EVALUATION OF VORTEX SETTLING BASIN (VSB) PERFORMANCE IN GRIT REMOVAL FROM SEWER LINES
Date
2022-12
Authors
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Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: This study was motivated by the prevalence of sewer spillages and sediment deposition in
sewers. The objective of this study was to validate the CFD (Computer Fluid Dynamics)
modelling of the sewer VSB (Vortex Settling Basin) design by Basson (2020) referred to as
VSB 1 and to investigate two modified VSB design configurations (referred to as VSB 2 and
VSB 3) including hydraulic aspects of the VSBs that could not be modelled in CFD. The flow
patterns and flow velocity contours in the VSB cylinder were observed in VSB 1 and 3 as
predominantly radial flow patterns; and axial flow patterns in the CFD VSB and VSB 2. The
observed VSB cylinder flow velocities decreased with increasing depth into the VSB cylinder.
The observed head losses increased with an increase in flow rate for all three VSB models.
The sediment entered the VSB cylinder in the flow as bed load; and settled into the VSB
cylinder against the outer sides for the CFD modelled VSB, for VSB 1 and 2. Sediment settled
along the centreline of the dominant radial flow pattern in the VSB 2. The highest sediment
trapping efficiency of 98.76% was observed for the VSB 1 for Fr=0.575 at PWWF. The
observed CFD modelling sediment trapping efficiency of 100% for Fr=0.696 at PWWF was
higher than sediment trapping efficiencies of 94.71% and 97.22% for VSB 1 and 2 respectively.
The observed maximum sediment storage was 0.093m3 at Fr=0.575 for PWWF in VSB 2. The
CFD modelling sediment storage capacity of 6.95m3 at Fr=0.696 for PWWF was lower than
the observed VSB model sediment storage capacities of 10.2m3 and 10.4m3 for VSB 1 and 2
respectively at prototype sediment storage volumes.
The VSB’s floating debris conveyance efficiency of plastic bottles, plastic bags filled with
garbage, and rags, posed no risk of obstructing the flow or causing blockages, even at high
concentrations. The VSB 1 and 2 conveyed a log length of 0.58m and 0.79m respectively.
Bernoulli’s equation was applied to calculate the water levels through the VSB bypass, and the
design was effective in conveying flow in the event of a blockage. The CFD modelling of the
VSB by Basson (2020) was partially validated due to practical constraints in sediment sizes,
densities and grading that resulted in differences in sediment trapping efficiency and sediment
storage capacity. The physical modelling of the VSBs successfully characterised their
hydraulic behaviour in terms of flow patterns, water levels, flow velocities, sediment, floating
debris and the VSB bypass hydraulic performance in the event of a blockage. The results from
this study could be used as guidelines in sewer VSB design considerations.
AFRIKAANS OPSOMMING: "Geen opsomming beskikbaar"
AFRIKAANS OPSOMMING: "Geen opsomming beskikbaar"
Description
Thesis (MEng) -- Stellenbosch University, 2022.
Keywords
Sewerage, Hydraulic structures, Combined sewer overflows, Waste spills, Vortex settling basin, Marine sediments, Separators (Machines), Grit