Earth's Resources, Potable Water and Life-Cycle Assessment
Natural Resources and Sustainability
Humans use the Earth’s natural resources to meet needs for warmth, shelter, food, and transport. Some resources are finite — they are being consumed faster than nature can replace them. Others are renewable — they replenish naturally and will not run out if managed carefully.
Sustainable development means meeting present needs without preventing future generations from meeting theirs. Chemistry contributes by finding alternatives to finite resources (biofuels, bioplastics), improving recycling, and designing more efficient processes that generate less waste.
Finite resources (e.g. fossil fuels, metal ores) will eventually run out. Renewable resources (e.g. timber, cotton, solar energy) replenish naturally. Sustainable development balances today’s needs with preserving resources for future generations.
Potable Water
Potable water is safe to drink. It is not pure water (which contains only H2O); it may contain low levels of dissolved minerals but must be free from harmful microorganisms and toxic substances. In the United Kingdom, most potable water comes from fresh water sources — rivers and reservoirs — and is treated before distribution.
The treatment of fresh water involves three main steps:
- Sedimentation: the water is allowed to stand in settlement tanks. Larger particles and fine debris sink to the bottom as sediment.
- Filtration: the water passes through filter beds of sand and gravel to remove smaller suspended particles, producing clear water.
- Chlorination: a controlled amount of chlorine is added to kill bacteria and other pathogens, making the water safe to drink.
UK potable water treatment: (1) sedimentation — particles settle; (2) filtration through sand/gravel — removes suspended particles; (3) chlorination — kills bacteria. The water is safe to drink but is not chemically pure.
Desalination
In regions where fresh water is scarce, desalination is used to convert sea water into potable water. Two main methods are used:
- Distillation: sea water is boiled; the steam (water vapour) condenses and is collected as pure water, leaving the salts behind. This requires large amounts of energy.
- Reverse osmosis: sea water is forced under high pressure through a membrane that blocks salt ions but allows water molecules through. This also requires significant energy input. This course names distillation as the way to make sea water potable, so treat reverse osmosis as background.
Treating fresh water from rivers and reservoirs (UK approach) is relatively low-cost and uses less energy because the water is already fresh and only needs sedimentation, filtration, and chlorination.
Desalination removes dissolved salts from sea water — a much more energy-intensive process. However, for countries with limited rainfall and no major rivers (e.g. parts of the Middle East), desalination is essential. The high energy cost also means higher carbon emissions unless renewable energy sources are used.
Waste Water Treatment
Used water from homes, hospitals, and industry is called waste water (sewage). It must be treated before being returned to rivers or the sea. The main stages of sewage treatment are: (1) screening — large solids are removed by metal screens; (2) sedimentation — fine suspended solids settle as sludge; (3) biological treatment — aerobic bacteria break down organic matter in the liquid effluent; (4) anaerobic digestion of sludge — the separated sludge is digested by bacteria in the absence of oxygen, releasing biogas (mainly methane), which can be used as a fuel. This course only asks how waste water is made potable by sedimentation, filtration and chlorination, so the sewage-works stages are background.
Recycling and Waste Reduction
Recycling conserves finite resources by recovering materials from waste products rather than extracting new raw materials. It generally requires less energy than extracting and processing virgin materials, and it reduces the volume of waste sent to landfill.
However, recycling is not without cost. Collection, sorting, transport, and reprocessing all require energy and produce some waste. Some materials (such as mixed plastics or composite materials) are difficult to recycle cost-effectively. Despite these challenges, recycling plays a vital role in sustainable development — particularly for metals such as aluminium, where the energy saving over primary production is very large compared to extracting fresh aluminium from its ore by electrolysis.
Life-Cycle Assessment
A life-cycle assessment (LCA) evaluates the environmental impact of a product at every stage of its life:
- Extracting and processing raw materials — energy used, land disturbed, waste generated.
- Manufacturing and packaging — energy and chemicals consumed in production.
- Using the product — energy consumed during use; repairs and maintenance.
- End-of-life disposal — whether the product goes to landfill, is incinerated (releasing CO2), or is recycled.
LCAs involve value judgements, making them potentially subjective. Different companies or researchers may choose to include different stages, use different data sources, or weight environmental impacts differently. For example, a manufacturer might minimise the extraction stage in their own product’s LCA but scrutinise it heavily for a competitor.
LCA results are most reliable when conducted by independent bodies using agreed methodologies. Students should note that LCA data can be presented selectively and should evaluate the source of any LCA before accepting its conclusions.
An LCA covers four stages: (1) raw material extraction; (2) manufacturing and packaging; (3) use of the product; (4) disposal (landfill, incineration, or recycling). LCAs can be subjective depending on what data is included and how impacts are weighted.