Tanning and Leather Finishing

Home , Deliming, Leather, Tanning (leather)

Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998

Industry Description and Practices etration of tanning solution, replenish oils in the hides, and impart color to the leather Hides and skins are sometimes preserved by dry- • Finishing to attain final product specifica- ing, salting, or chilling, so that raw hides and tions. skins will reach leather tanneries in an acceptable condition. The use of environmentally per- Waste Characteristics sistent toxics for preservation of raw hides and skins is to be avoided. The potential environmental impacts of tanning In the tanning process, animal hides and skins are significant. Composite untreated wastewater, are treated to remove hair and nonstructured amounting to 20–80 cubic meters per metric ton proteins and fats, leaving an essentially pure col- (m3/t) of hide or skin, is turbid, colored, and foul lagen matrix. The hides are then preserved by smelling. It consists of acidic and alkaline liquors, impregnation with tanning agents. Leather pro- with chromium levels of 100–400 milligrams per duction usually involves three distinct phases: liter (mg/l); sulfide levels of 200–800 mg/l; ni- preparation (in the beamhouse); tanning (in the trogen levels of 200–1,000 mg/l; biochemical oxy- tanyard); and finishing, including dyeing and gen demand (BOD) levels of 900–6,000 mg/l, surface treatment. A wide range of processes and usually ranging from 160 to 24,000 mg/l; chemi- chemicals, including chrome salts, is used in the cal oxygen demand (COD) ranging from 800 to tanning and finishing processes. 43,000 mg/l in separate streams, with combined The tanning and finishing process generally wastewater levels of 2,400 to 14,000 mg/l; chlo- consists of: ride ranging from 200 to 70,000 mg/l in indi- • Soaking and washing to remove salt, restore vidual streams and 5,600 to 27,000 mg/l in the the moisture content of the hides, and remove combined stream; and high levels of fat. Sus- any foreign material such as dirt and manure pended solids are usually half of chloride levels. • Liming to open up the collagen structure by Wastewater may also contain residues of pesti- removing interstitial material cides used to preserve hides during transport, as • Fleshing to remove excess tissue from the in- well as significant levels of pathogens. Signifi- terior of the hide cant volumes of solid wastes are produced, in- • Dehairing or dewooling to remove hair or cluding trimmings, degraded hide, and hair from wool by mechanical or chemical means the beamhouse processes. The solid wastes can • Bating and pickling to delime the skins and represent up to 70% of the wet weight of the origi- condition the hides to receive the tanning nal hides. In addition, large quantities of slud- agents ges are generated. Decaying organic material • Tanning to stabilize the hide material and im- produces strong odors. Hydrogen sulfide is re- part basic properties to the hides leased during dehairing, and ammonia is re- • Retanning, dyeing, and fat-liquoring to impart leased in deliming. Air quality may be further special properties to the leather, increase pen- degraded by release of solvent vapors from spray

404 Tanning and Leather Finishing 405 application, degreasing, and finishing (for ex- • Use photocell-assisted paint-spraying tech- ample, dye application). niques to avoid overspraying. • Precondition hides before vegetable tanning. Pollution Prevention and Control Through good management, water use can be reduced by 30–50%, to 25 liters per kilograms (l/ The design of new plants should address the fol- kg) of raw material. Recommendations for reduc- lowing process modifications: ing water consumption include the following: • Process fresh hides or skins to reduce the quan- • Monitor and control process waters; reduc- tity of salt in wastewater, where feasible. tions of up to 50% can be achieved. • Reduce the quantities of salt used for preser- • Use batch washing instead of continuous vation. When salted skins are used as raw ma- washing, for reductions of up to 50%. terial, pretreat the skins with salt elimination • Use low-float methods (for example, use 40– methods. 80% floats). Recycle liming, pickling, and tan- • Use salt or chilling methods to preserve hides, ning floats. Recycle sulfide in spent liming instead of persistent insecticides and fungi- liquor after screening to reduce sulfide losses cides. (by, say, 20–50%) and lime loss (by about 40– • When antiseptics or biocides are necessary, 60%). avoid toxic and less degradable ones, espe- • Use drums instead of pits for immersion of cially those containing arsenic, mercury, hides. lindane, or pentachlorophenol or other chlo- • Reuse wastewaters for washing—for example, rinated substances. by recycling lime wash water to the soaking • Flesh green hides instead of limed hides. stage. Reuse treated wastewaters in the pro- • Use sulfide and lime as a 20–50% solution to cess to the extent feasible (for example, in soak- reduce sulfide levels in wastewater. ing and pickling). • Split limed hides to reduce the amount of Waste reduction measures should include the chrome needed for tanning. following: • Consider the use of carbon dioxide in deliming to reduce ammonia in wastewater. • Recover hide trimmings for use in the manu- • Use only trivalent chrome when required for facture of glue, gelatin, and similar products. tanning. • Recover grease for rendering. Use aqueous • Inject tanning solution in the skin using high- degreasing methods. pressure nozzles; recover chrome from • Recycle wastes to the extent feasible in the chrome-containing wastewaters, which should manufacture of fertilizer, animal feed, and tal- be kept segregated from other wastewaters. low, provided the quality of these products is Recycle chrome after precipitation and acidi- not compromised. fication. Improve fixation of chrome by addi- • Use tanned shavings in leather board manu- tion of dicarboxylic acids. facture. • Recycle spent chrome liquor to the tanning • Control odor problems by good housekeeping process or to the pickling vat. methods such as minimal storage of flesh trim- • Examine alternatives to chrome in tanning, mings and organic material. such as titanium, aluminum, iron, zirconium, • Recover energy from the drying process to heat and vegetable tanning agents. process water. • Use nonorganic solvents for dyeing and finishing. Target Pollution Loads • Recover hair by using hair-saving methods to reduce pollution loads. For example, avoid Implementation of cleaner production processes dissolving hair in chemicals by making a and pollution prevention measures can yield both proper choice of chemicals and using screens economic and environmental benefits. The pro- to remove hair from wastewater. duction-related waste load figures presented in 406 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES

Table 1. Target Loads per Unit of Production Maximum upstream pollutant reduction is es- (kilogram per ton of raw material) sential for tanneries, but treatment is also required. Parameter Maximum value Emissions Guidelines BOD 40 COD 140 Emissions levels for the design and operation of Nitrogen 7 each project must be established through the en- Chromium 6 vironmental assessment (EA) process on the ba- (aim for 1.5) Sulfide 1 sis of country legislation and the Pollution Prevention Solid waste 500 and Abatement Handbook, as applied to local con- Effluent flow rate 30,000 ditions. The emissions levels selected must be (aim for 15,000) justified in the EA and acceptable to the World Bank Group. Sources: Indian Standards Institution 1977; UNEP 1991. The guidelines given below present emissions levels normally acceptable to the World Bank Group in making decisions regarding provision Table 1 can be achieved by implementing mea- of World Bank Group assistance. Any deviations sures such as those described above. The figures from these levels must be described in the World are for the waste loads arising from production Bank Group project documentation. The emis- processes before the addition of pollution con- sions levels given here can be consistently trol measures. These levels are derived from typi- achieved by well-designed, well-operated, and cal loads recorded in industry studies and should well-maintained pollution control systems. be used as maximum levels of unit pollution in The guidelines are expressed as concentrations the design of new plants. to facilitate monitoring. Dilution of air emissions Use of techniques such as water-based paint or effluents to achieve these guidelines is un- and roller coating can help achieve emissions of acceptable. volatile organic compounds (VOCs) from finish- All of the maximum levels should be achieved ing of less than 4 kg/t (aim for 2 kg/t). for at least 95% of the time that the plant or unit is operating, to be calculated as a proportion of Treatment Technologies annual operating hours.

Treatment of tannery wastewaters is always re- Air Emissions quired. Some streams, such as soaking liquor (which has high salinity), sulfide-rich lime liquor, Odor controls should be implemented to reduce and chrome wastewaters should be segregated. impacts on nearby residents. Preliminary screening of wastewaters is required because of the large quantities of solids present. Liquid Effluents Recovery of hair from the dehairing and liming process reduces the BOD of the process effluent. The effluent levels presented in Table 2 should Physical-chemical treatment precipitates metals be achieved. and removes a large portion of solids, BOD, and COD. Biological treatment is usually required to Solid Wastes reduce the remaining organic loads to acceptable levels (0.3 kg BOD, 2 kg COD, and 0.004 kg chro- Solid wastes and sludges must be disposed of in mium per metric ton of raw hide). a secure landfill. Good ventilation and minimization of solvent release can avoid the need to collect and treat Ambient Noise vapors in carbon adsorption beds. VOC emissions from finishing are approximately 30 kg/t Noise abatement measures should achieve either if pollution prevention measures are not adopted. the levels given below or a maximum increase in Tanning and Leather Finishing 407

Table 2. Effluents from Tanning and Leather- Monitoring data should be analyzed and re- Finishing Processes viewed at regular intervals and compared with (milligrams per liter, except for pH and bacteria) the operating standards so that any necessary cor- Parameter Maximum value rective actions can be taken. Records of monitoring results should be kept in an acceptable format. pH 6–9 The results should be reported to the responsible BOD 50 authorities and relevant parties, as required. COD 250 TSS 50 Key Issues Oil and grease 10 Sulfide 1.0 Chromium The key production and control practices that will Hexavalent 0.1 lead to compliance with emissions guidelines can Total 0.5 be summarized as follows: Nitrogen (NH4–N) 10 Phosphorus (total) 2 • Minimize chrome use; avoid the use of hexa- Coliform bacteria 400 MPN/100 ml valent chrome and use trivalent chrome instead; recover and recycle chrome. Note: Effluent requirements are for direct discharge to surface waters. MPN, most probable number. • Avoid the use of hides treated with persistent insecticides and fungicides. • Use nonorganic solvents for dyeing and fin- background levels of 3 decibels (measured on the ishing. A scale) [dB(A)]. Measurements are to be taken • Minimize storage of flesh trimmings and or- at noise receptors located outside the project ganic material. property boundary. • To reduce water use, monitor and control process waters; use batch instead of continuous Maximum allowable log washing; use drums for immersion of hides; equivalent (hourly reuse wash water and recycle floats; and seg- measurements), in dB(A) regate wastewater streams to simplify treat- Day Night ment. Receptor (07:00–22:00) (22:00–07:00) • Minimize solid waste by recovery and reuse Residential, of hide trimmings. institutional, educational 55 45 References and Sources Industrial, commercial 70 70 Danish Technological Institute. 1992. Possibilities for a Reduction of the Pollution Load from Tanneries. Final Monitoring and Reporting Report, Nordic Council of Ministers. Copenhagen. Indian Standards Institution. 1977. Guide for Treatment and Frequent sampling may be required during start- Disposal of Effluents of Tanning Industry. New Delhi. up and upset conditions. Once a record of con- UNEP (United Nations Environment Programme). sistent performance has been established, 1991. Tanneries and the Environment: A Technical Guide sampling for the parameters listed in this docu- to Reducing the Environmental Impact of Tannery Op- ment should be conducted monthly. erations. Paris. Annual monitoring for pesticides should be World Bank. 1996. “Pollution Prevention and Abate- carried out, and, if pesticides are present at lev- ment: Tanning and Leather Finishing.” Draft Tech- els of 0.05 mg/l and above, corrective actions nical Background Document. Environment should be taken. Department, Washington, D.C.