Potable Water and Treating Pollution
Why Water Treatment Matters
Access to safe water is one of the most fundamental public health challenges in the world. In the UK, water from rivers, lakes and reservoirs contains dissolved minerals, suspended solids and potentially harmful microorganisms. It must be processed before it is safe to drink. In many parts of the world — particularly arid regions and areas affected by drought or contamination — obtaining even basic safe water is a daily challenge that chemistry helps address. This lesson covers how potable water is produced from fresh water sources, how seawater is treated in water-scarce regions, and how pollution from human activity is managed.
What ‘Potable’ Means
Potable water is not the same as pure water in the chemical sense. Pure water contains only \(H_2O\) molecules. Potable water still contains dissolved mineral salts and trace amounts of other substances — but at concentrations low enough that it is safe for human consumption. Standards for potable water specify maximum concentrations of dissolved substances, levels of microorganisms, and other quality parameters. In the UK, these standards are set and enforced by regulators, and tap water routinely meets them.
Potable water is safe to drink but is not chemically pure — it still contains dissolved minerals. Pure water contains only \(H_2O\). The key measure for potability is that harmful substances are below safe thresholds.
Producing Potable Water from Fresh Water
In the UK and most countries with reliable fresh water sources, potable water is produced from rivers, lakes and reservoirs through a two-stage treatment process: filtration followed by sterilisation.
Step 1 — Filtration: Water is passed through beds of sand and gravel. The physical structure of these beds traps suspended particles — silt, organic debris and some microorganisms — leaving the water visually clear. This is a physical process, not a chemical one, and it does not remove dissolved substances or microorganisms that have passed through.
Step 2 — Sterilisation: Chlorine is most commonly added to kill remaining bacteria and other harmful microorganisms. Chlorine is favoured because it is effective, relatively inexpensive, and — crucially — it continues to protect water against microbial regrowth as it travels through the distribution network to homes and businesses. Alternatively, ozone (\(O_3\)) or ultraviolet (UV) light can be used; both kill microorganisms effectively but do not provide the lasting protection in the pipes that chlorine does.
Desalination: Water for Arid Regions
In countries or regions where fresh water is scarce — such as parts of the Middle East, North Africa and island communities — seawater or brackish water must be processed to make it potable. This is called desalination. Two methods are used:
Distillation: Seawater is boiled and the steam is collected and condensed. Because the dissolved salts remain in the boiling vessel, the condensed water is essentially pure. This method is energy-intensive, which makes it expensive, but it produces very pure water and is well understood.
Reverse osmosis: Water is forced at high pressure through a semi-permeable membrane with very small pores that block the dissolved salt ions but allow water molecules to pass through. This approach is more energy-efficient than distillation but the membranes require careful maintenance and can become fouled with impurities. Reverse osmosis is increasingly used in large-scale desalination plants because its energy cost is lower and the technology has improved considerably.
Both methods produce concentrated brine (salt water) as a by-product that must be disposed of carefully to avoid environmental damage to marine ecosystems near the outfall. In the UK, where rainfall is generally plentiful, desalination is rarely needed and fresh-water treatment is far more cost-effective. The choice of method is always a balance of economic cost, energy availability and environmental impact.
Desalination (by distillation or reverse osmosis) removes dissolved salts from seawater to make it potable. Both methods require significant energy, making desalination expensive — it is used where fresh water is scarce and no cheaper alternative exists.
Treating Waste Water and Controlling Pollution
Human activity produces large volumes of waste water from homes, industry and agriculture. This water cannot be returned to rivers or seas untreated without causing serious harm to aquatic ecosystems and human health. Waste water treatment is a multi-stage process involving physical and biological steps:
In sewage treatment, waste water is first screened to remove large solids. It then passes into settlement tanks where smaller solid particles sink as sludge. The remaining liquid undergoes biological treatment in which aerobic bacteria break down dissolved organic compounds. The cleaned water (effluent) is filtered and sterilised before being returned to rivers. The sludge removed at earlier stages is treated separately — it may be digested by bacteria (anaerobic digestion, which also produces methane as a useful biogas), dried, and in some cases used as agricultural fertiliser.
Industrial waste water may contain dissolved heavy metals, toxic organic compounds or acids that cannot be removed by standard biological treatment. These require specialist chemical treatment — for example, precipitation reactions to remove dissolved metals, or activated carbon filtration to adsorb organic pollutants. Agricultural run-off containing excess fertilisers is another serious problem. When nitrates and phosphates enter waterways, they cause eutrophication: algae grow rapidly on the excess nutrients, block sunlight from aquatic plants, then die and decompose. The decomposition uses up dissolved oxygen, suffocating fish and other aquatic life. Controlling fertiliser use and managing run-off from fields is therefore important not just for farming economics but for ecosystem health.
In waste water treatment, physical screening and sedimentation remove solids; biological treatment by aerobic bacteria removes dissolved organics. Untreated agricultural run-off can cause eutrophication, depleting oxygen and killing aquatic life.