**4. Management of Water Resources**

The primary source of freshwater for the use of human beings comes from the water that runs off after rain and feeds rivers, lakes and aquifers. However, freshwater is unevenly distributed on the planet: about three-quarters of the annual precipitations occur in specific areas and about 80% of the available freshwater is located in few basins, like in the Great Lakes of North America, in Africa and in the Baikal lake of Siberia; or in the five major fluvial systems: Amazon, Ganges with Brahmaputra, Congo, Yangtze and Orinoco. Large parts of the globe do not have enough water for human use and in many cases transport of water from nearby zones is required. In a growing number of cases freshwater is produced by desalination (see below).

The development of agricultural practices allowed the feeding of more people per hectare, thus inducing an increase in population size. Human beings soon realized that it was useful to move water to zones were land fertility could be increased through irrigation. Appropriate canals were constructed already a long time ago: remains of them, dating perhaps from the 6th millennium BCE, can be seen in Iran; others, built at least three millennia BCE, are found in India or China; and others, built one or two millennia BCE, have been found in Africa, departing from the Niger River. With the discovery of the New World, the Spanish *conquistadores* were astonished at the sight of a network of canals covering, on the Andes of Peru, an area of about 700 km2; these canals were dug into the rock with rudimentary stone tools and without the help of animals.

Many canals were built in modern times and today the irrigated areas of the planet are very large: the 2008 estimate was about 3 million km2, almost 70% of which are located in Asia, 15% in America and the rest distributed between Europe, Africa and Oceania. The following are few examples of modern structures built for the transfer of water. The aqueduct of Arizona is an open-air canal, 540 km long, constructed to provide irrigation of 400,000 hectares of land in Arizona. The aqueduct of California is a complex system of canals, tunnels and pipelines having a total length of about 1100 km. In Libya, the Great Man-Made River was built to transport water extracted from more than 1300 wells located in the South of the country in order to distribute it to the cities of Tripoli, Sirte and Benghazi. The transport of water required the assembly of large pipelines, for a total length of 3000 km. Unfortunately, the pipelines were damaged in 2011 during a war conflict and for this reason the Libyan cities are today facing a water shortage. This is a good example of the tight relationship between water needs and security [50]. Extensive use of water for irrigation has caused many cases of aquifers exhaustions, river depletion (for example the Yellow River, the Indus, the Colorado, the Nile) and a serious decrease in the volume of some lakes, as in the case of the Aral Sea [51]. Sometimes water sharing becomes a problem because of climate change or because two or more States do not agree in the sharing of the water of a lake or of a river or of an aquifer. A recent publication dealing with these problems is the World Water Development Report 2019 [52].

The managemen<sup>t</sup> and supply of water for irrigation is facilitated by the use of dams, mostly built in modern times. In 1997 the World Commission on Dams estimated that 800,000 dams exist in the world and contribute to 12–16% of the global agricultural production.

As stressed above, agriculture also changed the nomadic habits of people, who became sedentary and started new activities, like the production of artisan tools and their trading: this led to a social division of labour and a social stratification, thus generating the appearance of towns. Today the managemen<sup>t</sup> of water resources for the inhabitants of a city is a complex problem. It implies to provide enough water for drinking and for personal hygiene; to manage the disposal of wastewater and to use structures that avoid the spreading of waterborne diseases; at the same time, it is important to provide enough water for irrigation. In this respect, it is interesting to reflect on what happened during the development and expansion of ancient Rome. This city was founded in the 8th century BCE (the year 753, according to the legend) when it was inhabited by very few people, perhaps a few thousand; at the time of maximum expansion it approached (or perhaps exceeded) one million people. This remarkable increase was also permitted by the efficient managemen<sup>t</sup> of the freshwater used and by an accurate wastewater disposal. The city of Rome grew from small settlements, mainly located on the hills called *Palatinus*, *Capitolinus* and *Aventinus*, which were close to a ford on the river Tiber. The inhabitants used the river for procuring water, for fishing, for navigation and to exchange goods with the Etruscans that lived on the other bank of the river. They also used water coming from few springs that were located on the top of the hills and disposed of their waste material downhill. The Romans were open to immigration and therefore the number of inhabitants increased rapidly. As a consequence, they needed: (1) more space; (2) more water; and (3) a waste disposal structure. They began to settle in the space located between the hills and increased the provision of water by taking it from springs located far from Rome and transported through aqueducts built for this purpose. They soon realized that drinking water should not be mixed with wastewater and, to cope with the problems related to the needs of waste disposal, they started building a sewer. This was originally built around the year 600 BCE as an open-air canal that drained the sewage to the river Tiber; later on, the Romans covered the canal and turned it into a sewer system for the city, called the "Greatest Sewer" (*Cloaca Maxima*). In the city of Rome, therefore, we observe the first example of an accurate separation between clean water and wastewater (today many countries have built sewer networks and wastewater treatment facilities that have reduced the incidence of waterborne diseases). At the time of maximum expansion, the city of Rome utilized 11 aqueducts that carried 700,000 m<sup>3</sup> of water per day. At the same time, about 600 aqueducts were built throughout the Roman Empire (in Italy, Spain, Germany, France and North Africa) and contributed to the foundation or expansion, of many cities. The main reason to build the aqueducts out of Rome was to increase the efficiency of irrigation and thus to produce food to be imported (and cope with the problem of lack of water for irrigation). In fact, cities need land to grow vegetables and water to irrigate it. But cities have very little space for agriculture and therefore they need to import food. When they do it, they indirectly import the water that has been used to produce it, namely they import water-intensive commodities. The "water footprint" concept has been introduced by Hoekstra and collaborators. It refers to the total volume of freshwater used to produce a specific good, under standard conditions [53].

As discussed above, a long time ago people changed their nomadic habits, became sedentary and established themselves near the farms where they worked. Recently however, almost a century ago, a drastic social change took place: with the advent of the industrial revolution and the increased productivity in agriculture (the so-called green revolution), job opportunities in the country-side decreased and people were attracted by better salaries in the cities or by a higher standard of living and centralized services. The social, economic and environmental problems associated with a predominantly urbanized population are considerably different from those of the rural population of the past. Moreover, the population density is higher and therefore the spreading of contagious

diseases and of antibiotic resistant pathogens becomes more efficient; therefore, the quantity of water necessary for personal hygiene and sanitation becomes higher [54]. At the same time antibiotic resistant infections are becoming more frequent and their danger is discussed within WHO among the States member of this organization [55].

Large metropolitan areas encounter special problems that are not present in small or medium size cities. A megacity is usually defined as a metropolitan area with a total population in excess of ten million people. The new megacities experienced a recent and rapid urbanization, which sometimes modifies the territory and alters the pre-existing water fluxes, thus creating the necessity to adapt the infrastructures for distribution. Moreover, the administrative boundaries of a megacity may not coincide with those previously existing in the urbanized area and therefore the upgrading of the networks for water delivery and the infrastructures for sewage disposal is more difficult to manage. Finally, in megacities the percentage of impervious soil is high as compared to that of villages and therefore the sewerage system should be able to combine the managemen<sup>t</sup> of the usual type of urban waste with the abundant rain water that occasionally comes from the streets when it rains. In conclusion, in large cities water is distributed through complex and expensive structures; moreover, it is important to establish an accurate system of sanitation performed under the surveillance of a central authority.

Water supply policies and regulations are under the responsibility of the highest Authority of the country. In the ancient Rome the top officer, named "*curator aquarum*" reported directly to the Emperor. Today the Member States of the European Union conform their decisions to the European Directive number 60/2000. Water and sanitation policy in the USA is under the responsibility of the Environmental Protection Agency, which reports directly to the President. In most other countries the responsibility is entrusted directly to different ministries, as the Ministry of Environment or the Ministry of Health, of Public Works and so forth. In general, the political authorities publish each year a report on the diseases presumably caused by contaminated water and share this information with the World Health Organization. The publication of these data contributes to raise the awareness of the public opinion and thus it may induce the governments to increase the prevention of diseases caused by contaminated water.

Water supply for personal use should in principle reach all private houses, 24 h per day, at constant pressure. It should be clean, non-toxic and free from pathogenic microorganisms. For this reason, before distribution, the level of a detailed list of substances is analysed, as well as the presence of specific microorganisms; the analyses are repeated at predetermined intervals of time and numerous samples are taken at different points of the distribution chain. The results of the analyses are communicated to the authority entrusted for surveillance and are publicized. When a specific parameter increases over a certain value, predetermined precautions are enforced. It should be pointed out that taste cannot be determined through chemical and microbiological analyses and therefore water distributed by water companies, although safe, sometimes is not pleasant to drink. Furthermore, the parameters analysed are not necessarily complete, since new advances in technology may introduce into the environment new substances that may be toxic [56]. For this reason, the existing rules are sometimes revised and the analysis of one or more parameters is added.

Water has become very important from an economic point of view but the economic value of something that is essential for human survival and for human dignity is difficult to define. Moreover, lack of water and of sanitation structures favours the appearance of contagious diseases that later on spread to all sectors of the population: it is therefore in the interest of society to make available to everybody enough water for sanitation. One principle that is widely accepted is that the price should be affordable by all citizens, including the very poor. Moreover, when there is a shortage, water should be distributed with impartiality.

Wastewater managemen<sup>t</sup> for appropriate sanitation is more important today as compared to the past: in fact, our hunter-gatherer ancestors produced very little pollution per person as compared to what happens today and water contamination is today more frequent because of the presence of domestic animals [57]. Wastewater sanitation is today insufficient in many parts of the world and proper managemen<sup>t</sup> of sewer material should become a priority. This is important to preserve the quality of freshwater necessary for humanity: in fact, today, as the percentage of the total available freshwater required for human purposes is increasing (see below), it is becoming more important to preserve the water that is not used. The effluent of the wastewater treatment plants sometimes is reused for different purposes, as recovery of nutrients or irrigation or even drinking. Wastewater treatment is usually adapted to the type of material received by the depurator plant. After separation of solid material, a prolonged aeration is used to allow microbial digestion of organic substances and of nitrates, followed by disinfection to kill pathogenic bacteria. Membrane filtration may be used to remove some impurities. Thus, the wastewater treatment plants contain complex apparatus of different types. Wastewater treatment is more complex in a city as compared to that required for a small village [58]. Sewage treatment plants receive all types of hazardous waste from households, hospitals and industries. Combined sewers require much larger and more expensive treatment facilities as compared to plants serving small, well defined areas and therefore it is sometimes more practical and less expensive to use different waste disposal plants that are specialized for the substances produced in different portions of the city. In fact, there are potentially thousands of components of sludge that remain untested or undetected and are disposed of from modern society and that have been proven to be hazardous to both human and ecological health. Residents living in certain zones of the city sometimes show an increased risk for certain respiratory, gastrointestinal and other diseases [59]. Although correlation does not imply causation, these observations may lead to conclude that precaution is necessary.

Water desalination is widely used in different parts of the world [60]. According to the International Desalination Association, in 2015 there were more than 18,000 plants worldwide, mainly located in arid areas, providing water for 300 million people. Most of these plants use membranes to separate water (usually seawater) from the dissolved salts; energy is needed to apply pressure on one side of the membrane. Less energy would be required if the permeability of membranes to water is selectively enhanced and some private companies are beginning to try to use analogues of the aquaporin proteins to increase the efficiency of the process [61,62].

In conclusion, the managemen<sup>t</sup> of water availability has been a major factor in the development of humanity. Therefore, it is crucial to be prepared to make water available for a growing population and at the same time take care of appropriate sanitation.
