Industrial waste water may require removal of organic matter and harmful…

The idea

Industrial processes produce waste water that is often more variable and more contaminated than domestic sewage. It may carry organic matter, such as food, blood or solvents from factories, and harmful chemicals, including heavy metal ions, acids, alkalis or toxic compounds. Before this water can be returned to a river or reused, these substances must be removed or reduced to safe levels. Treatment usually combines physical separation, such as screening or settling, with chemical methods, such as neutralisation, precipitation or adsorption onto activated carbon. For example, acidic factory water is neutralised with an alkali, and dissolved metal ions are precipitated as insoluble hydroxides and filtered off. The specific method depends on the pollutant, so a factory must identify its waste before choosing treatment.

What earns the marks

• Industrial waste water can contain organic matter, such as food residues, oils or solvents, which must be removed because it can lower dissolved oxygen in rivers when decomposed by microorganisms.
• Harmful chemicals in industrial waste water include toxic metal ions, acids, alkalis and other dangerous compounds that can poison aquatic life or contaminate drinking water sources.
• Treatment methods are chosen to match the pollutant: screening removes large solids, sedimentation removes suspended particles, neutralisation treats acids or alkalis, and precipitation removes dissolved metal ions.
• Organic matter may be broken down by aerobic biological treatment, while some dissolved chemicals are removed by adsorption onto activated carbon or by chemical precipitation.
• Treated industrial waste water must meet safe discharge limits before release, so the level of treatment required depends on the original waste and the sensitivity of the receiving environment.

Common mistakes and how to put them right

• Treating all waste water as if it were sewage.

  Put it right: Recognise that industrial waste water varies widely and may need specific chemical treatment rather than only biological treatment.

• Assuming dilution alone makes harmful chemicals safe.

  Put it right: Toxic substances must be removed or chemically changed, because dilution does not destroy them.

• Confusing organic matter with harmful chemicals.

  Put it right: Organic matter is carbon-based material that microorganisms can decompose, whereas harmful chemicals include toxic metal ions and other dangerous substances.

Exam tips

• Link each treatment method to the type of pollutant it removes, rather than listing methods without a reason.
• Use the phrase harmful chemicals for toxic dissolved substances and organic matter for decomposable carbon-based material, so your answers match the specification language.
• When given a data table or flow diagram, identify which stage removes which pollutant before writing your explanation.