In the past, the pump regime of sewer pressure mains was based on the cycle frequency of the pump in combination with the storage capacity of the wet well of the pumping station. Modern pump units have higher pump cycle frequencies, enabling a reduction in size of the wet well of the pumping station. In view of construction costs of the pumping station, this trend of down-sizing is desired. A second development that took place in the Netherlands at the same time was the construction of mildly sloped, rather than steeply sloped pipe sections underneath obstacles such as railways and channels, due to a new drilling technique. A problem arises for these pressure mains when gas is present in the system. The long travel time through a mildly sloped pipe section, in combination with short pump operation, hinders the gas bubbles to travel through the sloped part. After pump stop, the gas bubbles return to the high point in the system, eventually a causing capacity reduction. The pumping station and its pump operating condition must be designed in close relation with the pipeline system. This paper describes and discusses new design aspects and proposes guidelines that have to be taken into account to avoid gas problems when designing a wastewater pressure main.@en

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.

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