|EP3 - Pollution Prevention Assessment for a Cattle Hide Tannery||Developing Country||1993||CP Audit|
TANNING AND DRESSING OF LEATHER # 6
This assessment evaluated a cattle hide tannery. The objective of the assessment was to identify actions that would:
|reduce the quantity of toxics, raw materials, and energy used in the manufacturing process, thereby reducing pollution and worker exposure,|
|demonstrate the environmental and economic value of pollution prevention methods to the tanning industry, and|
|improve operating efficiency and product quality.|
The assessment was performed by an Environmental Pollution Prevention Project (EP3) team comprising a US expert in hide tanning, a pollution prevention specialist, in-country EP3 staff, and local consultants.
This facility where the assessment was carried out is a cattle hide tannery producing chrome tanned and vegetable tanned leather from salted cattle hides. The wastes generated by the tannery come from the hides and the chemicals used in the production process. The tannery has a nominal production capacity of five hundred hides per day. The hides average 23-24 kg with the total weight of hides at 12,000 kg/day.
Chrome/vegetable retan leather accounts for 10,640 kg of hides per day, while vegetable tan leather accounts for the remaining 1,560 kg. The hides are domestic and imported from small slaughter - houses.
The processing of hides and skins in the facility is as follows:
The salted hides are inspected, resalted if needed, then weighed into production lots. The hides are placed in a conventional drum and soaked in cold water. Some minor chemicals may be added to assist in the soaking. The water use is approximately two liters of water per kg of hide. After the desired soaking time, the hides are washed in additional water.
The hair pulp process is carried out in the same drums as the soak. First, the hides are treated with lime and sulphides. Then, more lime is added and the hides are washed in cold water in the rotating drum.
The washed, limed hides are removed from the drum, wrung, then placed in a drum for deliming and bating.
This process involves the use of ammonium salts and enzymes, which remove most of the lime from the hide.
The next step is chrome tanning, which is conducted in drums. The hides are placed in a solution of salt and acid, followed by the chrome tanning agent (chromium sulfate). The pH of the solution is adjusted by adding magnesium oxide. When the chrome tannage is complete, the leather is washed with running water and the surface is then cleaned by adding a small quantity of acid. The tanned leather is then removed from the drum and wrung.
The hides to be vegetable tanned are separated from the other hides after liming. These hides are cut into section for bellies, shoulders, and bends in order to assure the most efficient use of the expensive vegetable tanning materials. The vegetable tanning is done without the use of chromium. Because the retan system for the chrome tanned leather is also a vegetable tannage, in most cases the effluent from the vegetable tannages is combined with the other effluents from the retan step.
The chrome tanned leather, after wringing, is split to the desired thickness. This results in two layers of leather : the grain and the split. The grain layer is the larger and more valuable layer. The split is trimmed and further processed in the same manner as the grain leather. Following splitting, the leather is precision to the desired thickness by shaving, which involves a high speed rotating blade that removes leather in small shavings (1 x 3 mm). These shavings are sold to be used in reconstituted leather products.
The chrome tanned leather is retanned in small batches to color and oil the leather as desired. The retan formulations vary widely depending on the leather desired. In most of the leather made in this tannery, vegetable tanning materials are used. In addition, dyes, speciality chemicals, and leather lubricating oils are applied. The retanning process produces large quantities of effluent with relatively low pollutant concentrations.
At the time of the assessment, there were a number of pollution problems at the facility, including excessive solid waste, chromium discharge, VOC discharge, water usage, sulphide waste, suspended solids in effluent, and BOD of effluent.
Cleaner Production Principle
Process modification; Housekeeping; Recovery, Reuse and Recycle; Material substitution
Cleaner Production Application
The largest sources of pollutants at the plant are from the soaking and hair pulp systems, which have very high concentrations of suspended solids and high BOD. The hair pulp system also contains sulphides and strong alkali as calcium hydroxide. Sulphides are toxic materials and must be destroyed chemically. The normal treatment system in the industry is to collect all the sulphide containing wastes, then oxidize the sulphides with air with a manganese sulfate catalyst. The lime solution, free of sulphide, can be used to neutralize the acid wastes to adjust the pH to the acceptable range.
The assessment identified pollution prevention opportunities that could address the problems identified, with significant environmental and economic benefits to the facility. Below are listed the opportunities recommended for the facility, along with the environmental benefits and implementation costs for each. Two of the recommendations can be implemented with no capital investment.
Recommendations for pollution prevention include:
Reducing VOC emissions by changing to water-based lacquer finishes.
Using fleshings for rendering: The hides, after washing, should be fleshed before the hair pulp step, improving the quality of the production and allowing the sale of fleshings.
Recycling some wash water to compatible processes: Recycling some water washes that are only slightly contaminated with process chemicals, where compatible with production processes, will result in water use reduction. The change in hide washing practices to produce better hair pulping and cleaner flesh is a relatively simple matter. There is no quality risk in this change and it should be taken as soon as possible.
Recycling the spent chrome tanning wastes: Recycling spent chrome tanning solutions will produce economic benefits for the tannery, decrease water use and prevent pollution of the effluent by chromium. In addition to pollution prevention benefits, recycling chrome tanning solutions has technical benefits for the tanner. This step is not simple, however, and will require some process adjustments. Required personnel should be hired and the project started as soon as possible.
Sulphide destruction by air oxidation : The pollution control regulations require sulphide oxidation; this step cannot be avoided. The engineering of the sulphide oxidation system should be started and the project implemented as soon as possible. Removing toxic sulphides from the waste stream by oxidation will effectively decrease the sulphide in the waste stream to less than 3 mg/l.
Reducing suspended solids by physico-chemical precipitation: The mixing of the acid and alkaline wastes at a controlled pH will result in a coagulation of the suspended solids.
Mixing the separated waste streams with pH control, after sulphide and chromium removal, will co-precipitate the suspended solids and decrease the BOD. The primary treatment system is the most extensive project recommended in this report. The suspended solids requirements make the primary treatment system essential. The design of the system should begin immediately and construction should start when planning is complete.
Instituting secondary treatment of the waste stream: Implementing secondary treatment, which will lower BOD, should be delayed until the primary system has been optimized. At that time, the most cost effective method for BOD reduction can be determined.
Using solid wastes from the waste stream as fertilizer was considered as a possibility of waste utilization.
Environmental and Economic Benefits
The implementation of the recommendations will be required in order to meet regulatory pollution abatement requirements.
1000-1500 kg of fleshings from soaked hides can be sold to a rendering facility. This decreases solid waste. Cost was estimated to about $3000 (US).
The chrome tanning wastes contain valuable chrome tanning materials. These spent solutions should be recycled to remove the chromium from the effluent and also reduce processing costs. Recycling of chrome tanning decreases chromium levels to less than 3 mg/l. Costs estimated was $20,000 (US) with a savings of $60,000 (US) per year.
Change to water-based lacquer finish was expected to decreases the VOC emissions by 60 to 90 percent.
Recycle of some wash and cooling water for compatible processes decreases water usage by 130 -150 cubic meters per day. Cost estimated for investment was $20,000 (US) covering for pumps, pipes and tanks.
Sulphide oxidation decreases sulphides in effluent to less than 3 mg/l. Cost estimated for investment was $30,000 (US) covering pumps, pipes and tanks.
Sludge from primary treatment can be used for land application as fertilizer. Cost was estimated as $20,000 towards the construction of sand filters.
Total estimated costs for implementation $240,000 (US) capital including some end of pipe measures with $60,000 (US) per year as the estimated savings.
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.