Potable water: passing the water through filter beds

The idea

After a suitable fresh water source has been chosen, the water is treated to make it potable. One key stage is passing the water through filter beds, which are layers of sand and gravel, sometimes with other materials, that trap solid particles suspended in the water. As water percolates through the beds, insoluble grit, soil, plant fragments and some microorganisms are removed, producing clearer water. Filter beds do not remove dissolved salts or all microbes, so the filtered water is then sterilised, for example by chlorination, ozonation or ultraviolet light, to kill remaining microorganisms. In the laboratory, filtration through filter paper separates an insoluble solid from a liquid, and the same principle applies on a larger scale in water treatment. Understanding filter beds helps explain why the order of treatment matters and why filtration alone cannot make water potable.

What earns the marks

• Describes filter beds as layers of sand and gravel through which water is passed to remove suspended insoluble particles.
• Explains that filtration separates insoluble solids from the water, producing clearer water but not removing dissolved substances.
• States that filter beds reduce some microorganisms but do not sterilise the water, so a later sterilising step is still needed.
• Links the filtration stage to the overall potable water process, recognising that it follows screening or sedimentation and precedes sterilisation.
• Applies the same separation principle as laboratory filtration through filter paper, where an insoluble solid is trapped and the liquid passes through.
• Uses correct vocabulary such as 'suspended particles', 'insoluble' and 'percolates' when describing the process.

Common mistakes and how to put them right

• Claiming that filter beds remove dissolved salts.

  Put it right by stating that dissolved substances pass through the filter beds and require other methods such as distillation or reverse osmosis.

• Believing that filtration alone makes water potable.

  Put it right by explaining that sterilisation is still needed to kill microorganisms that remain after filtration.

• Confusing filtration with sedimentation.

  Put it right by noting that sedimentation allows particles to settle under gravity, whereas filter beds trap particles as water flows through them.

Exam tips

• Name the materials in the filter beds, such as sand and gravel, and state what they remove rather than writing only 'it filters the water'.
• Sequence the treatment steps correctly, placing filter beds after screening or sedimentation and before sterilisation.
• Use the word 'insoluble' when explaining what filtration removes, and state that dissolved substances are not removed.