Water collected from rivers is used in the home for drinking and flushing toilets. Water used in the home must be potable. Potable water is safe to drink. Waste water produced after use in the home is called sewage. Figure 3 shows how water is collected from rivers and returned to rivers after use.

Explain what happens to water in Process A and in Process B in Figure 3. Do not refer to use of water in the home.

This question assesses knowledge of water treatment processes. **Process A: Treatment of ground water to make it potable.** The aim is to make water safe for human consumption. 1. **Filtration:** The water is passed through filter beds, typically made of sand and gravel. This removes insoluble particles like leaves, grit, and other suspended solids. 2. **Sterilisation:** Although the water is now clear, it may still contain harmful microorganisms (pathogens) like bacteria and viruses. To kill these, a sterilising agent is added. Chlorine is most commonly used because it is effective and has a residual effect, keeping the water safe in the pipes. Ozone or ultraviolet (UV) light are alternative methods of sterilisation. **Process B: Treatment of sewage.** The aim is to remove waste materials so the water can be safely returned to the environment (e.g., a river). 1. **Screening:** The incoming sewage is passed through a large metal grid (a screen) to remove large objects such as twigs, plastic bags, and grit. 2. **Primary Treatment (Sedimentation):** The screened sewage is then held in a large settlement tank. Solid organic matter, being denser, sinks to the bottom to form a layer of sludge. The less dense liquid part, called effluent, remains on top. 3. **Secondary Treatment (Aerobic Digestion):** The effluent is transferred to another tank where air is bubbled through it. This provides oxygen for aerobic bacteria to thrive. These useful bacteria feed on the organic waste and harmful microbes in the effluent, breaking them down into harmless substances like carbon dioxide and water. The treated effluent is now safe to be discharged.