Abstract: A heat exchanger system extracts heat from sewer waste water. Preferably, a first surface is narrow band in an interior surface of a pipe, and the band is preferably oriented transverse to a direction of fluid flow. In the case of a horizontal pipe, the band may be a partial band, and may be limited substantially to a wetted portion of the pipe in order to reduce the size of the band.

Abstract: The heat exchanger system for extracting heat from waste water comprises a surface area (5, 11, 21) made of copper, of a copper alloy or of a copper compound. The area of these surface areas amounts to solely a fraction of the heat exchanger surface area. In that waste water flows over the surface areas, the formation of a sewer biofilm on the heat exchanger surface is prevented or stopped. Preferably, the surface areas are formed by the surfaces of bands of copper, of a copper alloy or of a copper compound extending essentially in a direction transverse to the direction of flow.