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Journal of Cleaner Production 87 (2015) 39e49

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Review

Toxic hazards of leather industry and technologies to combat threat:a review

Sumita Dixit, Ashish Yadav, Premendra D. Dwivedi, Mukul Das*

Food, Drug & Chemical Toxicology Group, CSIR-Indian Institute of Toxicology Research, Mahatma Gandhi Marg, P. O. Box 80, Lucknow 226001, U.P., India

a r t i c l e i n f o

Article history:Received 25 February 2014Received in revised form24 September 2014Accepted 6 October 2014Available online 16 October 2014

Keywords:BatingClean technologyFinishing operationsPre tanning operationsPost tanning operationsWater management

Abbreviations: AOP, Advanced oxidation process;BOD, Biological oxygen demand; COD, Chemical oxygeffluent treatment plants; DBP, Di butyl phthalate; DBhexyl phthalate; NMP, N-methyl pyrrolidine; OPP, Ortdissolved solids; TBT, Tri butyl tin; VOC, Volatile orga* Corresponding author. Fax: þ91 522 2628227, þ9

E-mail addresses: mditrc@rediffmail.com, mditrc@

http://dx.doi.org/10.1016/j.jclepro.2014.10.0170959-6526/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Leather industry has significant economic influence; however, it suffers from the negative impact due toenvironmental pollution caused by tannery wastes produced during leather processing processes. Thetanning processes contribute significantly to chemical oxygen demand (COD), total dissolved solids(TDS), chlorides, sulfates and heavy metal pollution. The chemicals discharged into aquatic systems endup in highly polluted sediments and salinisation of rivers. European Chemical Agency (ECHA) hasprioritized some of the hazardous chemicals used in leather under Substances of Very High Concern(SVHC) and substances for Authorization. The situation has highlighted the need for greener technolo-gies. Out of the two broad categories of technical methods, the first group involves the introduction ofprocessing technologies by decreasing the effluent pollution load, avoiding the use of harmful chemicalsand producing solid wastes that can be used as by-products. The other category is related to the treat-ment of wastewater, handling and processing of solid waste in an environment-friendly manner. Bothmethods have been applied to prevent negative impact on the environment during leather production.The methods have been reviewed for their technical suitability and commercial feasibility and it was feltthat combination of both is essential. The technologies can have up-front additional costs but have to bebalanced against multiple benefits in terms of environmental cleanup, improved labour productivity,material quality consistency and better international image. By using the best available technologies andoptimized systems the leather industry can evolve as an environment friendly technology.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Leather industry, an age old enterprise provide a wide range ofconsumer goods such as shoes, garments, bags etc by turning thefood industry's waste product into a desirable, useful and sustain-able range of end products (Aloy et al., 1976). It has been estimatedthat about 1.67 � 109 m2 of leather is being made annually in theworld (FAO, 2001). The annual global trade in leather sector isestimated as seventy billion dollars (ITC, 1999).

Different manufacturing processes, ranging from cottage in-dustry to heavy industry are involved for the durable and flexibleleather material by the tanning of putrescible animal rawhide andskin (Heidemann, 1993). The different forms of leather depending

BBP, Benzyl butyl phthalate;en demand; CETPs, CommonT, Di butyl tin; DEHP, Diethylho phenyl phenol; TDS, Totalnic compounds.1 522 2611547.hotmail.com (M. Das).

upon various tanning processes are vegetable tanned leather(Madhan et al., 2001a), chrome-tanned leather (Heidemann, 1993;Chagne et al., 1996), aldehyde-tanned leather (Serra et al., 1991;Wojdasiewicz et al., 1992), synthetic-tanned leather (Dasgupta,1980), alum-tawed leather (Montgomery, 1987; Takenouchi et al.,1997) and rawhide (Bosnic et al., 2000). Cattle skin is generallyused for leather manufacturing, however, for soft leather items skinof other animals like lamb, deer, goats, etc is also used (Heidemann,1993).

2. Leather production processes

The manufacturing process for leather preparation can bedivided into three basic sub-processes: preparatory stage/beamhouse stage, tanning stage and crusting stage (Suresh et al., 2001;Sivakumar et al., 2010). Surface coating may be an additional stepinto the leather process.

Preparatory stage or the beam house operation stage is car-ried out when the hide/skin is prepared for tanning. This stageincludes preservation, soaking, liming, unhairing, fleshing, split-ting, reliming, deliming, bating, degreasing, bleaching, pickling and

Table 1Quantity of wastewater generated and pollution load of each step during processing1 tonne of skin/hide.

Pollution load Processing operations load kg/tonne hide

Soaking Unhairing/liming

Delimingandbating

Chrometanning

Posttanning

Finishing

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e4940

depickling (IL&FS, 2010). At the onset, the hides are trimmed andsoaked not only to restore the lost moisture during curing but to getrid of salt and other solids. Subsequently, the hides are fleshed toremove excess tissue, muscles or fat adhering to the hide so thatuniform thickness may be obtained. Dehairing is performed mostlyby liming process using a series of drums containing lime liquors(calcium hydroxide) and sharpening agents, however thermal,oxidative, and chemical methods may also be applied. The softnessand flexibility to the leather may be obtained by bating anddeliming, which is performed by keeping the hides in a solution ofammonium salt dissolved in water along with proteolytic enzymesat 27e32 �C (Bienkiewicz, 1983; EPA, 1982). Pickling is required asthe next step to adjust the acidity with the use of a brine solutionand sulphuric acid.

Tanning converts the raw hide or skin into a stable material thatdries out to a flexible form without putrefying and becomes suit-able for a wide variety of end applications (Krishnamoorthy et al.,2012). In general, depending upon the end application of theleather, two tanning methods is used; vegetable tanning or chrometanning.

The process of vegetable tanning usually requires 3 weeks sothat the dye penetrates to the hide. Further, the hides are dipped insodium bicarbonate or sulphuric acid drums for bleaching and forthe removal of tannins bound to the surface. Before set out to dry,lignosulfate, corn sugar and oils may be added to the leather andthen it undergoes further finishing steps.

Chrome tanning is done by the reaction between the hide andtrivalent chromium salt, most commonly basic chromium sulphate.At pH 3 the tanning materials are introduced and the pH is raised.The soaking, fleshing, liming/dehairing, deliming, bating, andpickling and the drying/finishing steps are essentially the same asin the case of vegetable tanning except that an additional processesof retanning, dyeing, and fat liquoring to produce usable leathers.Many a times oil is applied on the skin before leather drying toreplace the natural oil lost during beam house and tanyard pro-cesses; this step is called fat liquoring. The leather is thereafterwrung, set out, dried, and finished. The pH of chrome tannedleather finish between 3.8 and 4.2 (Heidemann, 1993).

Chrome-tanned leather is more superior to vegetable tannedleather due to its softness, high thermal and water stability and lesstime consuming. Almost all leather made from the skin of sheep,lambs, goats, pigs is chrome tanned.

Crusting takes place after the thinning, re-tanning and lubri-cating of the hide. Sometimes dye is also added in this process.During crusting, the chemicals added have to be fixed followed bydrying and softening process. Crusting involves several steps likewetting back, sammying, splitting, shaving, re-chroming, neutrali-zation, re-tanning, dyeing, fat liquoring, filling, stuffing, stripping,whitening, fixating, setting, drying, conditioning, milling, staking,and buffing (Bienkiewicz, 1983).

Finishing process consists of surface coating that include: oil-ing, brushing, padding, impregnation, buffing, spraying, rollercoating, curtain coating, polishing, plating, embossing, ironing,combing (for hair-on), glazing and tumbling (Heidemann, 1993).

Waste watergenerated(m3 or Kilolitre)

9.0e12.0 4.0e6.0 1.5e2.0 1.0e2.0 1.0e1.5 1.0e2.0

Suspended solids 11e17 53e97 8e12 5e10 6e11 0e2COD 22e33 79e122 13e20 7e11 24e40 0e5BOD 7e11 28e45 5e9 2e4 8e15 0e2Chromium e e e 2e5 1e2 e

Sulphides e 3.9e8.7 0.1e0.3 e e e

NH3eN 0.1e0.2 0.4e0.5 2.6e3.9 0.6e0.9 0.3e0.5 e

Total Kjeldahlnitrogen

1e2 6e8 3e5 0.6e0.9 1e2 e

Chlorides 85e113 5e15 2e4 40e60 5e10 e

Sulphates 1e2 1e2 10e26 30e55 10e25 e

3. Environmental impacts of leather industry

Environmental impact of tannery wastes containing waste-water; hazardous chemicals such as chromium, synthetic tannins,oils, resins, biocides, detergents; careless disposal of solid wastesand gaseous emissions creates a negative image of leather industry,although it has significant economic influence (Suresh et al., 2001;Nazer et al., 2006; Jerry, 2011; Sequeira et al., 2011; Shakir et al.,2012; Islam et al., 2014).

3.1. Waste water

Enormous amount of water and pollutants are discharged dur-ing the entire tanning process (Kaul et al., 2001). The details ofwater consumption for several steps and the characteristicpollutant loads for each operation are presented in Table 1. Con-ventional pre-tanning and tanning processes accounts for nearly90% of the total pollution from a tannery (Aloy et al., 1976). Pre-tanning process results in variations in pH and causes increase inchemical oxygen demand (COD), total dissolved solids (TDS),chlorides, sulphates in tannery wastewaters (Thanikaivelan et al.,2000a). The conventional dehairing process with sodium sul-phide and lime accounts to 84% of biochemical oxygen demand(BOD), 75% of COD and 92% of suspended solids (SS) from a tannery(Marsal et al., 1999). The use of sodium sulphide not only gives riseto unfavourable consequences on environment but also affects theefficacy of effluent treatment plants (Bailey et al., 1982). The pooruptake of 50e70% chromium during commercial chrome tanningmethod results in material wastage on one hand and createsecological imbalances on the other (Rao et al., 1997). The post-tanning processalso results in modifications in TDS, COD, andheavy metal pollution significantly (Simoncini and Sammarco,1995).

Highly polluted sediments resulting from discharge of chem-icals adversely affect the ecological functioning of rivers (Schillinget al., 2012). High concentration of heavy metals has been foundin sediments of the river Ganga and its tributaries (Singh et al.,2003; Tare et al., 2003). Increase salinisation of rivers andgroundwater has led to the loss of agricultural production andreduced the quality of drinking water in Tamil Nadu, India (Money,2008). It has been estimated that over 55,000 ha of land have beencontaminated by tannery wastes and around 5 million people areaffected by low quality of social environment and drinking water(CSIRO, 2001; Sahasranaman and Jackson, 2005).

3.2. Solid wastes

A great deal of sludge generated from the tannery plants(Ramasami and Prasad, 1991) render the solid waste managementsystem highly inactive due to non-biodegradability of the tannedleather (Dhayalan et al., 2007; Lofrano et al., 2007). Leather itself isslow biodegradable and treatment of different chemicals duringtanning process makes it resistant towards chemical, thermal, andmicrobiological degradation (Hagerman, 1980; Han et al., 2001).

Table 2Uses, Structures, LD 50 and toxicity of chemicals used in leather industry.

Name Uses Structure LD50 in rats, oral (mg/kg) Target organs

Benzyl butyl phthalate Used in process for the production ofa micro porous artificial leathercoating/water vapour-permeablesheet materials.

2330 Eyes, lungs, liver,reproductive system

Bis(2-ethylhexyl)phthalate (DEHP)

Used as plasticizers in a processingof shoes soles, and artificial leathermanufacturing

30,000 Liver and testes

Di butyl phthalate (DBP) Used as a Phthalate plasticizer inthe artificial leather industry

7499 Eyes, lungs, gastrointestinaltract, testes

Anthracene Used as a tanning agent(for leather)

16,000 Kidney, liver, fat andcarcinogen

short chain chlorinatedparaffin's (PBT)

Additive for the leather treatment(renders smoothness to leather),leather clothing and belts and asa leather oiling agent.

3090 Liver, kidney, thyroid andcarcinogen

Sodium dichromate Principal raw material used in theproduction of chrome tanningmaterials for the leather industrylike Chrome-tanning salts

Not available Blood, kidneys, heart,lungs, eyes andcarcinogen

Cobalt dichloride Used in leather dyeing andfinishing as well found intanned leather

80 Lungs, liver, kidney,heart, skin

Nonyl phenol Used in finishing 1475 Blood, lungs, eyes,skin, CNS, kidneys andlow biodegradability

Methyl isothiazolinone Biocide,microbiological protection 1800 Skin, eyes and carcinogen

N-Methyl pyrrolidone Coalescence, plasticizers, wettingagent

3914 Eyes, kidney, lymphaticsystem, liver, lung, testes

Formaldehyde Leather finishing HCHO 100 Eyes, lungs and carcinogenHeavy metalsArsenic As 763 Liver, kidneys, skin, lungs,

lymphatic system andcarcinogen

Chromium used for dyeing Cr 3250 Kidney, CNS,haematopoietic system

Organotin compounds(Dibutyl tin)

As a catalyst 175 Gastrointestinal tract,liver and carcinogen

Azo dyes (Orange II) Used for Dyeing 3418 Blood, liver, testesand carcinogen

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e49 41

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e4942

This in turn affects the agro based activities and degradesgroundwater system (Mwinyihija et al., 2012). These wastes are athreat to ecology and aquatic system in vicinity of tannery plants(Mwinyihija et al., 2010). Adding of pesticides for hide conservationduring transport also add to the problem (Pollution Prevention andAbatement Handbook, 1998).

3.3. Volatile organic compounds

Pollutants such as ammonia, hydrogen sulphide, volatile hy-drocarbons, amines and aldehydes are emitted to the atmospherefrom tannery plants as effluents (Fela et al., 2010). Ammoniaemissions may occur during deliming, unhairing, or drying pro-cesses, while, emissions of sulphides may be the result of liming/unhairing and subsequent processes. Hydrogen sulphide is releasedin tannery wastewater from alkaline sulphides if the pH is less than8.0. Particulate emissions contain chromium, which may occur(EPA, 1982; Streicher, 1987) due to reduction of chromate orthrough handling of basic chromic sulphate powder or from thebuffing process (EPA, 1982; Streicher, 1987; Telecon, 1996). Thussubstantial amount of volatile organic compounds (VOC) areemitted during different tanning processes which may pose threatto the atmosphere if not controlled properly.

3.4. Toxicity of chemicals used in leather industry

Awide variety of chemicals are used in order to bring the leatherin the usable form in the preparation of a variety of products.Table 2 describes the uses, structures, LD50 and toxicity of chemicalsused in leather industry. The regulatory bodies have put restrictionon the use of these chemicals in leather industry. Phthalates likebenzyl butyl phthalate (BBP), di-ethyl hexyl phthalate (DEHP) anddi-butyl phthalate (DBP) are used as plasticizers in micro porousartificial leather coating. Due to the reproductive toxic potential ofthese phthalates, EU (2003 a) has directed the companies to label ifthe products contain more than 0.5% of these phthalates. Nonylphenol, used in finishing of leather, should not be more than 0.1% infinished products as prescribed by EU (2003 b). The decision wastaken due to high persistence of these chemicals in the environ-ment because of its low bio-degradability. Additionally, nonylphenol shows oestrogens like activity. Biocides are used for themicrobiological protection of mainly water based finishing chem-icals. Some biocides like methyl isothiazolinone (MIT) and chlor-isothiazolinone (CIT) when used in combination act as an irritant(EPA, 1998).

The use of o–phenyl phenol (OPP) is restricted for leather fin-ishing chemicals due its carcinogenic activity (EPA, 2007). N-Methyl pyrrolidone works as coalescence, plasticizer, levellingagent, wetting agent and as a swelling material thus giving highperformance in finished leather. This compound has been classifiedas a reproductive toxin (OEHHA, 2001). The use of formaldehyde isinevitable in the manufacture of various types of leathers due to itsaction as cross linker for casein top coats (RamMohan et al., 2008).Due to the carcinogenic property of formaldehyde, its use isrestricted (EU, 1998). Inorganic pigments such as lead chromate,cadmium sulphate are used due to their fastness and brilliantcolour but these are toxic heavy metals (ATSDR, 2008; IARC, 2004;Louis et al., 2003). It has been shown that chromium (III) undercertain ligand environments leads to apoptosis by causing struc-tural modifications in proteins (Shrivastava and Nair, 2001;Balamurugan et al., 2002).

Azo dyes, the synthetic dyestuffs based on nitrogen are used inleather industry for dyeing the leather articles. The toxicity ofseveral azo dyes has been mentioned earlier (Khanna and Das,1991; Ramchandani et al., 1994). Many azo dyes on cleavage

produce carcinogenic and allergenic aromatic amines. The EU AzoColorants Directive (2002) has given a list of azo dyes that shouldnot be used in leather articles as they may release one or moreprohibited aromatic amines in detectable concentration above30 ppm in the finished articles or in the dyed components. Orga-notin compounds like dibutyl tin (DBT) used in leather finishing asa catalyst may contain tributyl tin as an impurity which is highlytoxic and has hormone like activity (Omura et al., 2001). No re-striction has been set yet for synthetic tannins although there havebeen extensive impact assessment and treatment applications forindividual chemicals such as cresols, phenols (Hohreiter and Rigg,2001; Wang et al., 2002).

4. Legislation scenario for leather industries

The discharge limit parameters are different from one country toother. Some legislative authorities have a check on the quality oftreated effluents; while others on the quality of the recipient waterbodies; some define the permissible levels of impurities to be dis-charged per day into the recipient water body, whereas in somecases specifications are linked to the total amount of waste waterdischarged (Bosnic et al., 2000).

In many countries, tannery effluents are subjected under overalllegislation of industrial waste discharge rather than specific limits(Bosnic et al., 2000). During 1990's several tanneries in India wereordered to close the units as they could not meet the standards ofpollution control systems, while many of them had to pay hugecompensation for the damage caused by salinisation of ground-water (CSIRO, 2001). Government has encouraged the tanneries tobuilt Common Effluent Treatment Plants (CETPs) to treat the toxicwastewater from tanneries by giving subsidies. Despite this initia-tive, many of the pollution problems are still unresolved due to thehigher cost associated to the treatment of effluents thereby causingillegal dumping (Beg and Ali, 2008). It was noticed that one of theUganda's main leather producing company directly dumped itswaste water in a wetland adjacent to Lake Victoria (The Monitor,2009). In Crotia the proper pollution abatement cost exceededthe compensation cost against irresponsible behaviour (EcoLinks,2001).

Some countries have made regulations related to production,import and sale of leather products with regard to hazardoussubstances. Table 3 describes the maximum permissible limits ofchemicals used in leather and leather products prescribed by someof the countries. Furthermore, in order to restrict the use of thesechemicals, European Chemical Agency (ECHA) has prioritized fewchemicals under Substances of Very High Concern (SVHC) whichare considered to be hazardous not only to the environment butalso to humans (UK REACH, 2009). It has been observed that out ofthese 30 SVHC substance list provided by ECHA, almost all of thechemicals are used in the leather industry (ECHA, 2010).

5. Technological options to combat the threat posed byleather industry

Environmental concerns over leather industries have beengrowing for the past two decades. High industrial and humanpopulation density and the use of old technologies all causeincreased levels of pollutant in the atmosphere (Maia, 1998). Thissituation has highlighted the need for greener technologies (Guptaand Babu, 2009; Sundarapandiyan et al., 2010, 2011;Krishnamoorthy et al., 2012). The protection of the environmenthas become a global issue throughout the world.

For reducing the negative environmental impact of hide pro-cessing, there are two broad methods. The first method is generallytermed as low-waste or cleaner technologies that avoid the use of

Table 3Mandatory requirements in leather products of some countries.

Mandatory residual substances limits (RSLs) EU Germany Austria Denmark France Netherlands Switzerland

Pentachlorophenol 30 ppm 5 ppm 30 ppm 30 ppm 30 ppm 30 ppmAzo dyes* 30 ppmChromium VI 3 ppm 10 ppmLead 90 ppmCadmium 100 ppm 75 ppm 100 ppm 100 ppmArsenic AbsentOrganotin Compounds AbsentSpecific Flame Retardants <0.1%Phthalates 0.1% 0.1% 0.05%PCBs and PCTs Not to be usedBiocides** 5 ppm 5 ppm 5 ppm 5 ppm 5 ppm 10 ppmFormaldehyde >1500 ppm >1500 ppm 200-400 ppm 120 ppm

Source: http://ec.europa.eu/enterprise/chemicals/legislation/markrestr/index_en.html.Azo dyes or the following aromatic amines on cleavage*: Biphenyl-4-ylamine; 4-aminobiphenyl xenylamine; Benzidine; 4-Chloro-o-toluidine; 2-Naphthylamine; o-ami-noazotoluene; 4-amino-20 ,3-dimethylazobenzene; 4-o-tolylazo-o-toluidine; 5-Nitro-o-toluidine; 4-chloroaniline; 4-methoxy-m-phenylenediamine; 4,40-methylenediani-line; 3,30-dichlorobenzidine; 3,30-dimethoxybenzidine o-dianisidine; 3,30-dimethylbenzidine 4,4-bi-o-toluidine; 4,40-methylenedi-o-toluidine; 6-methoxy-m-toluidine; p-cresidine; 4,40-methylene-bis-(2-chloroaniline); 4,40-oxydianiline; 4,40- thiodianiline; o-toluidine; 2-aminotoluene; 4-methyl-m-phenylenediamine; 2,4,5-trimethylaniline;o-anisidine 2-methoxyaniline; 4-amino azobenzene.Biocides**(23 approved): Human hygiene biocidal products; Private area and public health area disinfectants and other biocidal products; Veterinary hygiene biocidalproducts; Food and feed area disinfectants; Drinking water disinfectants; Preservatives; In-can preservatives; Film preservatives; Wood preservatives; Fibre, leather, rubberand polymerised materials preservatives; Masonry preservatives; Preservatives for liquid-cooling and processing systems; Slimicides; Metalworking-fluid preservatives; Pestcontrol; Rodenticides; Avicides; Molluscicides; Piscicides; Insecticides, acaricides and products to control other arthropods; Repellents and attractants; Other biocidalproducts; Preservatives for food or feedstocks; Antifouling products; Embalming and taxidermist fluids; Control of other vertebrates.PCBs: Polychlorinated biphenyls PCTs: Polychlorinated terphenyl.

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e49 43

harmful chemicals and produce solid wastes which can be used asby-products (EC, 1997a). The second method is related to thetreatment of wastewater and the environment-friendly handlingand processing of solid waste (EC,1997b; c). Themethods applied inboth groups can be used to prevent leather production with lessnegative impact on the environment.

5.1. Low waste or cleaner technologies

The cleaner processing options need to be cost-effective in orderto be economically viable and the success of these technologiesdepends on: (a) reduction of pollution in terms of quantity andquality, (b) leather quality improvement and/or cost reduction, (c)reproducibility of the process, (d) cost effectiveness to beeconomically viable, and (e) widemarket opportunities. A report byLudvik (2000) shows some possible options of reduction of pollu-tion load during processing of bovine hides into chrome tannedleathers by introducing advanced technologies based on low-wasteprocessing methods (Fig. 1).

Fig. 1. Advanced technological op

The recycling and reuse of spent liquor after the removal of thepollutants in leather processing provides better water management(Parthasarathy, 1995). It has been suggested that ideally zero ornear-zero discharge of waste liquors should be encouraged (Sykes,1997). Common salt or sodium chloride emerges largely from thecuring, pickling, and chrome tanning practices. Salt less or less-saltcuring may be an alternative to wet salting (Kanagaraj, 2005). Solardrying, freeze drying or microwave/dielectric drying should beconsidered (Komanowsky, 2000). Chemicals like potassium chlo-ride, boraxephenol, boric acid, zinc chloride, silica gel and metaloxinates can be used in place of sodium chloride (Bailey, 1995a;Kanagaraj et al., 2000, 2005). Processing green hides and skins,use of gamma and electron beam irradiation techniques andtransportation in refrigerated condition are some other options fora better cleaner processing (Bailey, 1995b, 1999; Bailey et al., 2001).

Soaking requires 25% of the total water consumption in con-ventional leather processing. But the chloride load is a hurdle inrecycling process as salt is not eliminated during physical, chemicaland biological treatment of waste water. The industrially proven

tions for leather processing.

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e4944

methods for reducing chloride load have been recommended byshaking the liquor in special drums (EC, 1997a); using wheel typede-salting machines (Rao et al., 2001) and counter-current soakingtechnique (Rao et al., 2003). Addition of environmentally accept-able antiseptics or commercial chemical bactericides such as ethyldithiocarbamate and isothiazolin (Buljan, 1995; EC, 1997a), use ofsodium sulphate (Vankar and Dwivedi, 2009), processing greenhides (Frendrup, 1995; EC, 1997a) have also been suggested todecrease the salt load. The recycling of spent floats in liming andunhairing processes showed substantial decrease in sulphide (70%),Ca(OH)2 (90%), BOD (7%) and COD (26%) load (Frendrup, 1995; EC,1997a).

Sulphide-lime based unhairing and liming achieves a significantdecrease in both total solids and dissolved solids (Frendrup, 1995;EC, 1997a). Enzymatic unhairing is an option for a clean beamhouse process (Frendrup, 1995; Thanikaivelan et al., 2000a; Valeikaet al., 2012). Enzyme-assisted unhairing followed by reliming withonce used relimed liquor ensures complete reduction of water(Thanikaivelan et al., 2000b; Shrewsbury, 2002). An economicallyviable option based on enzymatic dehairing and pickle-less chrometanning can lead to 67% and 78% decrease in COD and total solids(Aravindhan et al., 2007). Lime splitted hides save chromium andother chemicals and thewaste can be easily utilized as a by-product(Frendrup, 1995; EC, 1997a). Ultrasound not only decrease thewaste but also yield better leather quality (Valeika et al., 2009;Dettmer et al., 2013; Sivakumar et al., 2009).

A 97% decrease in ammonia load in effluents may be obtained byusing ammonia-free deliming and bating with commercial prod-ucts such as acids, esters of carboxylic acids, non-swelling aromaticacids (Frendrup, 1995; IUE, 2008). Carbon dioxide deliming in placeof ammonia decrease the pollution load of effluents to a significantextent (Palon and Marsal, 2002; IUE, 2008; Manfred et al., 2012;Gallego-Molina et al., 2013). Furthermore, it has been establishedthat deliming and bating can be effectively carried out as a floatlessoperation without impairing any physical/chemical or grain char-acteristics (John et al., 2001).

The spent pickle liquor contains high concentrations of neutralsalts (8e10%) and has an acidic pH (Chandrasekaran et al., 1989).The options of salt less pickling pickle less chrome tanning andpickle recycle are some solutions to overcome the problem (Pojerand Huynh, 1999; Burrows, 2001). These recycle methods reduceTDS of the effluent discharged from the identified stream(Sundarapandiyan, 2010).

Conventional chrome tanning processes use only 40e70% of thematerial (Warrier et al., 1995a, 1995b) and the disposal of theremaining wastes is a cause of concern (Ramasami, 1996; Fela et al.,2010). Intrinsically modified chrome tanning salts (Suresh et al.,2001; Thanikaivelan et al., 2002), use of chromium syntan or oxa-zolidine (Gupta and Babu, 2009; Morera et al., 2006;Sundarapandivan, 2011), high exhaustion tanning process(Frendrup, 1995; Covington, 1995; EC, 1997a), recycling/reusetechniques ((Rao et al., 1999a, 2002) would invariably help inincreasing the chrome utilisation and decreases the chromedischarge. The reuse of pickle liquor for the subsequent batches aswell as resorting to pickle free alumechrome combination fortanning result in reduction of significant levels of TDS (Rao et al.,2004; Burrows, 2001; Sivakumar et al., 2005). Use of Sargassumseaweeds to remove chromium from tannery effluent andmake useof this seaweed in the manufacture of basic chromium salt as areductant is another option (Aravindhan et al., 2004). A combina-tion between the chemical precipitation and the biological removalof chromium from tanning wastewater may render the tanningprocess eco-sustainable and environment friendly (Abdullah et al.,2010). Chromiumesilica-, chromium iron, aluminiumezinc-,chromiumezinc-, chromiumezincesilica-, zirconium oxychloride

and aluminiumetannic acidesilica- based tanning agents havebeen developed (Thanikaivelan et al., 2000b; Madhan et al., 2001b,2002, 2003; Fathima et al., 2003, 2004; Sunderarajan et al., 2003;Krishnamoorthy et al., 2012) for reducing the chromium emis-sion. The controlled incineration of tannery wastes in a starved airthermal incinerator and solidifying the calcined waste resulting in99.1% metal ion fixation is another alternative to reduce the chro-mium burden on the environment (Sekaran et al., 2007). The tan-nery wastes then can be used as fuel or nitrogenous source forleguminous plants (Famielec and Wieczorek-Ciurowa, 2011; Limaet al., 2010).

5.2. Solid waste treatment and management

The conventional disposal methods are not practicable for thedisposal of tanned leather wastes due to leaching of Cr3þ from thetanned leather wastes to groundwater and conversion of Cr3þ toCr6þ; emissions of nitrogen oxide (NOx); generation of hydrogencyanide (HCN); Cr3þ and NH3 during incineration (Rai et al., 1989).Less toxic soluble Cr6þ can be produced at low incineration tem-peratures (300e600 �C) (Rai et al., 1989). More promising resultscan be achieved by integrating the active microflora to degrade thelow molecular weight compounds and further degradation of themetabolites by anaerobic species (Beccari et al., 2002).

Anaerobic digestion of solid waste produce biogas nutrientenriched effluents for agricultural purposes (Gnanamani andKasturi Bai, 1992; Dhayalan et al., 2007). During leather process-ing, 850 kg of solid waste is produced per tonne raw hide and150 kg is converted into leather, the remaining waste can berecycled and utilized as a useful by product and raw materials(Colak et al., 2005; Ahmad and Ansari, 2013). Hydrolysate of keratinhas been employed in chrome tanning, retanning and exhaustionwithout altering the physical strength of leather (Chakraborty andSarkar, 1998). Fleshings have been explored for the possible utili-zation into useful end products such as soap, bio diesel and fat li-quor (Colak et al., 2005; Ravindranath and Gopalakrishnan, 2010).Modified fleshing hydrolysate not only has better uptake capacityof chromium in chrome tanning and rechroming (Kanagaraj et al.,2001a) but also, leather obtained has better physical and organo-leptic properties than conventionally produced method(Kanagaraj et al., 2001b). Fleshings hydrolysed by pancreatic en-zymes could be used as a feed formulation by mixing with otherfeed ingredients (Kumaraguru et al., 1998).

Glue, gelatine, reconstituted collagen, adhesives, films andchrome cake can be obtained from chrome tanned leather, splits,buffing dusts and trimmings (Brown et al., 1996; Cot et al., 2003;Cantera et al., 2000; Cantera, 2003; Taylor et al., 2002, 2003). Anew parchment-like material from chrome shavings has beenfound to be useful in the manufacture of home furnishing products(Sastry et al., 2005). The wet blue shavings completely digested byalkali protease have been used for casein formulation in leatherfinishing (Crispim and Mota, 2003). Chrome collagen residues havebeen used for the production of regenerated leather and variousarticles (Cot et al., 1986, 2003). Utilization of tannery waste as feed,fertilizer or cosmetic additive is a step for zero solid waste(Castilhos et al., 2002; Daudt et al., 2007; Gish, 2000; Konrad andCastilhos, 2002; Lima et al., 2010). Utilising tannery waste as aprotein source for poultry feed is a step for zero solid waste (Paulet al., 2013).

5.3. Treatment of waste water

Conventional tannery effluent treatment plants (ETPs) offerphysicochemical treatment followed by biological and tertiarytreatment to meet the standards (Rao et al., 1999b). Significant

Fig. 2. Environmental impact of leather industry and technologies to combat thethreat.

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e49 45

technological advancements are being made in the end-of-pipetreatment methods to achieve higher efficiency in meeting thestandards in a cost-effective manner. The anaerobic treatment oftanning waste water has included the application of technologiesbased on lagoons, contact filter, up flow anaerobic sludge blanket(UASB) reactor, and high-rate biomethanation (Rajamani et al.,1995; Van Groenestijn et al., 1995; Ramasami and Sahasranaman,2000; Lefebvre et al., 2006; El-Sheikh et al., 2011). Post treatmentmethodologies for tannery waste-water from anaerobic bio-digestion have included the use of aerators with and without theaid of aerobic microorganisms (Farabegoli et al., 2004; Ganesh et al.,2006). Membrane technologies such as activated carbon filters,reed bed, root zone techniques, crossflow microfiltration, ultrafil-tration and reverse osmosis methods are being utilized forproviding tertiary treatment of tannery waste water (Daniels, 1995;Rajagopalan and Thimmapuram, 1997; Labanda et al., 2009;Gallego-Molina et al., 2013; Wang et al., 2011). Membrane bio-reactors are being used for the treatment of tannery waste wateragainst conventional activated sludge process (Suganthi et al.,2013). The high-rate transpiration system and an acceleratedevaporation with crystallization of saline streams using flat-platecollectors have emerged as a possible method or treating salt-bearing tannery wastewaters (Rao et al., 2001; Buljan et al.,2001). Studies have shown that respirometry combined withsequencing batch reactor (SBR) may be an effective way for thebiodegradation of tannery waste water (Ganesh et al., 2006).

AOP treatment such as UV light, ozone (O3), photocatalyticoxidation and their combination (Schrank et al., 2004, 2005; Costaet al., 2008; Monteiro et al., 2009; Hoshyar et al., 2012) and Fentonreagent (Schrank et al., 2005; Lofrano et al., 2010; M�odenes et al.,2012) have been used as pre-oxidation or post-oxidation of tan-nery wastewater (Di Laconi et al., 2002, 2010). These treatments arebased on the principle of production and utilization of powerfuloxidants in a short time. An improved coagulation process byLofrano et al. (2007) gave an effective removal of toxic componentsfrom tannery wastewater. Integrated waste water ponds and con-structed wetlands (CWs) are other options studied for treatment oftannery effluents (Tadesse et al., 2004; Calheiros et al., 2008, 2012).Plants such as Arundo donax and Sarcocornia fruticosa in CWs canremove up to 51e80% COD and 53e90% BOD from the tannery ef-fluents (Calheiros et al., 2012). Recently, Dunn et al. (2013) hasshowed the use of tannery effluent for growth medium of Artho-spira (Spirulina) and retaining of chromium in wetlands with thehelp of non-specialized media (Dotro et al., 2012).

5.4. Development of some eco-friendly chemicals for leather andleather products

The consumer consciousness and strict regulations by theworldwide authorities require leather products with the lowestpossible risk for the environment. The chemical industry hasaccepted the challenge with the development of new eco-friendlyproducts. Phthalates used as nitrocellulose lacquer emulsions, canbe successfully replaced by castor oil which maintains the typicallacquer emulsion properties like glossy touch, softness and elas-ticity (Blach, 2004). Enzymes, amino acids or oxazolidine can beused as an alternative in tanning processes (Kanth, 2009;Krishnamoorthy et al., 2012; Sundarapandivan et al., 2011).

The ecological useful alternatives for nonyl phenol and nonylphenol ethoxylates are fat alcohol ethoxylates especially C12 andC14, the combination of which can successfully replace nonylphenol ethoxylates (Tox-Ecology, 2002). As an alternative for thesolvent N-Methyl pyrrolidone (NMP), high wear top coat acrylicsbeing free of solvents have been advocated (Blach, 2004). Formal-dehyde in leather finishing is not only used as a cross linker for

casein top coats but also as a biocide. Finishing chemicals maycontain formaldehyde even when no formaldehyde was used forproduction as many raw materials are preserved in formaldehydebase. To overcome this problem use of formaldehyde scavengerscan be adopted as in the case of some Wet End Chemicals (RamMohan et al., 2008). Inorganic heavy metal pigments like leadchromate, cadmium sulphide and others can be replaced by organicpigments or pigments from rare earth colorants which are free ofcarcinogenic aryl amines (Sreram et al., 2009).

6. Conclusion

A critical review on the conventional leather processes and theprinciples behind each step reveals that the bulk of the pollutionrests in pre-tanning and tanning processes, though post tanningand finishing steps also pollute the environment. The pre-tanningand tanning processes causes increase in COD, BOD, TDS, SS, chlo-rides and sulphates in tannery effluents. The post-tanning pro-cesses result in modifications in TDS, COD and heavy metalpollution. The sludge generated from the tannery plants effect theagro based activities and degrade the groundwater in the vicinity.Substantial amount of volatile organic compounds are emittedduring tannery process and cause a threat to environment if notcontrolled properly. The negative impact of tannery industry hascall attention to the need for green technology. The emergent greentechnologies have been divided into two broad methods; (i) Lowwaste or cleaner technologies that decrease the effluent pollutionload, avoid the use of harmful chemicals and produce solid wastesthat can be changed into useful by products (ii) treatment ofwastewater and the environment-friendly handling and processingof solid waste.

S. Dixit et al. / Journal of Cleaner Production 87 (2015) 39e4946

The present paper highlighted the environmental impact of theleather industry and technologies comprising of both advancedcleaner methods and environment friendly waste management tocombat the threat (Fig. 2). Zero discharge of waste liquor, salt lesscuring, enzymatic unhairing, CO2 deliming and salt less pickling aresome of the emerging technologies that are helpful in removing orreducing xenobiotics from the tannery effluents. The chemicalprecipitation or biological removal of chromium or the use of somealternative chemicals in place of chromium is required for eco-sustainable or eco-friendly tanning. These technologies come un-der the low waste or cleaner technologies. The solid waste utili-zation, chrome saving wastes, treatment of waste water bymembrane technologies, membrane bioreactors and advancedoxidation processes and constructed wetlands are the options ofenvironment friendly waste management method. It is clear that acombination of both cleaner and waste water management tech-nologies has to be followed by the tanning industries to sustain theecology and the environment. These processes may have theadditional cost but have multiple benefits in terms of environ-mental cleanup, improved labour productivity, material qualityconsistency and better international image. Benefits will alsoaccrue for the people living in the vicinity of the beneficiary tan-nery units and the working personnel. Though the steps presentedin the paper for cleaner technology in leather industry are quitestringent, nonetheless, regulators shall have to adopt these in orderto safeguard the environment and human health.

Acknowledgements

The authors are grateful to the Director, IITR, for his keen in-terest in the present study. One of us AY is thankful to CSIR/UGC forthe award of Senior Research Fellowship. The manuscript is IITRcommunication # 3249.

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