Ozone depletion describes two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere, a much larger springtime decrease in stratospheric ozone around Earth's polar regions. The latter phenomenon is referred to as the ozone hole. There are springtime polar tropospheric ozone depletion events in addition to these stratospheric events; the main cause of ozone depletion and the ozone hole is manufactured chemicals manufactured halocarbon refrigerants, solvents and foam-blowing agents, referred to as ozone-depleting substances. These compounds are transported into the stratosphere by turbulent mixing after being emitted from the surface, mixing much faster than the molecules can settle. Once in the stratosphere, they release halogen atoms through photodissociation, which catalyze the breakdown of ozone into oxygen. Both types of ozone depletion were observed to increase. Ozone depletion and the ozone hole have generated worldwide concern over increased cancer risks and other negative effects.
The ozone layer prevents most harmful UVB wavelengths of ultraviolet light from passing through the Earth's atmosphere. These wavelengths cause skin cancer and cataracts, which were projected to increase as a result of thinning ozone, as well as harming plants and animals; these concerns led to the adoption of the Montreal Protocol in 1987, which bans the production of CFCs, halons and other ozone-depleting chemicals. The ban came into effect in 1989. Ozone levels began to recover in the 2000s. Recovery is projected to continue over the next century, the ozone hole is expected to reach pre-1980 levels by around 2075; the Montreal Protocol is considered the most successful international environmental agreement to date. Three forms of oxygen are involved in the ozone-oxygen cycle: oxygen atoms, oxygen gas, ozone gas. Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing ultraviolet photons; this converts a single O2 into two atomic oxygen radicals. The atomic oxygen radicals combine with separate O2 molecules to create two O3 molecules.
These ozone molecules absorb ultraviolet light, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom joins up with an oxygen molecule to regenerate ozone; this is a continuing process that terminates when an oxygen atom recombines with an ozone molecule to make two O2 molecules. O + O3 → 2 O2 The total amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination. Ozone can be destroyed by a number of free radical catalysts; the dot is a notation to indicate that each species has an unpaired electron and is thus reactive. All of these have both man-made sources; these elements are found in stable organic compounds chlorofluorocarbons, which can travel to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are released from the parent compounds by the action of ultraviolet light, e.g. CFCl3 + electromagnetic radiation → Cl· + ·CFCl2 Ozone is a reactive molecule that reduces to the more stable oxygen form with the assistance of a catalyst.
Cl and Br atoms destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom to form chlorine monoxide and leaving an oxygen molecule; the ClO can react with a second molecule of ozone, releasing the chlorine atom and yielding two molecules of oxygen. The chemical shorthand for these gas-phase reactions is: Cl· + O3 → ClO + O2 A chlorine atom removes an oxygen atom from an ozone molecule to make a ClO molecule ClO + O3 → Cl· + 2 O2 This ClO can remove an oxygen atom from another ozone molecule. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere. A single chlorine atom would continuously destroy ozone for up to two years were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride and chlorine nitrate. Bromine is more efficient than chlorine at destroying ozone on a per atom basis, but there is much less bromine in the atmosphere at present.
Both chlorine and bromine contribute to overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, fluorine atoms react with water and methane to form bound HF in the Earth's stratosphere, while organic molecules containing iodine react so in the lower atmosphere that they do not reach the stratosphere in significant quantities. A single chlorine atom is able to react with an average of 100,000 ozone molecules before it is removed from the catalytic cycle; this fact plus the amount of chlorine released into the atmosphe
Water pollution is the contamination of water bodies as a result of human activities. Water bodies include for example lakes, oceans and groundwater. Water pollution results. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems. In turn, this can lead to public health problems for people living downstream, they may use the same polluted river water for bathing or irrigation. Water pollution is the leading worldwide cause of death and disease, e.g. due to water-borne diseases. Water pollution can be grouped into surface water pollution. Marine pollution and nutrient pollution are subsets of water pollution. Sources of water pollution are either non-point sources. Point sources have one identifiable cause of the pollution, such as a storm drain, wastewater treatment plant or stream. Non-point sources are more diffuse, such as agricultural runoff. Pollution is the result of the cumulative effect over time. All plants and organisms living in or being exposed to polluted water bodies can be impacted.
The effects can damage individual species and impact the natural biological communities they are part of. The causes of water pollution include a wide range of chemicals and pathogens as well as physical parameters. Contaminants may include inorganic substances. Elevated temperatures can lead to polluted water. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. Elevated water temperatures decrease oxygen levels, which can kill fish and alter food chain composition, reduce species biodiversity, foster invasion by new thermophilic species. Water pollution is measured by analysing water samples. Physical and biological tests can be done. Control of water pollution requires appropriate management plans; the infrastructure may include wastewater treatment plants. Sewage treatment plants and industrial wastewater treatment plants are required to protect water bodies from untreated wastewater. Agricultural wastewater treatment for farms, erosion control from construction sites can help prevent water pollution.
Nature-based solutions are another approach to prevent water pollution. Effective control of urban runoff includes reducing quantity of flow. In the United States, best management practices for water pollution include approaches to reduce the quantity of water and improve water quality. Water is referred to as polluted when it is impaired by anthropogenic contaminants. Due to these contaminants it either does not support a human use, such as drinking water, or undergoes a marked shift in its ability to support its biotic communities, such as fish. Natural phenomena such as volcanoes, algae blooms and earthquakes cause major changes in water quality and the ecological status of water. Water pollution is a major global problem, it requires ongoing revision of water resource policy at all levels. It has been suggested. Water pollution accounted for the deaths of 1.8 million people in 2015. India and China are two countries with high levels of water pollution: An estimated 580 people in India die of water pollution related illness every day.
About 90 percent of the water in the cities of China is polluted. As of 2007, half a billion Chinese had no access to safe drinking water. In addition to the acute problems of water pollution in developing countries, developed countries continue to struggle with pollution problems. For example, in a report on water quality in the United States in 2009, 44 percent of assessed stream miles, 64 percent of assessed lake acres, 30 percent of assessed bays and estuarine square miles were classified as polluted. Surface water pollution includes pollution of rivers and oceans. A subset of surface water pollution is marine pollution. One common path of entry by contaminants to the sea are rivers. An example is directly discharging sewage and industrial waste into the ocean. Pollution such as this occurs in developing nations. In fact, the 10 largest emitters of oceanic plastic pollution worldwide are, from the most to the least, Indonesia, Vietnam, Sri Lanka, Egypt, Malaysia and Bangladesh through the rivers Yangtze, Yellow, Nile, Pearl, Amur and the Mekong, accounting for "90 percent of all the plastic that reaches the world's oceans."Large gyres in the oceans trap floating plastic debris.
Plastic debris can absorb toxic chemicals from ocean pollution poisoning any creature that eats it. Many of these long-lasting pieces end up in the stomachs of marine animals; this results in obstruction of digestive pathways, which leads to reduced appetite or starvation. There are a variety of secondary effects stemming not from the original pollutant, but a derivative condition. An example is silt-bearing surface runoff, which can inhibit the penetration of sunlight through the water column, hampering photosynthesis in aquatic plants. Interactions between groundwater and surface water are complex. Groundwater pollution referred to as groundwater contamination, is not as classified as surface water pollution. By its nature, groundwater aquifers are susceptible to contamination from sources that may not directly affect surface water bodies; the distinction of point vs. non-point source may be irrelevant. Analysis of groundwater contamination may focus on soil characteristics and site
Air pollution occurs when harmful or excessive quantities of substances including gases and biological molecules are introduced into Earth's atmosphere. It may cause diseases and death to humans. Both human activity and natural processes can generate air pollution. Indoor air pollution and poor urban air quality are listed as two of the world's worst toxic pollution problems in the 2008 Blacksmith Institute World's Worst Polluted Places report. According to the 2014 World Health Organization report, air pollution in 2012 caused the deaths of around 7 million people worldwide, an estimate echoed by one from the International Energy Agency. An air pollutant is a material in the air that can have adverse effects on the ecosystem; the substance can be liquid droplets, or gases. A pollutant can be of man-made. Pollutants are classified as secondary. Primary pollutants are produced by processes such as ash from a volcanic eruption. Other examples include carbon monoxide gas from motor vehicle exhausts or sulphur dioxide released from the factories.
Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants interact. Ground level ozone is a prominent example of secondary pollutants; some pollutants may be both primary and secondary: they are both emitted directly and formed from other primary pollutants. Substances emitted into the atmosphere by human activity include: Carbon dioxide – Because of its role as a greenhouse gas it has been described as "the leading pollutant" and "the worst climate pollution". Carbon dioxide is a natural component of the atmosphere, essential for plant life and given off by the human respiratory system; this question of terminology has practical effects, for example as determining whether the U. S. Clean Air Act is deemed to regulate CO2 emissions. CO2 forms about 410 parts per million of earth's atmosphere, compared to about 280 ppm in pre-industrial times, billions of metric tons of CO2 are emitted annually by burning of fossil fuels. CO2 increase in earth's atmosphere has been accelerating.
Sulfur oxides – sulphur dioxide, a chemical compound with the formula SO2. SO2 is produced in various industrial processes. Coal and petroleum contain sulphur compounds, their combustion generates sulphur dioxide. Further oxidation of SO2 in the presence of a catalyst such as NO2, forms H2SO4, thus acid rain; this is one of the causes for concern over the environmental impact of the use of these fuels as power sources. Nitrogen oxides – Nitrogen oxides nitrogen dioxide, are expelled from high temperature combustion, are produced during thunderstorms by electric discharge, they can be seen as a plume downwind of cities. Nitrogen dioxide is a chemical compound with the formula NO2, it is one of several nitrogen oxides. One of the most prominent air pollutants, this reddish-brown toxic gas has a characteristic sharp, biting odor. Carbon monoxide – CO is a colorless, toxic yet non-irritating gas, it is a product of combustion of fuel such as natural coal or wood. Vehicular exhaust contributes to the majority of carbon monoxide let into our atmosphere.
It creates a smog type formation in the air, linked to many lung diseases and disruptions to the natural environment and animals. In 2013, more than half of the carbon monoxide emitted into our atmosphere was from vehicle traffic and burning one gallon of gas will emit over 20 pounds of carbon monoxide into the air. Volatile organic compounds – VOCs are a well-known outdoor air pollutant, they are categorized as either non-methane. Methane is an efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon VOCs are significant greenhouse gases because of their role in creating ozone and prolonging the life of methane in the atmosphere; this effect varies depending on local air quality. The aromatic NMVOCs benzene and xylene are suspected carcinogens and may lead to leukemia with prolonged exposure. 1,3-butadiene is another dangerous compound associated with industrial use. Particulate matter / particles, alternatively referred to as particulate matter, atmospheric particulate matter, or fine particles, are tiny particles of solid or liquid suspended in a gas.
In contrast, aerosol refers to gas. Some particulates occur originating from volcanoes, dust storms and grassland fires, living vegetation, sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes generate significant amounts of aerosols. Averaged worldwide, anthropogenic aerosols—those made by human activities—currently account for 10 percent of our atmosphere. Increased levels of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer. Particulates are related to respiratory infections and can be harmful to those suffering from conditions like asthma. Persistent free radicals connected to airborne fine particles are linked to cardiopulmonary disease. Toxic metals, such as lead and mercury their compounds. Chlorofluorocarbons – harmful to the ozone layer; these are gases which are released from air conditioners, aerosol sprays, etc. On release into the air, CFCs rise to the stratosphere.
Here they come in contact with other gases and
Chemical engineering is a branch of engineering that uses principles of chemistry, mathematics and economics to efficiently use, produce and transport chemicals and energy. A chemical engineer designs large-scale processes that convert chemicals, raw materials, living cells and energy into useful forms and products. Chemical engineers are involved in many aspects of plant design and operation, including safety and hazard assessments, process design and analysis, control engineering, chemical reaction engineering, biological engineering, construction specification, operating instructions. Chemical engineering degree is directly linked with all the majors of various engineering disciplines. A 1996 British Journal for the History of Science article cites James F. Donnelly for mentioning an 1839 reference to chemical engineering in relation to the production of sulfuric acid. In the same paper however, George E. Davis, an English consultant, was credited for having coined the term. Davis tried to found a Society of Chemical Engineering, but instead it was named the Society of Chemical Industry, with Davis as its first Secretary.
The History of Science in United States: An Encyclopedia puts the use of the term around 1890. "Chemical engineering", describing the use of mechanical equipment in the chemical industry, became common vocabulary in England after 1850. By 1910, the profession, "chemical engineer," was in common use in Britain and the United States. Chemical engineering emerged upon the development of unit operations, a fundamental concept of the discipline of chemical engineering. Most authors agree that Davis invented the concept of unit operations if not developed it, he gave a series of lectures on unit operations at the Manchester Technical School in 1887, considered to be one of the earliest such about chemical engineering. Three years before Davis' lectures, Henry Edward Armstrong taught a degree course in chemical engineering at the City and Guilds of London Institute. Armstrong's course failed because its graduates were not attractive to employers. Employers of the time would have rather hired mechanical engineers.
Courses in chemical engineering offered by Massachusetts Institute of Technology in the United States, Owens College in Manchester and University College London suffered under similar circumstances. Starting from 1888, Lewis M. Norton taught at MIT the first chemical engineering course in the United States. Norton's course was contemporaneous and similar to Armstrong's course. Both courses, however merged chemistry and engineering subjects along with product design. "Its practitioners had difficulty convincing engineers that they were engineers and chemists that they were not chemists." Unit operations was introduced into the course by William Hultz Walker in 1905. By the early 1920s, unit operations became an important aspect of chemical engineering at MIT and other US universities, as well as at Imperial College London; the American Institute of Chemical Engineers, established in 1908, played a key role in making chemical engineering considered an independent science, unit operations central to chemical engineering.
For instance, it defined chemical engineering to be a "science of itself, the basis of which is... unit operations" in a 1922 report. Meanwhile, promoting chemical engineering as a distinct science in Britain led to the establishment of the Institution of Chemical Engineers in 1922. IChemE helped make unit operations considered essential to the discipline. In 1940s, it became clear that unit operations alone were insufficient in developing chemical reactors. While the predominance of unit operations in chemical engineering courses in Britain and the United States continued until the 1960s, transport phenomena started to experience greater focus. Along with other novel concepts, such as process systems engineering, a "second paradigm" was defined. Transport phenomena gave an analytical approach to chemical engineering while PSE focused on its synthetic elements, such as control system and process design. Developments in chemical engineering before and after World War II were incited by the petrochemical industry, advances in other fields were made as well.
Advancements in biochemical engineering in the 1940s, for example, found application in the pharmaceutical industry, allowed for the mass production of various antibiotics, including penicillin and streptomycin. Meanwhile, progress in polymer science in the 1950s paved way for the "age of plastics". Concerns regarding the safety and environmental impact of large-scale chemical manufacturing facilities were raised during this period. Silent Spring, published in 1962, alerted its readers to the harmful effects of DDT, a potent insecticide; the 1974 Flixborough disaster in the United Kingdom resulted in 28 deaths, as well as damage to a chemical plant and three nearby villages. The 1984 Bhopal disaster in India resulted in 4,000 deaths; these incidents, along with other incidents, affected the reputation of the trade as industrial safety and environmental protection were given more focus. In response, the IChemE required safety to be part of every degree course that it accredited after 1982. By the 1970s, legislation and monitoring agencies were instituted in various countries, such as France and the United States.
Advancements in computer science found applications designing and managing plants, simplifying calculations and drawings that had to be done manually. The c
Renewable energy is energy, collected from renewable resources, which are replenished on a human timescale, such as sunlight, rain, tides and geothermal heat. Renewable energy provides energy in four important areas: electricity generation and water heating/cooling and rural energy services. Based on REN21's 2017 report, renewables contributed 19.3% to humans' global energy consumption and 24.5% to their generation of electricity in 2015 and 2016, respectively. This energy consumption is divided as 8.9% coming from traditional biomass, 4.2% as heat energy, 3.9% from hydroelectricity and the remaining 2.2% is electricity from wind, solar and other forms of biomass. Worldwide investments in renewable technologies amounted to more than US$286 billion in 2015. Globally, there are an estimated 7.7 million jobs associated with the renewable energy industries, with solar photovoltaics being the largest renewable employer. Renewable energy systems are becoming more efficient and cheaper and their share of total energy consumption is increasing.
As of 2015 worldwide, more than half of all new electricity capacity installed was renewable. Growth in consumption of coal and oil could end by 2020 due to increased uptake of renewables and natural gas. At the national level, at least 30 nations around the world have renewable energy contributing more than 20 percent of energy supply. National renewable energy markets are projected to continue to grow in the coming decade and beyond; some places and at least two countries and Norway, generate all their electricity using renewable energy and many other countries have the set a goal to reach 100% renewable energy in the future. At least 47 nations around the world have over 50 percent of electricity from renewable resources. Renewable energy resources exist over wide geographical areas, in contrast to fossil fuels, which are concentrated in a limited number of countries. Rapid deployment of renewable energy and energy efficiency technologies is resulting in significant energy security, climate change mitigation, economic benefits.
In international public opinion surveys there is strong support for promoting renewable sources such as solar power and wind power. While many renewable energy projects are large-scale, renewable technologies are suited to rural and remote areas and developing countries, where energy is crucial in human development; as most of renewable energy technologies provide electricity, renewable energy deployment is applied in conjunction with further electrification, which has several benefits: electricity can be converted to heat, can be converted into mechanical energy with high efficiency, is clean at the point of consumption. In addition, electrification with renewable energy is more efficient and therefore leads to significant reductions in primary energy requirements, because most renewable energy technologies do not need a thermodynamic cycle with high losses. Renewable energy flows involve natural phenomena such as sunlight, tides, plant growth, geothermal heat, as the International Energy Agency explains: Renewable energy is derived from natural processes that are replenished constantly.
In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, ocean, biomass, geothermal resources, biofuels and hydrogen derived from renewable resources. Renewable energy resources and significant opportunities for energy efficiency exist over wide geographical areas, in contrast to other energy sources, which are concentrated in a limited number of countries. Rapid deployment of renewable energy and energy efficiency, technological diversification of energy sources, would result in significant energy security and economic benefits, it would reduce environmental pollution such as air pollution caused by burning of fossil fuels and improve public health, reduce premature mortalities due to pollution and save associated health costs that amount to several hundred billion dollars annually only in the United States. Renewable energy sources, that derive their energy from the sun, either directly or indirectly, such as hydro and wind, are expected to be capable of supplying humanity energy for another 1 billion years, at which point the predicted increase in heat from the sun is expected to make the surface of the earth too hot for liquid water to exist.
Climate change and global warming concerns, coupled with high oil prices, peak oil, increasing government support, are driving increasing renewable energy legislation and commercialization. New government spending and policies helped the industry weather the global financial crisis better than many other sectors. According to a 2011 projection by the International Energy Agency, solar power generators may produce most of the world's electricity within 50 years, reducing the emissions of greenhouse gases that harm the environment; as of 2011, small solar PV systems provide electricity to a few million households, micro-hydro configured into mini-grids serves many more. Over 44 million households use biogas made in household-scale digesters for lighting and/or cooking, more than 166 million households rely on a new generation of more-efficient biomass cookstoves. United Nations' Secretary-General Ban Ki-moon has said that renewable energy has the ability to lift the poorest nations to new levels of prosperity.
At the national level, at least 30 nations around the world have renewable energy contributing more than 20% of energy supply. Na
Civil engineering is a professional engineering discipline that deals with the design and maintenance of the physical and built environment, including public works such as roads, canals, airports, sewerage systems, structural components of buildings, railways. Civil engineering is traditionally broken into a number of sub-disciplines, it is considered the second-oldest engineering discipline after military engineering, it is defined to distinguish non-military engineering from military engineering. Civil engineering takes place in the public sector from municipal through to national governments, in the private sector from individual homeowners through to international companies. Civil engineering is the application of physical and scientific principles for solving the problems of society, its history is intricately linked to advances in the understanding of physics and mathematics throughout history; because civil engineering is a wide-ranging profession, including several specialized sub-disciplines, its history is linked to knowledge of structures, materials science, geology, hydrology, environment and other fields.
Throughout ancient and medieval history most architectural design and construction was carried out by artisans, such as stonemasons and carpenters, rising to the role of master builder. Knowledge was retained in guilds and supplanted by advances. Structures and infrastructure that existed were repetitive, increases in scale were incremental. One of the earliest examples of a scientific approach to physical and mathematical problems applicable to civil engineering is the work of Archimedes in the 3rd century BC, including Archimedes Principle, which underpins our understanding of buoyancy, practical solutions such as Archimedes' screw. Brahmagupta, an Indian mathematician, used arithmetic in the 7th century AD, based on Hindu-Arabic numerals, for excavation computations. Engineering has been an aspect of life since the beginnings of human existence; the earliest practice of civil engineering may have commenced between 4000 and 2000 BC in ancient Egypt, the Indus Valley Civilization, Mesopotamia when humans started to abandon a nomadic existence, creating a need for the construction of shelter.
During this time, transportation became important leading to the development of the wheel and sailing. Until modern times there was no clear distinction between civil engineering and architecture, the term engineer and architect were geographical variations referring to the same occupation, used interchangeably; the construction of pyramids in Egypt were some of the first instances of large structure constructions. Other ancient historic civil engineering constructions include the Qanat water management system the Parthenon by Iktinos in Ancient Greece, the Appian Way by Roman engineers, the Great Wall of China by General Meng T'ien under orders from Ch'in Emperor Shih Huang Ti and the stupas constructed in ancient Sri Lanka like the Jetavanaramaya and the extensive irrigation works in Anuradhapura; the Romans developed civil structures throughout their empire, including aqueducts, harbors, bridges and roads. In the 18th century, the term civil engineering was coined to incorporate all things civilian as opposed to military engineering.
The first self-proclaimed civil engineer was John Smeaton. In 1771 Smeaton and some of his colleagues formed the Smeatonian Society of Civil Engineers, a group of leaders of the profession who met informally over dinner. Though there was evidence of some technical meetings, it was little more than a social society. In 1818 the Institution of Civil Engineers was founded in London, in 1820 the eminent engineer Thomas Telford became its first president; the institution received a Royal Charter in 1828, formally recognising civil engineering as a profession. Its charter defined civil engineering as:the art of directing the great sources of power in nature for the use and convenience of man, as the means of production and of traffic in states, both for external and internal trade, as applied in the construction of roads, aqueducts, river navigation and docks for internal intercourse and exchange, in the construction of ports, moles and lighthouses, in the art of navigation by artificial power for the purposes of commerce, in the construction and application of machinery, in the drainage of cities and towns.
The first private college to teach civil engineering in the United States was Norwich University, founded in 1819 by Captain Alden Partridge. The first degree in civil engineering in the United States was awarded by Rensselaer Polytechnic Institute in 1835; the first such degree to be awarded to a woman was granted by Cornell University to Nora Stanton Blatch in 1905. In the UK during the early 19th century, the division between civil engineering and military engineering, coupled with the demands of the Industrial Revolution, spawned new engineering education initiatives: the Class of Civil Engineering and Mining was founded at King's College London in 1838 as a response to the growth of the railway system and the need for more qualified engineers, the private College for Civil Engineers in Putney was established in 1839, the UK's first Chair of Engineering was established at the University of Glasgow in 1840. Civil engineers possess an academic degree in civil engineering; the length of study is three to five years, the completed degree is designated as a bachelor
The Romans constructed aqueducts throughout their Republic and Empire, to bring water from outside sources into cities and towns. Aqueduct water supplied public baths, latrines and private households. Aqueducts moved water through gravity alone, along a slight overall downward gradient within conduits of stone, brick, or concrete. Most conduits were followed the contours of the terrain. Where valleys or lowlands intervened, the conduit was carried on bridgework, or its contents fed into high-pressure lead, ceramic, or stone pipes and siphoned across. Most aqueduct systems included sedimentation tanks. Sluices and castella aquae regulated the supply to individual destinations. In cities and towns, the run-off water from aqueducts scoured sewers. Rome's first aqueduct was built in 312 BC, supplied a water fountain at the city's cattle market. By the 3rd century AD, the city had eleven aqueducts, sustaining a population of over a million in a water-extravagant economy. Cities and towns throughout the Roman Empire emulated this model, funded aqueducts as objects of public interest and civic pride, "an expensive yet necessary luxury to which all could, did, aspire".
Most Roman aqueducts proved durable. Methods of aqueduct surveying and construction are noted by Vitruvius in his work De Architectura; the general Frontinus gives more detail in his official report on the problems and abuses of Imperial Rome's public water supply. Notable examples of aqueduct architecture include the supporting piers of the Aqueduct of Segovia, the aqueduct-fed cisterns of Constantinople. Before the development of aqueduct technology, like most of their contemporaries in the ancient world, relied on local water sources such as springs and streams, supplemented by groundwater from or publicly owned wells, by seasonal rain-water drained from rooftops into storage jars and cisterns; the reliance of ancient communities upon such water resources restricted their potential growth. Rome's aqueducts were not Roman inventions – their engineers would have been familiar with the water-management technologies of Rome's Etruscan and Greek allies – but they proved conspicuously successful.
By the early Imperial era, the city's aqueducts supported a population of over a million, an extravagant water supply for public amenities had become a fundamental part of Roman life. The run-off of aqueduct water scoured the sewers of towns. Water from aqueducts was used to supply villas, ornamental urban and suburban gardens, market gardens and agricultural estates, the latter being the core of Rome's economy and wealth. Rome had several springs within its perimeter walls but its groundwater was notoriously unpalatable; the city's demand for water had long exceeded its local supplies by 312 BC, when the city's first aqueduct, the Aqua Appia, was commissioned by the censor Appius Claudius Caecus. The Aqua Appia was one of two major public projects of the time. Both projects had significant strategic value, as the Third Samnite War had been under way for some thirty years by that point; the road allowed rapid troop movements. It was fed by a spring 16.4 km from Rome, dropped 10 metres over its length to discharge 75,500 cubic metres of water each day into a fountain at Rome's cattle market, the Forum Boarium, one of the city's lowest-lying public spaces.
A second aqueduct, the Aqua Anio Vetus, was commissioned some forty years funded by treasures seized from Pyrrhus of Epirus. Its flow was more than twice that of the Aqua Appia, it entered the city on raised arches, supplying water to higher elevations of the city. By 145 BC, the city had again outgrown its combined supplies. An official commission found the aqueduct conduits decayed, their water depleted by leakage and illegal tapping; the praetor Quintus Marcius Rex restored them, introduced a third, "more wholesome" supply, the Aqua Marcia, Rome's longest aqueduct and high enough to supply the Capitoline Hill. The works cost 180,000,000 sesterces, took two years to complete; as demand grew still further, more aqueducts were built, including the Aqua Tepula in 127 BC and the Aqua Julia in 33 BC. Aqueduct-building programmes reached a peak in the Imperial Era. Augustus' reign saw the building of the Aqua Virgo, the short Aqua Alsietina that supplied Trastevere with large quantities of non-potable water for its gardens and to create an artificial lake for staged sea-fights to entertain the populace.
Another short Augustan aqueduct supplemented the Aqua Marcia with water of "excellent quality". The emperor Caligula began two aqueducts completed by his successor Claudius. Most of Rome's aqueducts drew on various springs in the valley and highlands of the Anio, the modern river Aniene, east of the Tiber. A complex s