LCA is a popular tool widely used to assess the environmental impact of waste management systems, which is illustrated by the substantial number of LCA computer models specifically addressing this subject. Due to the complex nature of waste management modelling and the range of country-specific data, as well as lack of harmonization, it has been observed that there are large discrepancies between the results using different models. Many studies have underlined the necessity of clearly identifying both the scope and methodological assumptions of LCAs in order to have confidence in the results. Therefore, the paper presented here reveals several methodology-related issues. The study tests two different pieces of LCA software, i.e. IWM-2 (designed specifically for MSW) and SimaPro (a generic and widely used LCA software). The pieces of software were used to LCA an MSW scenario and the results obtained (calculated using Ecoindicator’99 H/A) were compared to show the strengths and weaknesses of these tools, i.e., generic software usually treats the waste as a set of separate fractions, not as a whole mass, which means that the software is not highly sensitive to the composition of the waste and does not take into account the environmental impacts produced as a result of the interaction between the waste components after mixing. As waste composition is very important in planning, one study combines these two software packages to get final results, i.e., data generated by IWM-2 were entered into SimaPro. The discussion is built around a case study in Poland where waste management scenarios have been analyzed. The research carried out has shown that having the same initial inventory data collected on the basis of the same assumptions and with the same boundaries to the system model used and using the same method of LCIA to assess the impact on the environment, may not produce the same end results. In the presented study, the main differences in the LCIA results appeared in four output-related impact categories: carcinogens, climate change, ecotoxicity, and eutrophication/acidification, and for one input related impact category – fossil fuels. Four reasons responsible for these differences are identified:
(1) The IWM-2 program identified a smaller number of substances emitted to air and water associated with landfill and recycling than the Ecoinvent database (IWM-2 identified a total of 31 types of emissions to air and water for landfill while Ecoinvent identified 405 types, IWM-2 identified 39 types of emissions for recycling while Ecoinvent identified 403 types)
(2) The IWM-2 program did not cover emissions to the soil, while the Ecoinvent database identified 60 types of such impact for landfill and 58 for recycling
(3) The IWM-2 program does not cover consumption of resources, while the Ecoinvent database covered the use of 198 kinds of raw material (including 100 different minerals and fossil fuels)
(4) In each case a different total mass of emissions and resources consumed was identified in the analysis of the inventory included in both analyses.