The project was connected with the valorization of black liquor from paper pulp for leather treatment process. The main goal of the project was using the black liquor to prepare a compact product for leather retanning process and finding its environmental impact on leather treatment process in comparison with standard process. There were made 10 leather retanning trials. One of them was a standard retanning process, to which compact retanning processes were compared. Compact processes varied through amount of added compact product, kind of compact product, time of dye action (75 – 180min), temperature of retanning process (50 – 60ºC) and time of formic acid action (30 – 90min). The compact retanning process was distinguished by the fact that compact product was added instead the various products used in the standard retanning process and the quantity of water used was less. Leather and wastewater from the different processes were evaluated. Results showed that CP4 process represents the lowest values of COD (chemical oxygen demand): 9,37 mg/g of leather, TS (total solids): 86,04 mg/g of leather, SS (suspended solids): 3,14 mg/g of leather and the best values of mechanical resistance in comparison with standard process and remaining compact processes. Having these results, it can be concluded that connection of the lower amount of compact product, the lower temperature of retanning process, the shorter time of dye action and the longer time of formic acid action, allows to obtain a lower negative impact on environment and the highest tear and cracking resistance of leather.