Abstract
The study ‘Estudio comparativo Medioambiental en las Industrias de Curtidos de la Comunidad Valenciana’ carried out by CEC-FECUR in collaboration with INESCOP to promote environmental issues within the sector. The comparative study involving 18 tanneries in the Valencia region. The tanneries work with different types of skins, stages of the leather making process, volume of leather processed etc.
The study found that the following action was required: Reducing the quantity and danger levels of wastes and effluents generated; recycling and reuse of solid wastes; elimination of deposits in outflow; substitutions of dangerous products and adopting cleaner technology should be a priority. The sludge generated by tanneries is currently not suitable for use as fertiliser as the chrome levels it contains are too high. Salt levels in the wastewater remain a problem. Copies of the full report (in Spanish) can be obtained from CEC-FECUR.
Introduction
The Spanish leather industry has knowledge and experience of some of the most advanced tanning and processing technology and being within the EU can guarantee a high level of environmental safeguard. They see this as an advantage over their competitors in developing countries who do not offer such guarantees to the end user.
A study was carried out by Spanish tanners’ association CEC-FECUR with the collaboration of Footwear Technology Institute INESCOP with the aim of further promotion of environmental issues within the sector. The comparative study involved 18 tanneries in the Valencia region.
In recent years, legislation at both national and European level has forced tanners to invest heavily in environmental control.
It is important to note that investment on this scale has had an impact on the competitivity of the Spanish leather sector in comparison with other countries who do not have such stringent regulations. Therefore, measures which prevent pollution at source and internal recycling, so called minimisation methods, when efficiently applied can bring not only environmental benefits but also economic ones, reducing the cost of waste management and saving resources.
European overview
In parts of Europe, the leather industry is still an active sector of the economy, with a 25% share of the world market.
The majority of the European leather sector is concentrated in Italy, generating 30,000 jobs in the country. In second place is Spain with 206 companies, accounting for 6,500 jobs.
Spain has 206 tanneries, 86 of which are found in Catalonia, 72 in the Valencia region, 24 in Murcia, and five in Madrid. The number of tanneries has declined in the past six years (see Figure 1). The leather sector in Spain is mostly comprised of small and medium sized companies.
Environmental problems
The tanning industry is highly conscious of its environmental responsibility and transforms a degradable waste product from meat into something useful and beautiful. Its own byproducts must be eliminated or recycled. These include:
* Wastewater which contains organic material and traces of chemical products which have not been fixed to the skin during the industrial process
* Solid waste scraps of leather, remains of packaging, sludge
* Emissions into the atmosphere of particles from the finishing stage and gases from the drum/boiler
Wastewater
The amount of water consumed depends largely on which process is used and the type of leather to be treated. It can be said that around 20-40m3 of water is used per tonne of bovine skins processed and between 70-90m3 of water for sheepskins. Consumption of water by stage of the leather making process can be seen in Table 1.
The load of pollutants present in wastewaters comes from two different sources:
* From the skins, which is known as inherent pollution. This pollution contains protein, animal fat, blood, earth and other dirt. This waste brings organic material to the outflow.
* From products which are added to skins and not fixed, known as added pollutants. They are surplus to requirements and go into the landfill conserving their original organic structure or with a certain level of alteration, such as preservation salt.
Both types of pollution pose a problem for the company in relation to the final outflow of wastewaters. One of the first steps consists in identifying the main substances present as represented in Table 2.
It is important to note that not all leather industries generate wastewater which contains all of these contaminants, as for example sulfide which is not used in the production of doubleface and vegetable tanning which does not use chrome as a tanning agent.
Solid wastes
Ever increasing production levels have made the management of solid wastes a priority in environmental legislation and this requires swift action to halt any deterioration of the environment and the lack of use of the resources contained in residues.
By order of importance, the following areas were outlined as requiring action:
* Prevention: reduction of the quantity and danger levels of waste generated
* Value: regeneration, recycling, recovery and reuse
* Elimination: thermic destruction or deposit in the outflow
Methodology
The report involved is based on technical visits to 18 tanneries and asking them to fill in a questionnaire on waste, discharge and legal obligations. Samples of wastewater both before and after industrial purification were measured for pH, conductivity, suspended solids, COD, BOD5, total nitrogen, amoniacal nitrogen, total chrome, sulfides and oils and fats.
The 18 tanneries covered by the study are involved in various types of raw materials including cow, goat, sheep and reptile, and in all stages of the tanning process including beamhouse, tanning, post tanning and finishing. The tanneries involved ranged in size from seven to 180 employees and production levels varied from 600,000 and 10 million sq ft per annum.
Residues/wastes
Byproducts from the tanning process include liquid waste which carries organic material from the skins as well as traces of chemical products, solid waste (tanned or non tanned) produced by the mechanical operations of the process.
Half the companies surveyed do not take any steps to reduce the waste they generate. However, the majority of companies know the quantity of waste generated and the cost of its management.
Approximately 60% of tanneries surveyed consider themselves to be small scale producers of dangerous waste and are included in the register of small producers of dangerous waste (registro de pequenos productores de residuos peligrosos de la Comunidad Valenciana).
Non-tanned waste
Preservation salt: Skins arrive at the tannery from the slaughterhouse, generally in a salted state. Therefore the first process they go through is de-salting in a drum cage, which removes salt from the skins. Salt waste which contains other dirt from the skins can be used in the preparation of pickle/brine for salting hides if it is first submitted to thermic bacteriacide treatments. Alternatively it can be used to unfreeze motorways or in iron and steel works.
Cuttings of untreated skins: These can contain biocides, salt and other dirt which comes from the animal. They are usually mixed with fleshings.
Wool/hair: In tanning processes which do not require conservation of hair, this is destroyed chemically in the liming process. If, on the other hand, to reduce pollution the tanner decides to recover the hair, this is recovered after its immunisation. This liming bath is subjected to a process of filtration to recuperate the hair, allowing reductions in the COD and the nitrogen in wastewater. The recovered hair can be used as organic compost for agriculture due to its nitrogen content.
Proteins: These originate in the fleshing on separating the muscle tissue from the skins which had stuck on flaying the animal.
In Table 3 the average quantity in kg of non tanned waste generated in tanneries is shown.
In general the greatest quantities of non-tanned waste generated are cuttings of split pelts and offcuts, with averages of between 101.3 and 111.3kg/1,000 sq ft.
As can be seen in Table 4, the main destination for non tanned waste is landfill, followed by different ways of taking advantage of it such as composting or use as a byproduct for making gelatine and dietary fats.
Among non tanned wastes, the sector uses composting for hair and part of the tallow. Composting uses microorganisms to degrade organic material, with part of the material transformed into gases (CO2, CH4, nitrogen oxides). This technique is being extended to treat solid urban waste and generate compost which is used in agriculture.
The main destination for non tanned offcuts and flesh is as a byproduct for use in gelatine. Gelatine for human consumption is extracted and solubilised collagen from animal skin. This has historically been an important step in reducing waste in the sector. However this process must now be carried out in accordance with European regulation 99/724/EC.
Tanned waste
After tanning, a series of mechanical operations are carried out which generate tanned waste. These include shavings, cuttings of tanned skins, cuttings of finished leathers and dust from tanned and dyed leather.
Shavings: Generated during shaving to adjust the thickness of the leather to the requirements of the article to be made. Cuttings of tanned and finished leathers: Generated in cutting operations with the aim of eliminating wrinkled or defective areas such as holes. In the case of finished leathers, as it is a dry residue, it can be deposited in some landfill sites for non harmful waste on payment of the corresponding fees.
Dust from tanned leather: Generated in the buffing process, this dry residue can be deposited in landfills for non harmful waste.
As can be seen from Table 5, for each 1,000 sq ft of treated skins, the most important wastes are cuttings and shavings.
As shown in Table 6, companies manage almost all their finished leather waste by depositing it in a landfill site. As it is a non harmful, dry substance, many landfill sites admit this type of waste.
Some companies recycle part of the shavings of tanned leather to make reclaimed leather (known as leatherboard). This involves the shavings being ground/crushed and mixed with chemical compounds such as synthetic oils, natural latex, synthetic resins, dyes and retans.
Although the product obtained has technical characteristics which are inferior to those of leather, the superficial appearance is very similar and at a much lower cost. Another application which is becoming increasingly important is that of using leather cuttings to make small leather articles such as keyrings, purses etc.
Sludge in wastewater
Every day the tanning industry generates large quantities of wastewater which contains organic material, animal fats and chemical products which are not fixed to the hides or skins. In many cases these effluents are purified in the company’s own premises before final outflow, with the aim of reducing the level of pollutants in the atmosphere.
Purification of wastewaters creates a residue with a high percentage of humidity (around 90% according to Figure 2) known as sludge. It is important that this pollutant containing sludge is correctly managed by tanneries.
The Valencia study indicates that around 90% of the companies generate approximately 6.6 kg of sludge per m3 of treated water and 426.5kg of sludge for every 1,000 sq ft of processed leather.
The majority of companies send sludge to an authorised agent/manager with payment of the corresponding tax, which is between 0.04-0.1 €/kg. The agent is then responsible for its final destination (landfill, incineration, recycling).
For the purpose of deposition in landfill sites, this sludge is classified in the European Catalogue of wastes (annexe II of the order MAM/304/2002), in section 04, waste of the leather and textile industry, codes 040106 and 040107 as non dangerous wastes.
In cases where the level of toxicity is uncertain the sludge must be tested. Sludge can also be used as fertiliser in the agricultural sector.
However, as shown in Figure 3, the concentrations of chrome in the sludge analysed vary between 6,000 -11,000 mg chrome per kg of sludge, exceeding the legal limits for use in the agricultural sector.
When sludge contains a level of total chrome above the limits stated in Table 7, it cannot be used directly as fertiliser in agriculture. To reduce the chrome content of the sludge, measures such as recirculation of the tanning baths or non-chrome tanning methods (wet-white or vegetable) can be used.
Other waste generated by tanneries includes cardboard, wooden pallets, plastic drums, metal packaging, fluorescent tubes, glass, vegetable waste. Organic waste resembling household waste accounts for the largest amount.
With respect to water supply used in the industrial process, 78% of the tanneries surveyed used their own supply of water, whilst 22% used a mains supply. Those companies using underground water supplies claim to have authorisation to use them. Current legislation allows the consumption of water in a property (springs and underground waters) for exclusive use with the permission of the authorities.
Water consumption in the companies surveyed was between 12m3/day and 1,800m3/day. There are no significant losses of water in the tanning process. 70% of companies studied carry out periodic analyses of wastewaters from a daily to weekly basis.
Wastewaters from treating different types of hides and skins vary in their characteristics as can be seen in Table 8. Bovine hides create the most pollution in the effluent, but a lower volume of overall effluent than sheepskins.
As shown in Table 9, the wastewaters of companies carrying out the full process have high conductivity due to the soaking of salted hides and are the only ones which contain significant quantities of sulfur (which comes from the liming process).
At the opposite extreme are the wastewaters of companies which only finish leather which do not contain chrome and have a highly concentrated organic load as the process involves a small amount of water which is recirculated.
89% of the tanneries treat the wastewater in some way before it leaves their premises. The discharge of this water either directly to rivers or to a secondary treatment plant requires permission. 73% of companies release water into the sewerage system while the remaining 22% release them to rivers after treatment.
The study shows that the salt content (measured by conductivity), COD and BOD5 content and the presence of nitrogen as ammonia, are the main challenges presented by wastewater being released into the sewerage system.
Treatment of wastewater
The leather industry generates an effluent which carries a high level of contamination, which cannot be disposed of directly. Although 89% of the companies studied treat the water in some way before it is disposed of, not all use the same techniques or obtain the same results.
One of the first outflows to be treated comes from the dehairing baths which are contaminated with sulfur and lime which has not reacted with the skins, and generally has an alkaline pH of 12.5-13. If these baths have been mixed with the waters from the tanning process which have a pH of 3.5, they should be neutralised to minimise the risk of sulfuric acid formation. This is highly toxic for the workers and corrosive for the facilities. For this reason, companies using dehairing baths carry out a desulfuring treatment to eliminate the danger posed by sulfurs to the company and in 80% of cases these are treated independently to the rest of the company’s waters.
The desulfuring treatment consists of taking the dehairing baths to a pool/pond where sulfurs and sulfates are oxidised via aeration using superficial turbines or bubbling. To speed up the reaction manganese salts (sulfate or chloride) is added which acts as a catalyst and agitates the water. This treatment reduces the sulfur content by between 80 and 90%.
Prior to treatment of wastewaters, it is necessary to homogenise processes as these are carried out intermittently all through the day. This mix is carried out in an homogenisation pond with a shaking process. Around 45% of the companies carry out homogenisation prior to its treatment in the company.
Analysis of purification systems
In order to evaluate the performance of different purification systems, tests were carried out on entry and exit of each process. Among tanneries which carry out the complete process (from raw hides to finished leather), 10% of them carry out no purification of wastewaters. 50% perform a physico-chemical process. 10% perform a biological purification after physico-chemical processes and the remaining 30% have reached an agreement with the municipal purifier. Table 10 shows that biological treatment achieves larger reductions of the pollutant load than physico-chemical treatment.
These results suggest that in many cases biological treatment would purify the water sufficiently for release into the sewerage system. However, the excessive salt content cannot be sufficiently eliminated by either of the two processes by biological or physicochemical methods making it one of the biggest environmental problems for tanners.
The results are broadly similar for those companies involved in tanning to finished leather. 67% perform physico-chemical treatments and 10% choose biological treatment using active sludge after the physico chemical process.
Of those who post-tan and finish leathers, 72% use the physico-chemical process and the remaining 28% do not treat their wastewaters. In this group the physico chemical process is insufficient in reducing pollution to within legal limits, particularly in salt content and organic matter. Therefore, it would be necessary to carry out additional purification treatments (eg biological) to reduce organic matter or specific treatments to reduce salinity, such as reverse osmosis.
Recommendations
Recommendations for reducing danger to the environment following the study carried out by CEC-FECUR and INESCOP:
* Whenever possible, environmental efforts should be directed to minimising the quantity of pollutant containing wastes and effluents. The reduction of waste at source comes from changes in the product design and industrial process. The substitution of dangerous products for less dangerous ones should be the main objective.
* Recycling or reuse of the pollutant carrying effluent: Treatment technology should be applied to those wastes and effluents which cannot be reduced or reused.
* Methods of isolation and storage should be applied to all wastes remaining as such, minimising the risk to the environment and humans.
* The most efficient option would be adoption of cleaner technology. The industry must become more ecoefficient. Ecoefficiency in industry has the objective of reducing environmental damage at source through good environmental practice in already installed processes, the adoption of cleaner technologies, the eco-design and the environmental management.
The long term objectives are to promote the development of a sustainable system of production-consumption through a more efficient and reduced use of resources. That is to say, the development of a cleaner system of production and not simply the development of a system of recycling.
Many of these possibilities relate to the design stage of the product. This stage is the most important in terms of environmental effect (consumption of resources and emission of contaminants). Law 1/2002 for the integrated prevention and control of pollution (IPPC), states that the owners of companies such as tanneries whose production exceeds 12 tonnes of finished leathers per day, are obliged to adapt their technologies to the recommendations of Best Available Technologies (BAT) for the leather sector (see Table II).
The most important BATs for the leather sector include:
Measures to minimise volume and contamination of water
Milling salted hides before soaking: This method is carried out in the phase prior to soaking and consists of reducing the presence of free salt in excess in salted skins, to limit the salinity of the wastewaters generated during soaking.
To reduce salinity of wastewaters generated during soaking, the hides are shaken by hand before soaking. This simple operation reduces salt content in the skins by up to 30%. There are many advanced methods based on a perforated drum which shakes the skins.
The main problem is the post-processing management of salt waste. The possibility of reusing salts should be considered (although the treatment is very expensive) and that skins should be shaken with care to avoid possible damage to the raw material. This can achieve a considerable reduction of the level of salinity (conductivity) in the effluent.
Liming with recovery of hair: The elimination of hair in solid form substantially reduces the level of contamination caused by hair (for example suspended solids, BOD5) of the wastewater. This initiative allows easier recycling of the wastewater.
The operation is conducted in the following phases:
Immunisation: treatment of the soaked skins in a solution of alkaline lime and NaOH for 45-50 minutes.
Chemical shaving: sodium sulfide or hydrosulfate is added which attacks the hair root for 20 minutes and releases the whole hair immunised.
Recirculation of bath: Passing through a 1mm filter, which eliminates wet hair in solid form with 75% humidity. The hair can represent 17-20% of the skin’s weight.
More sodium sulfide or hydrosulfate is added to attack keratin in the epidermis and dissolve it.
Liming: Lime is added to produce a controlled alkaline swelling.
These methods require a higher control and vigilence. Furthermore, you have to avoid dissolving the hair with chemical products, select chemical products wisely and use mechanical filters to remove hair from wastewaters.
The environmental benefits which can be obtained by using this method are the reduction of water consumption, reduced generation of wastewaters, lower levels of chemical products in the effluent, the reduction of sludge and dangerous solid waste.
Recycling of wastewaters from liming: A large amount of the water from rinsing and washing in other processes where the low concentration of waste chemical products hardly affects the process can be reused. The water from the lime bath can be recycled, for example, to begin with a new liquor of lime. This operation can reduce water consumption considerably.
Another idea is a direct recycling of the lime liquors lost after each cycle. This system uses decantation using a 1mm brush filter to remove hair and solid waste. A 40% saving in sulfur and 50% saving in lime can be obtained. This method, therefore, reduces water consumption, the generation of wastewaters and chemical products.
Reuse of pickle-tanning baths: In order to limit the levels of chrome in tannery effluent the chrome liquor can be reused for pickling and tanning. This operation can reduce chrome content in the effluent by up to 90%.
INESCOP carried out a project (which was selected for the EU LIFE programme) in collaboration with a tannery.
The project established a demonstration plant for the treatment and recycling of tanning and pickle baths with a treatment capacity of 25m3 per day, the scheme for which is shown in Figure 4.
The main objective of the project was to demonstrate the viability of recycling the baths of pickle-tanning guaranteeing the quality of the end product and achieving A) reduction of the salt contamination found in pickling or tanning baths, and B) reduction of the quantity of chrome salt left in the tanning waste. The environmental benefits which can be obtained by using this method are reduction in water consumption, generation of wastewater, and the level of chemical products in the effluent as well as a reduction of the chemical products used.
Methods to minimise the production of waste
Solid waste generated in fleshing green hides (after soaking) does not normally contain any chemical products and can be used as a raw material in the industry to obtain industrial fats. This alternative will reduce the total quantity of solid waste generated in tanneries.
The byproducts generated in fleshing (after liming) can be reused for other purposes, reducing the total quantity of solid waste.
Some solid wastes of the tanning process have useful properties which can improve the quality of the soil and, therefore, be used as fertilisers in agriculture.
Furthermore, a low cost system of collecting the waste can be implemented minimising transport costs. Due to the possible decomposition of byproducts from fleshing, the conservation of this solid waste can cause problems if not processed immediately.
Recovery of fleshings and limed cuttings
Another option is to reduce the level of solid waste generated in the leather process consists of recovering fleshings and limed cuttings for the production of glue, gelatine, industrial fats and tripe.
Recovery of chrome shavings and cuttings
The solid waste generated in shavings and in chromed cuttings can be used as raw material for the regenerated leather industry, although current demand for this type of product is not high enough to absorb the large quantities generated by tanneries.
The shavings can also be used to obtain proteins and liquid fertilisers after their untanning, although the process is costly.
Regenerated leather can be used to make footwear components such as heels, as well as belts, covers for book binding etc.
Conclusions
* The Spanish government is exerting growing environmental pressure on the leather sector, exemplified by the increasingly regular inspections to which tanneries are subjected. The sector has responded to this by adopting measures which minimise the impact on the environment including the management of waste, purification of wastewater and the incorporation of clean technology in production processes.
* At present leather waste is undervalued and ends up as landfill rather than being re-used. However some types of waste are becoming valued, such as non tanned wastes, recycling of plastics, paper, cardboard and pallets.
* Sludge generated by tanneries, under current conditions is not suitable for use as fertiliser in the agricultural sector due to the high levels of chrome it contains.
* The characteristics of the wastewater from all of the participating tanneries necessitates the application of water treatment systems which are not yet present in all of the companies.
* The analysis in this study indicates that even after purification, wastewater still fails to comply with some regulations for outflow. The reduction of some of these elements, for example salt levels, is very difficult.
