Note, table and flow-chart completion

Most of the tap water in large cities begins its journey in a river, lake or reservoir. Although it may look clean, this water contains soil particles, tiny organisms and sometimes chemicals from farms and roads, so it must pass through several stages of treatment before it reaches people's homes.

The first stage is screening. As water is drawn from the river, it passes through metal screens that remove large objects such as branches, leaves and plastic litter. Fish are kept out in the same way. The water is then pumped to the treatment plant.

At the plant, chemicals called coagulants are added and mixed in quickly. Very fine particles of dirt carry a small electric charge, which keeps them apart; the coagulants cancel out this charge, so the particles begin to stick together. The water is then stirred gently for around half an hour, and the particles join to form larger, heavier clumps known as flocs.

Next comes sedimentation. The water flows slowly through large tanks, where the flocs sink to the bottom and form a layer of sludge that is removed regularly. By the time the water leaves these tanks, most of the solid material has gone, but very small particles and many microorganisms remain.

To remove these, the water is passed through filters made of layers of sand and gravel, and in some plants also charcoal, which helps to remove unpleasant tastes and smells. The filters gradually become blocked, so they are cleaned by pumping water backwards through them, a process called backwashing.

The final stage is disinfection, which kills any bacteria and viruses that have survived. Most plants use chlorine, although some use ultraviolet light or ozone instead. A small amount of chlorine is often left in the water deliberately, so that it continues to protect the water as it travels through many kilometres of pipes. The treated water is then stored in covered reservoirs or water towers until it is needed.

Throughout the process, samples are taken and tested in laboratories, and in many countries the results must be reported to a government regulator. If a problem is found, the plant can change the amount of chemicals it uses or stop taking water from a particular source until the cause has been identified.

Wind turbines and solar panels produce electricity only when the wind blows or the sun shines, and this does not always match the times when people need power. A cold, still evening, for example, may bring high demand but little wind and no sunshine. For this reason, electricity networks that rely heavily on renewable energy need ways of storing surplus power and releasing it later. Three very different technologies are used.

The oldest is pumped hydroelectric storage, which has provided most of the world's large-scale storage for decades. It uses two reservoirs at different heights. When there is spare electricity, it is used to pump water from the lower reservoir to the upper one; when demand rises, the water is allowed to flow back down through turbines, which generate electricity again. A single plant can store enormous amounts of energy and can operate for fifty years or more. Its main limitation is geography: it needs hills or mountains and large areas of land, and building a new plant can take many years.

Batteries work in a completely different way. Large banks of lithium-ion batteries, similar to those in electric cars but on a far bigger scale, store energy chemically. Their greatest strength is speed: they can respond to a sudden change in demand in a fraction of a second, which helps to keep the network stable. Their costs have fallen sharply in recent years. However, most battery systems are designed to supply power for only a few hours, and their performance gradually declines as they are charged and discharged thousands of times.

A third option, compressed air storage, uses spare electricity to pump air into large underground spaces, such as old salt caverns. When power is needed, the air is released, heated and used to drive a turbine. Because suitable caverns can hold huge volumes of air, this method can store energy for long periods. Its weakness is efficiency: compressing air produces heat, and unless that heat is captured and stored, a large share of the original energy is lost. Only a small number of such plants exist.

No single technology is likely to meet every need. Batteries are well suited to balancing the network from minute to minute, while pumped hydro and compressed air may be more useful for storing energy over days. Engineers expect future networks to combine several methods.