NA-06 - Cleaning: Flushing Methods

- Back to outline
- Back to Modules

Overview of the cleaning methods
Criteria for the selection of the cleaning process...
Flushing methods
Surge flushing
Back-up flushing
End of module

Practical example I

Use parameters

The use parameters for an automatic flush gate are determined by the dry weather water flow.

Image:

Average shear stress produced by a flush gate released surge wave [Dohma2002a]

Dohma2002a

Dohmann, M., Dettmar, J.: Untersuchungen zur optimierten Kanalreinigung im Hinblick auf eine Minimierung biogener Schwefelsäurekorrosion und eine Verhinderung von Schmutzfrachtstößen bei der Abwasserbehandlung und Mischwasserentlastung, Abschlussbericht zum Forschungsvorhaben des MUNLV NRW, Institut für Siedlungswasserwirtschaft der RWTH Aachen (2002).

The chart illustrates the average wall shear stresses in the invert caused by a downstream wave created by an automatic flush gate in a combined sewer DN 2,000. They are based on flow velocities and water level heights of the surge wave measured in three different locations.

/ 103

Under certain circumstances leakage rates that might impede the retention upstream from the flush gate have to be anticipated because a perfect seal cannot always be created. This situation can occur given a low dry weather flow, preventing the flush gate to function during this period. Furthermore, there is the risk of deposit formations in the area of the fixed metal frame and the swivel joint.

The shear stress distributions in the surge wave, with peak values between 6 and 13 N/m², are many times higher than the value determined in dry weather of τ_max = 0.9 N/m². In this case, the deposits could be removed within seven days from a 150 m long section with 120 surge waves [Dohma2002a].