Methodology
54
S: Wetted cross-section of the screen with : S =
𝑄 𝑅𝑤𝑝
𝑣
(m
2 ).
We have : 𝑸 𝑹𝒘𝒑 :Rainy weather peak flow (We have : 𝑸 𝑹𝒘𝒑 = 0,31m
3 /s).
V: Raw water flow velocity at inlet (weassume that V=1m/s).
α : Angle of inclination of the screen in relative to the horizontal (generally
α=60°)
𝑯 𝒎𝒂𝒙 : Maximum admissible height of water on the screen.With :
𝑯 𝒎𝒂𝒙 = 0.4m for 20000IE.
𝑯 𝒎𝒂𝒙 = 0.5m for 50,000 IE.
𝑯 𝒎𝒂𝒙 = 0.6m for 100000 IE.
In our case, we assume : 𝑯 𝒎𝒂𝒙 =0.5 m.
δ: Clogging coefficient of the screen (δ = 0.5 for a mechanical grille, and 0.25
for a manual screen, we chose δ = 0.5).
β : Fraction of surface occupied by bars. With:
β =
𝒃
𝒃+𝒆
we have :
b: Grid bar thickness (b=20mm).
e : Spacing between bars (e=50mm).
6.5.1.2.2. Fine screen
After lifting (an optional operation, depending on the site), the effluent will reach the
end of a screening channel equipped with an automatic fine screen. This screen, with its 15mm
spacing between bars, stops larger suspended matter escaping from the coarse screen.
To calculate the width of the screen, we apply the KIRSCHMER formula with:
W =
𝑺×𝒔𝒊𝒏 𝜶
𝑯 𝒎𝒂𝒙 ×𝜹(𝟏−𝜷)
With :
b = 10 mm
v = 1 m/s
α =60°
e =15 mm
𝑯 𝒎𝒂𝒙 = 0,5 m
𝑸 𝑹𝒘𝒑 =0,31 m³/s