Chapter 9 of 17 · Water and the Weather
Chapter 8. From the river to the tap, the French example
The man-made journey water makes between the natural environment and a tap, then from the tap back to the environment, described on the example of France.
A second cycle, built by human hands
Everything this volume has described so far happens with nobody there. The Sun evaporates, the air condenses, gravity brings things down: none of those mechanisms needs to be run. This chapter deals with something else, and that is why it changes vocabulary.
Between the resource and the tap, then between the tap and the natural environment, there is a second journey, entirely man-made, which the water agencies call the small water cycle. It is short, it short-circuits the great one, and it is built from end to end: pumps, treatment works, reservoirs, mains, plants. The Seine-Normandie water agency sums it up in one phrase: the small cycle runs "from the resource to the tap and from the tap to the environment".
The great cycle, for its part, is the one of the previous chapters: water in nature, at the scale of the planet. The two do not stand opposed. The small cycle borrows water from the great one, uses it, and gives it back. Everything is at stake in the quality of what it borrows and of what it gives back.
One warning about scope, and it holds for the whole chapter. The mechanisms described here are universal: everywhere water reaches a tap, it has had to be taken, treated, stored, distributed, then what comes back has had to be collected and cleaned. The INSTITUTIONS, the official words and the figures, for their part, are those of France, because they are published and can be checked. This chapter is therefore the French example of a general mechanism: read the workings as valid everywhere, and the names and the numbers as valid here. Chapter 10 will do the same for the weather warning scheme.
What chapter 7 announced is settled here: running short of water never means that the matter has disappeared, but that it is not in the place wanted, at the moment wanted, in the state wanted. The small cycle is exactly the construction that answers that problem, and it costs treatment works, networks and energy.
The objectives of this chapter
Each one is observable. By the end of the chapter, either you do it, or you know which section to read again.
- Tell the great cycle apart from the small cycle, saying what separates them.
- List in order the stages that run from the resource to the tap.
- List in order the stages that run from the tap to the natural environment.
- Tell a drinking-water treatment plant apart from a wastewater treatment plant, and say what each one receives and releases.
- Read a breakdown of fresh water abstraction by use, and spot the largest use.
What this chapter assumes is already known
Two prerequisites, installed by earlier chapters of this volume: groundwater and infiltration on the one hand, watercourses and the catchment on the other.
From the resource to the tap
The journey begins with an abstraction. Eaufrance describes the resource thus: raw water is taken from groundwater, from lakes or from rivers. Depending on the region, one origin or the other dominates, and that depends directly on what the local subsoil allows, as chapter 5 explained.
That raw water is not drinkable as it stands, and the Artois-Picardie water agency says so with a precision worth quoting: "the water pumped up is clean but it is not drinkable". Clean and drinkable are not synonyms. Water can be perfectly clear and hold micro-organisms or dissolved substances that no eye can detect.
So comes the treatment, in a works. Eaufrance names the two main operations of making water drinkable: filtration and disinfection. Filtration removes the particles in suspension; disinfection destroys micro-organisms. The Artois-Picardie water agency states that, in its own area, the water "is then made drinkable in a treatment works", passing through a filtration and a disinfection with ozone and chlorine.
The water, now drinkable, is then stored. The most visible reservoir is the water tower, and its height is not decorative: it is what puts the network under pressure. The water comes down from the reservoir to the houses by gravity alone, which makes it possible to supply without running a pump continuously, and to go on supplying for a while if the pumping fails.
Last comes distribution: the water is carried by a network of mains to houses, farms and industrial sites, under that pressure. The outward journey is over.
| Stage | What comes in | What is done there | What comes out |
|---|---|---|---|
| Abstraction | Water from the natural environment: groundwater, lake or river. | It is taken by pumping or by capturing a spring. | Raw water, clean but not drinkable. |
| Making it drinkable | Raw water. | It is filtered to remove the particles, then disinfected to destroy the micro-organisms. | Drinking water. |
| Storage | Drinking water. | It is held in reserve up high, most often in a water tower. | Drinking water under pressure. |
| Distribution | Drinking water under pressure. | It is carried through a network of mains. | Water at the tap. |
| Collection | Waste water after use. | It is gathered by the sewers, or directed to an individual system. | Waste water on its way. |
| Treatment | Waste water. | It is cleaned in a wastewater treatment plant, or in an on-site sanitation system. | Water given back to the natural environment, and sludge. |
From the tap to the environment
The return journey begins where the use ends. After use, waste water is collected, and the Seine-Normandie water agency distinguishes two routes: public sewerage and on-site sanitation.
Public sewerage goes through the sewers and through a wastewater treatment plant. The Artois-Picardie water agency describes it thus: "after use by consumers, waste water is carried to the treatment plant to be cleaned there". The plant removes the matter in suspension and treats the dissolved pollution, then releases the water into a watercourse or into the sea.
On-site sanitation concerns houses that are not connected to a network, often in thinly settled areas. Treatment there takes place on the plot itself, through a system belonging to the house, and the water returns to the environment locally.
One clarification is indispensable here, and Eaufrance gives it explicitly: drinking water "never comes from wastewater treatment plants". These are two separate chains, with two different works, two different networks and two directions. The drinking-water treatment plant takes water from the environment and makes it fit to drink; the wastewater treatment plant takes waste water and gives it back to the environment. Confusing the two is the commonest mistake on this subject, and it is the trap of this chapter.
The words to remember
Four words, of which two look alike and must never be confused.
- The great water cycle
- The natural cycle of water at the scale of the planet: evaporation, clouds, precipitation, runoff, infiltration. It works without any human intervention.
- The small water cycle
- The entirely man-made journey that runs from the resource to the tap, then from the tap to the natural environment. It borrows water from the great cycle and gives it back.
- The drinking-water treatment plant
- The installation that receives raw water taken from the environment and makes it fit to drink, mainly by filtration and by disinfection. It stands at the BEGINNING of the small cycle.
- The wastewater treatment plant
- The installation that receives the waste water collected and cleans it before giving it back to the natural environment. It stands at the END of the small cycle, and its water never goes back towards the taps.
How much France abstracts, and what for
The figures that follow come from the public report on the state of the environment and relate to the year 2022. They are useful because they shift a received idea: tap water is not the main use of the fresh water abstracted in France.
About 30 billion cubic metres of fresh water were abstracted in France that year. The breakdown by use is read as percentages, and it is those that must be remembered: cooling for electricity generation comes first with 45 % of the total, drinking water follows with 19 %, then navigation canals with 17 %, agriculture with 12 %, and finally industry and other activities with 7 %. Those five shares do come to one hundred.
This volume keeps to the percentages and does not recompute the corresponding volumes, for a reason of method that deserves stating: the published total is rounded, the published volumes are rounded too, and multiplying a rounded percentage by a rounded total produces a number that is no longer the one in the source. A reconstructed figure looks just as sure as a recorded one, and that is precisely what makes it dangerous.
An essential distinction goes with these figures: abstracting is not consuming. Water abstracted to cool a power station is largely given back to the environment shortly afterwards, a little warmer; water abstracted for irrigation is largely evaporated by the plants and does not come back to the river. The report states that between 12 % and 15 % of the fresh water abstracted is actually consumed, that is to say not given back to the environment.
A second piece of data throws light on the previous chapters: 80 % of abstraction is from surface water, rivers and lakes, and 20 % from groundwater. Be careful about exactly what this split says: it covers all uses taken together, and it does not carry over as it stands to any particular use. Cooling for power stations, which weighs the heaviest, is by its nature done from surface water; to infer the same proportion for drinking water would be a misreading, and this volume does not make it, for want of an open official figure for that use alone.
The trap of this chapter, and it is a double one. First: a wastewater treatment plant does not manufacture drinking water. The two chains never meet, and the water leaving a plant goes back into a watercourse, not into a tap. Second, and this lies outside the scope of this book: nothing in this chapter describes a method that can be used at home. Filtering water does not disinfect it, letting it stand does not clean it, and its appearance tells you nothing about what it contains. Whether water is fit to drink is a matter of regulated checks, made by laboratories, on parameters no eye can detect. Water is drunk because it is supplied as drinking water, never because a book has explained how a treatment works goes about it.
Worked example. Where is a village's water taken from?
Question: "A village is built on a cracked limestone plateau. There is no permanent river within fifteen kilometres, but several springs appear at the foot of the plateau. The water tower stands on the high point of the village." Where does the water probably come from, and why is the water tower there? Example guided up to the next-to-last step.
- Rule out what is impossible
There is no permanent river nearby: abstraction from surface water is therefore ruled out, or in any case very costly. The resource must lie elsewhere. - Read the subsoil
The plateau is limestone and cracked. Chapter 5 established that cracked limestone is permeable: rainwater soaks into it instead of running off. There is therefore, beneath this plateau, a body of groundwater resting on an impermeable bedrock. - Confirm from the springs
Springs appear at the foot of the plateau. A spring is the place where groundwater comes back out into the open air: their presence confirms that groundwater exists and that it flows towards the edges of the plateau. - Conclude on the resource
The village's water is very probably taken from that groundwater, either by a borehole on the plateau or by capturing one of the springs. This is consistent with what chapter 5 established: a water table lies near the surface, and its shallow depth makes it easily accessible. - Over to you: explain the position of the water tower
The water tower stands on the high point of the village. Say what its height is for, and what would happen if it were built down below. One sentence from the section "from the resource to the tap" is enough. Check it in the answer to the self-check, question 5.
Experiment 8. A particle filter, and what it does not do
What you need: a plastic bottle, gravel, sand, a piece of clean cloth, a glass, water dirtied with a little garden soil. SAFETY: this experiment is carried out with an adult's agreement, and the adult is present. It is the ADULT who cuts the bottle, with scissors or a knife, and never a child: cut plastic leaves sharp edges which must then be covered with tape. ABSOLUTELY FORBIDDEN, and it is the heart of this experiment: the water obtained is not drunk, not tasted, and serves no purpose other than being looked at, and then thrown away.
- Prepare the bottle
The adult cuts the bottle in two and turns the top part upside down, neck downwards, inside the bottom part. The upside-down top becomes the funnel, the bottom part becomes the collecting vessel. The adult covers the cut edges with tape. - Stack the layers
First put the cloth at the bottom of the funnel, against the neck. Add a layer of gravel over it, then a layer of sand. The order matters: the water must meet the finest grains last. - Rinse the filter before using it
First pour clear water through. It will come out cloudy, because it carries away the dust from the sand. Do it again until it comes out more or less clear. An unrinsed filter dirties instead of cleaning, and this is the step most often forgotten. - Dirty some water, then filter it
Mix a little garden soil into a glass of water and let it stand for a minute. Then pour that mixture slowly into the funnel and watch what arrives below. - Compare the two glasses
Set the glass of dirty water and the glass of filtered water side by side. The difference is clear: the particles in suspension stayed in the sand. You have just reproduced the FILTRATION stage of a drinking-water treatment works. - Note what the filter has not done
The filtered water is clearer, and that is all you know about it. What was DISSOLVED went through the sand without being held back: the filter removed no salt, no dissolved substance, no micro-organism. Clear water is not clean water, and clean water is not drinking water. - The proof by salt, without putting anything in your mouth
Do the test again with water in which you have dissolved a spoonful of salt, and above all taste nothing. To find out whether the salt went through, take a few drops of the filtered water, put them on a dark, dry plate, and let them evaporate near a window, as in experiment 1. The next day, a white ring is visible on the plate: the salt went through the sand without being held back. The filter could do nothing about what was dissolved. - Tidy up, and wash your hands
Pour all the water down the sink, without exception: nothing that comes out of this set-up is drunk or used. Empty the sand and the gravel into the bin or into the garden, and wash your hands with soap and water before touching anything else, as after experiment 6. It is precisely because filtration does not do everything that a treatment works adds a disinfection behind it.
What to remember from this chapter
- The great cycle is natural and planet-wide; the small cycle is man-made and runs from the resource to the tap, then from the tap to the environment.
- The outward journey comprises abstraction, making the water drinkable by filtration and disinfection, storage up high and distribution under pressure.
- The return journey comprises the collection of waste water, then its cleaning by public sewerage or on-site sanitation, then its release into the environment.
- Raw water that has been pumped up is clean but it is not drinkable: the two words are not synonyms.
- Drinking water never comes from wastewater treatment plants: they are two separate chains that meet nowhere.
- In France, in 2022, about 30 billion cubic metres of fresh water were abstracted, of which 45 % to cool power stations and 19 % for drinking water; between 12 % and 15 % of the total abstraction is actually consumed.
- The mechanisms in this chapter are universal; the institutions, the official words and the figures are those of France, and they are to be read again at their source.
Self-check for chapter 8
Answer in writing, using only the knowledge given by this volume so far. The answers are gathered at the end of the volume.
- List in order the six stages of the small water cycle, from the resource to the return to the environment.
- Someone states that tap water comes out of the wastewater treatment plant. Correct them, and say what each of the two installations receives and releases.
- A pumping operation gives perfectly clear water. Can it be drunk? Justify your answer with the distinction between clean and drinkable.
- In 2022, drinking water accounted for 19 % of fresh water abstraction in France and cooling for power stations 45 %. What has to be added to these figures so as not to conclude too quickly that the power stations are depriving people of water?
- Take up the worked example of the village on the limestone plateau again: say what the height of the water tower is for and what would change if it were built in the lower part of the village.