Step screens in Pakistan

Where this equipment fits

Sewage headworks requiring automatic fine screening.

For housing STPs in Lahore and mixed industrial drains, assess the actual rag, plastic and fibre load. Where stormwater can enter the sewer, check the peak hydraulic condition and a safe maintenance bypass. Specify how screenings will be washed, collected and removed from the site.

How to select step screens

Use these inputs to define the equipment duty before comparing proposals.

• 01Opening and channel geometry
• 02Capacity at maximum upstream level
• 03Grit exposure and waste handling

What affects the price in Pakistan?

The quoted price depends on opening and channel geometry, materials, controls and the agreed supply scope. For a useful comparison, identify freight, taxes, installation, commissioning and initial spares as separate proposal items where applicable.

For replacement equipment, share the existing nameplate, photographs and connection dimensions. For a new installation, share the design flow or loading, site location and available space. Final availability, delivery and performance terms are confirmed with the selected equipment.

Integration with your treatment plant

Peak flow and upstream / downstream channel levels. Required opening and downstream protection duty. Screenings handling, access and standby arrangements. These interfaces should be checked alongside the individual equipment specification.

Worked example: check clean and partially blinded opening velocity

Illustrative calculation, not a final hydraulic design. Assume peak flow is 100 m³/hour, or 0.0278 m³/second. At the design water level, a 0.9 m wide screen has 0.55 m of submerged screened height, giving 0.9 × 0.55 = 0.495 m² of gross submerged area.

If the clear slot is 6 mm and the supplier confirms 3 mm lamella thickness, the simplified clean opening fraction is 6 ÷ (6 + 3) = 0.667. Net clean open area is therefore 0.495 × 0.667 = about 0.330 m², and velocity through the clean openings is 0.0278 ÷ 0.330 = about 0.084 m/second.

If a screenings carpet and blinding leave 50% of the clean opening available before the next cleaning stroke, the simplified opening velocity rises to about 0.168 m/second. This does not predict headloss. Obtain the supplier's clean and loaded hydraulic curves using the proposed lamella geometry, installation angle, channel, water levels, cleaning sequence and actual wastewater.

Worked example: convert captured solids into the handling duty

Illustrative mass balance. At 1,000 m³/day, assume representative testing finds 80 mg/L of screenable dry solids and supports 50% capture by the selected step screen. Retained dry solids are 1,000 × 80 × 0.50 ÷ 1,000 = 40 kg/day.

If discharged screenings contain 20% dry solids after the proposed handling step, wet screenings mass is 40 ÷ 0.20 = 200 kg/day. If washdown and handling water are assumed at 0.4% of daily flow, return water is 4.0 m³/day. Replace these assumptions with measured site records, representative trials and supplier data before sizing chutes, presses, bins, wash water or drainage.

Define the protection duty and wastewater first

• Name the pumps, biological process, membranes or other equipment that the screen must protect and obtain their permitted opening and carryover requirements.
• Record minimum, normal and peak instantaneous flow, pump starts, batch discharges, upstream storage and downstream water-level changes.
• Characterize rags, wipes, hair, plastics, food pieces, fibres, grease, stones and mineral grit during normal and difficult operating periods.
• Use a coarse mechanical screen first where bulky debris could damage or overload the fine lamella grid.
• Use the broader fine-screen selection guide when the technology has not yet been chosen.

Match the lamella opening and screenings carpet to the duty

• State the clear opening, lamella thickness, fixed and moving lamella arrangement, total clean open area and the manufacturing tolerance that will be accepted.
• Do not treat the nominal slot as the complete capture specification. A retained screenings carpet can improve practical capture, but it also increases resistance and can release material when disturbed.
• Ask how the proposed stroke lifts, cleans and discharges the carpet at minimum, normal and peak flow without forcing unacceptable solids through the grid.
• Confirm whether a very clean grid at startup or after maintenance changes carryover to sensitive downstream equipment.
• For MBR duty, use the selected membrane supplier's current pretreatment requirement rather than assuming that every step screen or nominal slot is acceptable.

Survey the channel, levels and discharge height

• Survey channel width, wall condition, inlet and outlet inverts, operating levels, freeboard, available head, channel depth and the screen support points.
• Confirm installation angle, submerged grid length, discharge elevation and how the screen fits under the roof, covers or existing inlet-works structure.
• Check that the first moving step can collect screenings at channel-bottom level under the proposed geometry.
• Define upstream and downstream isolation, channel drainage, a safe removal route and one-unit-out flow path before fixing civil dimensions.
• Match the discharge point with the receiving conveyor, chute or screenings washer and compactor.

Check clean, loaded and one-unit-out hydraulics

• Use the actual submerged lamella area at each operating level, not the full installed grid area.
• Request clean and agreed loaded flow and headloss curves for the exact width, opening, lamella geometry, angle, stroke and channel.
• Check minimum flow for deposition, normal flow, peak instantaneous flow, cleaning cycles and the high-level condition if the screen stops.
• Prove required capacity with one channel isolated where continuity is required. Include usable gates and the effect on velocity and upstream surcharge.
• Use upstream and downstream level instruments to start cleaning, identify rising headloss and alarm before uncontrolled overflow or bypass.

Manage grit, stones, ragging and bottom cleaning

• A step screen does not remove fine mineral grit. Define the separate grit-removal duty and check abrasive exposure at the screen.
• Stones and hard debris at the channel bottom can obstruct or bend moving lamellae. Provide a practical inspection, flushing and removal method.
• Check how the bottom step prevents dead zones and how it is reached without unsafe entry into an operating channel.
• Inspect the top reversal and discharge area for ropes of rags, hair or fibres that can wrap, bridge or fall back into the channel.
• Where grease is present, confirm low-temperature behaviour, cleaning access and whether deposits can reduce slot area or stop the screenings carpet from releasing.

Compare step screening with other arrangements

Complete the screenings route and plant water balance

• Measure retained dry solids, wet mass, loose volume, free water and short peak discharge during representative operation or trials.
• Define the chute or conveyor interface, drop height, fall-back prevention, drainage, washing, compaction and bin position.
• State washdown and handling-water quality, pressure, instantaneous flow and daily volume. Return drainage to a controlled point without recirculating captured solids.
• Provide full-bin detection, spill containment, odour control, safe container change and the authorized disposal route.
• Do not assume that screening, washing, compaction, bagging and bins are all included in one supplier's package.

Controls, standby and safe failure response

• Coordinate the screen drive, conveyor, washer or compactor, bin alarm, level instruments and upstream pumps through the MCC and control panel.
• Define differential-level and timer starts, adjustable cleaning overrun, maximum idle period, high-level alarm and safe manual mode.
• Include overload or jam protection, motion or position confirmation where required, emergency stops and a fault response that prevents feeding an unavailable handling train.
• Define safe power-restoration behaviour. Do not restart into a jam, open guard, full bin or stopped downstream conveyor.
• Record run time, starts, differential level, overloads, high-level events and bypass so operating trends can guide maintenance.

Provide safe access and maintainability

• Provide non-slip platforms, gratings, handrails and lifting equipment around the channel, drive, top reversal and screenings discharge.
• Guard moving lamellae, drive arms, chains or other transmission parts and every pinch point. Provide clear lockout points.
• Allow external inspection of the lamella grid, bottom step, drive, guides, wear parts, discharge area, chute, drainage and level sensors.
• State the heaviest removable assembly, lifting points, clear vertical and horizontal removal path and laydown area.
• Provide lighting, ventilation, washdown and drainage suitable for the inlet works. Treat channel entry as a separately controlled hazardous task.

Specify the complete step-screen package

• Fixed and moving lamella packs, frame, bottom section, guides, drive, gearbox, transmission, supports, covers and discharge chute.
• Channel interfaces, anchors, side and bottom seals, isolation gates, drainage, upstream and downstream level instruments and protected overflow or bypass arrangement.
• Conveyor, washer or compactor, water supply, drainage return, bags or bins, enclosure and odour-control interfaces where required.
• Control panel, local station, alarms, interlocks, emergency stops, communications, cables, guards, platforms and lifting.
• Installation, alignment, dry checks, water testing, representative-wastewater commissioning, training, manuals, recommended spares and special tools.

Commissioning and acceptance checks

• Verify channel dimensions, anchors, alignment, lamella opening, side and bottom sealing, operating levels, clearances, guards and lifting provisions.
• Calibrate level instruments and confirm differential-level start, timer backup, overrun, alarms, interlocks, overload protection and emergency stops.
• Observe minimum, normal, peak and agreed loaded hydraulic conditions, including one-unit-out operation where specified.
• Use representative wastewater to confirm screenings-carpet formation, lifting, discharge, fall-back control, carryover and the complete handling sequence.
• Test bottom cleaning, high level, jam, stopped conveyor, full bin, bypass and power restoration under a safe agreed procedure.
• Accept against measurable flow, water levels, headloss basis, opening, carryover, reject condition, handling capacity and stable test duration.

Operating and maintenance requirements

• Trend flow, upstream and downstream levels, differential level, run time, starts, overloads, bypass and screenings quantity.
• Inspect fixed and moving lamellae for bending, wear, alignment and trapped debris. Keep the channel bottom free of stones and hard material.
• Inspect the top discharge for ragging and fall-back, then check drive components, guides, fasteners, seals, chute, drainage and corrosion.
• Investigate rising headloss, longer run time or increased carryover before changing control settings. Confirm the actual solids and mechanical condition first.
• Keep supplier-specific lamellae, wear parts, drive protection and critical spares identified instead of assuming generic parts will fit.

Information needed for a useful quotation

• Minimum, normal and peak instantaneous flow; pump or batch sequence; channel dimensions; inlet and outlet levels; available head and freeboard.
• Required opening and downstream protection duty; representative rags, fibres, grease, stones, grit and screenable-solids evidence.
• Clean and loaded hydraulic basis, duty and standby philosophy, one-unit-out requirement, isolation, bypass and high-level consequences.
• Installation angle, submerged grid length, discharge height, channel depth, bottom-cleaning method and available roof or removal clearance.
• Screenings mass and volume, chute or conveyor, washing, compaction, bin, drainage return and disposal interface.
• Controls, instruments, electrical supply, environmental conditions, access, lifting, commissioning, training, spares and acceptance terms.

For an existing screen with rising levels, bent lamellae, bottom obstruction, ragging or poor reject discharge, Aquenvia's upgrade and maintenance service can review the hydraulics, mechanics, controls and handling route together.

Common step-screen questions

Browse the screening equipment category or review Aquenvia's sewage-treatment service for the complete inlet-works duty.

Define the step screen from the complete fine-screen duty

The fine-screen selection guide connects aperture, clean and loaded hydraulics, the screenings carpet, bottom cleaning, grit, discharge, controls, standby and downstream protection.