Screw screens in Pakistan

Where this equipment fits

Compact STPs and constrained retrofit sites.

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 screw screens

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

• 01Screen opening and peak flow
• 02Inclination and discharge height
• 03Wash water and bagging

What affects the price in Pakistan?

The quoted price depends on screen opening and peak flow, 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 80 m³/hour, or 0.0222 m³/second. At the design water level, a screw screen has 0.55 m of effective basket width and 0.75 m of submerged screened length, giving 0.55 × 0.75 = 0.4125 m² of gross submerged area.

If the clear opening is 6 mm and the supplier confirms 3 mm of obstructing wire or support in the simplified flow direction, the clean opening fraction is 6 ÷ (6 + 3) = 0.667. Net clean open area is about 0.275 m², and velocity through the clean openings is 0.0222 ÷ 0.275 = about 0.081 m/second.

If blinding leaves 55% of the clean opening available before the next cleaning cycle, the simplified opening velocity rises to about 0.147 m/second. This calculation does not predict headloss. Obtain supplier curves for the exact screen geometry, inclination, water levels, opening, support bars, brush, screw position and agreed clean and loaded condition.

Worked example: convert captured solids into the discharge duty

Illustrative mass balance. At 800 m³/day, assume representative sampling finds 100 mg/L of screenable dry solids and supports 55% capture by the selected opening. Retained dry solids are 800 × 100 × 0.55 ÷ 1,000 = 44 kg/day.

If the proposed washing and compaction arrangement discharges material at 22% dry solids, wet screenings mass is 44 ÷ 0.22 = 200 kg/day. If washdown and handling water are assumed at 0.5% of daily flow, return water is 4.0 m³/day. If that water is used within one hour, the short water and drainage duty is 4.0 m³/hour. Replace every assumption with measured site records, representative tests and supplier data.

Define the wastewater and protection duty first

• Record minimum, normal and peak instantaneous flow, pump starts, batch discharges, upstream storage, downstream level and wet-weather conditions where relevant.
• Name the pumps, biological process, membranes or other equipment that needs protection and obtain its permitted opening and carryover criteria.
• Characterize rags, wipes, hair, plastics, food pieces, fibres, grease, stones and mineral grit during normal and difficult operating periods.
• Use a coarse mechanical screen ahead of the screw screen where bulky debris could damage or overload its basket, screw or discharge.
• Use the broader fine-screen selection guide when the cleaning mechanism or opening geometry has not yet been chosen.

Choose the opening, basket and cleaning arrangement together

• State whether the screen uses wedge wire, perforated plate or another verified geometry. Record the clear opening, tolerance, gross area, net clean open area and support obstruction.
• Confirm how the screw and replaceable brush clean the full active surface, lift screenings and limit material falling back into the channel or tank.
• Ask for the brush material, adjustment method, wear allowance and replacement procedure. A nominal opening cannot be maintained if the brush no longer contacts the screen correctly.
• Check whether the proposed basket is suited to long fibres, hair and grease, and how ropes of material are removed without unsafe manual contact.
• For MBR duty, use the selected membrane supplier's current pretreatment requirement rather than assuming that every nominal opening or screw-screen geometry is acceptable.

Survey the channel, tank, inclination and discharge height

• Confirm whether the screen fits an existing channel, requires a supplied tank or receives pumped flow. Survey inverts, widths, levels, freeboard, supports and pipe connections.
• Define the screen inclination, effective submerged length, inlet distribution, side and bottom sealing and the maximum upstream level if the screen stops.
• Check the discharge height, roof clearance, chute angle, container position and whether the screw can be removed without dismantling permanent structures.
• Provide upstream and downstream isolation, controlled drainage and a safe alternate flow path where continuous treatment is required.
• Coordinate the discharge with a separate or integrated screenings washer and compactor without assuming that washing, compaction and bagging are included.

Check clean, loaded and one-unit-out hydraulics

• Use the actual submerged active area at each operating level, not the full installed basket or trough area.
• Request clean and agreed loaded flow and headloss curves for the exact opening, basket, screw, brush, inclination, channel or tank and water-level range.
• Check minimum flow for deposition, normal flow, peak instantaneous flow, the cleaning cycle and the high-level condition after a drive, brush or reject-handling fault.
• Prove required capacity with one unit or channel isolated where continuity is required. Include usable gates and the effect on approach velocity and upstream surcharge.
• Use upstream and downstream level instruments to start cleaning, identify rising headloss and alarm before uncontrolled overflow or bypass.

Check screw, liner and drive wear

• Define the screw material, flight thickness, shafted or shaftless arrangement, support method, trough liner, allowable clearances and replaceable wear parts.
• Mineral grit, stones, glass and metal can accelerate abrasion or cause a jam. A screw screen does not replace a dedicated grit-removal process.
• Confirm allowable torque, overload detection, drive rating, reverse or clearing procedure and the parts that can be inspected without lifting the whole machine.
• Record baseline screw-to-trough clearance, brush contact, motor current and noise during commissioning so later wear can be identified.
• Do not change screw speed merely to clear repeated faults until feed solids, basket blinding, brush condition, bearings, liner and discharge restriction have been checked.

Compare screw screening with other arrangements

Complete the washing, compaction and drainage balance

• Measure retained dry solids, wet mass, loose volume, free water and short peak discharge during representative operation or trials.
• State whether the screw only transports screenings or also washes, compacts and dewaters them. Define each guaranteed function and discharge condition.
• Confirm wash-water quality, pressure, instantaneous flow and daily volume. Include strainers, solenoid valves, low-pressure alarm and freeze or heat protection where relevant.
• Return drainage to a controlled process point without recirculating captured solids or hydraulically overloading a small upstream sump.
• Provide fall-back control, full-bin detection, spill containment, odour management, safe container change and an authorized disposal route.

Controls, standby and safe failure response

• Coordinate the screen drive, wash valve, compaction zone, conveyor or chute, bin alarm, level instruments and upstream pumps through the MCC and control panel.
• Define differential-level and timer starts, cleaning overrun, maximum idle period, high-level alarm and a controlled manual mode.
• Include drive overload or torque protection, low wash pressure, downstream-unavailable and full-bin responses. Stop feeding equipment that cannot discharge safely.
• Define safe power-restoration behaviour. Do not restart into a jam, open guard, blocked chute, full bin or unavailable downstream unit.
• Record run time, starts, motor current or torque, differential level, overloads, high-level events and bypass so trends can guide maintenance.

Provide safe access and maintainability

• Provide non-slip platforms, gratings, handrails and lifting equipment around the drive, basket, screw, brush, wash system, compaction zone and discharge.
• Guard the screw, drive, couplings, discharge and every pinch point. Provide clear lockout points and prevent access to moving equipment.
• Allow inspection of both sides of the screened surface, brush contact, screw flights, liner, bearings, seals, spray nozzles, drainage and level sensors.
• State the heaviest removable assembly, lifting points, clear removal path, laydown area and the temporary support required during maintenance.
• Provide lighting, ventilation, washdown and drainage suitable for inlet works. Treat channel or tank entry as a separately controlled hazardous task.

Specify the complete screw-screen package

• Screen basket or trough, screw, brush, liners, frame, drive, supports, covers, discharge chute and any washing or compaction section.
• Channel or tank, anchors, seals, pipe or flange connections, isolation, drainage, level instruments and protected overflow or bypass where required.
• Wash-water strainer and valve, reject drainage, conveyor, 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 dimensions, anchors, alignment, inclination, opening, seals, screw and trough clearances, brush contact, operating levels, guards and lifting provisions.
• Calibrate level instruments and confirm differential-level start, timer backup, wash sequence, 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 capture, basket cleaning, lift, discharge, fall-back control, washing, compaction, carryover and drainage.
• Test high level, low wash pressure, jam or overload, stopped downstream equipment, full bin, bypass and power restoration under a safe agreed procedure.
• Accept against measurable flow, water levels, headloss basis, opening, reject mass and condition, handling capacity and stable test duration.

Operating and maintenance requirements

• Trend flow, upstream and downstream levels, run time, starts, motor current or torque, overloads, bypass and screenings quantity.
• Keep the screw clear of large debris and inspect the brush, wedge wire or perforated surface, trough liner, screw flights and clearances for wear.
• Check wash-water strainers, solenoid valves and nozzles, then inspect bearings, gearbox, seals, fasteners, chute, drainage and corrosion.
• Investigate rising headloss, longer run time, poor discharge or increased carryover before changing controls. Confirm the actual solids and mechanical condition first.
• Keep supplier-specific brushes, liners, seals, bearings, 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, tank or pipe dimensions; operating 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.
• Screen type, opening geometry, inclination, effective submerged area, discharge height, roof and removal clearance and environmental conditions.
• Screenings mass and volume, washing, compaction, discharge dryness basis, chute or conveyor, bin, drainage return and disposal interface.
• Controls, instruments, electrical supply, access, lifting, commissioning, training, spares and acceptance terms.

For an existing screen with rising levels, worn brushes, screw or liner wear, repeated jams or poor reject discharge, Aquenvia's upgrade and maintenance service can review the hydraulics, mechanics, controls and handling route together.

Common screw-screen questions

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

Check a compact screw screen against the full duty

Use the fine-screen package and acceptance guide to define hydraulic area, opening, screw and brush wear, wash water, inclination, discharge height, handling, access and one-unit-out operation.

Compare screw and step screening

Use the step-screen selection guide to compare clean and loaded hydraulics, screening and lifting mechanisms, bottom collection, grit, screenings condition, discharge height, controls and access.