Title: RECOVERY OF CHROMIUM FROM TANNERY WASTEWATERS

by : Dimitrios TSOTSOS Chemical Engineer

Ministry for Environment, Physical Planning and Public Works Environmental Planning Division Patission 147 112 51 Athens Greece

Abstract:

Chromium discharged as effluent in concentrations of 2.000 - 3.500 mg/l, is recovered and recycled back into the tanning process by using a method developed by the Dutch institute TNO: the collected wastewaters are treated with MgO (coagulation - flocculation - sedimentation) and the produced sludge is redisolved in sulfuric acid. This solution is fed back into the tanning drums by standartized technique, thus resulting in savings of "fresh" chromium quantities of up to 30 - 35%.

The removal. efficiency is almost complete, producing a clear supernatant with a chromium content of less of 2 mg/l which can be reused f o r washing and cleaning purposes.

This project is now in progress, being elaborated in a Greek tannery in cooperation of Greek authorities with Dutch counterparts and the EEC.

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INTRODUCTION

The tanning production process is practically the transformation of raw animal hides into leather and related products.

The hides are converted into leather (tanning) by means of tanning agents which are either natural organic substances (tannins) or Crg0’ - salts. The discharged effluents are some of the most polluting industrial wastes. Their treatment and disposal is therefore vital for the protection of the environment. This problem becomes more critical for countries where most of the plants are either small-size units with small economic background, or wherg they a r e located near watei- bodies and the effluents are discharged without the appropriate treatment.

DESCRITION OF THE PRODUCTION PROCESS - - WASTE SOURCES

The production process can be separated in two parts: preparation of the hides to receive the tanning agents (beam - house processes) and the tanning process itself.

BEAM-HOIJSE PROCESSES

1 . Washins - Soakinq

The hides are first washed to remove dirt and blood, and then soaked for the removal of salt and for scfteniny. The produced wastewaters contain a high organic load (BOD,) due to the dirt and blood along with a high concentration of salt.

2. Fleshinq

The next step is the removal of muscle and fatty tissue adhering to the corium l a y e r by means of revolving knives while a continuous stream of water

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carries away the produced impurities. The wastewaters from this process contain fat and fleshy particles.

3. Liminq

Liming is done to swell the hides for the better penetration of the tanning agents and for the hair removal. The hides are placed in vats containing a lime suspension. Small quantities of sodium sulfide are added for the acceleration of the process. These waters are heavily polluted with high concentrations of sodium sulfide, lime and organic matter.

4. Unhairins - Washinq

The unhairing of the hides is accomplished mechanically by means of rolling knives. Large amounts of water are used to flush out loosened hairs and excess lime. The related wastewaters contain fine hair epidermal particles and lime.

5. Delimins - Batinq

The hides are washed and then subjected to a process known as bating. During this process some pr-oteics, such as elastin, are hydrolyzed in order to avoid any negative effects on the quality of the final product.

TAN-YARD PROCESSES

After bating, the hides are ready for tanning. Two principal methods of tanning are used according to the related tanning agents: vegetable tanning and chrome tanning.

1. Vesetable tanninq

The tanning operation is carried out in vats where the hides are kept in contact with the tanning solutions for several days.

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The waste discharge is intermittent and heavily polluted with organic load and colour.

2. Chrome tanninq

For the chrome tanning process the bated hides are first soaked in a solution of sulfuric acid and salt, for 10-16 hours. This operation (picklinq) helps the better absorption of the chrome salts into the pores and tissues of the skins. After pickling, the hides are kept in contact with the tanning solution containing either salts of Cr:' or sodium dichromate which is reduced to Crxx* by reducing agents. The effluents contain high concentrations of chromium salts.

Fi ni shi nq

After tanning a series of operations known as f inishinq processes are carried out to produce different types of leather (oiling, dyeing etc.).

WASTEWATER CHARACTERISTICS

On Table 1, a rough distribution of the quantity and quality of the wastewaters in a chrome tannery is shown as a percentage of the total quantity.

TABLE 1

(lr:x.+~ Process Volume of BOD-, -..

wastewater ( % I ( 4 6 ) ( % I ( % I

Beam-house Ta n-yard F i n i sh i ng

- 90 100 5 5 - 99-100 5 5 - 0-1

90

Table 2 shows a similar distribution f o r a tannery using tannins as tanning agents.

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TABLE 2

Process Volume of wastewater BO DE^ ( % I ( % I

Beam-house Tan-yard Fi n i sh i ng

90 5 5

42 47 11

Table 3 shows a detailed flow and load distribution within a tannery.

TABLE 3

Qualitative and uuantitative data of tannery wastes

1%

! b

5

r J

5

5

2 . 4

5.0

3 . 2

3.9

The main pollutants found in tannery wastewaters

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are the high concentrations of organic matter, suspended particles and salt. Two other constituents, chromium salts and sulfides demand special attention in order to prevent their negative effect on the environment.

POLLUTION CONTROL TECHNOLOGY

Several treatment methods are generally applied with various efficiency grades.

Tables 4 shows the related figures.

TABLE 4

Treatment Removal Efficien-

Pollutant reduction efficiency (96)

BOD::? ss Chromium Sulfide

Screening 5 5-10 0 0

Sedimentation 25-62 69-96 5-30 5-20

Chemi ca 1 coagulation 41-70 70-97 50-80 14-50

Trickling filtration 65-80 85-90 25-75 75-100

Activated sludge 85-80 80-95 75 75-100

Preventive measures - Treatment methods. The elementary method mainly used in small tanneries consists of the by-products recovery such as flesh, hair etc. while the use of definite water amounts instead of a stream of water in the beam-house processes (washing) helps to reduce the produced wastewater volume.

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The equalization of the flow followed by sedimentation, helps to remove part of the suspended solids (40 - 50%) and a small fraction of BODs (30%).

Coaqulation with lime or iron and aluminium salts is also frequently used with various removal efficiencies. However, the produced amount of sludge creates another serious problem.

The Dutch research institute TNO has developed an alternative solution for the BODyd removal without any biolocrical treatment using the fact that 90% of the organic matter comes from the beam-house process and consists mainly of dissolved proteins: After the stream segregation from the tan-yard and the sulfide removal, the pH is requlated to 3.5 - 4.0 by adding hydrochloride or sulphuric acid. This acidification of the wastes helps the flocculation of the dissolved proteins and the production of a floating "cake" which can easily be removed.

Biolocrical treatment is a generally accepted method for tannery effluents concerning their organic load and sulfide content. There are controversial opinions about the chromium removal efficiency achieved by related biological treatment plants claiming that chromium accumulates in the biological sludge in the form of settleable solids thus being withdrawn from the liquid wastes, whereas the opposite assumption is based on the only slight alkaline environment of the biological treatment systems (pH 7 . 5 - 8 ) which prohibits the quantitative precipitation of chromium salts.

Combined treatment of tannery effluents with municipal wastewaters leads to acceptable results when the hydraulic load coming out from the tanneries is kept below 20% of the total quantity to be treated.

In any case the removal of chromium is a necessary step prior to any further treatment.

CHROMIUM RECOVERY

CrI I x salts are widely used as tanning agents for

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the production of durable and stable leather goods, however their discharge over the effluents can cause severe damages to the environment. Therefore strict effluent limitations have been adopted ( 2 - 5 mg/l) prior to their discharge either into sewers or directly to water bodies.

A n effective and attractive method for the complete removal of chromium constituents is their recovery from the effluents and their recycling in the production (tanning) process.

A related demonstration project is now in progress in a Greek tannery which is being elaborated in close cooperation o f Greek and Dutch authorities and institutions and the European Economic Community ( E E C ) , which is covering part of the cc3sts.

The main scopes of this project are the recvclinq of chromium and the reduction of water consumption in the tannery.

The applied technology is developed by the Dutch institute TNO which is the technical project leader together with the Greek Ministry for Environment, Physical Planning and Public Works.

The technical procedure is based on the following aspects:

The tan-yard effluents (Cr - content: 3200 mg/l) are separated from all other wastewaters and collected in a sedimentation tank where they are treated with magnesium oxide (MgO) . All chromium salts are then converted into chx-mium hydroxide (Cr - sludge which is accumulated on the bot.tom of the tank. After the discharge of t h e clear supernatant with Cr-content less than 2 m y / l , this sludge is redisolved in sulfuric acid (H: , .SOL~) and the produced solution is fed back into the tanning drums replacing the required "fresh" chromium quantities of up to 35%.

The clear effluent can be used for- washing and cleaning purposes whereas 13 further reduct ion of water consumption c a n be achieved by edjust ing the tanning receipe to skin wsshinq procedures instead o f rinsinq.

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t

Technical data

- Daily production

- Chromium consumption ( a s Cr-compounds)

- Chromium consumption ( a s Cr)

- Chromium quantity in the effluents (to be recycled)

- Chromium concentrc tion in the effluents (before treatment)

- Chromium concentra- tion i n the effluents (after treatment)

- Tanning effluent quant i ty

Economical data

Installation costs

Value of .recycled chromium' quantity (7.5 $/kg1

Operational costs (Chemicals, energy costs, manpower)

Net savings

8 tonnes

400 kg/day

2 2 . 4 kg/day ( 5 . 6 ton/year)

3.200 mg/l

7.500.000 Gdrs (50.000 $1

6.340.000 Gdrs (42 .000 $ )

1.500.000 Gdrs (10.000 $1

4 .840.000 Gdrs (32 .000 $1

Oruanizational arranaements

The demonstration project which is now being implenisnted in Greece is a good example of a smooth

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cooperation between several parties. According to the programme formulation the following organizational scheme has been developed:

Ministry for Environment (Greece)

Institute TNO (The Netherlands)

Hellenic Leather Center (Greece)

Ministry for Environment (The Nether 1 ands

Germanakos tannery (Greece)

European Community (EEC)

: Pro iect leader responsible for the whole project

: Project leader responsible for technical aspects

: Laboratory support - f inanciel arrangements

: Financial - Organizational support

: Construction - operation of the recycling installation

: Financial support

The total budget amounts 24.000.000 Gdrs (160.000 $ 1 covering all aspects of the project: collection of data, technical design, laboratory analysis, construction - operation, transfer of knowledge, travelling.

CONCLUSIONS

This demonstration project combines on a very effective way the requirements of minimized wastewater discharge with economical benefits for the industry, so that the proper functioning and operation of the related facilities is not only a matter of control of the environmental authorities but also closely linked with the industry's economic interest (raw material saving).

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In parallel it will help to overcome some misunderstandings which are widely spread concerning pollution control, namely that biolosical treatmes plants d1-e capable for the removal of any chemical substance from wastewaters and that coaqulation - flocculdtion techniques are the only existing alternative scslut ion.

As a matter of principle the reduction of pollution load at the source and the recycling - recovery of valuable materials from the effluents prior to discharge should always be the main concern of managers engineers etc. by tackling industrial pollution problems and only if all possible techno-economical possibilities are exhausted, end - of - pipe - technologies should be applied.

In countries where t he POLLUTER-PAYS-PRINCIPLE (PPP) has been widely adopted, the minimization of pollution load discharge into the sewer by using recycling -- recovery techniques will cut down the usual fees paid for being connected with the wastewater collection and treatment system. It should be kept in mind that the adoption of environmental taxes is a crucial aspect of equal competition conditions among industrial branches: the industrialized countries are increasingly complaining that industries in developing countries create favourable economic environment for their industries (lower costs) by neglecting to apply strict pollution control regulations. And this happens not because natural resources are abundantly available there, but mainly d u e to financial difficulties and ignorance of nature's restricted abilities to cope with man's disturbing activities.

Therefore the direction to be followed is clear: waste minimization at the source, recycling of valuable material, economy linked wit.h environmental protection.

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I

STRATEGIES IN THE FLEMISH

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