The International Union Environment Commission is concerned to take into account the technologies currently applied by the most advanced tanneries and not just to consider the latest developments from research units.

The general recommendations collected by the commission have to be adapted to local conditions and under the supervision of a leather specialist and taking into account the requirements of the production.

The commission understands that cleaner technology can be defined as the environmentally and economically best practicable technology. This document will be updated annually.

Rawstock

Treatment of fresh or cooled hides and skins

This exists in many countries and tanneries. Whenever possible, treatment of fresh hides and skins is the best solution to reduce salt pollution. Time elapsing between slaughtering and further treatment (e.g. beamhouse processing) must not exceed a few hours. Beyond this period, it is necessary to cool the hides and skins, either in ice or cold air. Cold air is interesting if hides are transported over long distance. Storage below 4°C yields good preservation up to three weeks. This system can be used only when the capacity of the slaughter-house is equivalent to that of the tannery.

It would be desirable to practise fleshing and trimming in the slaughter-house.

Drying

Shade drying of small skins is a low cost environmentally acceptable process in some climate. Controlled air drying using heat pump or other system is suitable for any climate.

Dry salting

Dry salting can minimise the amount of salt used for preservation of skins and hides.

Use of antiseptics

The use of antiseptics with low effect on the environment can help to increase storage time of fresh or chilled hides and skins. Suitable preservatives include: TCMTB, Isothiazolone products, potassium dimethyl dithiocarbamate, sodium chlorite, benzalkonium chloride, sodium fluoride and boric acid. Some of these are also appropriate for soaking, pickle and wet-blue preservation.

Partial salt elimination

It is possible by using hand shaking, mechanical brushes or a drum type shaker to eliminate up to 10 % of the salt added to hides and skins for preservation. The salt can be reused after dissolution and removal of solids for pickle processes. This method gives a partial answer to the salt pollution problem. Neither brine curing nor salt curing can be considered as cleaner technologies, even if pre-fleshing in slaughter-house on green hides gives an easier valorisation to this specific waste.

Beamhouse processing

The new drums and processors facilitate efficient draining and washing, and allow the routine use of low floats for processing, thereby resulting in significant savings in water consumption.

Soaking

Apart from the use of less harmful antiseptics, the only cleaner technology that could be applied at this stage is the fleshing of green hides after soaking. It yields a lower quantity, compared to lime fleshings, with a neutral pH, and better conditions for transformation into proteins and fats that are not contaminated with chemicals.

Classical unhairing-liming process

The enzymatic treatment of hides and skins can be considered as a cleaner technology only if the amount of sodium sulphide is reduced substantially. However it is not yet possible to use less than 1 % of sodium sulphide for bovine hides. Compared to a classical « hair-dissolving » process, 30 to 50 % of COD reduction, in beamhouse effluent, can result from enzymatic or another hair saving treatment.

Hair saving unhairing-liming methods

For traditional skin production, painting and sweating may be considered the cleaner technologies. Recovery of hair before dissolution, either when it is separated during the liming, or at the end of a hair saving process, can lead to a COD reduction of 15 to 20 % for the mixed effluent, and a total nitrogen decrease of 25 to 30 %.

It is an advantage to filter off the loosened hair as soon as possible and higher COD and nitrogen reduction can be obtained. This process can be considered as a cleaner technology if the hair is utilised, even as a nitrogen source.

Direct recycling can be applied when there is a good control level in the tannery. Resulting advantages are savings in sodium sulphide (up to 40 %) and in lime (up to 50 %). It could give a decrease of 30 to 40 % of the COD and 35 % of the nitrogen for the mixed effluent. The quality of the leather produced might be affected negatively through this recycling process, unless unhairing and opening up processes are used in two steps. The quality of the scudding can be improved during the subsequent phases of leather processing. This cleaner technology is industrialised in several large bovine tanneries for shoe upper leather.

Splitting limed hides

Faced with the difficulties of upgrading the chromium-tanned split waste, splitting on the lime can be considered as a cleaner technology as it saves chromium and yields waste that can be easily recovered for the production of gelatine.

Other ammonium-free deliming

Ammonium-free deliming agents, such as weak acids or esters, can totally or partially replace ammonium salts used for conventional deliming. However, in comparison with CO2 deliming the resulting COD is often higher.

Tanning operations

Chromium tanning salts are used today in 85 % of tanning processes. Only the trivalent form is used for tanning operations and this chemical cannot be replaced by another, except for special purpose, to give the same quality of leather. If its concentration in waste exceeds a certain level imposed by national regulations, it strongly limits any possibility of upgrading, or disposing the waste at acceptable costs.

When tanning and pickling floats are separated, the recycling of pickling floats can economise up to 80 % salt and 20 to 25 % of either formic or sulphuric acid. When they are associated, the greater economy can be made on the sulphuric acid. For wool-on sheepskins, recycling of pickling and eventually bating floats, using long floats over 150 %, is a current practice which gives good results. It is generally associated with chromium float recycling.

Salt concentrations in pickling floats can also be reduced by using non-swelling agents.

Degreasing operations

Solvent degreasing is still in use. This practice can lead to a cleaner technology when the solvent is recovered, the extraction brines are recycled, and the natural grease is commercialised. Discharge of solvents is unavoidable with solvent degreasing. Solvent degreasing is still in use but alternatives can be applied for the highest quality skin production.

On wool-on lamb skins, it is a common practice to undertake a dry solvent extraction when crusted.

The use of non-solvent methods implies the use of higher amounts of surfactants. Ethoxylated fatty alcohols should be recommended instead of the more widely used ethoxylated alkylphenols, given that they are more easily degraded. Nevertheless the effluents obtained by this method should equally be treated, given that its COD level may amount as much as 200,000 ppm, due to the content of natural grease and surfactants (1 g/l of natural grease is about 2,900 ppm COD, and 1 g/l ethoxylated alkylphenol is about 2,300 ppm COD).

Proteolitic enzymes can assist for degreasing pig skins and this reduces the amount of surfactant required.

Wet-white production

This process, giving the possibility to produce untanned and upgraded sheetings and shavings, can be considered as a cleaner technology when the chemicals used are not suspected of toxicity. Aluminium, titanium, zirconium are not listed as hazardous, although restricted in several countries. Modified aldehydes tanning agents can be considered as leading to a cleaner process, according to local regulations.

Direct recycling of chromium tanning floats

When this method is applied in the tannery, it gives the possibility to strongly limit the presence of chromium in the effluents arising from tanning. Savings can be obtained from the process, by a reduction of 20 % of the chromium used in a conventional tannery process, and up to 50 % for wool-on sheepskins, and substantial reduction in the amount of salt used.

Excess chromium containing liquor should be precipitated and recycled.

When large quantities of chromium floats are recovered, recycling after precipitation is the best adapted solution. Coagulants used are mainly sodium carbonate, sodium hydroxide, and magnesium oxide. The addition of polyelectrolyte can improve flocculation. Sludge obtained after sedimentation and optional filtration is re-dissolved with sulphuric acid, and basicity set at the right level. For conventional tanning, it is possible, with this process, to obtain a clarified effluent, with less than 10 mg/l of chromium expressed in Cr, which might be reused for the next pickling or tanning float. The clarified effluent can also be re-used for first soaking float.

High exhaustion tanning process

In order to reduce chromium concentration in the waste float, high exhaustion chromium salts, adapted basification products and/or temperature increase can be used. Tanning of lime splitted hides has to be preferred. It is very difficult to obtain a residual chromium float containing less than 500 mg/l Cr3+, however this process can be considered as a cleaner technology.

Chromium free tanning

In most cases, chromium tanning should be considered as the best available technology. Many alternative formulations have been proposed but the results obtained at the moment are not completely satisfying for all type of leather. Synthetic organic tanning agents, alone or in combination with a metallic cation can be considered as a substitute for chromium, provided that environmental and workers health regulations are complied with.

Tanning with organic tanning agents can produce a mineral free leather, but such leathers do not have the same characteristics as chromium tanned leather .

Vegetable tanning with a dry drum process, or in vats, in closed circuit, can minimise waste and must be included in these considerations. Due to the high pollution load and slow biodegradability conventional vegetable tanning cannot be considered more environmentally friendly than chrome tanning and vegetable tanned leather has limited application.

Recovery of vegetable tanning floats by ultrafiltration is used in several European tanneries and the recovered tannins may be used in the tanning process.

Vegetable plus aluminium tanning can produce chrome free leather.

When the use of chromium is required for retanning operations, the same consideration should be given as for chrome tanning. Absence of chromium during retanning, of environmentally risky dyestuffs and benzidine in dyes, of halogenated oils in fatliquors, are essential arguments in a cleaner process. High level of exhaustion for syntans, dyes and fatliquors are also to be considered.

In some cases, through feed dyeing with adapted dyes can be considered a cleaner technology.

Finishing operations

The use of water based finishes is fundamental for a cleaner process. Pigments must not contain any environmentally risky heavy metal or other restricted products. Water based formulations (containing low quantities of solvent) are available for spray dyeing. Finishing products have to meet the current limits imposed by environmental and workers health regulations. The equipment used is extensive. Roller coating or curtain coating machines are far more satisfying from the environmental point of view, but they cannot be used for all type of leather. For other types, spraying units with economisers and High Volume Low Pressure (HVLP) spray guns can reduce discharges to the environment.

Water management

Between 15 to 40 m3 of water is used for the production of 1 tonne of wet-salted raw hides and 110-260 litres per sheepskin. Measurement and control of consumption are an important and essential point of the water management

In many countries water has become a scarce commodity and the costs for the consumption and discharge of water increases regularly. Water has to be managed properly and several options are available to minimise the overall consumption of water.

Reduction: The first step is the reduction of water consumption with strict measurement and control of consumption. Low float processing, batch-type washing instead of rinsing and combining processes (compact recipes) are practical examples of technologies to reduce water consumption by 30% or more. Lower volume of water will result in a higher pollutants concentration.

Recycling: Certain specific processes are suitable for recycling of floats, although in most cases installations for treatment are necessary. Examples are; soaking, liming, unhairing, pickling and chrome tanning liquors, which can reduce the overall water consumption by 20-40%.

Re-use: Biologically treated effluent offers the opportunity of replacing a certain amount of the process floats such as, the beamhouse process floats, with treated water. Depending on the type and efficiency of the treatment process additional operations might be necessary, such as filtration and disinfection, to meet the required water quality standards.

Reduction in chemical use

Process should be optimised with regard to chemical use to minimise waste. Reduced floats allow reduction in chemical use (liming, deliming and pickling).

updated by

IUE Commission Barcelona (Spain)

11-12 April 2000