Waste water treatment: screening and grit removal

The idea

Screening and grit removal is the first physical stage of waste water treatment. Waste water arriving at a treatment works contains large objects such as rags, plastics, wet wipes and sanitary items, plus dense mineral particles such as sand, gravel and broken glass. Screening passes the water through bars or meshes that trap large solids, protecting pumps, valves and downstream equipment from blockage and damage. Grit removal lets dense particles settle or be spun out in a grit channel or vortex tank, because grit would otherwise erode machinery and accumulate in tanks. Screens and grit are removed as solid waste and sent to landfill or further processing. This stage is purely physical: no chemicals are added and no dissolved substances are removed, so it prepares the water for sedimentation and biological treatment rather than purifying it.

What earns the marks

• Screening removes large solid objects such as rags, plastics, wet wipes and sanitary items from the incoming waste water.
• Screens are bars or meshes that let water pass while trapping oversized solids, protecting pumps, valves and pipework from blockage or damage.
• Grit removal separates dense mineral particles such as sand, gravel and broken glass from the waste water.
• Grit is removed by settlement in a grit channel or by vortex separation, because these dense particles settle or are thrown outwards faster than lighter organic material.
• Screening and grit removal are physical processes: no chemicals are added and dissolved substances are not removed at this stage.
• The removed screen waste and grit are collected as solids for disposal or further processing, so they do not pass on to sedimentation and biological treatment.

Common mistakes and how to put them right

• Thinking screening removes dissolved pollutants.

  Put it right: Screening only traps large solid objects, so dissolved substances remain in the water and are dealt with later.

• Confusing grit with sewage sludge.

  Put it right: Grit is dense mineral material such as sand and gravel, whereas sludge is the organic solid that settles later during sedimentation.

• Believing chemicals are added at this stage.

  Put it right: Screening and grit removal are physical separation steps, so no coagulants or disinfectants are used here.

Exam tips

• Link each step to the problem it solves: screens stop blockages, while grit removal stops abrasion and accumulation in tanks and pumps.
• Use the phrase physical separation and name the property used, such as particle size for screening or density for grit removal.
• Keep the sequence clear: screening and grit removal come before sedimentation, so describe them as preparation for later stages.