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Effluent Guidelines, Leather Tanning:, and Pollution Prevention;
A Retrospective Study
Introduction
This study was undertaken to learn:
O in what ways and for what reasons a specific industry in the past
already was implementing pollution prevention in order to comply with
existing effluent guidelines; and
O to what degree the effluent guideline development document for that
industry had already previously projected that outcome.
The industry selected for this study was the leather tanning industry. The leather tanning
effluent guidelines were promulgated in 1982. This study looks at the steps taken in the past
to comply with those guidelines. Because of the confidentiality of the data obtained in this
study, this report will describe and present the findings in a general way that will not
compromise confidentiality.
It is important to note that this study is intended to serve as a case study within the
leather tanning industry and not as a study of the leather tanning industry as a whole.
Findings
1. Process changes and material substitutions were as important hi meeting the
guideline requirements as end-of-pipe treatment.
For example, in meeting chromium limitations a major step taken was to
modify the chrome tanning process to get more of the chrome into the tanned
leather and leave less of it in the tanning wastewater. This was done through
using different and more expensive chromium formulations as well as higher
temperature, longer times, and less water in tanning baths. Through these
measures, the amount of chromium fixation was increased from about 50% to
about 90%.
As another example, carbon dioxide was being used on a pilot basis as a
substitute for ammonium sulfate in the deliming process. This was considered
primarily as a cost reduction measure. But, if the change to carbon dioxide is
successful, it will substantially reduce discharge of ammonia and biochemical
oxygen demand (BOD).
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As a third example, a technique was adopted whereby different agents were
substituted for some of the sodium hydrosulfide used in the unhairing
operation. This reduced marginally the amount of sulfide in the wastewater.
2. Recycling was done to meet effluent guideline requirements. Several waste
streams were segregated so that portions of each could be recycled.
For example, there was recycling of some of the spent chrome tanning liquor.
It was collected, screened, and filtered, then recirculated back to provide acidity
and some initial chromium hi the pickling process.
As another example, there was recycling of the spent unhairing liquor. After
being screened, settled, and skimmed for removal of solids and grease, about
20% of the spent liquor is then withdrawn from the middle of the settling tank
and recirculated back to the unhairing process. This reduced by somewhat less
than about 20% the amount of sulfide and other compounds needed for
unhairing.
3. Measures were adopted to reduce water use by about 50%. These measures
included:
adoption of "closed-door" batch washing of skins rather than
constant flow washing;
changing the drums used for unhairing and tanning from
manually operated to automated;
recycling some of the wastewater from the final treatment plant
to various points in the production process;
recovery of waters from pasting and vacuum drying for reuse;
recycling of cooling water used in refrigeration and air
compressors.
4. Pollution prevention was adopted and implemented primarily because of the
effluent limitation requirements, even though there were other benefits as well
such as improved product quality, improved process efficiency, and reduced
costs of water supply, wastewater treatment, and sludge disposal.
5. The effluent guideline development document was generally quite accurate in
its projections. However, two major pollution prevention measures that were
implemented (the first two in 1. above) were not referenced in the development
document.
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Conclusions
1. The industry was implementing pollution prevention in the past to meet the
effluent guideline requirements.
2. The effluent guidelines were the driving force behind the implementation of
pollution prevention, even if not the only reason.
3. The effluent guideline development document projections, while quite accurate
generally hi the areas of end-of-pipe technologies, did not forecast all the
aspects of the pollution prevention creativity that was exercised by the industry.
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