The last decade has seen a rise in the use of centre-flow band screen technology in municipal sewage inlet works across Australia.

Centre-flow band screens use perforated plate manufactured from either stainless steel or non metallic materials such as polypropylene. The perforations provide two dimensional screening which increases screenings capture as compared to bar type or wedgewire profiles.

The centre-flow units proved their higher screenings capture at the UK WIR Test Facility, leading to increased usage of this technology in the UK, Australia, US, China and Europe.

Retrofitting this technology to existing inlet works poses a number of challenges, including:

  • Physical constraints of the existing channels (depth and width)
  • Hydraulic constraints (headloss, downstream process, plant bypass weirs)
  • Screenings handling design

An alternative to centre-flow screens is the through-flow band screen, also known as an escalator screen, which has been around longer and is also well proven – it also uses a perforated band, but it is aligned perpendicular to the flow path. Physically, these units are similar to step screens.

Hydraulic considerations

The total headloss of a centre-flow screen comprises of:

  • Headloss at inlet
  • Headloss through the screenings panel
  • Headloss at the two outlet channels

The total headloss of a through-flow screen comprises of:

  • Headloss at inlet
  • Headloss through the screenings panel

In many instances, the influence of the channel geometry can lead to headloss values where the loss through the panel itself is only a portion of the total headloss, the rest being attributable to the inlet and outlet area.

In other words, in some circumstances the increased surface area of the centre-flow profile has little influence on the screen capacity.

This is commonly experienced in retrofits and leads to many challenges in matching centre-flow hydraulics to an existing hydraulic profile within inlet works. Channel widths may need to be increased or changes made to the downstream water depths to accommodate centre flows.

The headloss at the inlet to a centre flow can be significant as its inlet width can be a lot less than the channel width. The reverse applies to a through flow.

For new inlet works, the integration of centre-flow hydraulics is a lot more straightforward, as the channels can be designed to fit the downstream hydraulic requirements (such as a vortex or longitude grit trap).

Whilst a through-flow screen cannot match the low headloss of a step or bar type screen, it can certainly provide higher capacity than a centre flow in some instances, and in particular during retrofits of small to medium sized inlet works (i.e. 100 to 1200 LPS).

When retrofitting fine band screens, designers need to consider the impact of the inlet and outlet headloss on centre-flow units and compare against a through-flow unit. It should not be assumed that the centre flow will always have the lower headloss due to the higher screening panel area.   

Ultimately both units provide screenings capture ratios greater than 80 per cent.

Hydroflux Epco provides both centre-flow and through-flow type band screens and can provide technical advice, hydraulic data and sizing on both options.

Lauren ‘LJ’ Butler is the Assistant Editor of Utility magazine and has been part of the team at Monkey Media since 2018.

After completing a Bachelor of Media, Communications and Professional Writing at the University of Wollongong in 2014, and prior to writing about the utility sector, LJ worked as a Journalist and Sub Editor across the horticulture, hardware, power equipment, construction and accommodation industries with publishers such as Glenvale Publications, Multimedia Publishing and Bean Media Group.

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