Optimization of waste water heat exchangers in the scope of building technology
11/2013 to 11/2015
Federal Ministry of Transport, Building and Urban Development (BMVBS)
The project on the optimization of waste water heat exchangers in the scope of building technology aims to study biofilm growth on heat exchangers and on the development of low-maintenance cleaning processes.
Without knowledge of the expected loss of heat transfer, it is not possible to install a suitable decentralized wastewater heat recovery system or to operate it efficiently. Also, a practical cleaning method is required to ensure comprehensive use of such a system. Therefore, the research project aims to study biofilms in domestic wastewater systems and to develop a practical fluidic cleaning method instead of high-maintenance mechanical cleaning processes. The central problem regarding heat recovery is to be seen in reduced heat exchange, caused by the biofilm that grows on the heat exchangers in the effluent stream. So far, biofilm growth has not been studied sufficiently. There is no accurate data on how domestic wastewater, gray water, and raw sewage contaminate heat exchangers – which is why the task of dimensioning heat exchangers is still quite problematic. Heat exchangers that are used centrally in a sewage system are designed with very low and therefore safe heat transfer coefficients. This leads to oversized heat exchangers. However, oversized heat exchangers lead to a less efficient central wastewater heat recovery, and, secondly, systems like this tend to be inefficient because the intermediate circulation pumps need more power. Therefore, the research project focuses on laboratory research and on processes to eliminate biofilm growth on heat exchangers. Due to high costs and considerable maintenance requirements, there are currently no mechanical cleaning processes that are suitable for heat exchangers in medium-size residential buildings. Apart from trying to determine characteristic properties of biofilms in domestic sewage systems, this research project also seeks to investigate possible cleaning methods that could serve to exploit the heat recovery potential that was detected in previous research.