Understanding the impact of biofilm growth on pipe roughness

Abstract

This paper will examine the increase in pipe roughness with time that can occur from biofilm build up in raw water, treated water and sewage pumping mains. Various methods exist which accurately predict the diameter of the pipe and the size of the pump needed for the most optimum and cost effective system. These methods use a specific range of pipe friction factors and it is assumed that this friction factor will represent the system roughness over the design lifetime of the system. However, pipes systems are subject to changes over time which can adversely affect performance and cause the system to run inefficiently. One of these changes is the build up of biological growth in the pipes and is most prevalent in systems used to transport raw or recycled water as the biofilms require nutrients to survive. Experiments were conducted in the hydraulics laboratory on small pipelines using raw water sources with high carbon content. The change in roughness and flow velocity were recorded with time. It was found that these biofilms can grow relatively quickly and depend upon initial flow velocity and turbulence in the pipe system. Experimentally observed growth curves are given to illustrate the rapid growth phase and then a maturing phase of the biofilm. Changes in the velocity illustrated that different ultimate friction factors can be achieved. Friction factors and equivalent roughness heights can be determined and compared with typical design values and also actual measurements of the biofilm thickness. Measurements over a range of Reynolds numbers of the friction factor of a fully developed biofilm illustrated that the friction factor initially decreased with increasing Reynolds numbers and then following a minimum value began to increase sharply with increasing Reynolds number. This behaviour can be explained by the thinning of the viscous sublayer and the exposure of the rough biofilm surface to the turbulent flow.