Mechanical bar screens in Pakistan

Where this equipment fits

Municipal headworks and industrial or housing STPs with recurring rag loads.

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 mechanical bar screens

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

• 01Channel width and water depth
• 02Opening and peak headloss
• 03Rake capacity, drive protection and bypass

What affects the price in Pakistan?

The quoted price depends on channel width and water depth, 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 channel and through-screen velocity

Illustrative calculation, not a final hydraulic design. Assume peak flow is 100 m³/hour, or 0.0278 m³/second. A 0.60 m wide channel with 0.50 m water depth has a gross flow area of 0.30 m², so approach velocity is 0.0278 ÷ 0.30 = about 0.093 m/second.

If the rack has 25 mm clear openings and 10 mm bars, the simple geometric open ratio is 25 ÷ (25 + 10) = 0.714. Ignoring side-frame and support blockage, clean open area is 0.30 × 0.714 = about 0.214 m², giving a clean through-screen velocity of 0.0278 ÷ 0.214 = about 0.130 m/second.

If debris blocks 35% of that open area, remaining open area is about 0.139 m² and through-screen velocity rises to about 0.200 m/second. This example does not calculate headloss. Obtain the supplier's clean and blinded hydraulic curves for the actual bar shape, screen angle, frame, channel, downstream level and debris condition.

Worked example: turn measured screenings into a handling duty

Illustrative operating basis. If site records show 0.30 m³/day of loose wet screenings over 12 operating hours, average handling rate is 0.30 ÷ 12 = 0.025 m³/hour. A short peak at four times the average is 0.10 m³/hour.

Two days of collection would contain 0.30 × 2 = 0.60 m³ before adding freeboard, drainage, compaction behaviour or contingency. Measure the real loose and compacted volume, mass, free water, rags, plastics and bulky objects. Do not select a rake, washer, compactor, conveyor, bagging station or bin from wastewater flow alone.

Measure peak flow, water levels and incoming debris

• Record minimum, normal and peak flow, storm or infiltration effects, pump starts, batch discharges and upstream storage. A daily average does not define the highest instantaneous screen duty.
• Survey channel width, invert, wall condition, upstream and downstream water levels, freeboard, available head, discharge elevation and the effect of downstream equipment.
• Inspect representative debris during normal and difficult periods. Record rags, wipes, plastics, wood, stones, grease, fibres and bulky objects, plus screenings volume and mass.
• Identify the downstream equipment to protect and the largest solid it can safely pass. Coordinate the screen with submersible sewage pumps and the complete inlet works.
• For an existing channel, document high-level events, manual raking, jams, bypassing, carryover, odour, corrosion, access restrictions and actual operator response.

Choose the opening for the protection duty

• State clear opening, bar thickness, bar shape and screen angle. Compare net open area, not channel width alone.
• Use a coarse mechanical bar screen to remove bulky material where that is the defined protection duty. Add a fine screen only where downstream treatment needs a smaller opening.
• Check the selected opening against pump solids passage, valves, downstream fine screens, biological process, membranes and the disposal route for extra captured material.
• Smaller openings can increase capture, headloss, rake cycles and screenings handling. They do not remove mineral grit or dissolved pollution.
• Do not assume that nominal spacing proves capture. Bar shape, orientation, carryover, flexible rags and the cleaning mechanism also affect performance.

Check hydraulics clean, loaded and with one channel unavailable

• Calculate approach and through-screen velocity at minimum, normal and peak flow using surveyed upstream and downstream levels.
• Request clean and partially blinded headloss curves for the proposed screen, opening and installation angle. Agree the blinded condition used for design.
• Check upstream surcharge, freeboard and overflow routes before the screen starts, while it cleans, if it jams and if one duty channel is isolated.
• Use upstream and downstream level instruments to calculate differential level where justified. A timer alone cannot detect every debris surge or blocked rack.
• Check low-flow operation for deposition and grit accumulation. A larger channel can reduce velocity enough to create a separate maintenance problem.

Match rake capacity and protection to the real solids load

• Define the number, spacing and speed of rakes or cleaning elements, plus the maximum screenings lift and discharge rate.
• Check engagement with the bar rack, bottom cleaning, carryover removal and behaviour with long rags, stones and bulky objects.
• Provide torque or overload protection, jam detection, safe reverse or clearing logic where the selected design permits it, and an alarm that reaches the operator.
• State chain, sprocket, bearing, comb, rake and guide materials and the parts kept outside wastewater. Confirm corrosion protection for sewage gases and splash.
• For deep channels or high solids loads, compare multiple-rake and other arrangements on hydraulic area, lifting duty, access and actual debris, not on motor power alone.

Plan duty, standby and bypass without hiding untreated flow

• Define whether the site needs two mechanical channels, a mechanical screen plus a safe standby manual bar screen, or another arrangement.
• Check the required peak duty with one screen isolated. Isolation gates, stop logs and lifting equipment must be usable without entering an unsafe channel.
• A bypass should protect people and the plant during a credible failure, but it should not become the normal flow path or send uncontrolled debris downstream.
• Provide high-level and bypass alarms, event records and a response plan. Confirm what happens during power failure and when the collection container is full.
• Coordinate emergency storage and pump controls so a screen fault does not cause an avoidable upstream spill or dry downstream pumps.

Complete the screenings handling route

• Show the discharge chute, conveyor or launder, drop height, enclosure, drainage and route to the receiving equipment.
• Where required, specify screenings washing and compaction using measured feed volume, free water and waste characteristics.
• Prevent bridging, windblown litter, unsafe manual transfer and liquid draining across the operating floor.
• Define bin or bag capacity, change frequency, full-container detection, access for removal, washdown and the authorized disposal route.
• Return wash water and drainage to a controlled point that the treatment plant can accept. Do not send captured screenings back to the inlet.

Controls, instruments and plant integration

• Define automatic start from differential level, upstream level, timer or a justified combination, with maximum idle time and overrun after the level recovers.
• Include local and remote mode, emergency stops, guarded isolators, run and fault status, overload, high level, failed cleaning, full bin and bypass alarms.
• Use the MCC and control-panel design to coordinate the screen, washer or compactor, conveyor, gates and upstream pumps in a safe sequence.
• Define safe power-restoration behaviour. Equipment should not restart into a jam, open guard or unavailable downstream handling system.
• Record running hours, starts, differential level and faults where trend data will help distinguish hydraulic overload from mechanical or debris problems.

Provide safe access and maintainability

• Provide non-slip platforms, gratings, handrails and lifting equipment around open channels, drives, covers and removable components.
• Guard chains, sprockets, rakes, scrapers, conveyors and pinch points. Interlock access where required and provide lockout points that isolate every moving component.
• Design jam clearing, rake replacement, chain tensioning, lubrication and bar-rack inspection from safe positions. Automatic screens still need hands-on maintenance.
• Provide ventilation, lighting, wash water, drainage and hygienic cleaning arrangements appropriate to sewage screenings.
• State the heaviest removable part, lifting point, clear removal path and laydown space before fixing roof, pipework or electrical trays.

Compare inlet screening options fairly

Specify the complete mechanical bar-screen package

• Screen frame, bar rack, rake or cleaning elements, chains or cables, guides, drive, bearings, scraper and discharge chute.
• Channel modifications, anchors, seals, isolation gates or stop logs, bypass or standby screen, level instruments and sample or inspection points.
• Screenings conveyor, washer, compactor, bags or bins, drainage, odour containment and disposal interface.
• Control panel, power and signal cabling, local controls, alarms, interlocks, emergency stops and integration with pumps and downstream equipment.
• Platforms, gratings, handrails, guards, covers, lifting, lighting, ventilation, wash water and floor drainage.
• Installation, dry checks, water testing, representative sewage commissioning, training, manuals, recommended spares and special tools.

Commissioning and acceptance checks

• Verify channel dimensions, invert, water levels, anchors, alignment, bar opening, rake clearance, lubrication, guards, covers and lifting before wet operation.
• Calibrate upstream and downstream level instruments and confirm displayed differential level. Test timers, permissives, alarms, emergency stops and isolation.
• Observe minimum, normal and peak hydraulic conditions, including the accepted partial-blinding basis and one-channel-out duty where required.
• Use representative debris to confirm rack cleaning, lift, discharge and the complete washer, compactor, conveyor or bin sequence without unsafe intervention.
• Demonstrate overload, jam, high level, full bin, unavailable downstream equipment, bypass alarm and power-restoration responses by an approved safe method.
• Accept against agreed peak flow, water levels, headloss basis, opening, screenings lift rate, operating sequence, safety functions, noise, vibration and stable test duration.

Operating and maintenance requirements

• Trend flow, upstream and downstream levels, differential level, starts, run time, overloads, jams, bypass events and screenings volume.
• Inspect the rack, rake engagement, comb or scraper, chains or cables, guides, sprockets, bearings, fasteners, seals, guards and corrosion.
• Clean ledges, the channel floor, discharge chute, drainage and level sensors before deposits create false readings or block the handling route.
• Investigate repeated overloads or high differential level. Possible causes include debris change, blinded bars, failed rake contact, inadequate capacity or a changed downstream level.
• Maintain bins, bags, washdown and disposal records. A reliable screen still fails operationally if captured waste cannot leave the site safely.

Information needed for a useful quotation

• Minimum, normal and peak flow; pump or batch sequence; upstream and downstream water levels; channel width, depth, invert, freeboard and available head.
• Required clear opening and downstream protection duty, plus representative debris types, loose and compacted screenings volume, mass and peak loading.
• Duty and standby philosophy, one-channel-out requirement, bypass basis, isolation method, high-level consequences and power-failure response.
• Screen angle, rack and frame materials, rake capacity, discharge elevation, enclosure, washer or compactor, conveyor, bin and disposal boundaries.
• Controls, instruments, alarms, pump and handling interlocks, electrical supply, ambient and corrosive conditions, ventilation and data requirements.
• Platforms, guards, lifting, removal clearances, civil modifications, installation, commissioning, training, manuals, spares and acceptance terms.

For an existing inlet with high level, bypassing, jams, carryover or unsafe manual cleaning, Aquenvia's plant upgrade and maintenance service can review the channel, hydraulics, debris, screen, handling system, controls and access together.

Common mechanical bar-screen questions

Browse related equipment in the screening equipment category or review Aquenvia's sewage-treatment service.

Protect sewage pumps without hiding the solids problem

Screen opening, peak headloss, bypass, screenings handling and cleaning frequency should match the pump station and incoming waste. Use the submersible sewage pump selection guide to coordinate screening with solids passage, wet-well levels, pump duty and maintenance access.

Remove bulky debris before a step screen

Use the step-screen selection guide to coordinate coarse-screen opening, fine lamella slots, clean and loaded hydraulics, stones, rags, screenings handling, standby and acceptance across the inlet works.

Remove bulky debris before a screw screen

Use the screw-screen selection guide to coordinate coarse and fine openings, clean and loaded hydraulics, rags, grit, brush and screw wear, reject handling, standby and acceptance across the inlet works.