Case study · CFD of wastewater structures

CFD study of an STP inlet screening building: Warsan, Dubai

Three-dimensional CFD of the coarse and fine screening building at a large sewage treatment plant. It assesses flow distribution across the screening channels, recirculation and dead zones, and where solids are likely to settle, then recommends improvements.

Client
Dubai Municipality

Location
Warsan STP, Dubai, UAE

Scope
Flow distribution, recirculation, sediment risk

Tools
ANSYS Fluent: k–ω SST, porous jump, DPM

Design flow
505 MLD average flow (baseline)

505 MLD

Average design flow modelled (baseline case)

1.91 M cells

Polyhedral mesh, refined at screens and outlet chamber

Porous jump

Screen losses calibrated to manufacturer data

DPM

Particle tracking to locate sediment-prone zones

The challenge

Even flow across parallel screening channels

Raw sewage arrives through gravity sewers and must split evenly between parallel trains of coarse and fine screens. Uneven flow overloads some screens and leaves others under-used, and slow zones collect grit and solids. The client needed to know how the existing arrangement performs and where it should be improved.

CFD flow domain of the screening building — Flow domain: inlet sewers, distribution chamber, screening channels and outlet chamber

Our approach

CFD methodology

Full hydraulic path modelled: inlet and outlet sewers, distribution and outlet chambers, screening channels and guide walls
Half-domain model with a symmetry plane, where the layout allowed, to save about 50% of the cells
Pressure-based steady-state solver with k–ω SST turbulence and gravity
Coarse and fine screens represented by porous-jump zones calibrated to the manufacturer’s head-loss data, and validated on a test channel first
Polyhedral mesh of 1.91 million cells, refined around screens, junctions and the outlet chamber
Suspended solids at 100 mg/L tracked with the Discrete Phase Model to find where particles linger and settle

Polyhedral CFD mesh of the screening building — Polyhedral volume mesh

Results

Key results

Velocity contours through the screening channels — Velocity magnitude through the facility

Velocity vectors showing recirculation — Velocity vectors: recirculation in the outlet chamber

Particle tracks coloured by residence time: long residence means settling risk

Key findings

The design flow arrives smoothly, with no separation upstream, and turns cleanly around the guide walls.
Screen head loss is reproduced within the manufacturer’s range, and both trains stay in service at design flow.
Flow is biased toward the central channel, so the side channels are under-loaded and the screens unevenly used.
Recirculation in the outlet chamber and vortices at the splitter walls create low-velocity zones where particles linger longest, the main sediment risk.
Recommendation: add baffling or rearrange the internal channels to even out the flow, and inspect and clean the outlet chamber regularly.

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