Résumé : |
In recent years, legisltators in many different countries have been pressing ever harder for the treatment of tannery effluents. Apart from the chromium III load, it is the protein Joad from the beamhouse effluents which is seen as a cause for alarm. This protein load consists of dissolved and undissolved substances like hair sludge and machine fleshings, e. g., which result in high COD. A review of the individual process steps quickly showed that the largest quantities of dissolved and undissolved organic substances are obtained during liming. The distribution of the COD values up to tanning inclusive is shown in Fig. 1.
The rising costs of waste water treatment and sludge disposai induced tanners to consider very carefully whether a hair-save liming process could substantially reduce the effluent load. Before we discuss the existing hair-save processes in terms of effluent load, technical outlay and feasibility, let us see whan an optimum hair-save process can achieve.
To answer this question, we took two pieces of cowhide. From one piece we removed the hair mechanically, whereas the other piece was subjected to a conventional hair-burn liming process. The results are shown in Fig. 2. The maximum reduction of the contamin-ants load COD to be expected from an optimum hair-save lime is about 55 - 60%, corres-ponding to an overall reduction of about 30% up to chrome tanning inclusive. Apart from reducing the COD value, however, hair-save processes will also cut down on sulphide, nitrogen and hair sludge. |