Case Study #244
1. Headline: EP3 - Pollution Prevention Assessment for a
Cattle Hide Tannery
What is EP3?
The United States Agency for International Development
(USAID) is sponsoring the Environmental Pollution
Prevention Project (EP3) to establish sustainable programs
in developing countries, transfer urban and industrial
pollution prevention expertise and information, and
support efforts to improve environmental quality. These
objectives are achieved through technical assistance to
industry and urban institutions, development and delivery
of training and outreach programs, and operation of an
EP3's Assessment Process
EP3 pollution prevention diagnostic assessments consist of
three phases: pre-assessment, assessment, and post-
assessment. During pre-assessment, EP3 in-country
representatives determine a facility's suitability for a
pollution prevention assessment, sign memoranda of
agreement with each facility selected, and collect
preliminary data. During assessment, a team comprised of
US and in-country experts in both pollution prevention and
the facility's industrial processes gathers more detailed
information on the sources of pollution, reducing this
pollution. Finally, the team prepares a report for the
facility's management detailing its findings and
recommendations (including cost savings, implementation
costs, and payback times). During post-assessment, the EP3
in-country representative works with the facility to
implement the actions recommended in the report.
This assessment evaluated a cattle hide tannery. The
objective of the assessment was to identify actions that
would: (1) reduce the quantity of toxics, raw materials,
and energy used in the manufacturing process, thereby
reducing pollution and worker exposure, (2) demonstrate
the environmental and economic value of pollution
prevention methods to the tanning industry, and (3)
improve operating efficiency and product quality.
The assessment was performed by an EP3 team comprised of a
US expert in hide tanning, a pollution prevention
specialist, in-country EP3 staff, and local consultants.
This facility 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 house production.
3. Cleaner Production Principle: The assessment identified
various cleaner production applications including: process
modification, good housekeeping, new technology,
recycling, and material substitution.
Overall, the assessment identified nine pollution
prevention opportunities at this facility. Recommendations
for pollution prevention include using fleshings for
rendering, recycling the spent chrome tanning wastes,
recycling some wash water to compatible processes,
oxidizing the sulfide containing wastes, reducing
suspended solids by physico-chemical precipitation, using
solid wastes from the waste stream as fertilizer,
instituting secondary treatment of the waste stream, and
reducing VOC emissions by changing to water-based lacquer
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
The hair pulp process is carried out in the same drums as
the soak. First, the hides are treated with lime and
sulfides. 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
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, specialty
chemicals, and leather lubricating oils are applied. The
retanning process produces large quantities of effluent
with relatively low pollutant concentrations.
Existing Pollution Problems
At the time of the assessment, there were a number of
pollution problems at the facility, including excessive:
(1) solid waste, (2) chromium discharge, (3) VOC
discharge, (4) water usage, (5) sulfide waste, (6)
suspended solids in effluent, and (7) BOD of effluent.
Pollution Prevention Opportunities
The assessment identified nine 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
Summary of Recommended Pollution Prevention Opportunities
Fleshings from soaked hides--use fleshings for rendering--
decreases solid waste by 1000-1500 kg per day. Cost is
about $3000 (US).
Chromium tanning--recycle chrome tanning decreases
Chromium to less than 3 mg/l. Costs estimated at $20,000
(US) with a savings of $60,000 (US) per year.
Solvent discharge-change to water-based lacquer finish -
decreases VOC discharge by 60 to 90 percent.
Water use--recycle some wash and cooling water to
compatible processes decreases water usage by 130 -150
cubic meters per day. Costs estimates of $20,000 (US) for
pumps pipes and tanks.
Sulfide waste- destroy sulfide by air oxidation -
decreases sulfides in effluent to less than 3 mg/l. Costs
estimated at $30,000 (US) for pumps pipes and tanks.
Primary treatment--Physico-chemical precipitation with
spent unhairing waste- decreases suspended solids by 60 -
80 percent; decreases BOD by 40 - 60 percent. Estimated
costs of $100,000 (US) for pumps pipes and tanks.
Sludge from primary treatment-dry sludge for land
application- disposes of sludge as fertilizer. Cost
estimates of $20,000 for sand filters.
Secondary treatment- treat primary waste - decreases BOD
by 60 - 80 percent. The cost is for $50,000 (US)
Total estimated costs for implementation $240,000 (US)
capital with $60,000 (US) savings.
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 sulfides and strong alkali as
calcium hydroxide. Sulfides are deadly toxic materials and
must be destroyed chemically. The normal treatment system
in the industry is to collect all the sulfide containing
wastes, then oxidize the sulfides with air with a
manganese sulfate catalyst. The lime solution, free of
sulfide, can be used to neutralize the acid wastes to
adjust the pH to the acceptable range.
The mixing of the acid and alkaline wastes at a controlled
pH will result in a coagulation of the suspended solids.
The removal of the coagulated materials by primary
treatment will result in a decrease of suspended solids by
about 80 percent. The primary treatment of tannery wastes
by coagulation and settling will also decrease the BOD by
50-70 percent. This approach has been successfully used in
at many tanneries.
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
It is recommended that several steps be taken by the
- The hides, after washing, should be fleshed before the
hair pulp step, improving the quality of the production
and allowing the sale of 1000-1500 kg of fleshings per day
to a rendering facility.
- Recycling spent chrome tanning solutions will produce
economic benefits for the tannery, decrease water use and
prevent pollution of the effluent by chromium.
- Recycling some water washes that are only slightly
contaminated with process chemicals, where compatible with
production processes, will result in water use reduction.
- Removing toxic sulfides from the waste stream by oxidation
will effectively decrease the sulfide in the waste stream
to less than 3 mg/l.
- Mixing the separated waste streams with pH control, after
sulfide and chromium removal, will co-precipitate the
suspended solids and decrease the BOD.
- 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.
Implementation of the Recommendations
The implementation of the recommendations will be required
to meet regulatory pollution abatement requirements.
Wash Water. 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.
Chrome Recovery. 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
Sulfide Oxidation. The pollution control regulations
require sulfide oxidation; this step can not be avoided.
The engineering of the sulfide oxidation system should be
started and the project implemented as soon as possible.
Primary Treatment. 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
5. Economics: See above.
6. Advantages: See above.
7. Constraints: See above.
EP3 Clearinghouse (UNITED STATES)
TEL: 1 (703) 351-4004
FAX: 1 (703) 351 6166
9. Keywords: leather, tanning, recycling, United States,
USAID, EP3, hide, chrome, sulfide, salt, biotechnology,
enzyme, VOC, BOD, solvent, water saving, sulfur
10. Reviewer's comments: This case study was carried out in a
developing country in which EP3 has an established
programme. It was submitted to UNEP IE and edited for the
ICPIC diskette in August 1995. It has not undergone a
formal technical review.