The thesis is based on independent chemical analyses, on-site analyses and field sampling. It includes seven selected wastewater treatment plants at mountain lodges. At each plant the author conducted three sets of winter analyses and two sets of summer analyses. The research includes 200 hours of field work in the mountains. Before conducting the analyses it was necessary to catalogue all wastewater treatment plants at Slovenian Alpine Association's mountain lodges and on this basis choose a representative sample of wastewater treatment plants for further research. The thesis interfer widely with Slovenian environment with exact, analytical review of Slovenian legislation about small wastewater treatment plants. The results are generally applicable to all small wastewater treatment plants while mountain-related are stressed separately. The thesis monitored 20 parameters at inflow and outflow of wastewater treatment plants all around Slovenia. Propositional improvements for concrete plants are made on this data. The great amount of data acquired in the research is of great help for further scientific research. The thesis also includes proposals and questions for improvement of Slovenian legislation, proposals for wastewater treatment plants producers as well as proposals to improve and to make on-site samplings and measures uniform. It also includes proposals for further investors of wastewater treatment plants in mountain environment.