Fine screens in Pakistan

Where this equipment fits

STP inlet works and industrial pretreatment.

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 fine screens

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

• 01Aperture and downstream protection duty
• 02Solids load and cleaning water
• 03Available hydraulic head

What affects the price in Pakistan?

The quoted price depends on aperture and downstream protection duty, 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: compare clean and partially blinded open area

Illustrative calculation, not a final hydraulic design. Assume peak flow is 120 m³/hour, or 0.0333 m³/second. A 0.80 m wide channel with 0.60 m water depth has a gross flow area of 0.48 m², so approach velocity is 0.0333 ÷ 0.48 = about 0.069 m/second.

For a simplified slotted screen with 6 mm clear opening and 3 mm support width, the geometric open ratio is 6 ÷ (6 + 3) = 0.667. Ignoring the frame and other supports, clean open area is 0.48 × 0.667 = 0.32 m² and clean through-screen velocity is 0.0333 ÷ 0.32 = about 0.104 m/second.

If screenings block 40% of that clean area, 0.192 m² remains and through-screen velocity rises to about 0.174 m/second. This simplified example does not calculate headloss or prove capacity. Request the supplier's curves for the actual perforation or slot, plate thickness, supports, angle, upstream and downstream levels, cleaning cycle and agreed blinding condition.

Worked example: include screen water in the plant balance

Illustrative operating basis. At 120 m³/hour for 12 hours, daily screened flow is 1,440 m³. If spray washing and screenings handling return water equal to an assumed 0.5% of screened flow, the return is 1,440 × 0.005 = 7.2 m³/day.

If that return occurs during two hours of actual wash cycles, its average rate while washing is 7.2 ÷ 2 = 3.6 m³/hour. Replace both assumptions with supplier data and field records. State the water source, minimum pressure, nozzle demand, wash sequence, drainage destination and effect of the short return peak on upstream pumping and treatment.

Define the downstream protection duty first

• Name every downstream item that needs protection, including pumps, aeration equipment, media-retention screens, membranes, valves and sludge equipment.
• Obtain the permitted opening and screen type from the selected downstream supplier. Do not copy an aperture from an unrelated project.
• State whether the required opening is a round perforation, square mesh or clear slot. A nominal dimension alone does not make different geometries equivalent.
• Use a coarse mechanical screen ahead of the fine screen where bulky debris could damage, jam or hydraulically overload the finer unit.
• Confirm whether fibres, wipes, hair, plastics, grease, food particles or another material controls selection. Representative debris matters as much as wastewater flow.

Check MBR screening as a supplier-specific requirement

• For an MBR treatment system, obtain the selected membrane-module supplier's current pretreatment requirement in writing, including opening, geometry and acceptable carryover.
• Check maximum instantaneous flow, fibres, hair, flexible material, bypass protection and what happens when the screen or wash system is unavailable.
• Do not present one aperture as universal. Module families can specify different pretreatment, and a future replacement may change the requirement.
• Prevent overflow or bypass from sending unscreened wastewater to the membrane tank. A high-level alarm does not replace a protected hydraulic route.
• Use the MBR module selection guide to coordinate screening with membrane area, tanks, air scouring, cleaning, integrity and acceptance.

Survey hydraulics at clean, loaded and unavailable conditions

• Record minimum, normal and peak instantaneous flow, pump starts, batch releases, storm inflow, upstream storage and downstream water-level changes.
• Survey channel width, invert, wall condition, approach, screen angle, upstream and downstream levels, freeboard, available head and discharge elevation.
• Compare gross channel area with net clean open area after perforations or slots, frames, supports and submerged obstructions are included.
• Request clean and agreed partially blinded headloss curves. Check surcharge, overflow and one-unit-out duty before accepting the layout.
• Use upstream and downstream level instruments where differential level starts cleaning or identifies blinding.

Match cleaning to the screen surface and wastewater

• For a perforated plate or belt, define brush contact, spray pressure, nozzle access, plate tracking, carryover removal and behaviour with grease or fibres.
• For a step screen, check the moving and fixed lamellae, bottom-step cleaning, screenings carpet, drive linkage and the effect of grit and stones.
• For a screw or drum screen, check the wetted screening area, spray coverage, screw or drum speed, seals, discharge zone and safe access to the internal surface.
• State wash-water quality, pressure, instantaneous flow, filtration if needed, control valve, isolation, low-pressure response and drainage.
• Agree cleaning start and stop logic from differential level, upstream level, timer or a justified combination. Include a maximum idle time where deposits could dry or compact.

Complete the screenings and return-water route

• Measure loose volume, wet mass, free water and short peak discharge during representative operation. Wastewater flow alone does not define the handling system.
• Show the chute, conveyor or launder, drop height, bridging points, enclosure, drainage and interface with screenings washing and compaction.
• Define bag or bin capacity, change frequency, full-container alarm, spill control, access, washdown and the authorized disposal route.
• Return wash water and drainage to a controlled point. Check the instantaneous return, organic load and any grit or fibres against that point's capacity.
• Do not return captured screenings to the inlet. If wash water separates attached organics, keep the retained solid and returned liquid paths distinct.

Compare fine-screen arrangements on the same duty

Plan duty, standby, bypass and controls

• Define whether the site needs parallel duty and standby screens, one screen with a protected standby channel, or another arrangement.
• Prove the required peak flow with one unit isolated where continuity is required. Include isolation gates or stop logs, not just a second machine.
• Prevent normal operation through an uncontrolled bypass. Record high-level, bypass and failed-cleaning events and give operators a clear response.
• Use an MCC and control panel to sequence the screen, wash system, conveyor, washer or compactor, full-bin alarm and upstream pumps.
• Define overload, jam, low wash pressure, high differential level, downstream unavailable, emergency stop and power-restoration responses.

Provide safe access and maintainability

• Provide non-slip platforms, gratings, handrails and lifting equipment around channels, drives, covers and removable parts.
• Guard moving plates, chains, belts, brushes, screws and pinch points. Provide lockout points for the screen and every linked handling item.
• Allow safe access to spray nozzles, brushes, lamellae, screen panels, seals, bearings, drives, level sensors, chutes and drainage.
• State the heaviest removable component, lifting point, hatch or door size, clear removal path and laydown area.
• Provide ventilation, lighting, washdown and drainage appropriate to sewage screenings, splash and odour.

Specify the complete fine-screen package

• Screen frame, screening surface, drive, chains or linkage, rollers, guides, bearings, brushes, scrapers, spray bar, nozzles, seals and discharge chute.
• Channel modifications, anchors, seals, isolation, standby or bypass screen, upstream and downstream level instruments and sample or inspection points.
• Wash-water supply, pressure control, filtration where required, valves, piping and drainage to the agreed return point.
• Conveyor, launder, washer or compactor, bags or bins, enclosure, odour control and final disposal interface.
• Control panel, cables, local station, alarms, interlocks, emergency stops, communications, platforms, guards and lifting.
• Installation, dry checks, water testing, representative-wastewater commissioning, training, manuals, initial consumables, recommended spares and special tools.

Commissioning and acceptance checks

• Verify channel dimensions, invert, water levels, anchors, alignment, opening geometry, screen angle, clearances, spray coverage, guards and lifting.
• Calibrate level instruments and confirm differential-level display, cleaning start and stop, timer backup, alarms, interlocks and emergency stops.
• Observe clean and agreed loaded hydraulic conditions at minimum, normal and peak flow, including one-unit-out duty where specified.
• Use representative wastewater and debris to confirm capture, cleaning, carryover removal, screenings discharge and the complete handling sequence.
• Measure wash-water pressure and flow, drainage and return rate. Demonstrate low-pressure, jam, high-level, full-bin, bypass and power-restoration responses safely.
• Accept against agreed aperture, geometry, peak flow, water levels, headloss basis, carryover, wash duty, stable test duration and downstream protection requirement.

Operating and maintenance requirements

• Trend flow, upstream and downstream levels, differential level, run time, starts, wash cycles, wash-water use, overloads, jams, bypass and screenings volume.
• Inspect the screen surface, panels or lamellae, brushes, scrapers, spray nozzles, chains or linkage, guides, bearings, seals, fasteners and corrosion.
• Clean nozzles, ledges, the channel floor, chute, drainage and level sensors before deposits create false readings or reduce capacity.
• Investigate rising differential level, longer run time or carryover. Causes can include changed debris, grease, damaged panels, poor brush contact, blocked sprays or a changed downstream level.
• Maintain the complete handling route. A screen is not operationally reliable if its conveyor, compactor, drainage or collection bin is unavailable.

Information needed for a useful quotation

• Minimum, normal and peak instantaneous flow; pump or batch sequence; upstream and downstream levels; channel width, depth, invert, freeboard and available head.
• Required aperture and geometry, downstream protection duty, selected MBR requirement if applicable, and representative debris or solids evidence.
• Clean and loaded hydraulic basis, duty and standby philosophy, one-unit-out requirement, bypass protection, isolation and high-level consequences.
• Screen type, materials, cleaning method, wash-water quality, available pressure, discharge elevation, screenings volume and handling route.
• Controls, instruments, alarms, pump and handling interlocks, electrical supply, ambient and corrosive conditions, ventilation and data requirements.
• Civil work, access, guards, lifting, installation, commissioning, operator training, manuals, spares and measurable acceptance terms.

For an existing screen with high level, carryover, bypass, nozzle blockage, panel damage or unreliable handling, Aquenvia's upgrade and maintenance service can review the hydraulics, screen, controls and access together.

Common fine-screen questions

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

Protect an MBR membrane system

If this screen serves an MBR, confirm the selected module supplier’s required opening, screen type, peak-flow capacity, bypass protection and screenings handling. The MBR membrane module guide explains why screening must be checked with the complete replacement or new-system scope.

Coordinate coarse and fine screening

The mechanical bar-screen selection guide explains how to remove bulky debris before a finer opening, compare clean and loaded hydraulics, and plan standby, bypass and screenings handling across the complete inlet works.