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Design of storage sedimentation
basins (Raf Bouteligier) |
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In accordance with the design method for
a sedimentation basin at a waste water treatment plant,
the design of a storage sedimentation basin is based
in practice on a steady state approach. A design discharge
is chosen and an ideal two-dimensional uniform flow
is assumed. Then, the three design parameters are chosen
: the surface load, the residence time and the length/width-ratio.
The surface load is the most important parameter and
is defined as the discharge divided by the horizontal
area of the basin. In an ideal laminar flow a particle
with a settling velocity equal to the surface load will
just settle. Often a surface load of 5 or 10 m/h is
chosen. For the residence time often a value between
20 and 30 minutes is taken. The residence time mainly
determines the storage effect. The length/width-ratio
must be at least 4 in order to limit the divergence
from uniform flow. With these three parameters the geometry
is fixed, but some other control parameters must be
checked, e.g. Reynolds and Froude number, depth, depth/length-ratio
and horizontal velocity.
In reality, storage sedimentation
basins rarely reach steady state conditions. At the beginning
of a storm the basin will be mostly (partially) empty
and the filling of the basin introduces very turbulent
flow with even waves moving through the basin. This has
a negative effect on the sedimentation. In order to still
the flow during the filling of the basin an adverse bottom
slope can be used. For this an adverse bottom slope of
2 % to 4 % is sufficient, which is often less than the
slope required for the cleaning of the basin. In addition
CFD calculations show that the placement of the entering
pipe can have a large influence on the sedimentation efficiency.
The placement of the entering pipe is important to obtain
a good uniform flow over the whole width of the basin.
The internal weir at the entrance of the basin must be
approximately 3/4 of the water height to optimise the
flow over the depth. A baffle at the beginning of the
basin can help to divide the flow better over the depth,
but the effect on the sedimentation is small.
Part of this research was
funded by IWT.
Full paper 'Design of storage
sedimentation basins' (1995) at 2nd International Conference
on Innovative Technologies in Urban Storm Drainage, Lyon,
France.
Full paper 'The efficiency
of a storage sedimentation tank : numerical simulation
and physical modelling' (1998) at 4th International Conference
on Urban Drainage Modelling, London, UK.
Full paper 'The
feasibility of flocculation in a storage sedimentation
basin' (1999) in Water
Science & Technology.
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Conceptual model for the impact
assessment of storage sedimentation basins (Patrick Willems,
Raf Bouteligier) |
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Storage sedimentation basins are in practice
designed based on steady state assumptions and for one
specific design discharge. However, for a reliable impact
assessment at combined sewer overflows, long term simulations
with statistical evaluation afterwards are necessary.
To make long term simulations possible for storage sedimentation
basins, a conceptual model has been developed. This
conceptual model is based on uniform flow, but takes
into account anomalies due to non-uniform flow and turbulence,
based on experiments in a physical model and simulations
with Computational Fluid Dynamics. This model can deal
with time varying input and can thus be used for continuous
simulations. The model has been verified with the measurements
on two real basins.
Part of this research was funded by
IWT.
Full paper 'A conceptual model for the
impact assessment of storage sedimentation basins' (1999)
at 8th International Conference on Urban Storm Drainage,
Sydney, Australia.
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Improved overflow structures (Raf
Bouteligier) |
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One possible measure which can be taken
to reduce the effect of combined sewer overflows on
receiving waters, is making use of improved overflow
structures. In the laboratory physical model tests have
been performed in order to find mathematical expressions
for the removal efficiency of such improved overflow
structures. Different types of structures have been
tested: a hydrodynamic separator, a high side weir overflow
(single and double) and a vortex overflow with peripheral
spill. The (high) side weir overflow is commonly used
in Flanders, although the interpretation of the “high”
side can differ from case to case. All devices were
thoroughly tested using sediments with different mean
settling velocities and efficiency relationships have
been found for all structures. For a given geometry
of the overflow device, the removal efficiency of a
certain sediment fraction is determined by the flow
conditions, by the settling velocity of this sediment
fraction and by the chamber geometry. The efficiency
relationships have been used to determine the dimensions
of the ‘optimal’ design. They also allowed
to make a comparison between the different types of
overflow structure, both from a technical and from an
economical point of view.
Part of this research was funded by the
Flemish water administrations (Aminal
Water and VMM).
Full paper 'Experimental
investigation on the efficiency on the efficiency of
a high side weir overflow' (1999) in Water
Science & Technology.
Full paper 'Comparison
between the separating efficiency of an improved high-side
weir overflow and a hydrodynamic Storm King separator'
(1999) in Water
Science & Technology.
Full paper 'Surface load as predominant
factor for CSO efficiency' (2002) at 9th International
Conference on Urban Drainage, Portland, US.
Full paper 'Combined sewer overflows :
solid separation performance' (2004) at 5th international
conference on sustainable techniques and strategies
in urban water management (Novatech), Lyon, France.
Nederlandstalig artikel 'Ontwerp
en optimalisatie van verbeterde overstortconstructies'
(2000) in @WEL
- Water, nr. 4.
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Flemish Interuniversity research on CSO impacts (Patrick Willems) |
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In the period 1995-1999 a research project on combined
sewer overflows and ancillary structures was carried out
by the different Flemish universities (Universities of
Leuven, Ghent, Brussels, Antwerp) and funded by the Flemish
water administrations (Aminal Water and VMM). Once all
sewer systems and treatment plants are built, the remaining
impact on the water courses will be due to combined sewer
overflows. The main question that needs to be answered
is : What is the impact of combined sewer systems on the
receiving waters and how can it be assessed ?
For this reason measurement campaigns were set up on three
sites and modelling of the sewer networks was carried
out. Furthermore, the impact on the river water quality
was investigated based on water quality measurements.
In the mean time, physical models were built to optimise
ancillary structures. Numerical models were built to identify
the behaviour of the upstream sewer systems and to perform
a long term assessment of the emissions.
The following conclusions can be drawn. More effort should
be paid to measurements, which should be more continuous
and accurate. For modelling purposes an optimal synergy
can be obtained by combining detailed and simplified models.
Continuous long term simulations are necessary in order
to assess the overflow impact in a statistically reliable
way (acute as well as chronic impact). Ancillary structures
can reduce the spilled pollutant loads if they are properly
designed.
Full paper 'Combined
sewer overflows : conclusions on the Flemish research
project and future prospects' (2000)
Nederlandstalig samenvattend
eindrapport (2000).
Nederlandstalig artikel 'Het
Vlaamse onderzoeksproject Riooloverstorten : randvoorzieningen'
(2000) in @WEL
- Water, nr. 4
Nederlandstalig artikel 'Riooloverstorten
: conclusies onderzoeksproject en toekomstperspectieven'
(2000) in @ WEL
- Water, nr. 4.
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