Inlet Screening Design for Sewage Works: Bar Spacing, Kirschmer Headloss, Screenings Quantities, Washing and Compaction

Screen types and where each belongs

Preliminary treatment removes material that would otherwise damage or clog everything downstream, and UK practice has settled into two stages. Coarse screens, with bar spacings of 25 to 50 mm (occasionally 75 to 150 mm on raw sewage pumping stations), protect inlet pumps and take out timber, bricks and bulky debris. Fine screens, now almost universally 6 mm two-dimensional screens using perforated plate, mesh or stepped laminations, remove the rag, wipes, cotton buds and plastics that one-dimensional bar screens let through edge-on. Smaller works often have only a fine screen; larger works put a coarse screen ahead of the pumps and fine screens on the pumped discharge, as at Wigan, where three 2.0 m wide, 50 mm coarse bar screens rated at 2,360 L/s each precede five 2.0 m wide, 6 mm escalator screens rated at 1,575 L/s each, with passive emergency bypass weirs built into the channel dividing walls.

BS EN 12255-3:2024, which replaced the 2000 edition, covers screens with mesh or aperture above 50 µm at works serving more than 50 population total; finer microscreens fall under Part 16. Its 2024 revision added design recommendations throughout, a normative annex on testing the washing performance of screenings wash-presses, and sections on screenings treatment, container loading, grit removal and treatment, grease separation and flow splitting. Any UK specification for new inlet works should cite it alongside the water company asset standard.

Channel sizing and velocities

Two velocities bound the design. Upstream of the screen the channel must stay self-cleansing, which means at least 0.42 m/s at minimum flow and preferably 0.6 to 0.75 m/s where the sewage carries grit, or deposits form in front of the bars. Through the screen openings the velocity must be low enough that captured screenings are not forced through or torn off the bars: 0.3 m/s at average flow for hand-raked racks and no more than 0.75 m/s at maximum flow for mechanically cleaned screens, with 0.6 to 1.2 m/s through a clean screen at peak flow acceptable in practice. Because the open area of a fine screen is only 30 to 50 percent of the gross panel, the submerged screen area including bars is usually two to three times the cross-section of the incoming sewer.

Flow to design for is the full flow arriving at the works, not flow to full treatment, since storm flows pass the screens before being split to storm tanks: for most UK works this is Formula A, roughly six times dry weather flow in a combined catchment, or the pumped capacity where inlet pumps set the limit. Provide at least two channels each able to pass the peak with one screen out of service, isolating penstocks upstream and downstream of each, a bypass weir or overflow so that a blinded screen cannot surcharge the sewer, and a straight approach of several channel widths so the flow reaches the screen evenly. A channel floor set 70 to 150 mm below the incoming invert, with a smooth entry transition, avoids grit accumulation and entrance losses.

Headloss through a clean screen: Kirschmer

Kirschmer's equation gives the loss through a clean bar screen from the bar geometry and the approach velocity head:

Clean-screen losses of 20 to 60 mm are the norm, and designers used to allow 150 mm for a hand-raked rack and accept 300 mm as the maximum before raking. Those figures are not what the hydraulic profile should carry, because the loss that matters is the one across a partly blinded screen just before the rake cycle starts, and across a fine screen whose open area is a fraction of the bar screen's.

Headloss through a blinded or fine screen

Once rag has bridged the openings the screen behaves as an orifice plate, and the loss follows the velocity through the remaining open area rather than the bar geometry. Two expressions are in common use:

Taking the example above: gross submerged panel area along the 75 degree incline is 1.5 × 0.9 / sin 75 = 1.40 m², of which the clear openings are 25/35 = 71 percent, so 1.00 m² open when clean. At 50 percent blinding the open area is 0.50 m² and the velocity through it 2.4 m/s. By the orifice form h = (2.4)²/(2 × 9.81 × 0.6) = 0.49 m; by the second form h = 0.0729 × (5.78 − 0.79) = 0.36 m. A 50 percent blinded coarse screen therefore costs 350 to 500 mm, twelve to eighteen times its clean loss, which is why mechanically cleaned screens are specified for a differential of 150 to 600 mm and why the rake is started on level difference (commonly 100 to 150 mm) rather than on a timer alone. For 6 mm fine screens the manufacturer's curves must be used: open area ratios of 30 to 45 percent and a thin captured mat give losses of 150 to 300 mm at design flow with the screen in its normal partly loaded state, and the allowance in the works profile should be the manufacturer's maximum operating differential plus the downstream channel loss, typically 400 to 600 mm for a fine screen and 300 to 400 mm for a coarse one. That allowance is often the single largest step in the inlet works profile; see the guide on building the works hydraulic profile.

Screenings quantities and composition

Screenings arisings depend on the aperture, on whether the sewer is combined or separate, on the catchment and on the season. Classic data for coarse racks give 0.0035 to 0.0375 m³ per 1,000 m³ of sewage, typically 0.015, rising to as much as 0.225 m³ per 1,000 m³ in the first flush of a storm on a combined system. Fine 6 mm screens capture several times more than a 25 mm rack on the same flow: UK and German experience puts coarse screenings at 2 to 5 litres per population equivalent per year, 6 mm two-dimensional screenings at 5 to 15 litres, and 3 mm screens higher still, measured as dewatered and compacted material at roughly 300 to 400 kg/m³. For a 50,000 population works on a 6 mm screen that is 250 to 750 m³ per year, or 75 to 225 tonnes, before washing.

Raw screenings are 80 to 90 percent water and about half of the dry mass is faecal and food organic matter trapped in the rag, which is what makes them putrescible, heavy to transport and expensive to dispose of. Washing returns that organic fraction to the flow, where it belongs, and compaction then takes the dry solids from 10 to 20 percent to 35 to 50 percent, cutting volume by more than 65 percent. Washed, compacted screenings attract lower gate fees and give far less odour in the skip.

Washing, compaction and handling

A screenings wash-press takes the discharge from the screen (usually by chute or a launder conveyor), agitates it with wash water in a hopper or along a shafted screw, drains the wash water back to the channel through a perforated section, and then compacts the solids in a tapering or extended tube against a restriction, delivering a plug into a bag or skip. Typical units handle 2 to 6 m³/h of raw screenings with 3 to 5.5 kW drives and wash water at 5 bar, and guarantee better than 65 percent volume reduction. BS EN 12255-3:2024 Annex A now defines a test for washing performance, so the specification can require a measured organic removal rather than a brochure claim; the usual target is a loss on ignition in the washed product below about 20 to 25 percent of dry solids, and dry solids after pressing above 40 percent.

Design points that recur in operational audits: the press must be rated for the screen's peak screenings rate in a storm first flush, not the annual average; wash water should be final effluent rather than potable where practicable, with a break tank and backflow protection; a shaftless screw launder allows several screens to feed one press but must be sized for rag without bridging; and the skip or bagging station needs covered, ventilated housing with odour control, since the inlet works is where hydrogen sulphide and the rest of the sewer atmosphere are released. Skip lorry access, washdown water and a hoist for the screen head belong in the first layout sketch. Packaged inlet works combine screen, wash-press and grit removal in one unit for works below about 10,000 population equivalent, which saves civil work but fixes the hydraulic profile around the package.

Commissioning and operating checks

Three measurements prove an inlet screen. First, the headloss at design flow with the screen in normal operation, logged against the upstream and downstream level sensors, should sit within the manufacturer's band and the rake should cycle at the set differential; a screen that runs continuously at low flow is either blinding with grease or has its level set point wrong. Second, a downstream sample of fine screenings captured on a 6 mm test sieve shows whether rag is passing edge-on through gaps, damaged panels or the bypass seal. Third, the washed screenings should be weighed and tested for dry solids and loss on ignition against the specification. A works that passes all three will see fewer pump blockages and a measurable drop in visible debris at the outfall. For the remainder of the preliminary stage see the guidance on grit removal and inlet works design.

How to size an inlet screen channel

1. Fix the design flow. Full storm flow to the works (Formula A or pump capacity), with each channel able to pass it when one screen is out.
2. Set the channel section. Approach velocity 0.6 to 0.75 m/s at peak and above 0.42 m/s at minimum flow; floor 70 to 150 mm below the incoming invert.
3. Choose the screen. 25 to 50 mm coarse bars ahead of pumps, 6 mm two-dimensional fine screen on all flow to treatment; through-opening velocity below 0.75 m/s at peak when partly loaded.
4. Compute headloss. Kirschmer for the clean bar screen, orifice form at 50 percent blinding, manufacturer curve for the fine screen; carry the maximum operating differential in the profile.
5. Size screenings handling. 5 to 15 L per PE per year for 6 mm screens, wash-press rated for storm peaks, dry solids above 40 percent after pressing.
6. Provide bypass and isolation. Penstocks both sides of each screen and a passive overflow weir so a blinded screen cannot surcharge the sewer.