35% in waste discharges.
Lipases are a type of enzyme that specifically degrades fat and so cannot damage the leather itself. Lipases hydrolyse not just the fat on the outside of the hides and skins, but also the fat inside the skin structure. Once most of the natural fat has been removed, subsequent chemical treatments such as tanning, retanning and dyeing have a better effect.
The Novozymes alkaline lipase has a pH range of 7-13 whereas the Novozymes acid lipase is recommended for acidic conditions, a pH of 3-7. Novozymes is the only company offering an acid and an alkaline lipase.
The main advantages of using lipases are a more uniform colour and a cleaner appearance. Lipases also improve the production of hydrophobic (waterproof) leather; makers of leather for car upholstery have commented that 'fogging' is reduced. This is the term for the build-up of a film of chemicals on the inside of car windscreens.
Lipases represent a more environmentally sound method of removing fat. For bovine hides, lipases allow tensides to be replaced completely. For sheepskins, which contain up to 40% fat, the use of solvents is very common and these can also be replaced with lipases and surfactants. Solvents tend to dry out the skin and give it a pale colour.
If surfactants are used for sheepskins, they are usually not as effective and may be harmful to the environment. Stronger surfactants such as nonylphenol ethoxylate have a better effect but they are more detrimental to the environment. When using Novozymes lipases, the original surfactant dosage can be reduced by at least 50% in the case of sheep and pigskins. In addition, nonylphenol ethoxylate can be substituted with more biodegradable surfactants.
To make the leather pliable, the raw material requires an enzyme treatment before tanning. This is called bating, whereby certain protein components are dissolved and can be washed away. The degree to which bating is applied is dependent on the desired character of the finished leather. Glove leather, for example, should be very soft and pliable, and is subjected to strong bating, whereas leather for the soles of shoes is only lightly bated. Leather for shoe uppers falls between these two extremes.
Traditionally, dog or pigeon dung was used as a bating agent. Besides being a difficult process to control with unpredictable results, the dung did not exactly contribute to the creation of a pleasant working environment. The dung bates owed their softening effect to the action of proteases, and it was heralded as a great step forward in 1908 when the German chemist Otto Röhm patented the first standardised bate. This was based on pancreatic enzymes extracted from slaughtered animals and turned out to be a great success. Today, bacterial proteases and trypsin (the traditional pancreatic protease) are used for bating.
Novozymes produces both types of enzyme. The company also offers a new acid protease that works well at pH 2.5-7.0. It should be applied during the second bating of pickled or wet-blue skin/hides.
Flexibility and synergy
In the bating and degreasing process steps, Novozymes's range offers the flexibility to add an enzyme suited to the conditions, whether acidic or alkaline.
In addition, two different enzymes may be added simultaneously. For example, Novozymes's unhairing enzyme (NUE) and alkaline lipase make a very good combination for liming. In trials, these two enzymes were found to give synergy effects. The Novozymes unhairing enzyme opens up the hide, allowing the Novozymes alkaline lipase to remove fat, giving improved unhairing and improved degreasing. Another good combination is the simultaneous use of a Novozymes bating enzyme and a Novozymes degreasing enzyme under acidic conditions. These are examples of new areas that Novozymes will be investigating in the future.