Discover why screening is critical in water and wastewater treatment
Introduction to Screening in Water Treatment
Screening is the first line of defense in any water or wastewater treatment plant. By removing large solids and debris, screening protects downstream processes, grit removal, biological reactors and membrane units from clogging and abrasion. An effective screening strategy boosts plant uptime, reduces maintenance costs, and improves effluent quality by up to 30 %.
1. Bar Screens (Coarse Screens)
-What They Are
Bar screens consist of parallel bars spaced 20–100 mm apart. They intercept large rags, sticks and plastics before they enter the plant.
– How They Work
– Static bar screens: Fixed bars; manually raked.
– Mechanical bar screens: Motor‑driven rakes lift debris into a conveyor.
* Key Benefits
Low headloss: Typically < 20 mm
High removal efficiency: Captures > 90 % of solids > 20 mm
Rugged construction: Durable stainless steel or alloy
2. Drum Screens (Rotary Screens)
Drum screens are cylindrical, stainless steel perforated drums that rotate slowly. Influent enters the drum; fine particles pass through the perforations while solids are carried to the top.
Advantages:
Handles flows from 50 m³/h up to 10 000 m³/h
Automatic self‑cleaning via spray nozzles
Available with perforations or wedge wire slots (0.5–10 mm)
3. Band Screens
Band (or belt) screens use a continuous, perforated mesh belt that moves in a loop. Debris is trapped on the belt and carried up for discharge.
Key Features:
Slot openings: 0.2–5 mm for fine screening
High throughput: Ideal for screening raw sewage
Wash‑down station: Ensures thorough cleaning
4. Fine Screens & Micro Screens
After coarse removal, fine screens (1–5 mm) and micro screens (0.1–1 mm) polish the water by removing grit, algae, and protozoan cysts. They’re critical upstream of:
Membrane bioreactors (MBRs)
Ultrafiltration (UF)
Reverse osmosis (RO)
Types:
Rotary drum micro‑screen
Screening presses
5. Traveling Screens
Often used in intake structures for large‑scale water treatment plants, traveling screens protect pumps and downstream equipment by intercepting debris continuously.
Mesh size: 3–10 mm
Frame speed: 4–10 m/min
Hydraulic cleaning: Spray bars at the discharge end
Design Considerations & Best Practices
Screen aperture vs. flow rate: Balance removal efficiency with headloss.
Materials of construction: Stainless steel 304/316 vs. hardened steel for abrasive influents.
Automation & controls: Remote monitoring, debris bin fill‑level sensors, and variable‑speed drives.
Maintenance schedule: Weekly inspections, monthly lubrication, annual overhaul.
