Biofilm refers to a population of various micro-organisms trapped in a layer of slime and organic matter that attaches itself to the surfaces of pumps, pipes, walls etc.

On the inside of sewer piping, biofilm may affect the transport and quality of wastewater. The biofilm takes up space, reducing the effective diameter of the pipe, and increases pipe friction. Both factors increase system resistance, which results in increased power consumption and decreased flow.

Biofilm begins to develop when free-floating micro-organisms attach to a surface. If they are not immediately separated from the surface, they can anchor more permanently. The first colonists facilitate the arrival of other cells and begin to build the matrix that holds the biofilm together.

Once colonisation has begun, the biofilm grows through a combination of cell division and recruitment of new cells. The micro-organism population forms a microenvironment where degradation of organic matter and nutrients provides the micro-organisms with energy and material for cell growth and proliferation.
If anaerobic conditions develop in the biofilm, certain micro-organisms produce foul smelling gasses, such as methane and hydrogen sulphide, as by-product of their anaerobic respiration.

This may cause odour and health problems in connection with the sewer network and at the wastewater treatment plant.