Jorn Tolle

Newtec Umwelttechnik GmbH

1 Abstract

Leather is produced by transformation of raw animal hides, a natural renewable resource, and can be considered a valuable byproduct of the food industry. The hides are processed by various steps of cleaning, tanning and dying. The leather tanneries in Europe and Latin America are typically small to medium-size industries, which are known to produce severe environmental problems, such as highly polluted wastewater, different kinds of solid wastes and gases. The following thesis is part of the research program INCO – DC 1997 EILT (Reduction of Environmental Impacts of Leather Tanneries) between various countries in Europe and Latin America, which is supported by the ALFA-program of the European Union Practical trials were carried out in cooperation with the Ecuadorian leather tannery E.C.S.

An integrated water management that includes segregation, depuration and reutilization of specific liquid streams expects considerable improvements. As a result, a minor use of water and chemicals, mainly sulfide and chromium, are considered to be reasonable. The main object of the proposed thesis is a depuration treatment of specific, already segregated effluents. Treatment operations found application on unhairing and chrome tanning effluent, which represent main environmental impact through high concentrations of suspended solids, organic material, chrome and sulfide.

2 Environmental impact of leather tanneries

To understand the environmental impact of leather tanneries it is necessary to explain the composition of animal hides.

An animal hide consists of water (61%), fibrous proteins (34%), globular proteins1 (1%), lipids (2%), mineral salts (1%) and other components, such as pigments. A hide can be divided into three structurally different parts, the epidermis, the dermis and the hypodermis. The epidermis contains hair and mainly consists of keratin. The dermis is the part that is normally considered to be skin and which is later transformed into leather; the main fraction of the dermis is collagen. The structure of the hypodermis is formed by horizontal fibers linked with blood vessels, muscles, fat, nerves, etc; the dominating fraction is flesh. For the transformation of a hide into leather, the epidermis, hair and flesh have to be eliminated, which is established in a production process of several different sub-processes. Every tannery follows the same scheme. The only variations are based on different combinations of subprocesses or use of different chemicals. The first phase of the process, called beamhouse, serves to clean the hide from hair, remaining flesh and prepare it for the actual tanning process. In the tanning phase the hide is treated with salts, acid and chromium to obtain resistant leather. Furthermore it undergoes the posttanning processes to raise the quality of the product.

2.1 Liquid stream segregation

The environmental impact of tanneries consists of solid, gaseous and liquid fractions, and is clearly dominated by the latter. While the solid wastes are manufactured into different kinds of economically usable products (dog toys, gelatin, shoes, etc.), the water effluents of Ecuadorian leather tanneries are simply released into rivers. The overall effluent of a tannery can be segregated into a minor stream with less contamination and two main streams, the unhairing and the chrome tanning effluent, which have to be treated separately. Recycling processes of these two effluents are reasonable in respect of high concentrations of unused chemicals, acidity or alcalinity.

2.1.1 Unhairing effluent

The processes reliming and unhairing are usually combined in a single bath with a single effluent. Main components are unused calcium and sulfide, which reach concentrations between 2.000 and 3.000mg/l justifying an effluent reuse.

The traditional unhairing process, which is used in the observed leather tannery E.C.S., destroys the hair completely by the use of lime and sulfides. The chemicals are in charge of breaking up keratin compounds, which are the principal components of epidermis and hair, in order to separate hair and epidermis from the hide without affecting its collagen negatively.

As a consequence, the effluent is characterized by high alkalinity, sulfides, lime, surfactants, destroyed hair and the main part of the organic material produced in the tannery. While high alkalinity, sulfides, lime and surfactants are useful in respect of an effluent reuse, the increased amount of COD and suspended solids hinder an effective recycling process.

2.1.2 Chrome tanning effluent

Based on conventional tanning processes, the collaborating leather tannery E.C.S. combines pickling with chrome tanning processes applying chromic sulfate (Cr(OH)SO4) as the main tanning agent. Components of the chrome tanning effluent are residual chromium and magnesium in concentrations between 500 and 1.000mg/l justifying an effluent reuse. In the pickling operation the alkaline hide has to be treated by acids, normally sulfuric and formic acid. The pH is lowered to approximately 3 because in a medium above pH 4 chromium starts precipitating. To avoid a hide swelling caused by osmosis, a high amount of salt, normally NaCl, is added until reaching a 5%-solution.

The actual tanning process follows after an hour reaction time. Chromic sulfate and a basifying agent (MgO or/and NaHCO₃) are added to the acidic solution which slowly elevates the pH up to 4 and hereby increases chromium binding. The chromic sulfate reacts with carboxyl groups of the collagen.

Synthetic and/or proteinaceous tanning agents, which are based on chromium containing shavings gained in one of the further subprocesses, can be used additionally but are not used in the conventional tanning process. Fungicides are conventionally added to protect treated hide if stored.

Since only about 70% of added chromium is chemically bound, the highly acidic effluent carries an elevated amount of chromium and salts. The more problematic component of the effluent, in respect of the recycling task, is again the increased amount of suspended solids.

3 Results and discussion

The idea of recycling unhairing and chrome tanning effluent implies changing important characteristics of the tanning effluents as little as possible, as far as their tasks in the actual tanning process is concerned. Concentrations of sulfide and chromium should be kept in a high range and the pH should remain the same during and after treatment processes to keep recycling and the conditioning of recycled effluents technically suitable and economic. Main task, nevertheless, was an effective removal of suspended solids to reach recyclable concentrations.

3.1 Unhairing effluent

The aim of the treatment was to find the most efficient way to reduce the suspended solid (TSS) concentration in every treatment step. In order to carry out various recycling cycles, the TSS concentration should not exceed 2.000 or 3.000mg/l [HIDALGO, 2000]. Efficiency comparisons of the primary treatment and floc removal results were made. Coagulation results were discussed in respect of efficiency of corresponding chemical and coagulation mechanism.

3.1.1 Coagulation/Flocculation

The application of coagulants Fe(III) and Al(III) in an effluent of a high sulfide concentration had a sulfide removal and color effect. A black precipitate reached its maximum when iron sulfate was applied. Iron(III) forms a sulfide precipitate of low solubility in the range of 10-88 mol²/l²[BENEFIELD, 1982]. After HIDALGO [2000], colored effluents are not recyclable in order to conserve effluent characteristics. A recycled colored effluent would have a negative quality effect on the hide. Iron chloride resulted in a minor colored effluent and less precipitation of iron sulfide. Sulfate seemed to favor precipitation of Fe₂S₃ in a highly alkaline medium.

Since precipitation and color through the input of iron sulfate exceeded a tolerable level, further coagulation/flocculation trials were only based on three remaining coagulants. The maximum TSS removal by coagulation/flocculation trials was achieved by a dosage of 80mg/l aluminum sulfate.

After [BENEFIELD, 1982] Fe(III) and Al(III) build the species Al(OH)₄^- and Fe(OH)₄^- in unhairing effluent based on the high pH of 12,3. Aluminate is soluble in a concentration of 10-2mol/l and ferrate in a concentration of about 10^-7mol/l at that pH. The excessive iron and aluminum precipitates as hydroxide, ferrate or aluminate. As far as coagulation mechanisms double-layer compression and adsorption are concerned, the negatively charged species are ineffective. The mechanism enmeshment in a precipitate is considered to be reasonable. Since iron has a much lower solubility at this pH, a greater removal effect was expected. Aluminum was more efficient at low dosages which might be based on the type of precipitate. Ferrate and aluminate are considered to build a precipitate with the excessive calcium of the unhairing effluent.

Table 3.1: Mass balance of metals iron, calcium and aluminum mainly involved in coagulation, flocculation experiments of unhairing effluent.

As presented in table 3.1, the calcium concentration of the solution decreased to a further degree by coagulation based on aluminum sulfate. Results showed that calcium aluminate precipitates better than calcium ferrate.

An additional removal effect after flocculant input was observed and is based on the flocculation mechanism interparticle bridging which resulted from coloumbic at-traction, ion exchange and/or hydrogen bonding. The long-chained polymers became attached to various particles and formed a bridge between them. The anionic polymer Praestol, an anionic modified polyacrylamide reached the best results and lowered the suspended solid concentration of the presettled effluent by a dosage of 4mg/l, to a residual concentration of 40%. The Cactus and the cationic polymer led to acceptable results at lower dosages, but did not reach residual concentration reached by the anionic polymer.

3.1.2 Conclusions

Treatment effort focused on the removal of suspended solids and organic material measured by the chemical oxygen demand. An efficient removal is given by the treatment combination shown in following figure:

Fig. 3.1: Optimum treatment combination of unhairing effluent

Primary sedimentation in combination with a coagulation treatment based on 80mg/l aluminum sulfate and 4mg/l Praestol, and a secondary sedimentation step, reached the aim of reducing suspended solids to a concentration between 2.000 and 3.000mg/l. The anionic polymer is preferred to the cationic and cactus polymer because of the higher efficiency reached, although treatment costs are raised through the higher optimum dosage. Sulfide and the pH were held in the range of raw unhairing effluent. To compensate the slight decrease of sulfide concentration, treated effluents will have to be conditioned before the reuse.

The overflow rate of both sedimentation steps has to be lowered to 0,3m/h in order to reach the stated aim in case of a continuous-flow system. The other possibility is to design a system of two parallel sedimentation tanks varying the application of each one with time.

3.2 Chrome tanning effluent

Treatment operations of the chrome tanning effluent were divided into pretreatment, filtration and floc removal phases, which have to be compared in respect of efficiency and practical application, focusing on the corresponding residual TSS concentration which should not exceed 2.000mg/l [HIDALGO, 2000].

3.2.1 Coagulation/Flocculation

Coagulation/flocculation trials were only applied on presettled effluent because screening experiments did not produce effective results. Both had an additional TSS removal effect on presettled chrome tanning effluent. The maximum removal percentage was achieved by a dosage of 400mg/l iron sulfate. Since both metals hydrolyze in water and form aquometal complexes, various soluble species are created. At the prevailing pH 4 in the chrome tanning effluent aluminum has a solubility of about 10^-1mol/l and iron dissolves in the molar concentration 10-6mol/l. Dissolved iron species are dominated by FeOH²⁺ and aluminum by species Al₁₃(OH)₃₄⁵⁺ and Al₆(OH)₁₅^3- . Coagulation results of aluminum species are probably based on mechanism adsorption and charge neutralization. Lower concentrations reached similar efficiency, and high aluminum dosages led to a charge reversal of the particles resulting in lower coagulation/flocculation efficiency. Since iron has a much lower solubility at pH 4, better results were expected due to iron precipitation (enmeshment in a precipitate). Results of coagulation/flocculation experiments confirm anterior assumptions. Coagulation efficiency increased with the coagulation dosage until it reached its optimum at a dosage of 400mg/l. The relatively high optimum dosage is probably due to the low TSS concentration of the presettled effluent. The large amount of coagulant was required to produce a good precipitation and to enmesh few particles. Chrome and magnesium only served as secondary coagulants as can be seen in table 3.2. Neither of the metal concentration were reduced significantly in spite of high concentrations in presettled effluent. Both metals form stable precipitates, but at a higher pH. Chrome starts precipitating above pH 4,5 and magnesium above pH 10. Iron only dissolved in a low percentage of the coagulant input. Excessive iron served as a coagulant through precipitation.

Table 3.2: Mass balance of metals chromium, iron and magnesium mainly involved in coagulation experiments in chrome tanning effluent

An additional removal effect of minor importance produced through polymer input was observed in flocculation trials. Flocculation was based on the mechanism interparticle bridging resulting from coloumbic attraction, ion exchange and/or hydrogen bonding. The long-chained polymers became attached to various particles and formed a bridge between them. The anionic polymer Praestol 650 TR, an anionic modified polyacrylamide, was the only polymer that increased coagulation/flocculation results based on aluminum sulfate without application of polymers.

3.2.2 Conclusions

Treatment effort focused on the removal of suspended solids and organic material measured by the chemical oxygen demand. Special observation was additionally put onto parameter pH and chrome concentration, which should not be changed in a far degree. A sufficient removal is given by the single treatment shown in following figure:

Fig. 3.2: Optimum treatment of chrome tanning effluent

Primary sedimentation reached the aim of reducing suspended solids to a concentration between 1.000 and 2.000mg/l. The most efficient secondary treatment, coagulation combined with a floc removal by raw unhairing effluent, is not considered to be necessary in addition to the primary sedimentation operation. The comparatively small additional reduction of TSS from 1.160mg/l to 610mg/l and of the chemical oxygen demand would be achieved by a high coagulant input and construction of an additional coagulation and sedimentation system. Since secondary sedimentation was extra-ordinary slow, two parallel or one large settling tank would be necessary, therefore increasing construction cost.

Primary sedimentation, counting on a continous-flow system with an overflow rate between 0,6 and 1,2m/h, seems to be the appropiate solution for the chrome tanning effluent. Chrome concentration and pH of the raw effluent stay constant during treatment process and prepare effluent well for the recycling task. The effluent still has to be conditioned before reuse because of chrome and calcium decrease throughout the chrome tanning process.

Special care must be put upon the sludge disposal because chrome belongs to the group of hazardous waste and has to be disposed of separately from organic unhairing sludge.

4 References

AMERICAN PUBLIC HEALTH ASSOCIATION ET AL., 1995: "Standard methods for the examination of water and wastewater", 19th edition, Washington, 1995

BENEFIELD, L. D.; JUDKINS, J. F.; WEAND, B. L., 1982: "Process chemistry for water and wastewater treatment", Prentice-Hall, Inc., New Jersey, 1982

BRETSCHNEIDER, H.; LECHER, K.; SCHMIDT, M., 1993: "Taschenbuch der Wasserwirtschaft", 7. Auflage, Verlag Paul Parey, Hamburg und Berlin, 1993

CLAAS, C., MORAES MAIA, R., 1994: “Manual basico de residuos industriais de curtume”, Porto Alegre 1994

CYTEC, 2000: “Personal information about product Superfloc”, Bogota, 2000

FRANKEL, A. M., 1989: "Tecnología del Cuero", Editorial Albatros, Buenos Aires, 1989

HACH, 1996: "Procedures manual, spectrophotometer" Hach Company, U.S.A., 1996

HAHN, H. H., 1985: "Physical and Chemical Aspects of Coagulation in Water Tech-nology", in WaBoLu 62, Gustav Fischer Verlag, Stuttgart, 1985

HENKEL, S. A., 1994: "Mecanismo de engraxe - principais parâmetros", Brasil, 1994

HIDALGO, D. H., 2000: "Información personal sobre técnicas de curtición", Quito, 2000

HOINACKI, E.; GUTHEIL, N. C., 1978: "Peles e Couros", Fundação de Ciência e Tecnologia (CIENTEC), Brasil, 1978

HUISMAN, L., 1981: "Sedimentation, flotation and mechanical filtration", Delft University of Technology, 2nd edition, 1981

IFFT, J. B.; ROBERTS, J. L., 1986: "Essentials of chemistry in the laboratory", 3rd edition, W. H.
Freeman and company, San Francisco, 1986

JEKEL, M., 1987: “Flockung”, DVGW-Fortbildungskurs 6, Wasseraufbereitungs-technik für Ingenieure, Bonn, 1987

JEKEL, M.; DREWES, J. E., 1994: “Skript zur Vorlesung Wasserreinhaltung II”, Berlin, 1994

JEKEL, M., 1999: "Persönliche Auskunft zur Flockung von Gerbereiabwässern", Berlin, 1999

LEWIS, G. F., 1985: "Analytical chemistry", Macmillan Publishers LTD, 2nd edition, London, 1985

MERCK, 1989: "The Merck Index", 11th edition, New York, 1989

MUÑOZ, M., 1996: "Manuscrito de la clase Plantas de Potabilización", Quito, 1996

MUÑOZ, M., HIDALGO, D., 1999: "Reducción de impactos ambientales en la industria del cuero", Quito, 1999

PATAKI, L.; ZAPP, E., 1988: "Basic analytical chemistry", Pergamon Press, Budapest, 1988

RENNER, G., 1994: “Abwässer aus Gerbereien und Möglichkeiten der Belastungs-verminderung”, Kolloquium an der TU Berlin “Abwässer aus der Zellstoffindustrie und der Lederherstellung”, Sonderforschungsbereich 193, 1994

ROJAS, C.; ZARATE, M., 1993: “Guia técnica para la minimización de residuos en curtiembres”, Lima,
1993

STOCKHAUSEN, 2000: “Ionicity of Praestol”, Degussa-Hüls Gruppe, Koefeld, 2000

TCHOBANOGLOUS, G., 1979: "Wastewater engineering - Disposal treatment", Metcalf & Eddy, Inc.,
New York, 1979

VALENCIA, J. A., 1992: "Teoría y práctica de la purificación del agua", Florida, 1992

VAN OLPHEN, H., 1977: "An introduction to Clay Colloid Chemistry", 2^nd ed., Wiley-Interscience, New
York, 1977

WEBER, W. J., 1972: "Physicochemical processes for water quality control", Wiley-Interscience, New
York, 1972

WHITE, A.; STRASSER, G., 1976: "Hierbas del Ecuador", ZIKR Publications, Quito, 1976