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Tanning of hides and skins to convert them into leather can have a considerable environmental impact. Wastewater treatment technology can successfully purify leather wastewater. The first part of this research investigated the influence of Al2(SO4)3 concentration on the velocity of tannery wastewater settling. The wastewater sample was taken from two of the most polluted wastewater flows after liming and after chrome tanning. These wastewater flows were mixed together in a ratio of 1:1. The second part was carried out with the samples of wastewater mixed together with different concentrations of Al2(SO4)3. The behaviour of settling was investigated after an addition of different concentrations of anionic polyelectrolytes in these samples. The pH, suspended solids mass, volume of sediment, ((Na2S), ((Cr2O3), chemical oxygen demand, along with turbidity in the supernatant were determined in the obtained sludge. The results demonstrate the influence of Al2(SO4)3 and anionic polyelectrolyte concentrations on the parameters studied and the improvement of environmental properties.

Fig. 3. Time dependency of sediment volume after Al2(SO4)3 + polyelectrolyte precipitation. and coagulants. According to the settling velocity measurement, it can be concluded that the precipitation process was finished after 30 min. After precipitation using Al2(SO4)3, the pH value decreased (from 8.30 to 7.60), but after adding the polyelectrolyte, that decrease was smaller (from 8.40 to 8.30). Consequently, as pH value decreased, the volume of sediment increased in all samples (with or without the polyelectrolyte). In samples after precipitation with a mixture of Al2(SO4)3 and polyelectrolyte, the volume of sediment obtained was smaller, but that process formed ticked sludge. The average value of suspended solids mass in a sample with polyelectrolyte was 0.3736 g, and in the sample without polyelectrolyte 0.3611 g.