Eutrophication of water courses, lakes, and marine environments is a major issue in most parts of the world. Looking back 150 years the urban situation in the emerging industrial part of the world led to the introduction of water-based systems for conveying and discharge of sewage. At first the wastewater was disposed into nearby watercourses and lakes. As the populations grow, this was not a sustainable solution - the natural wetlands became overloaded as evident from the odors. This untenable situation led to the development of more active treatment systems like shallow ponds and sand filters. In 1914 the activated sludge technique was introduced by Arden and Lockett, a technique that still probably is the most common technique for wastewater treatment (WWT) in the industrial part of world. In the 1960s eutrophication became evident due to the high amounts of plant nutrients discharged from sewage treatment plants. The first and maybe the simplest solution was to remove phosphorus by chemical precipitation. The European Commission and national authorities have gradually over the latest couple of decades sharpened the treatment demands, especially with regard to nitrogen, in order to avoid further eutrophication in the sea. Hence, WWT today probably is more focused on removing phosphorus and nitrogen than pathogens. It is still argued whether phosphorus or nitrogen is limiting for the eutro-phication process, that is, should either one or both of these elements be eliminated.

Simply put, biological WWT can be defined as a natural process in which organisms assist in environmental cleanup simply through their own life-sustaining activities. By studying the organisms in natural ecosystems the biologists have explored their function and capacity to degrade organic matter and transform nutrients. Such information has then been used by engineers to design effective WWT systems, that is, the biological processes have been concentrated into well-regulated units. In addition, knowledge of geochemistry, hydrology, etc., is essential component of a successful system for treating polluted waters. Hence, globally, WWT probably is the most common biotechnological process.

Though the same biological processes are the basis for most WWT systems, the number of technological solutions for achieving the goal probably is innumerable. The numbers of techniques are as many as there are sanitary engineers. However, the techniques may be categorized as follows: (1) soil filters and wetlands -terrestrial ecosystems working as natural filters; natural water courses, lakes, and wetlands; soils receiving irrigated wastewater; constructed wetlands and ponds; soil or sand absorption systems; and trickling filters; and (2) treatment plants - rotating biological contactors; fluidized beds; and activated sludge systems including sequencing batch reactors (SBRs). This array of techniques describes the systems on a scale from natural ecosystems at one end to high-technology solutions at the other end. In the choice of WWT system to be used many factors have to be considered like influent water characteristics, desirable effluent water quality, costs for building and maintenance, and population density and dimensioning.

In this article we have chosen first to give a general background on the microbial cell and biological processes important in all WWT and, second, to focus on the importance ofunderstanding the interaction between hydraulic performance and microbial processes to achieve effective nitrogen removal, and third, to outline the function oftwo common systems: the constructed wetland, requiring in-depth knowledge on hydraulic properties, and the activated sludge process, relying on advanced control and optimization. Finally, we give some perspectives on the future development of biological WWT systems and their use.

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