LeadNovel decentralized reactors can be the key to a more efficient and affordable wastewater treat-ment in cities. This project investigates, whether a new biological process - partial nitritation and anammox - is suitable for nitrogen removal in on-site reactors.BackgroundCombining sewers and centralized wastewater treatment has worked reasonably well for the industrialized part of the world, but for a number of reasons the success in developing and fast industrializing countries has been very limited. Recent research has shown that separation of wastewater streams at the source and decentralized treatment would be more resource-efficient, cheaper and easier to implement. Especially the separate treatment of urine would strongly reduce environmental pollution. Lately, a biological process - one-stage partial nitrita-tion and anammox - has been developed that removes nitrogen more efficiently than conven-tional processes. Lab experiments have shown that the process is suitable to treat urine, but a better understanding of the microbial transformation processes is needed to operate reliably on-site reactors.MethodsPilot reactors - rotating biological contactors - will be operated under controlled but changing environmental conditions. The main process parameters will be monitored continuously. Ge-netic information about the microbial community and the microbial activity will be measured with microbiological methods. To determine the requirements for process stability, the datasets will be evaluated with computer models.GoalsThe overall goal of the project is to determine the conditions for stable biological nitrogen re-moval from urine in small on-site reactors. We want to understand how the microbial commu-nity reacts on sudden changes in environmental conditions. We aim to analyze the importance of microbial diversity for process stability and we want to elucidate which factors determine whether the bacterial interactions support or destabilize the nitrogen removal process. Special emphasis will be put on the conditions that foster the release of harmful intermediates such as nitric oxide (NO) and nitrous oxide (N2O).SignificanceDecentralized reactors for nutrient removal can be essential tools to fight the deterioration of aquatic environments worldwide. From a scientific point of view, combining biotechnology and microbial ecology is a highly promising approach to better understand stress and stability in biological processes. A better understanding of the microbial interactions will not only help to increase process stability, it will also help to reduce emissions of environmentally harmful gases such as NO and N2O.