Pollution control of tannery effluents

From an End-of-Pipe Treatment System to In-Plant Methods

by Jurgen Pors

Depending on the raw materials, processes and end-products involved, the constituents of wastes from tannery operations vary in both quantity and importance. They include hair, hide scraps, pieces of flesh, blood, manure, dirt, salt, lime, soluble and insoluble proteins, sulphides, amines, chromium salts, tannins, soda ash, sugars and starches, oils, fats, greases, surfactants, mineral acids, dyes and solvents. The author discusses methods of controlling the most harmful pollutants.

Chrome tanning is carried out in rotating drums with chromium (III) sulphate hydrate, starting at pH3.5 or lower and ending at almost pH 4 as a result of basification. The chromium (Cr) penetrates the pelts, where it is bound and cross-links the hide collagen molecules. About 6080 % of the chromium in the bath is bound. The residual float contains about 6 g of Cr oxide per litre (20 - 40% of the initial amount if no further additives are used.
Under normal conditions the tanning bath is by no means exhausted. This is why measures to improve chromium fixation in the leather and to recover chromium from the residual float are of great interest from both an economic and an ecological point of view.
Chromium fixation can be improved by a higher level of basification, higher temperature, longer tanning periods and more sophisticated chromium compounds. These measures will lower the residual chromium concentration, but not sufficiently: chromium will still be found in the sludge residue after effluent treatment. Moreover, the quality of the leather produced by using these costly special agents (high exhaustion techniques) cannot be maintained in the same way as when the conventional chrome tanning process with 30% excess chromium sulphate is used.
This is why tanners aiming for high quality and environmentally safe processes use the conventional method, recovering the excess chromium from the used liquids by precipitation (addition of alkaline substances to increase the pH) and redissolution in sulphuric acid. The chromium can thus be reused for tanning.
After chromium recovery the residual effluents meet all national and international standard limits.
For the production of heavy leather, such as sole, mechanical and saddle leather, vegetable tanning is employed - the solution contains extracts from the bark of trees. The effluents from vegetable tanning consists of tannins which are difficult to break down, cannot be removed as easily as chromium and are intensively colored, causing a non-toxic but highly non-degradable pollution load.

Chromium and sulphide

Since chromium and sulphide are the two most important and harmful pollutants, they have to be removed from the effluents. This should be done as close as possible to the point where they are generated. The widespread belief that a spent chrome tanning bath is waste should also be refuted as quickly as possible. Such liquids usually contain a quantity of chemicals: chromium (III) salts are potentially toxic - the risk of oxidation to chromate cannot always be excluded - and are valuable substances which should not be wasted. Thus, not only environmental but also economic considerations are important.
Other constituents of waste water are said to be reduced by pretreatment in the plant itself: screening of solids and subsequent neutralization of alkaline partial streams, followed by presedimentation, can be accomplished without any costly or highly sophisticated techniques. Even small tanneries can implement these pretreatment steps without any major investment.

Effluent control

Only if every leather processing plant carries out these pretreatment stages can subequent chemical and biological treatment in an ordinary effluent treatment plant be efficient and economical.
The present state of the art in effluent control and treatment in Germany is a result of the principle of treating effluents and pollutants as close as possible to the point where they are generated. Only in this way can dilution or combination with other effluent streams be avoided. Treatment, recovery or detoxification of pollutants can be carried out with a minimum of equipment and at minimum cost immediately following the point of generation.
Regulations should therefore stipulate not only that waste water streams must be separated, but also how the separated streams are to be treated, naturally in combination with different standard limit values for the different constituents. Furthermore, regulations should define minimum technical requirements, specifying which technology has to be employed in order to meet them.
The approach to pollution control must therefore be changed, from end-of-pipe treatment systems to in-plant methods.
The recommendations in Table I were made for the treatment of effluents and solid wastes from a cluster of tanneries comprising numerous very small ("cottage"), some small and a few medium-sized tanneries in the vicinity of Bogota, Columbia.
All effluents containing chromium should be collected separately and the chromium recovered for reuse in the process after adding fresh chromium salt.

Chromium recycling

Larger tanneries can afford to buy a recycling unit of their own. The minimum size is normally for between 500 and 1000 cattle hides per day. A prefabricated unit from a western country costs at least US$ 60,000 - 100,000. However, a plant can be produced in practically any country using locally available materials.

Table 1: Principles of Treatment of Tannery Effluents

Chromium recycling is practiced with great success, whether in shared or decentralized plants: for example in Italy, where a large shared chromium recycling plant has been In operation for many years producing more than 21 tons of chromium a day, recovered from the collected spent floats of more than 150 tannieres in the area. The same system has recently been put into operation in Portugal. Large and medium-sized tanneries known to the author in India, China, Colombia and Brazil are also using this system successfully.
In Germany, a large and well known producer of top quality leather has been running a chromium recycling plant since 1923