Potable Water, Desalination, Wastewater Treatment & Life Cycle Assessments

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Potable Water & Fresh Water Treatment

Potable water is safe to drink (low dissolved salts, no pathogens), though not chemically pure. In the UK, rainwater (low in dissolved substances) collects in the ground (aquifers) and in lakes/rivers. Treatment: 1. Choose an appropriate fresh water source. 2. Pass through filter beds (removes insoluble solids). 3. Sterilise using chlorine, ozone, or ultraviolet (UV) light to kill microbes.

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Desalination of Seawater

Used in countries with scarce fresh water (e.g., Middle East) to remove high concentrations of dissolved sodium chloride from seawater. Two methods: 1. Distillation (boil seawater to produce steam, leaving salt behind, then condense the steam). 2. Reverse osmosis (pass salty water at very high pressure through a partially permeable membrane that only lets water molecules through, trapping ions). Both require huge amounts of energy.

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Required Practical: Water Analysis & Distillation

1. Test pH of water samples using a pH meter or Universal Indicator (must be pH 6.5-8.5). 2. Measure mass of dissolved solids: weigh an empty dry evaporating basin, add a measured volume (e.g., 50 cm³) of water, heat gently over a water bath until all water evaporates to dryness, reweigh, and subtract empty basin mass. 3. Distil seawater using a flask and Liebig condenser to collect pure distilled water (which boils at 100°C and leaves zero residue).

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Wastewater (Sewage) Treatment Steps

Sewage and agricultural wastewater contain organic matter and harmful microbes, while industrial wastewater may also contain toxic chemicals. Four stages: 1. Screening and grit removal (removes large debris, plastic, and grit). 2. Sedimentation (heavy solids sink in settlement tanks to form sewage sludge, while lighter liquid effluent floats on top). 3. Aerobic biological treatment of liquid effluent (air pumped in for aerobic bacteria). 4. Anaerobic digestion of solid sludge (produces methane biogas and agricultural fertiliser).

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Life Cycle Assessments (LCAs)

Carried out to assess the environmental impact of a product across cuatro stages of its life: 1. Extracting and processing raw materials. 2. Manufacturing and packaging. 3. Use and operation during its lifetime. 4. Disposal at the end of its useful life (including transport and distribution at every stage). While water/energy use and waste mass are easily quantified, assigning numerical values to pollutant effects is subjective (so selective/abbreviated LCAs can be biased for marketing).

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Reduce, Reuse & Recycle

Reducing use of finite resources, energy, and waste reduces environmental damage (such as quarrying and landfill). Glass bottles can be crushed, melted, and remoulded (recycled) or washed and refilled (reused). Metals are recycled by melting and recasting into new products, using far less energy than extracting fresh metal from ore.

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1. Q1

   Compare how potable water is produced from fresh groundwater in the UK with how it is produced from seawater in Saudi Arabia, explaining why the methods differ. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Fresh groundwater in the UK already has very low levels of dissolved mineral ions, whereas seawater contains a very high concentration of dissolved sodium chloride [1 mark]. 2. In the UK, groundwater is simply passed through filter beds (to remove insoluble solids) and sterilised with chlorine, ozone, or UV light (to kill microbes) [1 mark]. 3. Filtration and chlorination cannot remove dissolved salt from seawater, so Saudi Arabia must use desalination via distillation or reverse osmosis [1 mark]. 4. Desalination requires boiling water or applying very high pressures across membranes, which consumes far more energy and is much more expensive than UK fresh water treatment [1 mark].

2. Q2

   Describe what happens to domestic sewage after it arrives at a sewage treatment works, explaining how both the liquid effluent and the solid sludge are made safe and useful. [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. First, the raw sewage undergoes screening (passing through metal grids) and grit removal to filter out large solid objects, twigs, rags, and grit [1 mark]. 2. Second, the screened sewage enters sedimentation tanks where it stands still, allowing heavy suspended solids to sink to the bottom as sewage sludge while lighter liquid effluent remains on top [1 mark]. 3. Third, the liquid effluent is piped into aeration tanks where compressed air is pumped through it; aerobic bacteria carry out aerobic biological digestion to break down dissolved organic matter and harmful microbes before the clean water is discharged to rivers [1 mark]. 4. Fourth, the solid sewage sludge from the bottom of the tank is transferred to sealed tanks for anaerobic digestion (in the absence of oxygen) by anaerobic bacteria [1 mark]. 5. Anaerobic digestion breaks down the organic matter in the sludge, releasing methane (biogas) which can be burned to generate electricity, while the remaining dried solid cake is used as agricultural fertiliser [1 mark].

3. Q3

   A supermarket wants to decide whether to supply customers with single-use paper shopping bags or reusable poly(ethene) plastic bags. Carry out a comparative Life Cycle Assessment (LCA) across all four life-cycle stages to evaluate the two bags. [6 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Stage 1 (Raw materials): Paper bags are made from timber (trees), which is a renewable resource if forests are replanted, whereas poly(ethene) bags are made from crude oil (via cracking and polymerisation), which is a finite, non-renewable resource [1 mark]. 2. Stage 2 (Manufacturing and packaging): Pulping wood and bleaching paper requires huge volumes of water and more energy than polymerising ethene, and paper bags are heavier so they require more fuel to transport [1 mark]. 3. Stage 3 (Use during lifetime): Paper bags have low tensile strength and tear easily when wet, so they are typically used only once, whereas tough, waterproof poly(ethene) bags can be reused dozens of times [1 mark]. 4. Stage 4 (Disposal): Paper bags are biodegradable (decompose naturally via microorganisms in soil/compost) and can be recycled easily [1 mark]. 5. Poly(ethene) bags are non-biodegradable, so if littered or sent to landfill they persist for hundreds of years, taking up space and harming wildlife (though they can be recycled or incinerated for energy) [1 mark]. 6. Conclusion: If customers actually reuse a plastic bag many times and recycle it, its lifetime energy/water footprint is lower than using dozens of heavy single-use paper bags [1 mark].

4. Q4

   A student is given two unlabelled colourless liquids: one is pure distilled water and the other is potable tap water. Describe a chemical/physical experiment to determine which sample is tap water and measure the concentration of dissolved solids in g/dm³. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Weigh an empty, clean, dry evaporating basin using an accurate electronic balance and record its mass [1 mark]. 2. Use a measuring cylinder to measure 50 cm³ (0.050 dm³) of the first water sample into the evaporating basin and heat gently over a water bath until all the water has evaporated [1 mark]. 3. Allow the basin to cool, reweigh it, and subtract the initial mass of the empty basin: pure distilled water will leave zero solid residue (0.00 g change), whereas potable tap water will leave a small white residue of dissolved mineral salts [1 mark]. 4. To calculate the concentration of dissolved solids in g/dm³, divide the mass of solid residue (in g) by the volume in dm³ (0.050 dm³) [1 mark].

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