CASE STUDY #235
1. Headline: Pollution Prevention Assessment for a Goatskin
Tannery
2. Background:
What is EP3?
The amount of pollutants and waste generated by industrial
facilities has become an increasingly costly problem for
manufacturers and a significant stress on the
environment. Companies, therefore, are looking for ways to
reduce pollution at the source as a way of avoiding costly
treatment and reducing environmental liability and
compliance costs.
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
information clearinghouse.
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.
Objectives of this Assessment
This assessment evaluated a facility that tans goatskin.
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 battery industry, and
(3) improve operating efficiency and product quality.
The assessment was performed by an EP3 team comprised of
an expert in hide tanning and a pollution prevention
specialist.
Facility Background
This facility is a goatskin tannery producing chrome
tanned suede and grain shoe, garment, and fancy leathers.
Dry and green salted skins, as well as wet blue goatskins
are used. The tannery produces leather from approximately
1,000 kg of dried goatskins per day.
The wastes generated by the tannery come from the hides
and the chemicals used in the production process. Tannery
wastes are discharged in a number of batches during the
production day.
3. Description of Cleaner Production Application: The
assessment identified various cleaner production
applications including: process modification, good
housekeeping, new technology, recycling, and material
substitution.
Overall, the assessment identified eight pollution
prevention opportunities at this facility. Recommendations
for pollution prevention include recycling of the spent
chrome tanning wastes, oxidation of the sulfide containing
wastes, decreasing the volatile organic discharge by
changing finishing materials, decrease of water use by
batch washing, and use of solid wastes from the waste
stream as fertilizer.
Manufacturing Process
In the production of leather from dry goatskins, the dry
skins must be thoroughly re-wet and the dirt, salt and
undesirable hide substances removed. Soaking and washing
the skins is done in three steps. The first step removes
dirt, salt, and some organic matter, while the other two
are rinses. The waste water is nearly neutral, and
contains salt and some suspended solids.
Next, the skins are unhaired by treatment with lime and
sulfides. The waste water is very alkaline, contains toxic
sulfides, and is the main cause of the high BOD and
suspended solids in the total waste stream.
The next step is de-liming and bating to remove the lime
in the skins and soften them by enzymatic action. The
first dump of this process contains ammonium sulfate,
enzymes, and some protein. The subsequent washes are very
dilute, nearly neutral pH solutions.
The skins are then tanned. The chrome tanning process is
standard for the industry: the solutions contain chromium
as chromium sulfate salt and some free acid. About 75
percent of the chromium present combines with the hide.
Finally, the retan, color, and fat liquor steps are
employed to color and oil the leather to make it as soft
or firm as desired. A number of chemicals are sued in
these steps, and about 90 percent of the load is fixed to
the leather. The spent solutions are mildly acidic with a
pH of between 4 and 6. BOD and suspended solids are
relatively low.
Existing Pollution Problems
At the time of the assessment, there were a number of
pollution problems at the facility, including (1)
excessive chromium discharge, (2) excessive VOC discharge,
(3) excessive water usage, (4) excessive leather waste,
(5) excessive sulfide waste, (6) excessive suspended
solids in effluent, and (8) excessive BOD of effluent.
Normally, the unhairing wastes are dumped during the
morning, while the balance of wastes from the washes of
the untanned skins and the retan come as a fluctuating
flow of mildly acidic wastes. The variation in the flow
and the changing acidity or alkalinity of the combined
waste stream is a problem for treatment of the pollution.
Pollution Prevention Opportunities
The assessment identified eight pollution prevention
opportunities that could address the problems identified,
with significant environmental and economic benefits to
the facility. Below are listed the opportunities for
pollution prevention recommended for the facility, and the
environmental benefits and implementation costs for each.
Two of the recommendations can be implemented with no
capital investment.
--Chromium tanning--Recycle chrome tanning - decreases
Chromium to less than 3 mg/l. At the time of the
assessment the costs had not been calculated. Financial
benefits were estimated at $5,000 (US) from savings in
chemicals.
--Solvent discharge-- Change to water-based lacquer finish
- decreases VOC discharge by 60-90 percent. No costs are
involved.
--Water use-- Change to batch washes - decreases water
usage by 20-40 percent. No costs involved.
--Solid waste-- Save leather trimmings for reconstituted
leather - decreases leather waste by 60 - 80 percent.
Costs were estimated at $5,000 (US).
--Sulphide waste-- Destroy sulfides by air oxidation -
decreases sulphide waste by 95-98 percent. Cost were
estimated at $20,000 (US).
--Primary treatment--decreases suspended solids by 70-85
percent. Costs were estimated at $50,000.
--Sludge from effluent-- Dry sludge for land application -
allows for disposal of sludge as fertilizer. Costs were
estimated at $10,000.
--Secondary treatment-- Treat primary waste decreases BOD
by 60-80 percent. Tickling filters were estimated at
$50,000.
The total costs for the actions were estimated at $135,000
(US) in capital costs. The total financial benefits were
estimated at $5,000 (US) plus 7 unquantified
opportunities.
Effect on the Environment
The recommended actions are based on cost effective
methods that have been proven in commercial applications.
These actions will have a number of positive environmental
impacts.
1) Chromium recycling will decrease the chromium in the
discharge by 80-90 percent. The spent chromium solutions
contain about 25 percent of the total chromium used in the
tannage. The loss of this valuable material can be
decreased and the chromium concentration lowered by
recycling. Some of the spent chromium solution can be
directly used to make the pickle solution without
affecting the quality of the leather. The remainder can be
saved, and the chromium precipitated with the addition of
an alkali. The recovered chromium can be dissolved in acid
for use in the tannage.
2) The suppliers of finishing products have developed
water-based lacquers with significantly lower volatile
solvent contents. The reduction of volatile solvents will
decrease VOC releases to the atmosphere by 60-75 percent.
3) In some hide wetting processes there is an opportunity
to recycle the final rinses. In the goatskin process,
extensive washing of the bated skin is common. The water
from this wash could be used for rinse water in the
original soaking, as the final rinse wastewater is
compatible with fluids used for the first wetting of
hides. The judicious recycling of rinse waters and
automated systems in a tannery could result in savings of
up to 50 percent of water consumed.
4) Elimination of solid leather waste discharges by using
trimmings to make reconstituted leather will ease the
burden on landfills.
5) Eliminating sulfide discharges is very important as
sulfides can corrode pipes, cause objectionable odors, and
cause fatal accidents. The sulfide-lime solution, and
washes from this process can be easily collected, placed
in a tank, and the sulfides oxidized by air with a
manganese sulfate catalyst. This method is effective and
can destroy the sulfide in 4-8 hours. The oxidized wastes
are kept for use in controlling the pH of the effluent
stream.
6) Decreasing by 80 percent the suspended solids
discharged, and instituting secondary treatment will serve
to decrease BOD. With primary and secondary treatment, the
BOD can be reduced by 75 percent. In addition, the
reduction of suspended solids creates a useable by-product
in the form of an organic fertilizer, thus eliminating
possible high disposal costs.
Implementation Plan
The schedule and timing of implementation of the
recommendations will depend on the relative costs and
benefits and the availability of personnel and capital.
The recycling of the chrome tanning solutions is the most
cost effective recommendation in that the company will
have a large saving in material costs in addition to
significant pollution prevention.
Chrome Recycling. The value of chrome recycling to the
tannery and its low capital costs should make it an
attractive option. Implementation may be accomplished in 2-
6 months.
Solvents. The shift to low VOC finishes should occur
between 6-12 months as the system is accepted by the
company for each of the leathers it makes.
Process Water. The decrease in the volume of process water
used can be accomplished without a capital investment. The
batch washing of coloring and fat liquoring batches could
result in savings of about 50 percent in these operations.
With the recommended pretreatment system, a decrease in
flow would decrease the capital and operating costs of the
treatment system. The shift to lower water use should come
gradually over the next 12 months.
Solid Wastes. At present, the tannery has useful disposal
of most of its solid wastes in the form of fleshings,
trimmings, and leather shavings. With the introduction of
the pretreatment system, a new source of solid waste --
about 1000 kg of solid sludge containing 120-150 kg of
nitrogen-- will be generated. This organic nitrogen and
the other materials in the sludge have been proven to be
very valuable as fertilizers and soil conditioners. The
implementation of the primary treatment system over the
next 6-18 months will not only clean the liquid tannery's
wastes, but result in an environmental benefit to the
community.
Pretreatment. The implementation of the pretreatment
system will require design data and engineering and
construction of the system. Total time to completion
should be 12-15 months.
5. Economics: See above
6. Advantages: See above.
7. Constraints: No information provided.
8. Contact:
EP3 Clearinghouse (UNITED STATES)
TEL: 1 (703) 351-4004
FAX: 1 (703) 351 6166
Internet: apenderg@habaco.com
9. Keywords: tanning, leather, goatskin, recycling, leather
tanning, EP3, chrome, solvent, raw material saving, water
saving, sulfur, BOD, nitrogen, good housekeeping, process
modification, material substitution, USAID
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.