Bar screens and fine screens for wastewater: a selection guide

Screening is the first step at almost every treatment plant inlet. It takes out rags, plastics, sticks and other solids that would block pumps, wrap around mixers and settle in tanks. The right screen depends on how fine the screening must be, how much flow and solids it must handle, the channel or sump it sits in, and how much operator attention is available.

Coarse and fine screening

Coarse screens have wide gaps and catch large solids that would damage or block pumps. Fine screens have narrow openings and remove the smaller material that would otherwise collect in tanks, foul diffusers or blind membranes and filters. Many plants use both: a coarse screen first to protect the fine screen, then a fine screen to protect the rest of the plant.

The screen types

• Manual bar screen: an inclined bar rack cleaned with a hand rake onto a drip platform.

• Basket screen: a removable basket on guide rails, lifted out with a hoist and emptied.

• Multi rake bar screen: several rakes on twin chains clean the bar rack continuously.

• Step screen: fixed and moving stepped lamellae lift screenings up the incline step by step.

• Honeycomb screen: a rotating drum with hexagonal mesh and a spray bar to keep it clear.

• Rotary brush screen: a rotating brush sweeps a curved screen and moves solids to a trough.

Comparison

Screen	Coarse or fine	How it is cleaned	Suits
Manual bar screen	Coarse	Hand rake	Small plants, bypass channels, temporary works
Basket screen	Coarse	Basket lifted and emptied	Pump sumps, wet wells, drains
Multi rake bar screen	Coarse to medium	Several rakes on chains	Large inlets, heavy loads, storm flows
Step screen	Fine	Moving lamellae; a mat of screenings improves capture	Municipal and industrial inlet works
Honeycomb screen	Fine	Rotating drum and spray bar	Fine screening, intake water, food and paper effluent
Rotary brush screen	Fine	Rotating brush	Small and medium plants with little space

A typical arrangement

At a larger plant, flow passes a coarse mechanical screen and then a fine screen. A bypass channel fitted with a manual bar screen lets flow continue, still screened, while a mechanical screen is stopped for maintenance. Screenings drop into a chute and onto a conveyor or into a washer press, which washes out organic matter and squeezes out water, so the waste is smaller, cleaner and less smelly. Sumps and wet wells are often protected by a basket screen on guide rails.

What drives the choice

• Opening size. Set it by what you need to protect downstream: pumps need coarse protection, while membranes, diffusers and sludge processes need fine screening.

• Flow range. The screen must pass the peak flow, including storm flow, without the upstream level rising too high.

• Head loss. As a screen collects solids, the water level upstream rises. Check the channel has enough freeboard. Mechanical screens usually start cleaning when the level difference across the screen rises, as well as on a timer.

• Type of solids. Rags, fibres, grease, grit and plastics each behave differently on a screen.

• Channel or sump. Width, depth and the height to the discharge point fix the size and angle of the screen.

• Materials. Stainless steel for wetted parts gives a long life in effluent.

Checklist: what to send us

• Channel width and depth, and the water depth at peak flow

• Average and peak flow

• Required bar spacing or opening size, or the equipment you need to protect

• Type of solids expected

• Discharge height for screenings, and whether a conveyor or washer press is needed

• Power supply and control requirements

• Indoor or outdoor installation

Frequently asked questions

What bar spacing do I need?

It depends on what the screen protects. Tell us the equipment downstream and the type of wastewater, and we will recommend the opening size.

Do I still need a manual screen if I have a mechanical one?

A manual screen in a bypass channel keeps the plant protected while the mechanical screen is stopped. Most designers include one.

Step screen or drum screen?

Step screens cope well with heavy loads of rags and fibres and use the mat of retained screenings to catch finer material. Drum screens give a large screening area in a compact unit and suit fine screening of steadier loads. Your flow and solids decide.

Can a screen be fitted into an existing channel?

Usually, yes. Send us the channel drawing or measurements and photos, and we will check the fit.

Each screen is sized to the channel and flow. Send us your details and we will come back with a proposal. Customers in the European Union can also order through our European company, VDT Europa OÜ in Tallinn, Estonia.