Centralized or decentralized systems? This question is raised time and again by planning officers and local authorities. When adding together the costs for sewer connection and cleaning with those for the wastewater charges, it becomes apparent that in rural areas with lower population densities the connection to a central sewer treatment plant is the most expensive solution. Smaller decentralized treatment units made from precast concrete are usally a more economic alternative. Ecological treatment processes that are in harmony with the natural processes and are almost self-regulating after a short start-up phase and purify the wastewater biologically are particularly cost-efficient and compatible with the environment.