Capacity reduction caused by air intake at wastewater pumping stations

In the Netherlands, wastewater is normally collected in combined sewer system and pumped to the WWTP through pressure mains. These pressure mains are a part of the system that did not receive much attention lately with respect to monitoring of performance and maintenance. A recent inventory showed that about half of the pressure mains suffer from increased pressure loss for no obvious reason. Reduction of the systems nominal capacity may be caused by many things, such as increased wall roughness, scaling and occurrence of free gas in the pipeline. Free gas may be caused by degassing of dissolved gas, but also by air entrainment at the pump's inlet or at air valves. Experiments have been conducted with DN200 pipes to investigate the influence of entrapped gas on the head loss of wastewater systems on the transport phenomena of gas. The critical flow velocity to transport gas in downward inclined pipes is investigated as a function of the pipe angle and water flow rate. This paper describes the first results of the experiments.