Highly polluted wastewater is formed in dairies, pig farms and slaughterhouses. Before released into watercourses, wastewater should be properly processed with different treatment procedures in wastewater treatment plants. The thesis deals with the descriptions of mechanical, physical and chemical, and biological wastewater treatment procedures and the description of the factors which affect the reactions in wastewater treatment plants. I give special emphasis on anaerobic wastewater treatment. Its advantages over aerobic processes are smaller production of highly stabilized sludge, reduced need for nutrients, smaller necessary volumes of reactors, and production of methane, which is a potential source of energy. In this thesis I present water treatment plants of various companies Pomurske mlekarne d.d. (Prekmurske Dairies PLC), Panvita MIR d.d. (Panvita MIR PLC) and Panvita – prašičereja Nemščak d.o.o. (Panvita – pig farming Ltd).