Flow resistance of wastewater pumping mains

A key requirement for the effective and economic design of sewer pumping stations is reliable information on the flow resistance of pipes carrying wastewater. The hydraulic resistance of such pipes can depend on a variety of factors, including the thickness of the biological slimes that build up in the pipe walls over a period of time, the size of the pipe, the velocity of the flow and characteristics of the sewage being pumped through the system. Current knowledge on this problem is limited and, therefore, a field study was undertaken of 16 pumping mains to collect data on in-service flow resistance. Data from each site were analysed to determine the hydraulic resistance of the pumping mains and the importance of parameters such as flow velocity and pipe diameter. It was found that the most important parameter affecting the hydraulic resistance in wastewater pumping mains is the flow velocity: as the flow velocity increases the hydraulic resistance decreases. Other parameters such as pipe material and diameter were also investigated, but the results showed no clear overall effect on the resistance values. From these results a predictive equation was developed that relates the flow velocity to the best-fit value of hydraulic resistance. The variability of the results, due to pipe sliming and other uncertainties, is considered by identifying upper and lower bounds for the prediction equations based on the spread of the data collected.