Uncontrolled or illegal waste tips are an important environmental problem. The volume of waste is increasing, and there are serious difficulties in finding land for new controlled tipping, in particularly in densely populated areas.
 Approximately 80% if the world's waste ends up in tips. These are large holes dug into the ground where rubbish is deposited, which must meet specific standards for reducing environmental impact. Most countries deposit solid waste in these places. In many nations this system is combines with incineration or recycling plans. Illegal or poorly managed tips pose a continuous threat to the environment. The decomposition of organic waste generates lixiviates (liquids) which can contaminate water supplies. This risk is greater when dangerous waste is mixed in with domestic waste. Many illegal solid waste deposits take place on riverbanks with the intention of their being eroded and carried away by the water flow.
 The lixiviate from a rubbish tip contains partially decomposed organic material, bacteria and by-products formed during the process of degradation, and which may in turn be contaminated by other chemical substances present at the tip. If this filters into underground water channels, it causes serious danger. Filtrations from non-regulation tips may contain carcinogens and synthetic chemical substances; if these liquids run into the drinking water system, they cause contamination and threaten its safety for human consumption.
 Towns and cities with serious waste management problems, due to a lack of land for depositing waste, have opted for incineration in order to reduce the solid volume of waste. However new incinerator projects face the protests of local people due to a fear of the smoke emitted by this waste which can be harmful to their health. Uncontrolled burning of rubbish at tips also contaminates the atmosphere.
 One third of domestic rubbish is organic: vegetables, peels, fruit, leftover food, and garden rubbish. If these are not recycled at source as compost, they end up in a tip. Composting plans which use incorrectly separated or selected waste are destined to fail, as the market does not accept poor quality compost which cannot fertilise the soil.
 This process of decomposing organic waste and other elements from rubbish tips releases gases such as methane - a strong greenhouse gas which contributes to global warming - which can be collected for thermal use and the generation of electricity. There are more and more installations being build to trap methane in this way and achieve energy recovery.
 Preventing dangerous substances from reaching rubbish tips and landfill sites has become a priority. Everyday objects such as certain types of batteries, contain a mix of heavy metals, including cadmium and mercury, which means it is important for special collection schemes for this type of waste must operate efficiently. New standards require the amount of toxic substances in batteries to be reduced in order to prevent contamination if deposited in the environment. Rechargeable batteries prevent the use of toxic substances. Experts consider that the reduction of waste is the only real alternative for a sustainable future. In this regard more efficient recycling programmes are being developed. For example, in some countries, local plans for producing fertilisers prevent biodegradable material from reaching tips.

A compound formed by one atom of oxygen and two of hydrogen, with the molecular formula H2O. At room temperature it is liquid, odourless, tasteless and colourless, and it is essential for life. No form of life is known to exist without this molecule, which covers 72% of the surface of the planet Earth and forms between 50% and 90% of all living bodies. It can be found in practically any part of the biosphere and in its three different states: solid, liquid and gas. It is found in liquid form in seas, rivers, lakes and oceans; in solid form - snow or ice-, on the polar ice caps, on mountain tops and anywhere on earth where the temperature is below 0ºC; and in gas form, as water vapour in the atmosphere.
 Some scientists believe that the chemical components of water (oxygen and hydrogen) were already present in the primitive cloud which gave rise to our solar system, some 4500 million years ago. There is another theory which suggests that a great deal of the water on earth was brought by comets attracted by the earth's gravitational force, which ended up crashing into our planet and, due to their huge momentum, vaporised completely upon impact injecting hydrogen and oxygen to the Earth's atmosphere. Water has no smell, taste, or colour. It can dissolve a large number of chemical substances, including gases. Its boiling point at a pressure of one atmosphere, which is the pressure habitually found at sea level, is 100ºC, and its freezing point is 0ºC.
 It is calculated that each year, 500 million tonnes of industrial waste is discarded around the world. Water can no longer be considered a natural, renewable, low-cost and easily accessible resource, but, in coming years and in contrast to crude oil, it will become our "blue gold". Water is a strategic resource for many countries. Wars are fought over the control rivers and seas, such as the Six Day War in the Middle East. Given the increasingly scarcity of water, many more such conflicts are expected to arise in the future, given the increase in the world's population, pollution, and the issue of global warming.
 The maximum density of liquid water in 1g/cm3, reaching this point at a temperature of 3.8 ºC. It is known as the "universal dissolvent" as no other liquid dissolves as many substances. Almost all naturally-occurring chemical processes, not only in living organisms but also on the natural surface of the Earth, plus those carried out in laboratories and in industry, take place between substances dissolved in water.
 This is the name given to the processes involved in the extraction, treatment and sanitary control of waste products which are tipped into the water from homes, industries and business activities of all kinds. It is estimated that each year, around the world, some 500 million tonnes of industrial waste is poured into the earth's water supply, and therefore purification systems are becoming increasingly important in modern society. 

Water purification systems date back to ancient times: there are archaeological remains of drainage systems in Crete, and in the ruins of Assyrian cities; and the sewer systems built by the Romans, which were mainly designed to redirect the waste thrown onto the street by water channels, still operate correctly. In Medieval Europe, Roman sewers were replaced with private underground tunnels, and later, with cesspools, until, several centuries later, open air channels or street gutters were built. With the introduction of local mains water supplies and distribution pipes, came lavatories and the first modern sanitary systems.
 In the early 20th century some cities and industries began to recognise that direct tipping into rivers, water tables or lakes caused health problems. This led to the construction of purification facilities. At around this time came the septic tank, used as a mechanism for treating domestic residual water in suburban and rural areas. Since the 1970s, chlorination has spread through the industrialised world, another key step in chemical water treatment.
 Residual waters can come from homes, industries, underground sources and the climate, and is known, respectively, as domestic, industrial, infiltration and rain water. All this water is transported to purification plants via pipelines. Upon arrival at the purification plant, the waste is passed through a series of screens, chambers and chemical processes which reduce its volume and toxicity. This reduction requires three phases: primary, secondary and tertiary. In the first phase a large proportion of suspended solids and inorganic material is removed; in the second organic material is reduced by accelerating the natural biological process; the third phase is only carried out when the water is to be reused for urban, farming or industrial purposes. The final disposal of the treated water is completed in a number of ways, but the most common is for it to be deposited into a river or lake.
 

Water resources are one of the most important renewable natural resources for life on earth. This would explain why recent research into our solar system is aimed at searching for signs of water on other planets and moons as indicators of the possible existence of life.
 Water resources are understood to be the part of the water cycle which runs towards rivers and filters into water tables, clearly referring to the rain which falls on land and does not evaporate. According to this definition, water resources are renewable and their value can be calculated on a yearly basis.
 Global water resources inland are calculated at around 40,000 km3/year. However, this overall figure does not reflect the large regional variations in water resources or the enormous increase in demand, factors which have led to a shortage in many parts of the world. For example, Central Africa, which represents 18% of the surface area of the continent, receives 49% of water, while Northern Africa, of a similar size, receives only 1.2% of the water on the entire African continent.
 The imbalance in water distribution on earth means that there are areas with excess water, such as central and southern America, which have one of the largest concentrations of fresh water in the world (46.5%), and others such as the Middle East and Northern Africa, suffer the greatest shortage of water. 6% of the world's population is concentrated into these areas and yet they only have 2% of the global fresh water resources.
 By sectors, farming and agriculture uses the largest proportion of fresh water on the planet, with a world average of 69%, followed by industry (23%) and domestic and urban use (8%). Agriculture is the sector registering the lowest rates of return on investment and where water is used most wastefully.

In environmental engineering, the term "water treatment" refers to the different physical, chemical and biological operations applied in order to remove or reduce contamination of undesirable parts of water, be it natural, mains, processed of residual water (also known as "foul" or "black")
 Residual water can come for industrial or farming activities and domestic use in cities. In the case of urban water, treatments tend to start with a preliminary process, which aims to condition residual water in preparation for the remaining processes, preserving the installations from erosion  and blockages. The next step is the primary treatment, also known as physical-chemical treatment, which reduces the material suspended in the water by precipitation or sedimentation, be it with or without reactors, or using different types of chemical oxidation. The secondary treatment, known as biological treatment, is use on a large scale to remove all diluted organic contamination, which is difficult to eliminate using physical and chemical methods. Finally, a third treatment can be applied which mixes the two previous techniques in order to refine the final result, improving its quality. When used intensively, the resulting water is appropriate for farming and industrial use, and even for further treatment to make drinking water. The installation where this sequence of treatments is carried out is known as a Residual Water Treatment Station.
 It was in England, in 1848, when people began to become aware for the first time of the importance of guaranteeing water quality and the need to treat it in order to achieve acceptable and generalised public health levels. In Spain, the first treatment station was installed in Madrid in 1932. Until that time, there were no more that a few cesspits and other simple purification systems, most foul water being thrown out into the street or, in the case of Madrid, into the Manzanares River, which became a true sewage collector where people nevertheless bathed and washed their clothes. Today, Madrid has one of the best sewage systems in Europe, guaranteeing the treatment of practically all the residual water produced. For this purpose it has eight main regeneration stations and over 60 secondary plants. The model treatment system in this city is supported by over 3000 kms of sewer system, of which 40% can be visited.