This doctoral thesis was written within the framework of the INCO-DC project “EILT” (Reduction of Environmental Impact of Leather Tanneries). Members of the EILT working group investigated the input and output figures of 6 different tanneries (4 Chilean, 1 Ecuadorian and 1 Spanish). Based on the data which had been obtained by these input/output analyses, a flow sheet of a chromium tanning bovine hide tannery was drawn up and average consumption figures and quantities of waste water and solid waste were computed. All specific input and output figures (fresh water, chemicals, waste waters and solid wastes) are based on one ton of wet salted hide input. To reduce the dilemma of unavoidable inaccuracies, minimum and maximum figures are presented. The given ranges are practical rules of the thumb to estimate the input and output figures of a SME - tanning process. The obtained figures are discussed and proper segregation proposed. It is suggested to segregate effluents into 4 waste water streams: low polluted, high polluted, chromium bearing and sulphide bearing waste waters. Prevention, recycling & reuse and finally treatment opportunities are discussed for the respective effluent streams. Solid wastes might be segregated into 3 fractions: spent salt, non-chromium containing solid waste and chromium containing solid waste. Main stress is put on the treatment of the non-chromium containing solid waste fraction, which can be up to 80% of the global solid waste that is generated at a tannery. Aerobic biological treatment (composting) is proposed as a proper treatment technology. Practical trials on reactor composting of hair residues, fleshings and thermally treated fleshings were carried out. The results are presented in this work. The main parameters - substrate, bulking material, amendments, volume ratio, aeration rate, water balance and turning frequency – were investigated and their impact on the composting process is discussed. Finally, the product quality were analysed. Plant tolerance and the chemical composition of compost made out of hair residues, fleshings and thermally treated fleshings were investigated. That composting is a feasible alternative to other treatment technologies under certain conditions, is discussed in the final conclusion.