TANNING AND DRESSING OF LEATHER # 4

This assessment evaluated a sheep hide tannery. The objective of the assessment was to identify actions that would:

The assessment was performed by an EP3 team comprised of an expert in hide tanning and a pollution prevention specialist.

This facility is a tannery producing leather from sheep and goat hides. The facility tans approximately 2,000 sheep hides per day, for a total of 600,000 hides per year. Between 90 and 95 percent of the annual production hides are sheep hides for clothing, while the rest are goat hides for shoes. The wastes generated by the tannery come from the hides and the chemicals used in the production process. The facility provides on-site effluent pre-treatment prior to discharging the waste water to the municipal sewer system. The tannery operates two staggered 8-hour shifts, and employs 45 permanent and 30 seasonal workers.

The leather making process at this facility begins with delivery of sheep or goat hides by truck. Fresh hides and dried hides are sorted into piles according to size and wool color; black is separated from all other colors. The fresh hides go immediately to the paddle vat for soaking and washing; the dry hides enter the process on an as-needed basis.

The soaking paddle vat typically contains 1,000 raw hides to which ambient temperature fresh water is added. The vat mixes the hides and water for 17 to 48 hours to remove blood, manure, and dirt from the hides.

Next, the hides enter the lime painting process, which prepares the hides for wool removal. After the hides are painted with a lime and sodium sulfate solution and allowed to cure for three to seven hours, a mechanical hair-pulling machine is used to remove the wool. The hides then move to another paddle vat for liming.

The liming step dissolves the epidermis, any remaining hair, and any other undesirable compounds in or on the hide. The hides usually remain in the vat for 24 hours. The wastewater from this step generates hydrogen sulfide when mixed with other wastewater streams.

After liming and washing, the hides move to a fleshing operation where they are pressed by a roller and shaved by a blade adjacent to the roller. The fleshing waste averages 500 kilograms per batch; the tanning runs two batches per day.

Next, the hides move to a rotating, wooden drum for neutralization with a solution of fresh water, ammonium salts, enzymes, and organic acids. After neutralization, a series of steps add emulsifiers, non-solvent degreasing agents, salt, formic acid, and sulfuric acid to lower the pH to the 2.8 - 3.0 range.

The hides are then tanned. Chromium sulfate salts and sodium bicarbonate are added sequentially and mixed for two hours and six hours, respectively. This step stabilizes the collagen in the hides by blocking further chemical reactions. The waste tanning solution drains into the pre-treatment system.

After aging for about 24 hours, the hides undergo post tanning, which consists of a rinse and degreasing. Depending on the quality of the hides and their final use, the hides may be re-tanned, neutralized and washed.

The hides are then dyed; over 80 percent are dyed black. After dyeing, fat liquoring agents which give the leather its supple feel are added. Upon completion of this step, wastes are sent to the pretreatment system, and the hides receive a final wash before moving on to an air drying area.

After drying, the hides go to the finishing steps where they are first trimmed, stretched out and pinned onto a light metal grill, and dried to the desired final moisture. Depending on the final use of the hide, an automatic system sprays pigments, resin binders, and waxes onto the grain side of the hide. The finished hides are shaved on the flesh side to produce equal thickness, and then are ironed, inspected, and hand trimmed to their final size. Finally, each hide is measured by machine, bundled, and sent to the warehouse for shipment.

At the time of the assessment, there were a number of pollution problems at the facility, including,

Process modification; Housekeeping; Recovery, Reuse and Recycle; Material substitution

Below are listed the opportunities recommended for the facility, and presents the environmental benefits and implementation costs for each.

Pretanning:

1. Recycle improved secondary treatment waste effluent.
2. Liming - Segregate from other waste streams and oxidize separately
3. Liming wash water Reuse effluent from liming wash

Tanning:

1. Chromium Fixation -- Increase temperature and control pH.
2. Chromium Effluent Recycling - recycle used chromium effluent with addition of 1/3 of initial requirements reduces chromium in water.
3. Chromium recovery - precipitate chromium wastes with hydroxide, settle, and re-dissolve with acid and re-use chromium in wastewater.

Chromium reuse and recovery, black dye recycling, and water recycling will produce equal quality tanned hides while simultaneously reducing the quantity of chemical toxics released into the environment. Effluent from each virgin tanning bath can be reused up to five times by adding one-third the normal amount of chromium sulfate before each tanning bath. In addition, chromium recovery will reduce emissions and reduce production costs.

The recommendations can reduce the amount of wastewater treated by 2,000 m³ per year, reduce the loading of toxic chromium salts and dye baths, and isolate incompatible waste streams for separate treatment. While it is not practical for the tannery to eliminate the need for its wastewater pre-treatment facility, the recommendations will enable the treatment system to operate more efficiently. Further, separation of waste waters will avoid the generation of foul smelling and toxic hydrogen sulfide gas.

Pretanning

1. Recycle improved secondary treatment waste effluent yields a 30 percent reduction in water consumption. Implementation cost of $17,000 (US).
2. Liming - Segregate from other waste streams and oxidize separately - eliminates the generation of H₂S. There are no costs to implement and the payback period is immediate.
3. Liming wash water Reuse effluent from liming wash - reduces wastewater. Implementation costs of $1000 (US), with a financial benefit of $1000 (US) and a payback period of 1 year.

Tanning

1. Chromium Fixation -- Increase temperature and control pH reduces chromium in waste water. Implementation costs of $2000 (US).
2. Chromium Effluent Recycling - recycle used chromium effluent with addition of 1/3 of initial requirements reduces chromium in water. Implementation costs of $2000 (US) with financial benefits of $5000 (US) per year and a payback period of 5 months.
3. Chromium recovery - precipitate chromium wastes with hydroxide, settle, and re-dissolve with acid and re-use chromium in waste water. Implementation costs of $2000 (US) and a financial benefit of $1000 (US) per year and a payback period of 2 years.

Total costs for the options are estimated at $24,000 (US) with financial benefits of $7000 (US) per year.

Two major environmental issues for the tannery are the extensive use of chromium in the black dye and tanning salts, and direct worker contact with these toxic materials. Successful implementation of the recommendations would cut chromium sulfate purchased by 55 percent or 25 metric tons per year, and black dye purchases by 25 percent or 6 metric tons per year.

Overall, the assessment identified five pollution prevention opportunities at this facility that can save as much as $ 95,000 (US) in the first year after implementation for an overall investment of at most $ 22,000 (US). If implemented, these pollution prevention changes will reduce the amount of chromium sulfate used for tanning by 25 metric tons per year, eliminate the generation of H₂S, reduce the amount of waste water generated by 2,000 cubic meters per year, and reduce the chemicals needed to treat waste water. All five of the options identified can be quickly and easily implemented by the plant's staff. None require complicated, expensive, or new technologies.

None repported.

Contacts

EP3 Clearinghouse (UNITED STATES)

Tel : 1 (703) 351-4004

Fax : 1 (703) 351 6166

Internet: apenderg@habaco.com

This case study was carried out in a developing country in which EP3 has an established Program. It was submitted to UNEP IE and edited for the ICPIC diskette in August 1995. Subsequently the case study has undergone a technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.