Timelines in the history of Atmospheric Water Harvesting, Water Purification and Air Purification
Sushant Shetty, Saloni Bedi, Fatema Hirani and Subodh Deolekar
REDX Weschool lab, Welingkar Institute of Management Development and Research, Mumbai 400019, India.
With an extreme need for water in majority parts of the world due to the
increasing population, it has become imperative to find alternative sustainable methods to draw out and conserve this water. Our major sources of renewable freshwater have been extremely contaminated due to human activities and have thereby become dangerous to consume due to the countless pollutants present in it. Air pollution over the past century has drastically increased due to the increasing industrial activities post the first industrial revolution back in the 18th century and the recent rise in globalization. Studies suggest that an enormous amount of moisture is locked up in the atmosphere as humidity which could become a major source of freshwater, beneficial for various essential purposes, if the right methodologies are put in place. This study briefly highlights atmospheric water harvesting methods and purification techniques for both, water and air along with a detailed timeline of various advancements in the above-mentioned domains, right from the oldest known records to the most recent and popular ones. With this, we also take a closer look at the diverse futuristic technologies currently being developed or formulated for atmospheric water harvesting and purification techniques for water and air. This study also unfolded a need for the development of an atmospheric water harvester, which purifies water and air in an indoor setting for personal use and could help combat the scarcity of clean drinking water and pure air.
Keywords: atmospheric water generation, water purification, air purification,
Water is one of the most essential resources required to sustain human life and all the activities associated with our daily lives. Human water consumption data reveals that freshwater consumption for human activities has gone up from roughly 600 billion m3 in 1900 to roughly 4 trillion m3 in 2014 (Ritchie 2017). An inhabitant of a developed or developing country consumes around 100–400 litres of water daily. This figure drastically drops down to 20–30 litres per day when looking at poorer economies. Furthermore, annual water consumption has increased fourfold over the last 50 years and statistics indicate that this trend is unlikely to decrease (Beysens and Milimouk 2000). This increasing trend compounded with the exponential growth in human population has given rise to extreme water scarcity in several parts of the world. According to a recent study, around 67% of the global population (approximately 4 billion people) live with extreme water scarcity for at least one month every year with more than 50% of them living in highly populous countries like India and China. The study also states that enough freshwater is available to meet the growing demands but because spatial and temporal variations of water demand and availability are highly variable, many parts of the world face water scarcity during specific times of the year (Mekonnen and Hoekstra 2016). Data reveals that the Earth is 71% water (USGS 2019). Fig. 1 describes the percentage of water available from different sources on earth (USGS 2019). To simplify this image and better explain the data, if the earth has 10,000 litres of water, only 0.6417 litres of freshwater is available to humans from their major sources i.e., rivers and lakes. These statistics give us an idea about how little the amount of water the entire human race has to use. A promising source which can complement the existing sources of water, but which has not been fully utilised yet is atmospheric humidity. An estimated 12,900 km3 of freshwater exists in the atmosphere as water vapour (98%) and condensed clouds (2%) (Beysens and Milimouk 2000). Humidity in the air exists as fog and dew. While fog is occasional and requires specific conditions to form, humidity is always present in the air (quantity differs as per location and temperature) and can be extracted by manipulating the device parameters. From Fig 1. we can see that atmospheric humidity makes up 3% of the total freshwater reserves (0.0009% of total water) compared to 0.49% from rivers (0.000147% of total water). These numbers suggest that if better and efficient techniques are developed for atmospheric water harvesters and are used in conjunction with the existing sources of freshwater, it can really benefit domestic and agricultural uses on a large scale.
Human water use can be broadly classified into 3 categories: industrial, agricultural and domestic purposes which consumptively makes up for more than one-third of the freshwater use (Schwarzenbach et al. 2010). These activities lead to water contamination which eventually finds its way into rivers, lakes and groundwater, our major sources of renewable freshwater. And hence water collected from these sources might not always be safe for consumption or direct use. Countless chemicals and microorganisms in varying concentrations and numbers are present in water bodies. According to the American Chemical Society’s CAS Registry, around 23 million organic and inorganic substances present in water have been indexed as of March 2004 (Daughton 2004) and are a result of anthropogenic and natural activities. The most important water pollutants causing degradation of water sources around the world are pathogens, nutrients (majorly from agricultural runoff), heavy metals, oxygen consuming materials, pesticides and suspended sediments (UNESCO WWAP 2009). Water which is contaminated with pathogens can transmit diseases such as diarrhoea, cholera, dysentery, typhoid, and polio. Some 829,000 people are estimated to die each year from diarrhoea as a result of unsafe drinking-water, sanitation, and hand hygiene (WHO 2019). According to a Global Burden of Disease study around 1.2 million people died prematurely in 2017 as a result of unsafe water (Ritchie 2019) and over 220 million people required preventative treatment for Schistosomiasis — an acute and chronic disease caused by parasitic worms contracted through exposure to infected water (WHO 2019). To tackle the water crisis, world bodies and governments all across the globe are proactively formulating policies and setting up milestones to be followed by the industrial and agricultural sectors. In 2015, the United Nations announced 17 Sustainable Development Goals to help countries end all forms of poverty, fight inequalities and tackle climate change, while ensuring inclusiveness for everyone. One of the goals among them is to improve and provide clean water and sanitation facilities to every stratum of the society (UN SDG 2015). One of the key points from the Joint Monitoring Programme of the WHO and UNICEF which started back in 1990 states that in 2015, 91% of the global population has access to an improved source of drinking water, up from 76% in 1990. Although, according to this statistic, 6.6 billion people get water from improved sources, 663 million people still lack access to safe water for drinking and domestic purposes (WHO/UNICEF JMP 2015) thus making the UN SDGs’ an important checkpoint for countries to evaluate their progress on providing clean water to their citizens.
The composition of the air we breathe is roughly 78% Nitrogen, 21% Oxygen, 0.92% Argon, 0.03% Carbon Dioxide and trace elements like water vapour, methane, nitrous oxide etc. accounting for the rest. However, due to air pollution rising in the past few decades the percentage of harmful air pollutants have gone up compared to the pre-industrial era causing serious environmental damage and degrading human health as well (Fenger 2009). Human beings breathe in oxygen while giving out carbon dioxide, but the speed of inhaling varies from person to person. Kids generally tend to breathe faster than adults thus taking in more air than adults. Due to this, the effects of harmful air pollutants tend to be more severe in children than in adults (WHO 2018). A study on Air pollution and child health states that pregnant women who are exposed to polluted air, are more likely to give birth prematurely with babies having significantly low birth-weight, abnormal birth length and head circumference among other complications (Buka et al. 2006). Polluted air can also impact neurodevelopment, cognitive ability and also trigger asthma attacks (WHO 2018). The WHO’s website on Air Pollution states that, “From smog hanging over cities to smoke inside the home, air pollution poses a major threat to health and climate. The combined effects of ambient (outdoor) and household air pollution cause about seven million premature deaths every year, largely as a result of increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer and acute respiratory infections. More than 80% of people living in urban areas that monitor air pollution are exposed to air quality levels that exceed WHO guideline limits, with low and middle-income countries suffering from the highest exposures, both indoors and outdoors” (WHO 2019). Air pollution can be broadly classified into two categories: indoor and outdoor pollution. Major causes of outdoor pollution include smoke generated from thermal power plants, industrial smoke, forest fires, waste burning etc. Indoor air pollution can majorly be attributed to kitchens burning firewood in rural areas and dust and pollen in urban areas. Although deaths due to indoor pollution are rare and indirect, they can cause serious lifelong respiratory diseases and generally affect women and kids. Among the 7 million deaths that happen due to air pollution estimates suggest 4.2 million deaths due to exposure to outdoor pollution and 3.8 million deaths every year as a result of household exposure to smoke from dirty cookstoves and fuels (WHO 2019). Until World War II and for some time after it the most important urban pollutant was sulphur dioxide along with soot from the use of fossil fuels. This problem was partly solved using cleaner fuels, higher stacks and flue gas cleaning in cities. However, in recent times growing traffic due to affordable cars has given rise to pollution from nitrogen oxides and VOCs along with particulate matter from car exhausts and industries (Fenger 2009). However, due to increasing deaths because of air pollution related diseases in recent times and the pressure that it puts on the healthcare expenditure, countries all over the world have started taking measures to curb air pollution. Strict industrial emission laws, ban on use of cars on weekends, odd even car schemes, free public transport, use of CNG instead of petrol and diesel as fuel are some of the measures taken by governments to keep air pollution under check (Vidal 2016).
In the following section, popular techniques used for atmospheric water harvesting, water purification and air purification are explained in brief. Along with this a timeline of the events is also presented to understand humankind’s journey in tackling the major issues of water and air pollution along with finding sustainable sources of water for the ever-growing populace. Finally, recent advancements and future research being conducted in these domains is discussed to understand where these technologies are headed.
2.1 Atmospheric Water Harvesting
The simplest way of classifying atmospheric water harvesters is on the basis of what they collect and hence can be broadly classified into two groups: Fog collectors and Dew collectors.
Fog generally forms when water vapour in high humidity areas condenses into miniature water droplets on microscopic particles like dust and hangs in the air. Fog is generally referred to as clouds on the ground and they affect visibility as they can be very dense. Fog collectors are fine mesh structures made up of steel, nylon or polypropylene clothing erected against the wind through which fog passes. Water droplets carried by the wind are trapped on the mesh structure which then trickles down due to gravity and is collected in storage tanks or supplied via pipes. Typical conditions required for successful fog collection are liquid water content around 0.1–0.5 g/m3, 40% fog immersion time and 50% collection efficiency with wind speeds of 3 m/s (Tu et al. 2018).
Fog collectors are of 2 types: Standard Fog Collectors (SFC) that are used in experimental setups to study and evaluate the amount of water that can be collected and Large Fog Collectors (LFC), used for actual harvesting. SFC’s are typically 1 square meter in size and raised at a height of 2 m from the ground. LFC’s are bigger than SFC’s and generally vary in size and produce up to 150 to 750 L of water depending on the site (Klemm et al. 2012). The two key factors in determining the efficiency of fog collectors are Mesh Topology and Surface Wettability. Mesh topology is based on the fibre radius R and half spacing of the fibres D which are used in the calculation of Stokes coefficient, a number which defines the collection efficiency of a fog collector device (Jarimi et al. 2020). To increase the surface wettability, researchers have tried coated meshes which have in experiments yielded results which are higher than standard meshes (Park et al. 2013).
Dew forms when water vapour condenses on cold surfaces when the temperature falls below the dew point. The dew point is the temperature at which air cannot hold excessive water vapour and it condenses into water droplets and it varies widely, according to the location, weather and time of the day. Dew Collectors rely on either material like desiccants which have hygroscopic properties or cooling surfaces to temperature below the dew point to capture dew. Surfaces can be cooled either using passive/radiative measures or using active techniques. Radiative cooling Dew collectors are inspired from dew formation on plants in the morning and use materials like low density polyethylene foil which have hydrophilic properties. Such condensers work mostly during the night since daytime radiation results in surface temperature higher than the dew point temperature (Tu et al. 2018). Another passive atmospheric water harvesting system is the Solar Regenerated Desiccant method. This method is also known as the sorption-regeneration-condensation method. Desiccants are hygroscopic meaning they can absorb moisture in the air through adsorption and absorption. The first stage in this method is 1. the absorption process at night where the desiccant will absorb moisture from the humid air followed by 2. desorption of desiccant during the day using heat from solar radiation which regenerates the desiccant by extracting the water vapour and finally 3. the evaporated water vapour is condensed into water droplets (Jarimi et al. 2020). The major advantage of the desiccant system over radiative cooling method is that they can capture more moisture due to their hygroscopic nature, thereby extracting more water from the humid air. Multiple designs using the solar regenerated desiccant technique have been published and patented. Some of them include the Glass pyramid collector, Trapezoidal prism, Solar glass desiccant box type system, MOF porous metal-organic framework etc (Kabeel 2017; William et al. 2015; Park et al. 2013; Kim et al. 2017). As condensation technology advanced after the commercialization of refrigerators around the 1980s, active cooling dew collector technology developed rapidly. These work on the principle of dehumidifiers, wherein moisture from the air is extracted by condensation onto a surface which is cooled using either compressors or thermoelectric cooling (TEC) devices also known as peltier modules. Since compressor-based systems can be large and bulky, and since TEC devices can be as small as 40 x 40 mm, TEC based methods are being actively investigated to develop compact water harvesting devices. The thermoelectric effect is a phenomenon in which applied electric voltage is converted into a temperature difference at the junction of two dissimilar semiconductors and vice versa. So, when voltage is applied across a TEC device, one side becomes hot while the other becomes cold. In TEC water harvesters, a heat sink along with fans is used on the hot side to help in heat rejection by increasing the convection heat transfer. An internal sink on the cool side is used to increase the condensation surface area. When moist air flows over the cool sink, heat is absorbed at the cool side by the peltier module. Due to this, the temperature of the moist air starts falling below the dew point and condensation begins on the cool sink (Joshi et al. 2017). One of the major disadvantages of peltier based devices is the poor efficiency in removing heat resulting in high energy consumption. To tackle this, solar photovoltaic powered thermoelectric condensers are being tested. Such systems find applications in providing water to young saplings after transplantation (Munoz-Garcıa et al. 2013) and possibly for individual person use (Joshi et al. 2017).
2.1.1 Timeline of Atmospheric Water Harvester Development
Humidity or water vapour is present in varying quantities in the air around us. In principle, the amount of water vapour that the air can hold increases as temperature rises. Hence, the absolute humidity increases from the poles towards the equator. Data shows that the absolute humidity at the poles is 2 to 3 g/m3, and at the equator the value reaches 19 to 20 g/m3 (Hellstrom 1969). This humidity then condenses onto surfaces when the temperature of the surface drops below the dew point temperature. Our early ancestors observed this phenomenon and then built simple structures which were quite successful in converting humidity into liquid water. However, recent developments in this field, in terms of technology as well as materials, have increased the water output compared to what our ancestors could do with the limited knowledge that they had.
6th Century B.C.
Air Wells constructed in the city of Theodosia (modern day Feodosia), on the Crimean Peninsula
As per a popular myth, in the city of Theodosia in the Crimean Peninsula, people constructed aerial wells above the ground surface to extract water vapour from the atmosphere (Jumikis 1965).
2000 years ago
Dew mounds constructed in Negeb Desert area
Two thousand years ago, in the Negeb desert area, where the average rainfall per year was only about 3 inches, dew mounds were constructed from limestone and sandstone pebbles to condense morning dew in the desert which would then trickle down to the seeds planted below it (Jumikis 1965).
Between 500 A.D. — 1500 A.D.
Artificial dew ponds constructed in England
Other records of atmospheric water harvesters are from the middle-ages. During these times artificial dew ponds were used in England. Bowl shaped ponds were dug in the ground and covered with a layer of dry straw and clay which was then covered with a layer of stone pebbles. People used to draw water from these dew ponds in the evening, emptying them. However, by morning they were full of water again due to dew accumulating in them (Beysens and Milimouk 2000).
Russian engineer, Friedrich Zibold constructs a water condenser at Mount Tepe-oba
The old and popular myth about aerial wells in Theodosia can largely be attributed to a Russian engineer named Friedrich Zibold who discovered large conical piles of stones approximately 600 m3 in volume in the forests of Feodosia (Feodosia was called Theodosia during the Greek times) in 1900 and mistakenly identified them as aerial wells from that period (Beysens and Milimouk 2000). He did some excavations and found the remains of ancient pipes and water channels near a few of the mounds. This led him to the conclusion that they were built by the Early Greeks and used as dew condensers. Zibold even theoretically calculated that each mound can produce up to 55400 litres of water every day (Jumikis 1965). However, upon further investigation of 80 such mounds it was discovered that they were tombs (also called tumuli) either of ancient Greeks or Scythes and dated back to 4th — 3rd century B.C. and most of them had no signs of pipes or a surrounding water supply system. The water supply tubes were probably laid out during the middle-ages when Fedosia became a Genoa city called Caffa and hence weren’t a part of the water supply system of Theodosia as mentioned by Freidrich Zibold. However, it is also possible that the inhabitants of Caffa replaced the tubes of the Early Greeks (Nikolayev et al 1996). Although the origins and purpose of the mounds cannot be ascertained, the findings really propelled future research into the topic of water harvesting from the air. Inspired by his discovery and not being able to operate the mounds, Zibold built a model condenser similar to the ancient ones. It was constructed in the shape of a bowl, 1.15m deep and 20m wide at an altitude of 288m from the sea level at the top of mount Tepe-Oba (Beysens and Milimouk 2000). The construction of the bowl began in 1907 and consisted of rounded and polished pebbles from the sea shore instead of the calciferous crushed stones found in the original structure and had a volume of 1117 m3. The structure was operational in 1912 (Beysens and Milimouk 2000) and during hot summers, produced around 335 litres of water but produced negligible water during the more humid winters in the region. However, due to design flaws in the construction, the efficiency of the aerial well wasn’t as high as expected and consequently the committee investigating the project deemed it a failure. The structure slowly started to disintegrate as the concrete base started to crack due to heat from the sun and later was out of operation due to structural damages. Although the overlooking committee saw it as a failure, Leon Chaptal (head of the Agricultural Physics and Bioclimatology Station in Montpellier) stated that up until 1917, Zibold’s aerial well produced decent enough water to satisfy the needs of the caretaker’s family in charge of maintaining the well (Jumikis 1965).
Leon Chaptal builds a pyramid structure to collect dew in France
Inspired by Zibold’s work, Director of the Station of Agricultural Physics and Bioclimatology of Montpellier, L. Chaptal began construction of a pyramid in 1929 with a 9 square meter base which was made up of calcareous stones enclosed in a cube of concrete. The structure was built in such a way that the walls of the cube had a few holes to permit flow of air into the calcareous stones and also had holes in the floor with pipes to collect water into a small reservoir. The structure produced 2.5 liters of water per day during the summers but generated negligible water during the winters (Gottmann 1942).
Belgian Engineer, Achille Knapen builds a huge tower built as a dew condenser in France
Inspired by L. Chaptal, Belgian engineer Achille Knapen started construction of a dew condenser in Trans-en-Provence, France. The construction was completed in 1931 and a huge tower was built which was over nine meters high and 1 meter in diameter. However, the results of this experiment were way below what was expected from such a big structure. The best it could collect was a bucketful of water during the summers and due to dismal results the condenser gradually stopped operations (Beysens and Milimouk 2000).
The Hydraulic Laboratory of the Royal Institute of Technology, Stockholm uses an ordinary commercial air conditioning plant to generate water from atmospheric humidity
In 1953, efforts were made to collect dew water using simple, inexpensive and non-mechanical methods at the Hydraulic Laboratory of the Royal Institute of Technology, Stockholm. Since the results of these experiments were found to be promising, similar research was conducted in Egypt towards the end 1953 and in 1954. Now that non-mechanical methods were researched, the Hydraulic Laboratory in Stockholm tried using mechanical methods to extract water from the air. Due to limited funds to build a special set up for the experiment, the lab instead used an ordinary, commercial air-conditioning plant. Air differing in humidity and temperature was drawn inside the unit through a fan at the input side and air devoid of humidity came out at the other end. The unit produced around 50–170 litres of water per 24 hrs depending on air supplied at the input. Although the feasibility of the method was proven by this experiment it was understood that the extracted water was expensive as compared to non-mechanical methods due to the high-power consumption required by the air conditioning unit. And hence the method was intended only for places where water by traditional methods was very costly or required a lot of effort to extract and was not required in large quantities (Hellstrom 1969).
Fog condensation nets were experimented in Northern Chile
More recent advances in non-mechanical methods of extracting water from the atmosphere came during 1956 when the Catholic University of the North in Antofagasta, Chile experimented with nets to condense fog and collect the water accumulated in them (Beysens and Milimouk 2000).
Large Scale atmospheric humidity extraction plant which uses sea water to cool the radiative condenser is proposed
Around the 1960’s, another large-scale method to extract potable water by condensing moisture near the shores was proposed. Water from the greater depths greater than 600 m of the sea would be pumped to cool the large radiative condenser situated on the shore. The temperature of the water at such depths is generally less than 5°C and is sufficient to cool the highly humid air intercepted by the radiative condensers on the shore. The proposed system would run on renewable power generated by windmills thereby reducing the cost for electricity. Given the scale of the system, it would be able to harvest around 3,790,000 litres of water on a daily basis. The only cons of such a system would be the high initial investments and the huge power requirements (Gerard and Worzel 1967).
Previous experiments with fog nets in Chile were now conducted on a large scale in different parts of the world
Fog net experiments in Chile were conducted on a larger scale by Robert Schemenauer and his colleagues in 1987 to provide water for a small fishing village. The nets had an area of roughly 48 m2 made up of plastic mesh. Even during the dry seasons, the structures would generate around 11000 litres of water every day, or roughly 36 litres for the 300 inhabitants of the small fishing village. Owing to the success of these experiments in Chile, a lot of countries set up fog nets in areas where water was scarce and fog in abundance. For example, in the 1990’s fog nets were set up in many places by a major European programme in the Arequipa region of southern Peru and small scale, fog-collecting nets were erected in the Canary Islands and Namibia towards the end of the 20th century (Mekonnen and Hoekstra 2016; Fessehaye et al. 2014).
Dew condensation using polythene foil is used to study the effects of radiative cooling in Sweden
In 1993, researchers at the Chalmers University of Technology and Uppsala University, Sweden tried dew condensation using polythene foil as the base material and pigments consisting of a silica core and a titanium dioxide coating to study the effects of radiative cooling to collect dew. The pigments have a high thermal emittance meaning it has a high thermal power loss leading to a low temperature at the surface. The total area of the foil was 1.44 m2 with a 5 cm styrofoam insulation, tilted at a horizontal angle of 20 degrees. The maximum water collected by the system during a single night was 0.12 liters/m2. Although the amount of dew water condensed is very little, the required materials are inexpensive and can be produced in large quantities by standard methods. One of the possible applications for such a system includes small-scale irrigation in arid regions (Nilsson et al. 1994).
A new method using the absorption-desorption process with desiccants is proposed to extract atmospheric humidity
Towards the end of the 20th century a new method using desiccants was proposed for condensing atmospheric humidity. In 1996, P. Gandhisan and H.I. Abualhamayel used an analytical procedure to calculate the mass of water absorbed by a liquid desiccant from the atmosphere as a function of meteorological quantities (Gandhidasan and Abualhamayel 2016). In the coming years they built a wet desiccant unit with CaCl2 as the desiccant. The unit operated in such a way that it would absorb moisture during the night followed by desorption during the day time. The unit produced upto 1.92 kg of water per m2 of the unit (Abualhamayel and Gandhidasan 1997).
2011 A setup using TEC (Thermoelectric Cooling) devices is tested to generate water from atmosphere using the dehumidification process
TEC devices are compact devices which can fit in the palm of a human hand thereby making the final dehumidification devices more compact. The initial investigations concluded that the cost of generating 1000 L of water using TEC devices would be approximately 82$, which was calculated using a modest energy charge. Around 95% of this is the energy cost which is due to the poor efficiency of TEC devices in moving heat from the cold to the hot side (Milani et al. 2011).
2.2 Water Purification
Water purification refers to the process of removing unwanted chemicals (e.g., arsenic, lead, cadmium, mercury), disease causing microorganisms, suspended solids to produce water fit for humans typically for industrial, domestic and agricultural purposes. Water purification can be simply classified into Simple and Complex/Technology based methods. Simple techniques like boiling, sedimentation and filtration using cloth, sand and gravel have been used by our ancestors for a long time.
WHO states that boiling water for a few minutes is sufficient to inactivate pathogenic bacteria, viruses and protozoa thereby eliminating the risks of water borne diseases. In case of turbid water, it is recommended to let the water stand still for a few minutes and allow the particles to settle at the bottom(sedimentation) and then filter it using a cloth or a combination of sand, gravel and activated charcoal as a multilayer filter. However, apart from micro-organisms and turbidity, water from lakes or coming out of a tap may contain multiple unwanted contaminants and dissolved solids which cannot be removed by the simple techniques mentioned above. This is where techniques like Distillation, Reverse Osmosis, UV light disinfection, Advanced Oxidation Process, and Chlorination become necessary.
Distillation involves boiling water at 100°C till all of the water is converted into steam. The steam is then condensed in a separate container. The principle involved here is that not all contaminants present in water have a boiling point of 100°C and hence remain in the boiling solution. Although expensive due to the amount of fuel required, distillation provides 99.9% pure water which can be used where ultra-pure water is a necessity. Osmosis is a process in which solvent tends to pass through a semipermeable membrane from a less concentrated solution to the more concentrated section thereby balancing the concentrations on both sides. Reverse Osmosis on the other hand is the complete opposite of this process wherein mechanical pressure greater than the osmotic pressure is applied to a concentrated solution (basically containing impurities) to force pure water through a semipermeable membrane. Most consumer water purifier products are based on this technique due to the simple mechanism and low maintenance. However, due to the unselective nature of the semipermeable membrane, useful minerals added to water by regulatory bodies can also be flushed out. UV-C light has the potential to kill or inactivate microorganisms at the DNA level. The level of inactivity or death depends on the concentration and the time for which the light is used. Similar to UV light treatment, chlorination is also effective in killing micro-organisms as it is highly toxic. The doses are adjusted by the water treatment plants making it safe for human consumption. However, since UV light treatment and chlorination is only effective in killing microorganisms, it has to be used in conjunction with a separate technique to remove other water contaminants. In general, waste water produced by industries and from domestic use has to be treated before releasing it into water bodies. This industrial waste water has very high concentrations of contaminants and are difficult to remove by any of the above-mentioned techniques. Advanced Oxidation Process is a technique which shows promise in wastewater treatment. The process refers to a set of reactions wherein organic as well as inorganic materials to some extent are removed through the process of oxidation. Chemicals like Ozone and Hydrogen Peroxide are used which generate highly reactive hydroxyl radicals, combined with the pollutants which are then removed via mineralization. Although effective in wastewater treatment, AOP is expensive due to the scale at which the treatment takes place and the continuous use of expensive chemicals.
2.2.1 Timeline of Water purification techniques
Ideally, water that is delivered to people around the globe by their respective governments should be clear, colourless and more importantly safe to drink. However, it wasn’t until the early 1900s that standards for water quality came into existence. Prior to this, people had observed that consuming water from certain sources led to various disease outbreaks while other sources did not and they gradually realized that their basic senses alone were not sufficient to judge water quality (USEPA 1999). Looking back in time, civilizations settled around water sources when they finally began farming for food. While it was apparent to our ancestors that it was important to have ample amounts of potable water, an understanding of the quality of drinking water was not well known or documented. Earlier, people used to only treat water to improve its aesthetic qualities, typically colour, smell, taste and clear turbidity if any. The earliest records of water purification or treatment are found in the Sanskrit writings and Egyptian hieroglyphics.
Sanskrit writings mention simple techniques to purify water
Sanskrit writings as old as 4000 BC, mention methods to purify foul water by boiling it in copper vessels and then straining to remove sediments. Other methods include exposure to sunlight, filtering through charcoal and cooling in earthen vessels (USEPA 2000).
Egyptian writings mention the use of alum for water treatment
To clear water, ancient Egyptians used alum as an effective means of removing suspended particles as early as 1500 B.C. (USEPA 2000).
400 B.C. — 300 B.C.
Ancient scholars propose methods to purify water
Around 400 B.C., Hippocrates developed a water filter which used a big cloth to remove large particles and clear water turbidity up to a certain extent. Later, this filter came to be known as the ‘Hippocratic sleeve’ (Sanghavi and Balaji 2013). Around the same time, the Greek scientist Aristotle, described a desalination method in which non potable water was converted into potable liquid using evaporation (Zotalis et al. 2014).
The first proper report on water quality is submitted in the Roman empire
The first known documentation or engineering report on water supply and its treatment was done by Sextus Julius Frontinus, a water commissioner of Rome in AD 98 (Verma et al. 2015).
8th Century A.D.
An Arab writer produces the first treatise on distillation
In 8th century AD, an Arab writer, produced a treatise on distillation which included a methodical discussion of the facts and principle involved and the conclusions derived from it (The Editors of Encyclopædia Britannica 2019).
Multiple innovations in filtration techniques of water are proposed by scholars
Sir Francis Bacon, a great scientist and philosopher from England, conducted a few experiments pertaining to water treatment which included topics like percolation, filtration, boiling, distillation, aeration, infiltration and clarification. From his experiments he concluded that “clarifying water tends to improve health and increases the pleasure of the eye”. Although he contributed a lot to the scientific literature, his hypothesis that digging a pit on the seashore would produce fresh water from the sea by removing salt, proved to be incorrect. This hypothesis however, paved the way for future studies on better filtration techniques (Symons 2006). Towards the later part of the 17th century, two unrelated events led to more future work on studies of water treatments. A Dutch inventor, Anton van Leeuwenhoek’s experiments with optics, lead to the invention of the microscope. It was with this invention of his that he discovered the existence of tiny animals in water, which he termed as the animalcules. However, this discovery of his was deemed as unimportant and it wasn’t until the 19th century that scientists established the connection between animalcules, water and health. About the same time an Italian physician named Luc Antonio Porzio designed the first multiple filter. He gave a detailed description of the filter in his book on mass sanitation in army camps (1685). The process used simple sedimentation and straining through gravel, followed by filtration through sand (Symons 2006).
French Inventor, Joseph Amy receives a patent for a water filter which uses sponges instead of sand
The first patent on a water filter came in the year 1745. Joseph Amy, a French inventor, developed a filter which used sponges instead of sand and lead or earthen containers instead of copper containers. Although Joseph Amy tried to eliminate copper poisoning by replacing copper with lead, he was unaware of the ill effects of lead on human health. Despite his efforts to promote the invention, the filter wasn’t widely accepted by the masses and disappeared from the markets by 1760 (Symons 2006).
Water treated using John Gibb’s slow sand filter is supplied to the town’s residents, becoming the first instance of a scalable water treatment method
The first instance of modern water treatment on a semi-large scale dates back to 1804, when John Gibb built a slow sand filter for his bleachery in Paisley, Scotland and sold the surplus treated water to the masses for half a penny per gallon. The design closely resembled the modern Dorrco-Aldrich-Perifilter. It was made up of 3 concentric circles, the first one being a settling basin, the next circle consisted of a gravel filter followed by a sand filter and then finally the clear water basin at the centre (Symons 2006).
World’s first municipal water treatment and distribution system designed
In 1807, English engineer Thomas Telford designed the world’s first municipal water treatment and distribution system. However, the filters failed within a year of installation due to the high turbidity of the Clyde River. In the late 1820s, Robert Thom, a cotton mill operator, solved this problem by designing a self-cleaning filter with a false bottom and a reverse-flow action for washing (Symons 2006).
A public water supply system is setup in London
In 1829, following public outcry in London against foul drinking water, James Simpson, took cues from Telford’s and Thom’s design to set up a public water supply system. The system was constructed as an installation to treat the water supplied by the Chelsea Water company in London. Estimates suggest that this system would filter nearly 90,000 gallons every 24 hours (Symons 2006).
The Metropolis Water Act passed in London
By 1852, due its advantages, the practice of treating water with a slow sand filter had become so largely widespread, that the Metropolis Water Act was passed stating that all water derived from the River Thames requires to be filtered before being supplied to the public (Huisman and Wood 1974).
English epidemiologist, John Snow’s experiments conclude that the outbreak of Cholera in London was caused due to pathogens present in water
Although Anton van Leeuwenhoek had observed ‘animalcules’ in water under the microscope, as early as the 17th century, no significant research was done to understand the sources and effects of water contaminants which weren’t visible to the naked eye until the 19th century. However, in 1855, John Snow, an epidemiologist concluded that the outbreak of cholera in the vicinity of a public well, which was contaminated by sewage in London was waterborne and was caused due to ‘materies morbi’ or pathogens present in water which infect people when ingested. Following this finding, frequent examinations of water bodies including their chemical analysis were conducted in London from 1858 (USEPA 2000; Huisman and Wood 1974). Later, this fact was reinforced when in the late 1880s, Louis Pasteur demonstrated the “germ theory” of disease, which explained how microbes could transmit diseases through a media like water (USEPA 2000). From the year 1885, these tests were extended to include bacteriological examinations of water supplies as well as following the discoveries of Pasteur and other scientists like Koch and Escherich (Huisman and Wood 1974).
British officer, Vincent Nesfield proposes the possible use of chlorine to treat water
A British officer, Vincent B. Nesfield working in the Indian Medical Service, had put forward a method to purify water in his paper — “A Chemical method of sterilizing water without affecting its potability” in 1903. In this paper, Vincent Nesfield proposes using liquefied chlorine or chlorine tablets to treat water which might contain diseases causing bacteria. In separate experiments that were conducted, he found that chlorine when added in certain quantities would sterilize B. typhosus, B. coli, and Shiga’s dysentery bacillus broth in the water (Nesfield 1902).
Chlorine is used as primary disinfectant to treat water in New Jersey
Once it was understood that chlorine can be used to get rid of disease-causing microorganisms in water, a lot of water treatment systems around the world started using chlorination as the preferred method of disinfection along with sedimentation and filtration. One such recorded instance of this is the drinking water facility in New Jersey, that used chlorine as the primary disinfectant for the first time in 1908 (USEPA 2000).
UV disinfection is used to treat water for the first time in Marseille, France
The theory behind using specific light rays was provided by the Danish physician where he used different kinds of chemical light to treat diseases like smallpox and lupus vulgaris. His experiments and treatments finally concluded that certain wavelengths of light when used at a particular concentration have bactericidal effects (Gøtzsche 2011). Following these findings, UV light was then used as a means of water disinfection at a water treatment plant in Marseille, France (Cotton et al. 2005).
Public Health Service act passed by the United States government
Governmental regulation of drinking water quality was first initiated in 1914, when the Public Health Service act was passed in the United States, authorizing regular surveys and studies of water pollution. However, the standards were applied only to water systems carrying water to interstate carriers like ships and trains and only checked for contaminants capable of causing contagious diseases (USEPA 2000).
American Civil Engineer, Abel Wolman and chemist, Linn Enslow establish a controlled Chlorination method for water treatment
Although chlorination was being used as a way of treating municipal water supplies, it wasn’t until 1919 that a controlled method was established. In 1919, Abel Wolman, a Civil Engineer along with a chemist, Linn Enslow, published an article describing a controlled method for chlorine absorption, which later transformed water treatment and helped municipal systems deliver safe water to a larger part of the world. This was a major milestone for water treatment as it meant water safe from diseases causing microbes could now be delivered to the common public without risk of chlorine overdosing, which could be really harmful (Fee 2011). Due to the widespread use of chlorine, the typhoid death rate in the United States had dropped from 36 per 1,00,000 to 0.1 per 1,00,000 (USEPA 2000).
First large-scale desalination plant built in Aruba
Another major milestone in water purification techniques was the ability to desalt seawater. Seawater is an abundant source of water, however, due to dissolved salts it has a corrosive effect on everyday equipment and is unfit for drinking. The first patent for a desalination plant was granted in England in 1869, however, the first large scale plant to provide desalinated water on a commercial scale was built in 1930 in Aruba, near Venezuela (The Editors of Encyclopædia Britannica 2019).
Reverse Osmosis technique for desalination was demonstrated using a cellulose acetate membrane at UCLA (University of California, Los Angeles)
Although the mechanisms of reverse osmosis were known since 1748 thanks to Jean Antoine, a French physicist, it wasn’t until researchers at UCLA developed the membrane that RO was put to use to obtain usable water from the sea. This membrane was able to selectively reject salt and some other dissolved solids under pressure while allowing water to pass through it (Wiles and Peirtseqaele 2018).
Students at Thayer school, US successfully demonstrate a RO water purification system
Given a jar of brackish water, students Dean Spatz and Chris Miller successfully use a RO system with a semipermeable membrane to clear the water of the impurities and thus make it potable. The research eventually led to Spatz getting multiple contracts to develop low pressure RO systems (Michaelides 2011).
US scholar William Glaze presents the Adaptive Oxidation Process to remove organic pollutants from water
William Glaze along with environmental engineer Joon-Wun Kang presents AOP as a technique to remove organic impurities from water using a combination of ozone and hydrogen peroxide. The process relies on the production of highly reactive hydroxyl radicals which unselectively react with contaminants and are then removed via mineralization (Glaze and Kang 1989).
American scientist and inventor, Ashok Gadgil receives patent for a device that allows for UV treatment of water at homes
A small device uses UV treatment wherein water is exposed to the UV radiation from a lamp (around 20 watts) thereby killing micro\
organisms present in water (Gadgil et al 2005).
2.3 Air Purification
Air pollution at the consumer level can be treated using various techniques like use of masks, using HEPA or electrostatic air purifiers and air purifying plants inside homes.
HEPA or High Efficiency Particulate Air filters consist of a mesh structure made up of fine glass fibres (0.25 mm) and were developed and used during World War 2. Following the war, due to their high efficiency in filtering air, they saw huge demand in industries like computers, electronics, aerospace, and nuclear power industries. In the coming decades HEPA filters were commercially introduced in aircraft and hospitals to remove or stop the spread of airborne fungi, viruses, and bacteria along with pollen and dust particles. Today most air purifiers use HEPA filters along with a pre filter which removes large dust particles, hair and dirt particles. HEPA does not refer to a specific design but defines the level of efficiency with which a filter can clean air. Typical HEPA filters can clean or capture 99.97% of particulate matter less than 0.3 mm (Kte’pi 2015). On the other hand, an electrostatic precipitator uses an electric charge to remove impurities from the air. The device consists of a row of thin vertical wires followed by large vertical plates. A very high negative voltage (several thousand volts) is applied between the wire and the plate which ionizes the contaminant particles in the air passing horizontally through them. These ionized particles are then attracted towards the collection plates and are removed from the air stream. This method is different compared to wet scrubbing technique which applies energy directly to the fluid medium and is predominantly used for air purification in industries. Use of masks is another simple method which the general population can use to safeguard themselves from air pollution. The consumer market provides different kinds of masks with different levels of air purifying capacity. N95, N99, N100, P95 and R95 are some of the different mask variants available for general public use. These filters generally consist of a primary filter which removes large dust particles followed by a particle filter which protects against PM 2.5 pollutants. Expensive filters also use a carbon filter which removes gaseous pollutants and filters bacteria and viruses present in the air (Johnson 2018). People can also use certain houseplants along with an activated carbon plant filter to remove certain impurities in the air. A NASA study concluded that certain low light requiring houseplants when used with filters are effective in removing trace organic pollutants with the root zone being the most effective in removing volatile organic chemicals (Wolverton et al. 1989).
2.3.1 Timeline of Air purification techniques
The Air pollution that we see today is majorly a consequence of the Industrial Revolution that began in England in the 18th century and later spread to other parts of the world. Although it brought positives like more employment, cheaper products, mechanization of agriculture and automation along with tons of other benefits, negative effects like air pollution, water contamination and overcrowded cities have started to outweigh the benefits in the recent decades. The use of specialized equipment to protect human lungs from the thick industrial smoke began as early as the 19th century. Today people use multilayer face masks and air purifiers to shield themselves and indoor spaces from harmful air pollutants.
British inventor Charles Anthony Dean along with his brother John develops a fire-fighting helmet which is later patented
British inventors and brothers, Charles Dean and John Dean develop a copper helmet (which is supplied with air) along with a bodysuit to help firefighters breathe safely when entering smoke filled areas (Newton and Partington 1825). Although not an air purifier, this was one of the first inventions to pave the way for the development of air purification devices.
US Engineer Lewis P Haslett receives patent for a protective gas mask also referred to as the “lung protector”
Scottish chemist, John Stenhouse exhibits a charcoal-based air filter to remove odour and chemical gases
From multiple experiments John Stenhouse found out that different kinds of charcoal are capable of absorbing different amounts of gases like chlorine, hydrogen sulphide, carbonic acid gases, ammonia, sulphurous acid among others. Using these findings, he developed powdered wood charcoal masks/respirators which could benefit painters from paint vapours, gunners from gases of exploding powders and travellers passing through unhealthy air. His findings were published as articles between 1854–1858. Stenhouse also invented a charcoal ventilator device to purify air entering inside rooms. It was made up of 2 sheets of wire gauze filled with charcoal in between and was used to deodorize air entering several government offices (Royal Institution of Great Britain 1858).
Irish physicist, John Tyndall exhibits fireman’s respirator, a combination of the Stenhouse mask and other breathing equipment
John Tyndall after conducting several experiments develops a masked respirator with a mouthpiece and two valves. Through the first valve inhaled air passes through cotton wool (dry or moistened) and then through a charcoal filter for better removal of gases. The second valve acts as an outlet for carbon dioxide given out during breathing. Due to their efficiency in stopping poisonous gases while breathing in smoke filled rooms these respirators were in considerable demand in the Metropolitan Fire Brigade, London (Tyndall 1874).
Hutson Hurd patented an improved version of the respirator or inhaler
this device made use of a cup shaped mask made of soft rubber or other flexible material to fit properly over the user’s face and was designed to prevent the entry of poisonous gases along with dust and other suspended air particles into the throat (Hurd 1879).
American Chemist, Frederick Cottrell patents the electrostatic precipitator
F. Cottrell observed that fine droplets and solid particles in smoke are held in suspension due to the repulsive forces of electric charges on different surfaces. This observation along with a few other experiments led to the invention of the electrostatic precipitator. Earlier designs of the electrostatic precipitator were used in acid making and smelting industries to remove sulfuric acid droplets and lead oxide fumes (White 1957; Hosansky 2016).
The first description of HEPA (High-efficiency Particulate Air) filters is found in the Handbook of Air Cleaning, published by the U.S. Atomic Energy Commission in 1952
The book described the newly declassified filter (originally acquired from German masks) as a high efficiency filter which was then developed by the Chemical Corps for use in gas masks. A mesh made up of asbestos fibre is used for air filtration while cellulose fibres are mixed to provide mechanical strength. This filter had an efficiency of 99.9% for all particles till 0.1 mm diameter. To reduce imports, soon filters made of glass fibres as small as 0.25 mm in diameter were developed. These filters had better filtration characteristics as compared to the asbestos cellulose combination (First 1998). According to the United States Department of Energy, the current HEPA filters are any set of filters that can capture at least 99.97% of particles greater than 0.3 mm. In reality, HEPA systems were developed around the 1940s’ during the World War and used first in the Manhattan project to contain the spread of airborne radioactive contaminants (Kte’pi 2015).
Klaus and Manfred Hammes start selling the first residential air purifiers
Late 20th Century
The Cloud Chamber Wet Scrubber technology is developed by Dr. Clyde Richards
Research into charged droplet scrubbing had been going on from the 1950s upto 1980s without satisfactory results. Dr. Clyde Richards while investigating the role of charged droplets in initiating lighting, worked on optimizing the size and charge of droplets thereby increasing the efficiency of charged droplets in removing particulate matter (Tri-Mer n.d.).
3. Recent Advancements
3.1 Advancements in Atmospheric Water Harvesting technology
In recent years fog collector designs inspired from plants and animals which have better humidity capturing capabilities have been developed. Also known as biomimicry, designs have been inspired from the Namib desert beetles, Moloch horridus, a lizard species native to hot and arid regions, a spider species, Uloborus walckenaerius and plants like the cactus and Lychnis sieboldii from Japan (Jarimi et al. 2020).
When it comes to dew collectors, water vapour is not the only component present in air. At 30°C,100% RH the amount of water vapour present is 30.4 g/m3. So, to obtain 30.4 gms of water 1 m3 of air has to be cooled which is quite inefficient and expensive in terms of energy costs. To improve on such a system, researchers have developed a water vapour selective membrane as a pre-filter which thus prevents cooling of other atmospheric gases. Tests with such a membrane have shown 50% improved efficiency over systems without the membrane (Jarimi et al. 2020).
HVAC systems are used worldwide and use compressors to cool the air during summers. The temperature of this cooled air is mostly below the dew point temperature. These systems generally generate a lot of condensed water but due to no proper setup for the water, most of it ends up in the sewage systems or evaporates into the atmosphere (Tu et al. 2018). Researchers have found out that condensate water from large scale HVAC systems in a hotel can provide around 56% of the hotels daily water demand (Jarimi et al. 2020).
3.2 Advancements in Water Purification technology
Nanotechnology in general seems to have a lot of applications for advanced techniques of water purification. Use of nano filters in water filtration systems seems to be more efficient at removing toxic elements like arsenic along with sediments and bacteria. Carbon Nano Materials, Carbon Nanotubes and Graphene in particular have shown excellent results in adsorption due to their high surface area, porosity, stability and excessive functionalities and find applications in desalination and membrane technologies (Gusain et al. 2020).
3.3 Advancements in Air Purification technology
In 2018 China built an experimental smog tower over 100 meters in height in Shaanxi province. Dubbed as the world’s biggest air purifier, the smog tower built in northern China has undergone testing by researchers from the Institute of Earth Environment at the Chinese Academy of Sciences. The tower has greenhouse coverings at the base with an area about half the size of a football field, through which polluted air is sucked in. The heated air then rises through the tower where it undergoes filtration through multiple filter layers. The test results state that improvements in air quality were reported over an area of 10 sq. kms with a 15% reduction in PM2.5 levels during heavy pollution (Chen 2018).
China is also planning to build a forest city in South China near Liuzhou. Expected to house 30000 inhabitants initially, the city would have around 40000 trees and 1 million plants to absorb around 10000 tons of CO2 and 57 tons of pollutants per year and generate around 900 tons of oxygen. The city would run on solar and geothermal energy to reduce the use of CO2 emitting fossil fuels to curb more air pollution (Moore 2018).
In addition to the above mentioned three domains, another area which could be further explored is the combined application of atmospheric water harvesting, water purification and air purification. The technologies in all three domains have matured to the stage where low-cost products have been developed to cater to the individual consumer. Water purifiers and air purifiers are commonplace, however atmospheric water harvesters haven’t become mainstream due to their disadvantages such as high electricity consumption in case of dew collectors and low daily yields for fog collectors. This can be potentially solved using advanced materials and biomimicry inspired designs in the case of fog collectors and solar energy for dew collectors. This would generate more water and at a lower energy price compared to conventional techniques. Decentralized energy from solar panels combined with the assimilation of the three technologies in the form of a consumer product can have a huge impact on solving the water and air crisis in many parts of the world.
With advancements in science and technology over the centuries, we have developed sophisticated techniques and technologies to help us in better management of water and air as a resource. Today’s techniques of atmospheric water extraction and water and air treatment are far more superior and advanced as compared to the methods of our ancestors. In this paper, we have presented a brief assessment of techniques currently used in atmospheric water harvesting and treatment of polluted water and air. The techniques mentioned have been tried and tested in several countries before full-scale deployment. The timelines presented in each section gives us an idea about the progress humans have made towards protecting important resources while making it safer to use. The timelines also give us an idea about where the technologies in each domain are headed and help people decide areas which might require more focus. The penultimate section describes future research in these domains, especially atmospheric water harvesting, as the scarcity of clean water due to high levels of pollution and over-exploitation of available water resources is a huge problem in many countries and needs to be tackled at the earliest. Furthermore, this paper opens up new possible avenues for atmospheric water harvesting with integration of water and air purification which is a small step towards solving the water scarcity problem worldwide.
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