Screening is the first treatment step at almost every wastewater treatment plant, and it decides how reliably everything downstream runs. Rags, wipes, fibres and hair that pass the headworks end up wrapped around pump impellers, blocking aerators and clogging membranes. This guide explains the main fine screen types for wastewater, how opening size affects capture, what matters for channel design and screenings handling, and which data you need to choose the right screen.
The role of screening in the headworks
Wastewater screening removes coarse and fibrous solids at the inlet, before grit removal and biological treatment. EN 12255-3, the standard for preliminary treatment, treats screens as part of the headworks together with grit and grease removal. A well-designed screen protects pumps, mixers and aerators, reduces deposits in tanks and keeps the activated sludge free of rag balls. Screening is usually arranged in stages:
- Coarse screening with bar spacings of typically 10 mm and more, mainly to protect inlet pumps from large objects.
- Fine screening with openings of about 1–6 mm, which removes most rags, fibres and hygiene products and is the standard protection for biological treatment.
- Micro screening below 1 mm, used ahead of sensitive processes or, with mesh drum filters, for polishing treated effluent.
A combined grit and grease removal unit typically follows the fine screen, removing more than 95% of grit particles larger than 0.2 mm and 50–80% of oil and grease. Good screening upstream keeps that unit free of rags.
Fine screen types for wastewater
Four designs are established for fine screening. Each has its strengths depending on flow, channel geometry, solids load and required opening.
Rotating drum screen
A rotating cylindrical basket of perforated plate or wedge wire retains the solids, lifts them out by rotation and is cleaned by spray nozzles or brushes. Drum screens achieve fine openings in a compact unit and are often combined with an integrated screw that conveys, washes and compacts the screenings.
Step screen
Alternating fixed and moving lamellae, arranged like a staircase, lift the retained material step by step out of the channel. The screenings form a mat that improves capture of fine material. Step screens handle high solids loads and wide channels well and are common in medium and large municipal inlet works.
Band and perforated plate screen
A continuous belt of perforated panels moves through the channel and carries the solids to a discharge point. Round holes retain fibres and hair more effectively than slots of the same width, because elongated particles cannot slip through lengthwise. This is why perforated band screens are preferred ahead of membrane bioreactors.
Spiral or screw screen with integrated compactor
A screen trough and a screw are combined in one inclined unit. The screw clears the screen, conveys the screenings upwards and dewaters them in a pressing zone. Spiral screens are compact and economical for small plants, pumping stations and industrial duties with moderate flows and narrow channels.
| Screen type | Screening element | Strengths | Points to consider |
|---|---|---|---|
| Rotating drum screen | Perforated plate or wedge wire drum | Compact, fine openings, often with integrated washing and compaction | Flow per unit limited; spray water needed |
| Step screen | Fixed and moving lamellae (slots) | High solids loads, wide channels, screenings mat improves capture | Slots let some fibres pass lengthwise |
| Band / perforated plate screen | Moving belt of perforated panels | Very high capture of fibres and hair, suitable for membrane protection | Higher headloss; more moving parts |
| Spiral / screw screen | Screen trough with screw and press zone | Screening, conveying and compaction in one unit | Best suited to smaller flows and narrow channels |
Our mechanical fine screens are available as rotating drum, step and band screens with slot or hole openings from 0.5 to 6 mm, for flows from 5 to 2,000 L/s and channel widths from 200 mm to 3,000 mm.
Bar spacing, perforation and capture rate
The opening, given as slot width or hole diameter, is the key design parameter. It determines how much material is retained, how high the headloss becomes and how much screenings must be handled.
- 6 mm is common for conventional biological treatment where robust protection is sufficient.
- 3 mm gives noticeably higher capture of fibres and hygiene products and reduces rag formation in tanks.
- 1–2 mm perforated plate is typical ahead of membrane systems; the membrane supplier’s requirements set the limit.
Capture rate is the share of screenable material the screen retains; our fine screens reach more than 98% at the rated opening. For sensitive processes such as MBR package units, finer perforated openings are chosen because even small amounts of hair can braid on membrane modules.
Micro screening is a different duty. Drum filters with 10–100 µm stainless steel mesh polish secondary effluent, removing 70–90% of incoming suspended solids ahead of UV disinfection, membrane filtration or reuse. They do not replace a fine screen in the headworks.
Headloss and channel design
Every screen creates a headloss that rises as the screen surface loads with solids, so hydraulic design should consider a partially blinded screen at peak flow. Key points are:
- Approach velocity high enough to prevent grit settling in front of the screen, typically around 0.4 m/s or more.
- Velocity through the screen limited, typically to about 1.0–1.2 m/s at peak flow, so solids are not forced through.
- Freeboard sufficient for the upstream level rise without flooding the inlet.
- Level-differential control that starts cleaning when the upstream/downstream difference reaches a set point.
- Bypass via an overflow weir or a second screen line for storm events and maintenance.
Screenings washing, compaction and transport
Raw screenings contain much water and organic matter, which causes odour and adds disposal weight. Washing returns the organic fraction to the wastewater, where it serves as carbon for biological treatment; compaction removes water. An integrated compacting screw press and washer reduces screenings volume by up to 60% and allows hygienic disposal.
Where several screens feed one container, or the container is further away, screw conveyors and compactors in stainless steel transport the material in enclosed troughs over 1–30 m at inclinations of up to 45°. The compactor reduces screenings volume by 4–6 times before disposal, which means fewer container changes.
Materials, operation and maintenance
Headworks are aggressive: hydrogen sulphide, acids and abrasive grit attack equipment continuously. Fine screens are therefore built in 304 or 316L stainless steel, with 316L recommended for more aggressive conditions. Motors are rated IP55 or IP65, ATEX Zone 1/21 versions are available, and drive power is low at 0.37–3.0 kW. For reliable operation:
- Check screen surface, brushes, spray nozzles and seals regularly.
- Monitor cleaning cycles; frequent cycles at normal flow point to blinding or worn cleaning elements.
- Keep spray water pressure and nozzles in good condition, especially with greasy industrial wastewater.
- Provide good access, lifting points and space to withdraw the screen.
Industrial vs municipal use and typical mistakes
Municipal plants face fluctuating flows, storm peaks and many wipes. Industrial wastewater, for example from food processing, textiles or paper production, often carries specific solids such as fibres, skins, fats or plastic particles, sometimes at high temperature. Opening, material grade and cleaning method must match the actual solids. Typical mistakes include:
- Sizing for average flow only and ignoring peak flows.
- Choosing slots where perforated openings are needed, for example ahead of membranes.
- No bypass or redundancy.
- Omitting washing and compaction.
- Insufficient approach velocity, causing grit deposits.
For sizing, we need average and peak flow, channel width, depth and water level, wastewater type, required opening, washing and compaction requirements, indoor or outdoor installation and any ATEX zone.
Frequently asked questions
What is the difference between a coarse screen and a fine screen?
Coarse screens have bar spacings of typically 10 mm or more and protect pumps from large objects. Fine screens with 1–6 mm openings retain most rags, fibres and hygiene products and protect aerators and biological treatment.
Which screen opening do I need upstream of an MBR?
Membrane systems need more protection than conventional activated sludge. Perforated openings of 1–2 mm or finer are typical; always check the membrane supplier’s specification.
Is a drum screen or a step screen better?
Neither is better in general. Drum screens are compact and suit smaller channels with integrated compaction, while step screens handle wide channels and high solids loads. Flow, channel geometry and opening decide.
How much can screenings volume be reduced?
An integrated compacting screw press and washer reduces screenings volume by up to 60% and removes much of the organic content, which reduces odour.
Planning a new inlet works or replacing an existing screen? Send us your flow data, channel dimensions and wastewater type via our contact page, and we will prepare a proposal within 48 hours.